Wk 8- Ch15-Chem of Carbon

8/9/2019 Wk 8- Ch15-Chem of Carbon

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1515 TheThechemistry ofchemistry of

carboncarbon


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Alkanes are hydrocarbons that contain only carboncarbon singlebonds. Such hydrocarbons are said to be saturated hydrocarbons

- refer to alkanes as aliphatic hydrocarbons ,

Alkenes are hydrocarbons that contain one or more carboncarbondouble bonds.

Alkynes are hydrocarbons that contain one or more carboncarbontriple bonds.

Arenes are cyclic structures containing carboncarbon bonds thatimpart special stability.

Alkenes, alkynes and arenes are unsaturated hydrocarbons


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15.2 Alkanes15.2 Alkanes

Single carbon-carbon bonds

All carbon atoms tetrahedral All bond angles approximately 109.5


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R epresenting alkanes



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IUP AC nomenclature names of all alkanes end in ane

e.g. meth ane (CH 4), eth ane (C 2H6)

Alkane general molecular formula C nH2n+2


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Structural and condensed structural formulae are useful for showingthe order of attachment of atoms, they do not show

three-dimensional shapes

The relationships between the structure and properties of molecules ,it is crucial to understand the three-dimensional shapes of molecules

Carbon atoms can be twisted into a number of differentthree-dimensional arrangements by rotating around one

or more carboncarbon bonds


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Conformations result from carbon-carbon bond rotation

Staggered conformation e.g. ethane



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Eclipsed conformation e.g. ethane

Lowest energy conformation:staggered



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D raw Newman projections for one staggered conformationand one eclipsed conformation of propane

The following are Newman projections and ball-and-stick models of theseconformations :

WORKED EXAMPLE 15.1


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P hysical properties of alkanes

nonpolar compounds with weakinteractions between molecules weak dispersion forces boiling points

alkanes 14 carbons are gasesalkanes 517 carbons are liquidsalkanes 18+ carbons are waxy solids


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P hysical properties of alkanes

melting point and densityaverage density 0.7 g/m L (110 carbons)

isomeric alkanesconstitutional isomers: same molecular formula but different atom connectivitydifferent physical properties


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Constitutional Isomers of Various C n H2 n +2 Hydrocarbons

Molecularformula

No. of

constitutionalisomers

CH 4 0

C 5H 12 3

C 10 H 22 75

C 15 H 32 4347

C 25 H 52 36797588

C 6H14 , three different molecules are possible:


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Ph ysical P roperties of t h e Isomeric Alkanes wit h t h e Molecular Formula C 6H14

Name Melting point (C) Boiling point (C) Density (g mL -1 at 0 C)

hexane - 95 69 0.6593-methylpentane - 118 63 0.664

2-methylpentane - 153 60 0.653

2,3 -dimethylbutane - 128 58 0.662

2,2 -dimethylbutane - 100 50 0.649

The more branching there is, the lower the boiling point .

- differences in boiling points are related to molecular shape.

As branching increases, the shape of an alkane molecule becomes

more compact, and its surface area decreases. As the surface area decreases, the area of contact betweenmolecules are decrease.

This decrease leads to weaker dispersion forces, so boiling points alsodecrease


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As branching increases, the shape of an alkane molecule becomes

more compact, and its surface area decreases. As the surface areadecreases, the strength of the dispersion forces decreases,and the boiling point also decreases.


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Cycloalkanes carbon-carbon single bonds in a ring

five-membered and six-memberedrings are the most common general formula: C nH2n



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W rite the molecular formula and IUP AC name for the following cycloalkanes.

(b)(a)

(a)The molecular formula of this cycloalkane is C8H16. Because thereis only one substituent on the ring, there is no need to number theatoms of the ring. IUP AC accepts two names for this compound:

isopropylcyclopentane and 1-methylethylcyclopentane

(b) Number the atoms of the cyclohexane ring beginning with t er t -butyl, thesubstituent of lower alphabetical order. The compound's name is

1- t er t- butyl-4-methylcyclohexane and its molecular formula is C 11 H22 .IUP

AC also accepts 1-(1,1-dimethylethyl)-4-methylcyclohexane


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cis -t ran s isomerism in cycloalkanes same molecular formula same order of attachment of atoms arrangement of atoms in space that cannot be

changed by rotation around sigma bonds ( )


In cis isomers, the groups are on the same side; in t ran s isomers, thegroups are across from each other.


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Carboncarbon bonds of the ring that project forwardsare shown as heavy lines

The cyclopentane ring can be viewed from above, with the ring in the planeof the paper. Substituents on the ring then either project towards you (that is,they project above the plane of the page) and are shown by solid wedges,

or they project away from you (they project below the plane of the page)and are shown by hashed wedges


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15.3 Alkenes and alkynes15.3 Alkenes and alkynes

Alkenes 1 or more carbon-carbon double bond bond angles around each carbon atom

in double bond ~120 o

Alkynes

1 or more carbon-carbon triple bond bond angles around each carbon atomin triple bond ~180 o


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A carboncarbon double bond consists of one bond and one bond.

E ach carbon atom of the double bonduses its three sp 2 hybrid orbitals to form

bonds with three atoms.The unhybridised 2 p atomic orbitals, whichlie perpendicular to the plane created by theaxes of the three sp 2 hybrid orbitals,

combine to form the bond of the carboncarbon double bond.


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O rbital pictures of the bonding in ethene from three perspectives. Note thatthe two electrons in the bond are delocalised over both lobes of thebond.

The bond forms from the end-on overlap of two hybrid

orbitals, and the bond forms from the side-by-sideoverlap of two atomic p orbitals

CH APTER 5.6


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Alkenes: cis -t ran s isomerism restricted rotation about carbon-carbon double

bond different compounds with different physical and

chemical properties


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Because of repulsion between alkyl substituents onthe same side of the double bond in the cis isomer,

cis alkenes are less stable than their t ran s isomers.

This can be seen from the forcing together of themethyl hydrogen atoms in the space-filling model

of cis -but-2-ene.


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Nomenclature names of alkenes end in ene

names of alkynes end in yne find the longest carbon chain containing thealkene/alkyne and number such that thedouble/triple bond has the lowest set of numbers



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W e number the carbon atoms, locate the double bond, locateand name substituent groups, and name the main chain.


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The physical properties of but-2-ene dependon the orientation of the substituents or bonds to the substituents in relation to thedouble bond .

W e need a precise way of describing the different isomersthat can arise when a double bond is present in ahydrocarbon


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D esignating configuration in alkenes cis -t ran s system

orientation of atoms in parent chain determines

whether the alkene is cis or t ran s

E,Z systemused for tri- and tetrasubstituted alkenes



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D esignating configuration in alkenes E,Z s y s t em

assigns priorities to the substituents on each side of

the carbon-carbon double bondZ configuration if higher priority groups are on thesame sideE configuration if higher priority groups are on

opposite sides



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E ,Z system priorities priority is based on atomic number

priority is assigned at the first point of difference if itcannot be assigned on the basis of atoms bondeddirectly to double bond



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Cycloalkenes cyclic hydrocarbons containing double bonds



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In naming cycloalkenes, we number the carbon atoms of the ring doublebond 1 and 2 in the direction that gives the substituents the smallestpossible numbers. W e name and locate substituents and list them in

alphabetical order, as in the following compounds.


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(a)

(b)

(d)

(c)

The active ingredients in echinacea extracts

The chemical make-up of herbs and

other natural remedies is remarkablycomplicated and it is important toassess accurately the levels of activeingredients, some of which may varyfrom season to season and even fromplant to plant.


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W rite the IUP AC names for the following cycloalkenes.

(a)(b) (c)

(a) 3,3-dimethylcyclohexene

(b) 1,2-dimethylcyclopentene

(c ) 4-isopropyl-1-methylcyclohexene


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Cycloalkenes cyclic hydrocarbons containing double bonds cis -t ran s isomerisation

not possible to have t ran s configuration incycloalkenes of 7 or fewer carbons (angle strain)



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In these representations, the configuration around eachdouble bond is cis . Because of angle strain, it is not possibleto have a t ran s configuration in cycloalkenes of seven or fewer carbons.


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D ienes, trienes and polyenes alkadienes (2 double bonds)

alkatrienes (3 double bonds)

polyenes (multiple double bonds) n carbon-carbon double bonds:

maximum 2 n cis -t ran s isomers possible

Physical properties of alkenes/alkynes non-polar compounds

dispersion forces (only attractive forces) physical properties similar to alkanes


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15.4 Reactions of alkanes15.4 Reactions of alkanes

Very little reactivity strong sigma bonds

Can react with oxygen (oxidation) under certain conditions forms carbon dioxide and water energy sources for heat and power


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Ch aracteristic Addition Reactions of AlkenesR

eaction Descriptive name(s)hydrochlorination (anexample of hydrohalogenation)

hydration

bromination (an example of halogenation)

hydrogenation (reduction)

15.5 Reactions of alkenes15.5 Reactions of alkenes


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The basis of reactivity is the attraction between positive andnegative species.

The double bond in alkenes is an electron-rich (i.e. negative)target for positive species.

These positive species are called electrophiles


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E lectrophilic addition reactions

hydrohalogenationaddition of HCl, HBr or H I to alkeneaddition obeys Markovnikovs rule- hydrogen adds to carbon with most hydrogens



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Name and draw a structural formula for the major product of each of thefollowing alkene addition reactions.

(a) (b)

U sing Markovnikov's rule, we predict that 2-iodo-2-methylpropaneis the product in (a) and 1-chloro-1-methylcyclopentane is theproduct in (b).

(a)(b)

EXAMPLE 15.14


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E xplain why one product predominates over other P roducts?

- we need to understand the mechanism of the process

-breaking of the single bond in the hypothetical AB moleculeby using curved arrows:

The arrow begins at the bond that is being broken and the headof the arrow shows the destination of the pair of electrons.

W e use arrows to show the movement of electrons in eachbond-forming and bond-breaking step in a mechanism.

Example


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The reaction of but-2-ene with hydrogen chloride to give 2-chlorobutane

Step 1: The reaction begins with the transfer of a proton from HCl to

but-2-ene, as shown by the two curved arrows on the left side of thefollowing equation

The first curved arrow shows the breaking of the bond of the alkene and itselectron pair now forming a new covalent bond with the hydrogen atom of HCl.

The second curved arrow shows the breaking of the polar covalent bond in HCl

A mechanistic arrow should never start from a hydrogen atom in an organic molecule

C C C C


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Step 2: The reaction of the cation (a Lewis acid) with a chlorideion (a Lewis base) completes the valence shell of carbonand gives 2-chlorobutane:

S tep 1 results in the formation of an organic cation. O ne carbon atom in this cationhas only six electrons in its valence shell and carries a charge of +1. A species

containing a positively charged carbon atom is called a carbocation (c arbon + c a t i on ).

Carbocations are classified as primary (1), secondary (2) or tertiary (3), dependingon the number of carbon atoms bonded to the carbon bearing the positive charge.

All carbocations are Lewis acids . They are also electrophiles.


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E lectrophilic addition reactions hydrohalogenation mechanism carbocation stability 3 o>2 o>1 o


The 2 carbocation is formed in preference to the 1 carbocation.


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Reaction of HC l with 2-methylpropene,the transfer of a proton to the carboncarbon double bond might formeither a 1 carbocation (isobutyl cation) or a 3 carbocation ( t er t -butyl cation

The 3 carbocation forms in preference to the 1 carbocation.


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The order of stability of four types of alkyl carbocations.

The order of stability of four types of alkyl carbocations


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R elative Stabilities of Carbocations

Arrange the following carbocations in order of increasing stability.

a) b) c)

Carbocation (a) is secondary, (b) is tertiary and (c) is primary. In order of increasing stability, they are (c), (a) and (b).


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P ropose a mechanism for the addition of H I to methylenecyclohexane,which gives 1-iodo-1-methylcyclohexane

W hich step in your mechanism is rate determining?

Step 1 : A rate-determining proton transfer from H I to the carboncarbondouble bond gives a 3 carbocation intermediate


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Step 2: R eaction of the 3 carbocation intermediate (a L ewis acid) withan iodide ion (a L ewis base) completes the valence shell of carbon andgives the product


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Addition of W ater: Acid-catalysed Hydration

In the presence of an acid catalyst most commonly, sulfuric acid

water adds to the carboncarbon double bond of an alkene to give an alcohol.The addition of water is called hydration .

H is added to the carbon atom of the double bond with the greater number of hydrogen atoms and O H is added to the carbon with the lower number of hydrogens.Thus, H O H adds to alkenes in accordance with Markovnikov's rule

Th h i f th id t l d h d ti f lk


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The mechanism of the acid-catalysed hydration of alkenesis similar to the mechanism for the addition of HCl, HBr and H Ito alkenes

Step1: P roton transfer from the acid catalyst to propene gives a 2 carbocationintermediate (a L ewis acid):

Step 2 : R eaction of the carbocation intermediate (a L ewis acid) with water (a L ewis base) completes the valence shell of carbon and gives anoxonium ion :


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Step 3 : P roton transfer from the oxonium ion to water gives the alcoholand generates a new molecule of the catalyst:


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P ropose a mechanism for the acid-catalysed hydration of methylenecyclohexane to give 1-methylcyclohexanol. W hich step in your mechanism is rate determining ?

The mechanism involves three steps, similar to that for the acid-catalysedhydration of propene. The formation of the 3 carbocation intermediate instep 1 is rate determining.

Step 1 :P

roton transfer from the acid catalyst to the alkene gives a 3 carbocationintermediate (a L ewis acid):


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S t e p 2: R eaction of the carbocation intermediate (a L ewis acid) with water (a L ewisbase) completes the valence shell of carbon and gives an oxonium ion:

S t e p 3: P roton transfer from the oxonium ion to water gives the alcohol andgenerates a new molecule of the catalyst:


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Addition of Bromine and Chlorine

Chlorine, Cl 2, and bromine, Br 2, react with alkenes at room temperature by theaddition of halogen atoms to the two carbon atoms of the double bond, forming twonew carbonhalogen bonds:

Note : CH2Cl2 is dichloromethane a solvent


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E lectrophilic addition reactions halogenation (addition Br 2 or Cl 2)

t ran s addition to cycloalkenesBr 2 addition test for C-C double bond



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E lectrophilic addition reactions reduction of alkenes (hydrogenation)

reaction with molecular hydrogen (H 2) and

transition metal catalysts yn addition most common (hydrogens add tosame side)



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Similar chemistry to alkenes reduction/oxidation, addition reactions

R eduction choice of catalyst controls outcome

15.6 Reactions of alkynes15.6 Reactions of alkynes


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Aromatic compoundsAromatic compounds

Benzene aromatic, parent arene, highly unsaturated does not undergo addition, oxidation or reduction,

reacts by substitution Kekuls model of benzene


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The carbon skeleton of benzene forms a regular hexagon with C C Cand H C C bond angles of 120. For this type of bonding, carbonuses sp 2 hybrid orbitals. E ach carbon forms bonds to two adjacentcarbons by the overlap of sp 2 sp 2 hybrid orbitals and one bond tohydrogen by the overlap of sp 2 1 s orbitals

1s2 2s2 2p2


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O rbital overlap model

R esonance


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Hckels aromatic rules to be aromatic, a ring must

have one 2 p orbital on each atom

be planar or nearly planar (continuous overlap of all 2 p orbitals)have (4 n + 2) electrons (n = 0, 1, 2 etc.)


Benzene meets these criteria. It is cyclic, planar, has one 2 porbital on each carbon atom of the ring, and has six electrons (an aromatic sextet) in the cyclic arrangement of its 2 p orbitals


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Nomenclature monosubstituted benzenes

named as benzene derivativescertain common names retained by theIUP AC system


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The five-membered-ring compounds furan, pyrrole andimidazole are also aromatic

The unsaturated rings may also contain atoms other than carbon, andthese molecules are called heterocyclic compounds


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Monosubstituted Benzenes

The IUP AC system retains certain common names for several of the simpler monosubstituted alkylbenzenes. E xamples are toluene (rather thanmethylbenzene) and styrene (rather than phenylethene):

R etained by the IUP AC system

Th b i d i d b h l f H f b i h l


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The substituent group derived by the loss of an H from benzene is a phenyl group(P h); that derived by the loss of an H from the methyl group of toluene is a benzylgroup (Bn):

D isubstituted Benzenes disubstituted benzenes

locate substituents by numbering atoms of ring or using locators or th o (1,2-), me t a (1,3-)and para (1,4-)


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disubstituted benzeneslocate substituents by numbering atoms of ring or using locators or th o (1,2-), me t a (1,3-)and para (1,4-)


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Nomenclature polysubstituted benzenes

for 3 or more substituents, use numbering to locatesubstituentsnumber to give the smallest set of numbers and listsubstituents in alphabetical order


Wk 8- Ch15-Chem of Carbon

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