Hydrocarbons

HYDROCARBONS CHEMISTRY

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HYDROCARBONSINTRODUCTION

Compounds containing only two elements carbons & hydrogen are known as hydrocarbons. On thebasis of structure & properties, hydrocarbons are divided into two main classes viz. aliphatic & aromatic.Aliphatic hydrocarbons are further divided into different families namely alkanes, alkenes, alkynes andtheir cyclic analogs (cycloalkanes, cycloakenes and cycloalkynes etc.)

ALKANES:Alkanes are open chain (acyclic) hydrocarbons comprising the homologous series with the general

formula n 2n 2C H where ‘n’ is an integer. They have only single bonds & therefore are said to be saturated.Because of their low chemical activity, alkanes are also known as Paraffins.NOMENCLATURE:

According to IUPAC system of nomenclature open chain aliphatic compounds are named as al-kanes.

CH3 CH3 CH3CH3

CH3

CH3

CH3CH3ethanepropane 2,2-Dimethyl propane

H5C2 CH3

CH3CH3 CH3CH3

2,2,4,4-tetramethylhexane

CH3 CH3

CH3

C2H53-ethyl-2-methylpentane

CH3CH3

CH3

CH3

CH3

3,4,7-trimethylnonane

CH3CH3

CH3CH3

CH3

CH3 CH3CH3

CH3

2, 3-dimethylpentane 2,5, 6-trimethyloctane

Names of several common alkyl groups


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CH3

CH3 CH3

CH3

CH3

CH3

isopropyl isobutyl sec-butyl

CH3CH3

CH3CH3

CH3

CH3

tert butyl neopentyl

CONFORMATIONS OF ALKANESThe different arrangement of atoms in space that results from the free rotation of the groups about C –C bond axis are called conformers or conformational isomers and the phenomenon is known asconformational isomerism.

Conformations of EthaneOf the infinite number of possible arrangements of ethane two conformations represent the extremes.These are called the eclipsed conformation (I) and the staggered conformation (II). Any other arrangementwhich will be between these two extreme positions are known as Gauche or Skew form.

Sawhorse Projection

HH H

H

H H

H

HHH

H H

Eclipsed form (I) Staggered form (II)

Newman Projection

H

H HH

HHH

H HHH

H

Eclipsed form (I) Staggered form (II)dihedral angle = 0 dihedral angle= 180°

Relative Stabilities of the conformations of EthaneThe potential energy of staggered form is minimum and that of eclipsed form is maximum for ethane.The difference in energy content between the eclipsed and staggered form is 12 kJ mol–1. This smallbarrier to rotation is called Torsional Barrier. This energy is not large enough to prevent the rotation.Hence the conformers keep on changing from one form to another. The eclipsed conformation is leaststable where as the staggered conformation is most stable.



H

H HH

HH

H

H HHH

H

Eclipsed

12 kJ mol-1

Staggered

H

H HH

HH

StaggeredRotation

PE

Conformations of n-Butanen-Butane molecule can be considered as a dimethyl derivative of ethane in which one H-atom of eachcarbon atom is replaced by a methyl group as shown below:

CH3 CH

HCH

HCH3

1 2 3 4

If one of these central carbon atom (C2 or C3) is fixed and the other is rotated round the C2 – C3 bond, thefollowing important conformations are obtained.

CH3

H HH

HCH3CH3

H HHCH3

H

Staggered (Anti) (I)

CH3

H HCH3

HH

Eclipsed (II) Gauche (III)CH3

H HHH

CH3

Fully eclipsed (IV)

CH3

H HH

CH3H

Gauche (V) Eclipsed (VI)

CH3

H HCH3H

H



Relative stability of the conformation of n-ButaneAnti Gauche Eclipsed Fully eclipsed

CH3

H HH

HCH3

16 kJ mol-1

CH3

H HCH3

HH

Rotation

PE

CH3

H HH

CH3HCH3

H HCH3

HH

CH3

H HHCH3

HCH3

H HHH

CH3 CH3

H HCH3H

H

3.8 kJ mol-1

19 kJ mol-1

3.8 kJ mol-116 kJ mol-1

0 60° 120° 180° 240° 300° 360°

PREPARATION OF ALKANES:Reduction of Alkenes and AlkynesReduction of alkenes: alkenes are reduced into alkanes by hydrogen in the presence of catalyst [Pt,Pd, Ni/S. 2PtO ]

catalyst2 2 2R — CH CH — R H R — CH — CH — R

When catalyst is Ni the reaction is known as Sabatier - Sanderen’s reactionReduction of alkynes: Alkynes also reduced by hydrogen in presence of catalyst.

catalyst2 2 2R — C C — R 2H R — CH — CH — R

First alkene forms thereafter alkane.Reduction of alkyl halides:Alkyl halides undergo reduction with nascent hydrogen to form alkanes

R X 2H R H HX Alkyl halide Alkane

The yields are generally good and the hydrocarbons obtained are pure. The nascent hydrogen for reduc-tion may be obtained by using any one of the following :

(a) Zinc and dilute hydrochloric acid(b) Zinc and acetic acid; Zn and NaOH(c) Zinc-copper couple in ethanol(d) Red phosphorus and hydrogen iodide



(e) Al-Hg in ethanol

2 5Zn HCL

3 4orZn Cu / C H OHCH I 2H CH HI

2 5Zn HCL

2 5 2 6orZn Cu / C H OHC H I 2H C H HI

0Re dP

3 2 3 3 2150 CCH CH Br 2HI CH CH HBr I The purpose of use of red phosphorus is to remove iodine. alkyl halides may also be reduced catalyti-cally by hydrogen using catalyst like nickel, palladium, platinum etc.

Pd2R X H R H H X

Primary and secondary alkyl halides may be conveniently reduced with lithium aluminium hadride in adry organic solvent

Organic4 4 3Sovent4RX LiAlH 4RH LiAlX (LiX AlX )

4LiAlH is not useful for 03 alkyl halide which is converted into alkenes. In such case 4NaBH or TPHis used.

3CH3CH3CH

CCl

4LiAlH2CH3CH

3CHC

Cl 4NaBH

03 2-methylpropaneAlkyl halides 0 0 01 ,2 ,3 can also be reduced to alkanes with TPH (Triphenyl tin hydride, 3Ph SnH) .Reduction of alcohols: Alcohols can be reduced to corresponding alkane as

2P/I /R — OH R — H 2P / I /

3 2 3 2CH CH OH CH CH I Wurtz Reaction: When alkyl halide is treated with sodium in presence of dry ether, higher alkane isformed.

Nadry etherR — X X — R R — R 2NaX

R — X R — X R — R R — R R — R4CH Cannot be prepared by this method.



3° R—X do not give this reactionWhen a mixture of two different alkyl halides is used a mixture of three alkanes is obtained

Ether3 2 5 3 2 5CH Br 2Na BrC H CH C H + 2NaBr

Ether3 3 3 3CH Br 2Na BrCH CH CH 2NaBr

Ether2 5 2 5 2 5 2 5C H Br 2Na BrC H C H C H 2NaBr

The separation of the mixture into individual members is not always easy. Thus Wurtz reation is notsuitable for the synthesis of alkanes containing odd nubmer of carbon atoms but the method is useful forthe preparation of symmetrical alkanes

Mechanism : .Mechanism:

R — X Na R NaX R R R — R

Frankland’s reactionThis method is similar to Wurtz reaction. When alkyl halides is heated with zinc in inert solvent higheralkanes are formed.

2 5Zn/C H OH2R — X R — X R — R ZnX

2 5Zn/C H OH3 3 3 3 2CH — Br CH — Br CH — CH ZnBr

Decarboxylation of Salts of Carboxylic AcidsSodium salt of carboxylic acids undergo decarboxylation when heated with sodalime.

R– COONa + NaOH CaO2 3R H Na CO

CH3—COONa + NaOH CaO4 2 3CH Na CO

Mechanism

OH R CO

OR

O

OOH R H O C

O

O23CO R H

Corey-House synthesisA superior method for coupling is the corey-house synthesis which could be employed for obtainingalkanes containing odd number of carbon atoms. An alkyl halide (R–X) which may be primary, secondaryor tertiary is first converted into alkyl lithium by treating with lithium. The alkyl lithium is then reactedwith cuprous halide to get lithium dialkyl cuprate. The complex is then treated with another alkyl halide(R—X) which must be preferably primary. The reaction follows SN2 mechanism. With secondary alkylhalides the reaction leads to partly substitution forming alkane and partly elimination forming alkene,with tert alkyl halide only elimination takes place.

dry etherR — X 2Li R — Li LiX 22R — Li CuI R CuLi LiI



2R CuLi R — X R — R R — Cu LiX

Li CuI3 3Methyl bromide Methyl lithium

CH Br CH Li CH3 CuLiCH3

Lithium dimethylcopper2 5C H Br

3 2 3Pr opaneCH CH CH

CH3Cl

CH3

sec-Butyl chloride

Li CuI (CH3CH2CH-)2CuLi

CH3

CH3Br

n-Pentyl bromide

CH3

CH3

CH3

3-methyloctane

Illustration - 1 : Prepare 2-methylbutane from chloroethane and 2-chloropropane using Corey-Housesynthesis.

Solution:

3 2CH CH Cl1. Li3 3 2 3 2 32. cuICH CHCl (CH CH) CuLi CH CHCH CH

3CH 3CH 3CH2-Chloropropane Lithiumdiisopropyl cuprate

From organo metallic compounds: Grignard reagents and alkyl lithium reacts with water and othercompounds having acidic hydrogen to give hydrocarbon corresponding to alkyl group of the organometallic compounds.

R — Mg — X H — OH R — H R — Li H — OH R — H

Kolbe ElectrolysisAlkanes can be prepared by the electrolysis of a concentrated solution of sodium or potassium salt ofcarboxylic acid.

2H O electrolysis2 2R — C — O — K R — C — O — K R — R 2CO H 2KOH

O O

4CH can not be prepared by this method.



––eAnode (Oxidation)R C

OO R C

OO 2R CO

R R R — R PHYSICAL PROPERTIES

Physical Statei) All are colourless and possess no characteristic odour.ii) Lower alkanes (C1 to C4) are gases, middle one (C5 to C17) are liquids and highers are solids.Boiling Pointsi) The boiling point of alkanes increases with increase in molecular weight due to increase in van derWaals forces with increase in molecular weight i.e.Boiling point: pentane hexane heptaneii) Also the branching in alkanes gives a decrease in surface area (as the shape approaches to spherical)which results in decrease in van der Waals forces. The boiling point of isomeric alkanes show theorder: pentane isopentane neopentane.Melting PointsThe melting points of alkanes do not show a regular trend. Alkanes with even number of carbon atomshave higher melting point than their adjacent alkanes to odd number of carbon atoms.

Melting order: ( 172.0 C) ( 182.5 C)( 187.7 C)

propane ethane methaneThe abnormal trend in melting point is probably due to the fact that alkanes with odd carbon atomshave their end carbon atom on the same side of the molecule and with even carbon atom alkane, theend carbon atom on opposite side. Thus alkanes with even carbon atoms are packed closely in crystallattice to permit greater intermolecular attractions.Density: The density of alkanes increases with increase in molecular weight and becomes constant at0.76g/ml. Thus all alkanes are lighter than water.Solubility:i) Alkanes being non polar and thus insoluble in water but soluble in non polar solvents

e.g. C6H6, CCl4, ether etc.ii) The solubility of alkanes decreases with increase in molecular weight.iii) Liquid alkanes are themselves good, non polar solvents.

CHEMICAL PROPERTIES OF ALKANES:Halogenations: One of the important chemical reactions of alkanes is halogenation which is definedas the replacement of hydrogen atom of the molecule by halogen atom. A mixture of alkanes andchlorine remains unaffected in the dark but a vigorous reaction occurs when it is exposed to light or ata higher temperature.

hv or2R — H Cl R — Cl HCl

As pointed above the reactions takes place by free radical chain mechanism which proceeds in thefollowing three distinct steps.a) Chain initiations

hv orCl — Cl 2Cl [ H 58 kcal / mole] b) Chain propagation

3 3Cl H — CH H — Cl CH [ H –1 kcal / mole]



3 3CH Cl — Cl CH — Cl Cl [ H –23 kcal / mole] c) Chain termination

Cl — Cl Cl — Cl3 3CH Cl CH — Cl 3 3 3 3CH CH CH — CH

The ease of substitution at various carbon atoms is of the order tertiary > secondary > primary whichis the same as the stability of various alkyl radicals.Bromination of alkanes has close similarities to chlorination except the rate of reaction is slow.Fluorination of alkane is so violent that it results in the cleavage of C—C as well as C—H bonds. Reactivity of 2 2 2 2 2X :F Cl Br IThe potential energy curve for the halogenation (chlorination) of alkane is shown as

Potent

ial ene

rgy

Progress of reaction

R H Cl 2R Cl

R Cl Cl

actE

REACTIVITY SELECTIVITY PRINCIPLE:In more complex alkanes, the abstraction of each different kind of H atom gives a different isomericproduct. Three factors determine the relatives yields of isomeric products are

1. Probability factor: This factor is based on the number of each kind of H atom in the molecule. Forexample, in CH3CH2CH2CH3 there are six equivalent 1° H’s and four equivalent 2° H’s. The ratio ofabstracting a 1°H are thus 6 to 4, or 3 to 2.

2. Reactivity of H*: The order of reactivity of H is 3° 2° 1°3. Reactivity of X*: The more reactive Cl* is less selective and more influenced by the probability factor.

The less reactive Br* is more selective and less influenced by the probability factor, as summarized bythe Reactivity-Selectivity Principle. If the attacking species is more reactive, it will be less selective,and the yields will be closer to those expected from the probability factor.



2oCl /hv25 C CH3

CH3 CH3 Cl CH3CH3

Cln-butane 28%72%

CH3

CH3 CH3

CH3

CH3Cl CH3

CH3

CH3Cl

36%64%

2oCl /hv25 C

CH3CH3 CH3 Br CH3

CH3

Br

2 oBr , light127 C

2% 98%

CH3Br

CH3CH3

CH3

CH3BrCH3

CH3

CH3 2 oBr , light127 C

trace over 99%

The rate of abstraction of hydrogen atoms is always found to follow the sequence 3° 2° 1°. At roomtemperature, for example, the relative rate per hydrogen atom are 5.0 : 3.8: 1.0. Using these values wecan predict quite well the ratio of isomeric chlorination products from a given alkane. For example:

CH3 Cl CH3CH3

Cl2o

Cl /hv25 CCH3

CH3

o oo o

n butylchloride no. of 1 H reactivity of 1 Hsec butylchloride no. of 2 H reactivity of 2 H

6 1.0 6 28%equivalent to4 3.8 15.2 72%Inspite of these differences in reactivity, chorination rarely yields a great excess of any single isomer.The same sequence of reactivity, 3° 2° 1°, is found in bromination, but with enormously largerreactivity ratios. At 127°C, for example, the relative rates per hydrogen atom are 1600: 82:1. Here,differences in reactivity are so marked as vastly to outweight probability factors. Hence brominationgives selective product.In bromination of isobutene at 127°C,



o oo o

Isobutylbromide no. of 1 H reactivity of 1 Htert butylbromide no. of 3 H reactivity of 3 H

9 11 1600 9

1600Hence, tert-butyl bromide happens to be the exclusive product (over 99%).

How many monochlorinated products are obtained by the chlorination ofisopentane and what is the percentage of each assuming the reactivity ratio of 3°H : 2°H : 1°H= 5 : 3.8 : 1.

Solution: Isopentane on chlorination in presence of sun light gives four different monochlorinated products.hv

3 2 3 2 2 2 3CH – CH – CH – CH Cl CH Cl – CH – CH – CH 3CH 3CH (A)

3 2 3CH — CCl — CH — CH 3CH 3CH(B)

*3 3CH — CH — C HCl — CH

(C)

3CH3 2 2CH—CH—CH—CH Cl

(D)A and C are optical isomers.

Product Reactivity Factor

Probability factor

= Number of parts

Percentage (A) 1 6 = 6 27.8% (B) 5 1 = 5 23.2% (C) 3.8 2 = 7.6 35.2% (D) 1 3 = 3 13.8%

Total number of parts

= 21.6

NitrationThe displacement of an atom of hydrogen by a nitro group ( 2–NO ) is called nitration. Alkanes can benitrated with nitric acid in the gas phase generally at 400–500°C.

400–500 C2 2 2R — H HO — NO R — NO H O



2HNO3 2 3 400–500 CCH — CH — CH

3 2 2 2 3 3CH — CH — CH — NO CH — CH — CH2NO

3 2 2 3 2CH — CH — NO CH — NOSulphonationSulphonation of a hydrogen atom of the alkane by sulphuric acid group 3(–SO H) is called sulphonation.The ease of substitution is tertiary secondary > primary..

2H OCH3CH3

CH3H CH3

CH3

CH3SO3H2 4H SO

IsomerisationHeating of straight chain alkanes with anhydrous aluminum chloride (lewis acid) affords branchedchain hydrocarbons. This process is termed isomerisation.

3AlCl3 2 2 3CH — CH — CH — CH CH3

H

CH3CH3

The process of isomerisation has been of immense utility in petroleum industry for raising the octanenumber of a particular petroleum fraction.Example

3AlCl (RCl)25 C

Rearrangement of alkanes are thermodynamically controlled

AromatisationAromatisation of n-alkanes containing six or more carbon atoms into benzene and its homologuestakes place at high temperature (600° C) in presence Cr2O3—Al2O3 as a catalyst.



2 3 2 3oCr O Al O

600 C 23H

2 3 2 3oCr O Al O

600 C 23H

n-hexane cyclohexane benzene

n-octane ethyl cyclohexane ethyl benzene

OxidationAll alkanes readily burn in excess of air or oxygen to form carbon dioxide and water.

CnH2n+2 + 2 2 2(3n 1) (2n 2)O (g) nCO (g) H O(l)2 2

On the other hand, controlled oxidation under various conditions, leads to different products. Extensiveoxidation gives a mixture of acids consisting of the complete range of C1 to Cn carbon atoms. Lessextensive oxidation gives a mixture of products in which no chain fission has occurred. Under moderateconditions mixed ketones are the major products and oxidation in the presence of boric acid producesa mixture of secondary alcohols. The oxidation of alkanes in the vapour state occurs via free radicals,e.g., alkyl (R*), alkylperoxy (ROO*) and alkoxy (RO*). Oxidising agents such as potassium permanganatereadily oxidizes a tertiary hydrogen atom to a hydroxyl group. For example, isobutene is oxidized totert butyl alcohol.(CH3)3 CH + [O] 4KMnO

3 3(CH ) COHALKENES:

Alkenes are unsaturated hydrocarbons having carbon-carbon double bonds. The general molecularformula for alkene is n 2nC H .Alkenes and cycloalkanes are isomeric compounds because both have same general molecular formula.

NOMENCLATURE:The IUPAC names of alkenes are given by replacing the –ane ending of the corresponding alkane with–ene.

2 2IUPAC EtheneCommen Name: ethylene

H C CH 3 2IUPAC : propeneCommen Name: Pr opyleneCH — CH CH

1 2 3 4

2 2 31 Butene(not 1 2 butene)H C CH — CH — CH

6 5 4 3 2 1

3 2 2 32 Hexene(not 4 hexene)

CH — CH — CH — CH CH — CH



4 3 2 13 2CH — CH — C H C H

3CH3-methyl-1-butene

6 5 4 32 2 2 2 2 3CH — C H — C H — CH — CH — CH — CH

2 12CH C H

6-Bromo-3-propyl-1-hexeneBr

STRUCTURE AND BONDING IN ALKENES:

117.2°

121.4°

134 pm

110 pmH

H H

H

Ethylene is planar, each carbon is sp2 hybridized and the double bond is considered to have a component and component. The component arises from overlap of 2sp hybrid orbital along a lineconnecting the two carbon atoms, the component via a side by side overlap of two p orbital. Regionsof high electron density attributed to electrons, appear above and below the plane of the moleculeand are clearly evident in the electrostatic potential map.RELATIVE STABILITIES OF ALKENE:The stabilities of alkene are governed by —

a) Degree of substitution: Double bonds can be classified as monosubstituted, disubstituted, trisubstitutedand tetrasubstituted. According to the number of carbon atoms directly attached to the C=C structuralunit.Alkenes with more highly substituted double bonds are more stable than isomers with less substituteddouble bond.

b) Vander Waal’s strain: Alkenes are more stable when large substituents are trans to each other thanwhen they are cis.When two or more atoms are close enough in space that a repulsion occurs between them is one typeof steric effect and therefore trans isomers are more stable than cis isomer.

c) Heat of hydrogenation: The addition of hydrogen is called hydrogenation. Hydrogenation reactionsare exothermic (negative H0).

CH3CH3

CH3

2-methylbut-2-enePt / C

2H CH3CH3

CH3

0H 26.9 kcal/mol

CH2CH3

CH3

Pt / C2H CH3

CH3

CH3

0H 28.5 kcal/mol 2-methylbut-1-ene



CH3CH2

CH3

Pt / C2H CH3

CH3

CH3

0H 30.3 kcal/mol 3-methylbut-1-ene

The preceding three catalytic hydrogenation reaction all from the same alkane. So the energy of theproduct is the same for the each of three reactions. However, three reactants must have differentenergies. The reaction associated with the least negative H° has the most stable reactant and thereaction with the most negative has the least stable reactant.

CH3CH2

CH3

0H 30.3 kcal/mol

CH2CH3

CH3

CH3CH3

CH3

Potent

ial Ene

rgy

0H 28.5 kcal/mol 0H 26.9 kcal/mol

The greater the number of alkyl groups attached to the doubly bonded carbon atoms the morestable the alkene.Stability of alkenes

R2C = CR2 R2C = CHR R2C = CH2 RCH = CHR RCH = CH2 CH2 = CH2PREPARATION OF ALKENES:Dehydration of Alcohols: Alcohols when heated in presence of H2SO4, H3PO4, P2O5 or Al2O3 undergoloss of water molecule with the formation of alkene.

H2H — C — C — OH C C H O

Alcohol Alkene

2 4H SO3 2 2 2 2160 C EthyleneEthyl alcohol

CH — CH — OH H C CH H O

2 4H SO140 C

OH2H O



2 4H SOCH3 C

CH3

OHCH3 CH2

CH3

CH32H O

2 4H SO and 3 4H PO are the acids most frequently used in alcohol dehydration.Regioselectivity in Dehydration of Alcohol

2 4H SO80 CCH3

CH3

OHCH2 CH3 CH2

CH3

CH3 CH3

CH3 CH3

2-Methyl-1-butene (10%)

2-Methyl-2-butene (90%)

Dehydration of this alcohol is selective in respect to its direction

3 4H PO

CH3

OH

CH3 CH3

84% 16%

Regioselectivity of elimination is governed by Zaitsev’s Rule.Stereoselectivity in Alcohol Dehydration: In addition to being regioselective, alcohol dehydrationare stereoselective. A stereoselective reaction is one which a single starting material can yield two ormore sterioisomeric products, but give one of them in greater amount than other.

2 4H SO

OH

CH3CH3 CH3

H H

CH3

CH3

H

H

CH3cis-2-pentene (25%) trans-2-pentene (75%)

Illustration - 2 : Predict the major product of dehydration of each of:(A) (CH3)2C(OH)CH2CH3 (B) (CH3)2CHCH(OH)CH3(C) (CH3)C(OH)CH(CH3)2

Solution: CH3 CCH3

CHCH3 C CCH3

CH3 CH3

CH3(A) and (B) (C)

Dehydrohalogenations of Alkyl Halide: Dehydrohalogenation is the loss of hydrogen and a halogento form an alkene.



H — C — C — X C C HX The reaction is carried out in presence of strong base such as sodium ethoxide 2 5(NaOC H ) in ethylalcohol as solvent.

2 5 2 5H — C — C — X NaOC H C C C H OH NaX

2 3NaOCH CHH

H

Cl

H

H

H

Similarly 3 3 3NaOCH , KOC(CH ) are the preferred bases used in this process.

3 3KOC(CH )3 2 15 2 2 3 2 15 2DMSO 25 CCH – (CH ) – CH – CH – Cl CH – (CH ) – CH CH

The regeoselectivity of dehydrohalogenation of alkyl halide follow zaitsev rule of elimination predominatesthe direction.

CH3CH3

Br

CH3CH2

CH3

CH3

CH3

CH3 CH3

2-methyl-1-butene (29%)2-methyl-2-butene (71%)

Illustration - 3 : Predict the various product formed by dehydrohalogenation of

CH3 CH3

Br

Solution:

CH3CH3 CH3

CH3CH2

CH3

(major) (minor) (minor)

Dehalogenation of Vicinal DihalidesThere are two types of dihalides namely gem (or geminal) dihalides in which the two halogen atoms are



attached to the same carbon atom and vic. (or vicinal) dihalides in which the two halogen atoms areattached to the adjacent carbon atoms.Dehalogenation of vic dihalides can be effected by either NaI in acetone or zinc in presence of aceticacid or ethanol.

3 2 5

Zn dust3 2 3 2CH COOH or C H OHCH CHBr CH Br CH CH CH NaI

3 3 3 3AcetoneCH CHBr CHBr CH CH CH CH CHThe reaction mechanism involves loss of the two halogen atoms in two steps. The two halogen atomsalign themselves at 180° and in the same plane before they are lost.i) With NaI in acetone:

XCX

CX

C CC CXI

IXI

ii) With Zn-dust and acetic acidZn Zn2+ + 2e–

XCX

CX

C C XC CX2e

Partial Reduction of AlkynesReduction of alkyne to alkene is brought about by any one of the following reducing agents.i) Alkali metal dissolved in liquid ammonia.ii) Hydrogen in presence of palladium poisoned with BaSO4 or CaCO3 along with quinoline (Lindlar’scatalyst).iii) Hydrogen in presence of Ni2B (nickel boride).

R—CH2 —C CH (i), (ii) or (iii) R—CH2CH = CH2Alkali metal dissolved in liquid ammonia produces nearly 100% trans alkene by the following mechanism:Na + liq. NH3 Na+ + es (solvated electron)



R C C R C CR

R2H NH C C

R

R

H

sese

C CR

R

H2H N HC

R

R

H

H2NH ( - 100%)

(trans-alkene)

The dissolution of alkali metal in liquid NH3 produces solvated electrons. The reaction is initiated bythe attack of sp hybridized carbon atom of alkyne molecule with a solvated electron (es) when the -electrons move to the other sp hybridized carbon atom. In order to acquire greater stability the singleelectron on one carbon atom and the pair of electrons on the adjacent carbon atom orient themselvesor far away as possible forcing the two alkyl groups to acquire the farthest position. The carbanion thenpicks up a proton from NH3 to produce a vinylic radical. Attack by another solvated electon gives vinylicanion which produces trans-alkenes by picking up a proton from NH3.Hydrogenation of alkynes by Lindlar’s catalyst or nickel boride produces nearly 100% cis-alkene. Thecatalyst provides a heterogeneous surface on which alkyne molecules get absorbed. Hydrogen moleculescollide with adsorbed alkyne to produce cis-alkene in which both the hydrogen atom come from thesame side.

2Lindlar 's cat.

2 or Ni BR C R H H

R R

H

Hofmann Degradation MethodAlkenes can be prepared by heating quaternary ammonium hydroxide under reduced pressure at atemperature between 100°C and 200°C

3 3 2 2 2OH (CH ) N CH CH H O CH3 NCH3

CH3CH2 CH2 H

This is the final step of the overall three step reaction called Hofmann degradation. In the first step,primary, secondary or tertiary amine is treated with enough CH3I to convert it to the quaternary ammoniumiodide. In the second step, the iodide in converted into hydroxide by treatment with Ag2O.

32CH I 2Ag O NH

CH3 N CH3CH3 CH3N CH2CH3 CH3

H OH NCH3 CH3I I



The OH ion invariably removes proton from -carbon atom. If two or more alkyl groups have -carbonatoms, OH removes proton from that -carbon atom which gives more stable carbonion. That meansif tetra alkyl ammonium halide contains ethyl group as one of the alkyl groups, then ethene will be themajor product in this reaction. For example

OH 2 2CH CH CH3 CH

CH3CH2N

CH3

H2CCH2CH3CH2CH3

CH3 CHCH3

CH2NCH3

H2C CH2CH3

PHYSICAL PROPERTIES OF ALKENE:a) All are colouress and have no characteristic odour. Ethene has pleasant smell.b) Lower members (C2 to C4) are gases, middle one (C5 to C17) are liquids, highers are solids.c) The boiling points, melting points, and specific gravities show a regular increase with increase in

molecular weight, however less volatile than corresponding alkanes.d) A cis isomer has high boiling and melting point than trans because of more polar nature.e) Like alkanes, these too are soluble in non polar solvents.f) Alkenes are weak polar. The polaritiy of cis isomer is more than trans which are either non polar or less

polar (e.g. trans butene-2- is non polar; trans pentene-2-is weakly polar).CHEMICAL PROPERTIES OF ALKENES:

Addition of Hydrogen: Alkenes reacts with hydrogen in the presence of platinum, palladium, rhodiumor nickel catalyst to form the corresponding alkane.

Pt,Pd,Rh or Ni2 2 2 2 2R C CR H R CH CHR

2HPt

Addition of Halogens: Bromine and chlorine adds to alkenes to form vicinal dihalide. A cyclic haloniumion is an intermediate stereospecific anti addition is observed.

2 2 2R C CR X RR

X

X

RR

CH2

CH3 2BrBr

Br

CH3



MechanismStep 1:

CH3

CH3 CH3

CH3

Br+CH3 CH3

CH3

CH3

–

Br

BrBr

Bromonium ion Bromide ion

A bromine molecule becomes polarized as it approaches the alkene. The polarized bromine moleculetransfers a positive bromine atom (with six electrons in its valance shell) to the alkene resulting in theformation of bromonium ion.

Step 2:

Br

Bromonium ion vic-dibromide

C CBr+

CH3CH3CH3

CH3 C CCH3CH3

CH3CH3

Br

BrBromide ion

A bromide ion attacks at the back side of one carbon (or the other) of the brominium ion in an SN2reaction causing the ring to open and resulting in the formation of vic-dibromide.Addition of Hydrogen HalideHydrogen halides (HCl, HBr and HI) add to the alkenes to form alkyl halides.The addition of HX to alkenes is an electrophilic addition reaction. The order of reactivity of hydrogenhalides is HI HBr HCl.

HX R

X

R

HMechanismStep – 1

slowRDSH X C C

R R X

Step – 2



FastX CR

CHX

CR

C

The addition takes place according to Markownikov’s rule.In the ionic addition of an unsymmetrical reagent to a double bond, the positive part of the reagentattaches itself to the carbon atom of the double bond so as to yield more stable carbocation asintermediate. Because, this is the step that occurs first, it is the step that determines the overallorientation of the reaction.The reaction in which rearrangement of carbocation occurs, the overall addition of HX to alkenes doesnot follow Markownikov’s rule.For example the addition of HBr to 3-methylbut-1-ene gives a mixture of 2-Bromo-2-methylbutane and2-Bromo-3-methylbutane.

BrHBr CH2CH3

CH3CH3

CH CH3

CH3

H shift

Br CH3CH3

CH3

Br

CH3 C CH3

CH3Br CH3 CH3Br

CH3

2-Bromo-3-methyl butane

2-bromo-2-methylbutane

Addition of HBr in presence of peroxide (Peroxide effect)Addition of HBr to unsymmetrical alkenes in presence of organic peroxide is an anti Markownikov’saddition

ROORHBr CH3

H H

HCH3 CH2 CH2

BrMechanismIn presence of peroxide addition of HBr to alkenes follows free radical mechanism R O O R RO OR R O HBr R OH Br



3 2CH CH CH Br

CH3 CH CH2Br

CH3 CH CH2Br

(more stable)

(less stable)

H Br CH3 CH CH2Br

CH3 CH CH2Br

Br

(2°)

(1°)

Note:– Peroxide effect is not observed by HCl and HIAddition of sulphuric Acid: Alkene reacts with sulphuric acid to form alkyl hydrogen sulphates. Aproton and a hydrogen sulphate ion adds to double bond in accordance with Markovnikov’s rule.

2 2 2 2 2R C CR HOSO OH R — CH — CR 2OSO OH

Halohydrin formation: When treated with bromine or chlorine in aqueous solution alkenes are convertedto vicinal halohydrin. A halonium ion is an intermediate. The halogen adds to the carbon that has thegreater number of hydrogens. Addition is anti.

2 2 2R — CH CR X H O OHR R

RX HX

22

BrH O CH2

OHBr

Mechanism 2 2H O X HO X HX

CH3

CH3 CH3

CH3

X

CH3 CH3CH3

CH3

–

X

XX

Halide ion

C CX

CH3CH3CH3

CH3 C CCH3CH3

CH3CH3

O

X

HH

O HH

O HH



Here however a water molecule is a nucleophile and attacks a carbon of the ring causing the formationof a protonated halohydrin.

C CCH3CH3

CH3CH3

O

X

H

O+ HH

H

The protonated halohydrin loses a proton (it is transferred to molecule water). This step produces thehalohydrin and hydronium ion.Epoxidation: Peroxy acids transfer oxygen to the double bond of alkenes to yield epoxides the reactionis a sterospecific syn addition.

2 2 2 2R C CR R — C — OOH R C — CR R — COH O

O

O

3CH COOHCH3 CH3

O3CH — COOOH

Mechanism

CC

CH3CH3

CH3 CH3

OO

H O

R C

CO

CH3

CH3

CH3

CH3

CO

H

RO

Alkene Peroxy acid Epoxide Carboxylic Acid

The peroxy acid transfers an oxygen atom to the alkene in a cyclic single step mechanism. The resultis the syn addition of the oxygen to the alkene, with formation of an epoxide and a carboxylic acid.Acid-Catalyzed Hydration: Addition of water to the double bond of an alkene takes place in aquousacid. Addition occurs according to Markovnikov’s rule. A carbocation is an intermediate and is capturedby a molecule of water acting as a nucleophile.

H2 2 2 2RCH CR H O R — CH — CR

OH H

2 3 2 2 3 3H C C(CH ) H O (CH ) COHMechanism



Step 1:

CH2

CH3 CH3 H O+H

H CH3 CCH3

H

O HH

slow

The alkene accepts a proton to form more stable 3° carbocation.

Step 2:CCH3

CH3 CH3 O HH

CH3 CCH3

CH3

O+H

Hfast

The carbocation reacts with a molecule of water to form a protonated alcohol.

Step 3:fast O H

HCH3 C

CH3

CH3

O H CH3 CCH3

CH3

O+ HH

H O+H

H

A transfer of proton to a molecule of water leads to a product.Hydroboration-Oxidation: The two step sequence achieves hydration of alkene in a stereospecificsyn manner with a regeoselectivity opposite to Markovnikov’s rule. An organoborane is formed byelectrophilic addition of diborane to an alkene. Oxidation of organoborane intermediate with hydrogenperoxide completes the process. Rearrangement do not occur.

2 6 –2 2B H

2 2H O , OHR — CH CR R — CH — CH — CR OH

3 –2 2H B. THF

3 2 2 3 2 2 2H O . OH(CH ) — CH — CH CH (CH ) — CH — CH — CH — OH



Mechanism

CH3

CH3 H

H

pi-complex four centered transition state

CH3

CH3 H

H

BHH

H

B

HH

H

CH3

CH3 H

H

H- B HH

–

CH3CH3 HH

H BH H

22 –22BH

HO,OHCH3CH3 HH

H OH

Addition takes place through the initial formation of a complex which changes into a cyclic fourcenter transition state with the boron atom adding to less hindered carbon atom. The dashed bonds inthe transition state represent bonds that are partially formed or partially broken.The transition state posses over to become an alkylborane. The other B—H bonds of alkylborane canundergo similar additions, leading finally to a tri-alkylborane.

-3 2 2 2(BH ) H O , OH CH3

methylcyclopentane

CH3

H

H

OHtrans-2-Methyl-1-cyclopentanol

syn-Addition

Oxymercuration-demercurationAlkenes react with mercuric acetate in the presence of water to give hydroxymercurial compoundswhich on reduction yield alcohols.

2 2Hg(OAc) H O OH HgOAc

4NaBHOH H

Markovnikov additionMercuric acetate

The first stage, oxymercuration, involves addition of –OH and HgOAc to the C–C double bond. Then, indemercuration, HgOAc is replaced by H. The reaction sequence amounts to hydration of the alkene,but is much more widely applicable than direct hydration.Oxymercuration-demercuration gives alcohols corresponding to Markovnikov addition of water to the



carbon-carbon double bond. (anti addition). For example:

2 2 4Hg(OAc) , H O NaBH Norbornene

OHexo-Norborneol

Syn-HydroxylationHydroxylation with permanganate is carried out by reaction at room temperature of the alkene andaqueous permanganta solution; either neutral or slightly alkaline. Hydroxylation is a very good methodfor the synthesis of 1, 2-diols.

2 2 4 23CH CH 2KMnO 4H O 3 22MnO2KOH CH2 CH2OH OH

(ethylene glycol)4

32(1)OsO

(2)NaHSO/HO CH2 CH2OH OH

CH3

(propylene glycol)

CH – CH = CH3 2

The mechanism for the formation of glycols by permanganate ion and osmium tetroxide involves theformation of cyclic intermediates. Then in several steps cleavage at the oxygen-metal bond takesplace ultimately producing the glycol and MnO2 or Os metal. The course of these reactions is syn-hydroxylation.Hydroxylation can also be carried out with peroxy acids such as peroxy formic acid (HCO2OH) mixtureof hydrogen peroxide and formic acid allowed to stand for few hours. Sequently the product is heatedwith water give glycol.



2cold

4 H O/OHMnO O MnO

O O2

OHH O

Several steps

2MnOOH OH

2cold

4 H O/OHMnO

OH

H

OH

H

Oxidative cleavage of AlkenesAlkenes are oxidatively cleaved by hot alkaline permanganate solution or higher conc.of KMnO4 or acidmedium (bayer’s reagent)The terminal CH2 group of 1-alkene is completely oxidized to CO2 and water. A disubstituted atom of adouble bond becomes C = O group of a ketone.A monosubstituted atom of a double bond becomes aldehyde group which is further oxidized to salts ofcarboxylic acids. = CH2 part get oxidized to CO2 + H2O = CHR part get oxidized to RCOOH

= CR R part get oxidized to RCORExample

4KMnO , OH3 3 heat(cis or trans)

CH CH CHCH 2 H3 22CH CO HCH3

O

O

4KMnO , OHH 2 2CO H O CH3

CH2

CH3CH3

OCH3

Ozonolysis of AlkenesA more widely used method for locating the double bond of an alkene involves the use of ozone (O3).Ozone reacts vigorously with alkenes to form unstable compounds called initial ozonides, which rearrangesspontaneously to form compounds known as ozonides. Ozonides, themselves are unstable and reduceddirectly with Zn and water. The reduction produces carbonyl compounds that can be isolated andidentified.

3O 2Zn/H O 2O O OO

O OO

Ozonolysis can be of either of reductive type or of oxidative type. The difference lies in the fact thatproducts of reductive ozonolysis are aldehydes and/ore ketones while in oxidative ozonolysis, the



products are carboxylic acids and/ore ketones (this is because H2O2 formed would oxidise aldehydesto carboxylic acids but ketones are not oxidized). In reductive ozonolysis, we add zinc which reducesH2O2 to H2O and thus H2O2 is not present to oxidise any aldehyde formed.

Zn + H2O2 ZnO + H2OFor example,

(CH3)2CH – CH = CH2 3 2 22

(i) O , CH Cl(ii) Zn/H O (CH3)2CHCHO + HCHO

(CH3)2C = CH –CH3 3 2 22

(i) O , CH Cl(ii) Zn/H O (CH3)2C = O + CH3CHO

Illustration - 4 : Identify the products (x, y) of following reaction:-

3 2O /H O,Zn (X) (Y) CH3 CH3

CH3 CH3

CH3

Solution: CH3 CCH3

O H CO

CCH3

CH3CH3(X) : (Y) :

Substitution Reactions at Allylic PositionCl2 + H2C = CHCH3 High

temperature H2C = CHCH2Cl + HCl

Br2 + H2C = CH–CH3 2

Low concentrationof Br CH2 = CH – CH2Br + HBr

These halogenations are like free radical substitution of alkanes. The order of reactivity of H-abstractionis allyl 3° 2° 1° vinyl.Allylic substitution by chlorine is carried out using Cl2 at high temperature and alkene (with -carbon)in gaseous phase. Allylic bromination can be carried out using N-Bromosuccinimide. Propene undergoesallylic bromination when it is treated with N-bromosuccinimide (NBS) in CCl4 in the presence of peroxidesor light.

4LightorROOR

2 2CClN Br CH CH CH Br CH = CH – CH2 3

O

O

O

O

NH

N-Bromosuccinimide (NBS) 3-Bromopropene (allyl bromide) Succinimide

Dimerization



acidCH3CH2

CH3 CH3

CH2

CH3CH3 CH3

CH3

CH3

CH3

2,4,4-trimethylpent-2-ene

CH3 CH3

CH3

CH2

CH3

2,4,4-trimethylpent-1-ene

ALKYNES:Introduction:

Unsaturated hydrocarbons having a carbon-carbon triple bond are called alkynes. They correspond togeneral formula n 2n–2C H i.e. they have smaller proportion of hydrogen than alkene. Alkynes are isomericwith cycloalkenes and alkadienes.

NOMENCLATURE:.HC C– CH – CH = CH2 2pent1-ene4-yne

CH354

32

CH1

CH3

4-methylpent-1-yneCH31

23

45 CH36

Cl

2-chlorohex-3-yne

CH3CH

CH3CH

Cyclopentylethyne5-methylhex-1-yne

PREPATATIONS OF ALKYNES:Dehydrohalogenation of vic dihalides: Vicinal dihalides having hydrogens on carbon gives alkyneswith strong base.

2/NaNHR —CH—CH—R R —C C—R X X

Kolbe hydrocarbon synthesis: Potassium salt of maleic acid and its alkyl derivatives gives alkyneson electrolysis.

R —C —COOKR —C —COOK

electrolysis2 2R —C C —R 2CO 2KOH H



Hydrolysis of CarbidesSome of the lower members of alkyne series can be synthesized by the hydrolysis of carbides. Forexample, calcium carbide on hydrolysis gives acetylene and magnesium carbide on hydrolysis givespropyne.

CaC2 + 2H2O HC CH + Ca(OH)2 Mg2C3 + 4H2O CH3–C CH + 2Mg(OH)2

The difference in the behaviour of calcium carbide and magnesium carbide is due to the differencesin their structures. Both the carbides are ionic in nature. In calcium carbide, the anion exists as C Cwhile in magnesium carbide, the anion exists as 3C C C . It is pertient to note that aluminiumcarbide (Al4C3) and beryllium carbide (Be2C) do not form any alkyne on hydrolysis, instead they formmethane on hydrolysis. This is due to the fact that their anions exist as C4–.PHYSICAL PROPERTIES:

i) Lower members (C2 to C4) are gases; middle one (C5 to C12) are liquids; highers are solids.ii) The boiling point, melting point and specific gravity of alkynes show a regular increase with increase in

molecular weight; however less volatile than alkene. The order of boiling point in hydrocarbons hasbeen explained in terms of polarity. Alkynes possess more polarity and thus have higher boiling point.alkyne Alkene Alkane

iii) All are colourless and possess no characteristic odour; however C2H2 has garlic odour due to theimpurities of PH3, H2S etc. Pure C2H2 has ethereal odour.

iv) Soluble in organic solvents like acetone, alcohol and sparingly soluble in water.v) The boiling point of acetylene is –84°C. Liquid acetylene is dangerously explosive and therefore storage

and transportation of liquid acetylene is prohibited by law. That is why acetylene is stored and transportedby dissolving it in acetone soaked on porous material like asbestos packed in steel cylinders, underhigh pressure.CHEMICAL PROPERTIES OF ALKYNES:Acidic Character of alkynesThe hydrogen bonded to the carbon of terminal alkyne is considerably more acidic than those bondedto carbons of alkenes and alkanes. The pKa values for ethyne, ethene and ethane illustrate this point

H

H H

HH

H

H

H

HHH C C H

pKa 25 pKa 44 pKa 50The order of basicity of their anions is opposite that of their relative acidity

Relative BasicityCH3CH2

– CH2 = CH– HC C–

From the pKa values it can be concluded that there is a vast difference in the stability of the carbanions.This difference can be readily explained interms of the character of the orbital occupied by the lone pairelectrons in three anions. In ethyl anion the lone pair electrons are in sp3 hybrid orbitals, in vinyl anionthe lone pair is in the sp2 hybrid orbital and in acetylide ion the lone pair is in sp hybrid orbital. Thepercentage s-character of the orbital increases in the order sp3 sp2 sp. Greater is s-character of thehybrid orbital containing a lone pair of electrons, the less basic is that anion and the more acidic thecorresponding conjugate acid.



Relative Acidity

2 2 2 316 17 2515.7 38 44 50pKa :H OH H OR H C C H H NH H CH CH H CH CH

Relative Basicity 2 2 2 3OH OR C C H NH CH CH CH CH

Reactions due to Acidic Hydrogeni) Reaction with Sodium : Terminal alkynes react with sodium in liquid ammonia or sodamide to form

sodium alkynide

R – C C – H + Na 3NH (l) R – C C Na + 21 H2

R – C C – H + NaNH2 3NH (l) R – C CNa + NH3This reaction is utilized for the preparation of higher alkynes.R – C C – H 2

3NaNHNH (l) R – C C – Na

R – C CNa R X R – C C - R + NaXii) Reaction with ammonical cuprous chloride solution :- When acetylene is passed through ammonical

cuprous chloride solution, a red precipitate of copper acetylide is formed.

H – C C – H + Cu2Cl2 + 2NH4OH copper acetylide(Red ppt )

Cu C C Cu + 2NH4Cl + 2H2O

R – C C – H + Cu2Cl2 + 2NH4OH Copper alkynideR C C Cu + 2NH4Cl + 2H2Oiii) Reaction with ammonical silver nitrate solution :- When acetylene is passed through ammonical

silver nitrate solution (Tollen’s reagent) a white precipitate of silver acetylide is formed.H – C C – H + 2AgNO3 + 2NH4OH

Silver acetylide (white ppt)Ag C C Ag + 2NH4NO3 + 2H2O

The alkynes can be regenerated from the insoluble salts and the overall process serves as a method forpurifying terminal alkynes.

iv) Reaction with Grignard reagent: These two reagents reacts with terminal alkyne to form hydrocarbon and new organometallic compound respectively.

R — C C — H 3 4CH MgBr R — C C — MgBr CH 3CH Li

4R — C C — Li CH Addition of HydrogenAlkynes can be reduced directly to alkanes by the addition of 2H in the presence of Ni,Pt or Pd asa catalyst. The addition reaction takes place in two steps. It is not possible to isolate the intermediatealkene under the above reaction conditions. By using Lindlar’s catalyst , nickel boride or palladisedcharcoal, alkynes can be partially hydrogenated to alkenes

Ni Pt or Pd3 2 3 2 3CH C CH 2H CH CH CH



Lindlar ' s catalyst3 2 3 2CH C CH H CH CH CH

2-butyne when reduced with Lindlar’s catalyst gives nearly 100% cis-isomer while Na in liquid ammoniagives nearly 100% trans-isomer.

3CH

3CH

3CH3CH 3CH

3CH

C

C

C

C

C

C

H

H

H

HNa in 3liq .NH

+ 2H(Cis)

(trans)

lindlar’s catalyst

Addition of Halogen AcidsAddition of halogen acids to alkynes occur in accordance with Markovnikoff’s rule. Addition of onemolecule of halogen acid gives an unsaturated halide, which then adds another molecule of hydrogenhalide to form gem dihalides. For example, additon of HI to propyne first gives 2-iodopropane and then2,2-iodopropane

3CH C CH HI 3 2CH C CH- =I HI 3 3CH C CH- -

I

I2-iodopropene 2,2-diiodopropane

The order of reactivity of halogen acids towards addition reaction is H I H Br H Cl Addition of HalogensOne or two molecules of halogens can be added to alkynes giving dihalides and tetra halides respec-tively. Chloride and bromide add readily to the triple bond while iodine reacts rather slowly

2Cl2 2 2CH CH Cl CHCl CHCl CHCl CHCl

2Br3 2 3 3 2 2CH C CH Br CH CBr CHBr CH CBr CHBr

Addition of water (Hydration)Alkynes cannot be hydrated more easily than alkenes because of their low reactivity towards electro-philic addition reactions. Further when acetylene is passed through 40% 2 4H SO containing 1%

4HgSO at 080 C , a water molecule adds upto give acetaldehyde.



CH

CH+ HOH 2 4

440%H SO1%HgSO

2CH

CHOH Vinyl alcohol

Tautomerises

3CH

CHO Acetaldehyde

R C CH HOH 24

H SO4 dilHgSO

CR 2CHOH

(Enolic)

TautomerisesCRO

3CH

ketoneAddition of Boron hydridesDiborane, the simplest hydride of boron reacts with alkyne to form trialkenylborane. Diborane splits intotwo 3BH units and the addition of 3BH takes place following Markovnikoff’s rule. The addition contin-ues as long as hydrogen is attached to boron atom.

2 6 22R C CH B H 2R CH CH BH 2 2R C CH R CH CH BH R CH CH BH

2 3R C CH R CH CH BH R CH CH B Trialkenylborane on hydrolysis gives alkene 3CH COOH

2Hydrolysis3 3R CH CH B 3R CH CH B OH Internal alkynes give rise to alkene where geometrical isomerism is possible. Hydroboration followed byhydrolysis of alkynes gives cis alkene as the major product.

2 6 3 2B H CH CO H3R C C R RCH CR B C CH H(cis)

R R

Oxidation of trialkenylborane with alkaline 2 2H O results in the formation of carbonyl compounds.Terminal alkynes give rise to aldehydes whereas internal alkynes gives rise to ketones.



2 2H O / NaOH3R CH CH B CH CH

OHR 2R CH CHO

2 2H O /NaOH3R CH CHR B CH CH

OHR

OCR R2CH

OxidationAlkynes are oxidised by allkaline 4KMnO . which causes cleavage of C C resulting in theformation of salts of carboxylic acids. The salts on acidification are converted into acids. internal alkynesgive mixture of carboxylic acids while terminal alkynes give a carboxylic acid and the terminal C atom isoxidised to 2CO and 2H O .

4(i)KMnO / OH3 3 2 2(ii)HCH C CH CH COOH CO H O

4(i)KMnO / OH

3 2 3 3 3 2(ii)HCH C C CH CH CH COOH CH CH COOH

OzonolysisReaction of alkynes with 3O gives rise to the formation of ozonide. Hydrolysis of ozonide with 2H Ogives a mixture of two carboxylic acids. This is called oxidative ozonolysis.

3 3CH C CH O

O

O O3CH C CH

2H O 3CH COOH HCOOH

However, if ozonide is hydrolysed with Zn and 2H O , a diketone is formed. This is called reductiveozonolysis

3 2 3 3CH C C CH CH O O

O O3CH C C 2 3CH CH

2Zn /H O

O

O3CH C C 2 3CH CH

Ethane behaves differently. Ozonolysis followed by oxidative hydrolysis of ethyne gives a mixture ofglyoxal and formic acid.



32

1.O2.H OHC CH CHO CHO HCOOH

polymerisation : Acetylene undergoes polymerisation yielding different types of polymeric compoundsunder different conditions:(i) Cyclic polymerisation : When acetylene is passed through a red-hot metallic tube at 0600 C ,cyclic polymerisation takes place with the formation of benzene

3HC CH 0Heat

600 C

BenzenePropyne on heating trimerises under similar conditions and forms mesitylene (1,3,5-trimethyl benzene).

Cu Tube3 Heat3CH C CH

3CH

3CH3CH Mesitylene

(ii) Acetylene dissolved in tetrahydrofuran polymerises into cyclo-octa-1,3.5,7-tetrene in presence of 2Ni CN and under high pressure.

2Anhydrous Ni CNTetrahydrofuran(Solvent)

4HC CH

CH CH

CH CH

CHCH

CHCH

Cyclo-octatetrene(iii) Linear polymerisation : When acetylene is passed into cuprous chloride solution containing

4NH Cl linear polymerisation occurs forming monovinyl acetylene ad divinyl acetylene2 24

Cu Cl2NH ClHC CH HC CH CH CH C CH

Monovinyl acetylene

HC CH 3rd molecule

2 2CH CH C C CH CH Divinyl acetylene

Vinyl acetylene on reduction with 2H /Pt in presence of 4BaSO forms buta-1,3-diene.



24

H / Pt2 2 2BaSOH C CH C CH H C CH CH CH

Vinyl acetylene 1,3-ButadieneKEY CONCEPTSAlkanes Alkanes are saturated hydrocarbons. They are called paraffins. General formula of alkanes is CnH2n+2 Hydrogenation of alkenes or alkynes in presence of Nickel catalyst at 250°C gives alkanes. Alkyl halides on reduction with Zn/HCl, Zn/CH3COOH or red P/HI or of subjected to Wurtz reaction or

Frankland reaction give alkanes. Alkanes are formed by the reduction of alcohols, aldehydes, ketones & carboxylic acids with HI & Red

P. Decarboxylation of sodium or potassium salt of carboxylic acid gives alkanes. Grignard’s reagent react with compound containing active hydrogen like H2O; C2H5OH, CH CH, NH3

etc give alkanes. Alkanes containing odd number of carbon atoms can be prepared by corey-house synthesis. Melting points, boiling point and density of alkanes increase with the increase in molecular weights. Straight chain alkanes have higher boiling point than branched chain alkanes. Due to strong and non polar C—C and C—H bond alkanes are not very reactive. Halogenation, nitration and sulphonation of alkanes follow free radical mechanism. In the halogenation of alkanes the reactivity with respect to halogens follows the order F2 Cl2 Br2

I2. Reactivity of with respect to C—H follows the order teretiary hydrogen secondary hydrogen primary hydrogen.

Alkenes Alkenes are unsaturated hydrocarbons having one carbon-carbon double bond. Alkenes are called olefins. General formula of alkenes is CnH2n. Dehydration of alcohols leads to the formation of alkenes. Dehydrating agents are H2SO4, P2O5, Al2O3,

H3PO4 and ZnCl2. The ease of dehydration follows the order 3° 2° 1°. Dehydrohalogenation of alkyl halides gives alkenes. The ease of dehydro-halogenation is 3° 2° 1°. Alkenes can be prepared by the controlled partial hydrogenation of alkynes with Lindlar’s catalyst. Dehalogenation of vic-dihalides with Zinc dust gives alkenes. The formation of more substituted alkene as major product is called Saytzeff elimination. The formation of less substituted alkene as major product is called Hofmann elimation. The stability of alkenes follow the order R2C = CR2 R2C = CHR R2C = CH2 RCH = CHR R—CH

= CH2 CH2 = CH2. Trans-but-2-ene is more stable than cis-but-2-ene. Olefines show electrophilic addition reaction. Alkenes give addition product with halogens to form vic-dihalides. Alkenes decolourised by

i) Baeyer’s reagent ii) Br2 water.



These reactions are the test of unsaturation. Alkenes react with HX to form alkyl halides. The order of reactivity of HX is HI HBr HCl HF. Addition of HX to unsymmetrical alkenes takes place according to Markownikov’s rule. Addition of HBr to unsymmetrical alkenes in presence of peroxide follows anti-Markownikov’s addition. HCl and HI do not give anti Markownikov’s addition. Alkenes on reductive ozonolysis give carbonyl compounds. Hydroxylation of alkenes by cold aqueous alkaline KMnO4 solution (Baeyer’s reagent is a syn-addition.) Hydroboration-oxidation of alkene is an anti markownikov’s addition of water to a double bond. Oxymercuration-demercuration is a markownikov’s addition of water to a double bond. In Oxymercuration-demercuration reaction and hydroboration-oxidation reaction no rearrangement takes

place. Oxidation of alkenes by acidic KMnO4 or acidic K2Cr2O7 give ketone or carboxylic acid or both. Periodic acid (HIO4) or leadtetra acetate [(CH3COO)4Pb] oxidizes alkenes into glycols and finally gives

aldehydes or ketones depending upon the nature of alkene. Oxidation of alkenes by SeO2 affect the allylic position. NBS (N-Bromosceccinimide) is used for the bromination of alkenes at the allylic position.Alkynes Alkynes are unsaturated hydrocarbons with one carbon-carbon triple bond. General formula CnH2n-2. Dehydrohalogenation of vic-dihalides give alkynes. Dehalogenation of 1, 1, 2, 2-tetrahalides with Zn dust and alcohol give alkynes. Acetylene can be prepared by the action of water on calcium carbide. Magnesium carbide on hydrolysis give propyne. The acidic character of Ethyne, Ethene and Ethane follows the order H – C C – H CH2 = CH2 CH3

– CH3. The basic character of their conjugate base follow the order CH C– CH2 = CH– CH3—CH2

–

Alkynes are less reactive towards electrophilic addition as compared to alkenes. Acetylene and terminal alkynes react with ammonical silver nitrate solution (Tollen’s reagent) to give

corresponding alkynides (white ppt.). Acetylene and terminal alkynes react with ammonical cuprous chloride solution to give a red precipitate

of corresponding alkylnides. Hydrogenation of alkynes in presence of Lindlar’s catalyst give cis-alkene where as hydrogenation by

Na/Liq. NH3 give trans alkenes.

OOzonolysis [O]

3 2 2 3 3 2(F)(D) CH CH — C — CCH CH CH CH COOH

O(E)

Hydrocarbons

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