1

2

• Unsaturated hydrocarbons enhance our lives in many ways:

1. Polyethylene plastic bags and bottles

2. Polystyrene Styrofoam cups

3. Plastic wraps

4. Essential oils in plants contain multiple bonds between carbon atoms.

– Cosmetics, medicines, flavorings, perfumes

5. Hydrocarbons also form rings of carbon atoms (aromatics)– Detergents, insecticides, and dyes

Fragrant Alkenes

3

4

Bonding in Unsaturated Bonding in Unsaturated HydrocarbonsHydrocarbons

5

• The unsaturated hydrocarbons consist of three families of homologous compounds that contain multiple bonds between carbon atoms.

• Alkenes contain carbon-carbon double bonds.• Alkynes contain carbon-carbon triple bonds.• Aromatic compounds contain benzene rings.

6

Schematic hybridization of 2s22px12py

1 orbitals of carbon to form three sp2 electron orbitals and one p electron orbital

The four orbitals available for bonding in alkenes are three sp2 orbitals and one p orbital.

7

(a) A single sp2 electron orbital and (b) a side view of three sp2 orbitals all lying in the same plane with a p orbital perpendicular to the three sp2 orbitals.

8

• The carbon-carbon pi () bond is much weaker and, as a consequence, much more reactive than the carbon-carbon sigma () bond.

9

10

11

• The formation of a triple bond between carbon atoms, as in acetylene, CHCH, may be visualized as shown below.

• These pi bond electrons are not as tightly held by the carbon nuclei as the sigma bond electrons. Acetylene, consequently, is a very reactive substance.

12

13

Nomenclature Nomenclature of Alkenesof Alkenes

14

The general formula for alkenes is:

CnH2n

15

IUPAC Rules for Naming Alkenes

1. Select the longest continuous carbon-carbon chain that contains the double bond.

2. Name this parent compound as you would an alkane, but change the –ane ending to –ene.

CH3CH2CH3 propane CH3CH=CH2 propene

16

IUPAC Rules for Naming Alkenes

3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name.

CH3CH=CHCH3 2- butene

CH3CH2CH2CH=CH2 1-pentene

17

IUPAC Rules for Naming Alkenes

4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded.

H3C CHH2C

HC CH2

CH3

4-methyl-1-pentene

12345

H2C

H2C CH

HC CH2

12345H3C

H2CH2C CH3

3-propyl-1-hexene

6

18

How would we write the structural formula for 4-methyl-2-pentene?

• The name indicates:– Five carbons in the longest chain

– A double bond between carbons 2 and 3

– A methyl group on carbon 4

19

Write a structural formula for: 7-methyl-2-octene

• Octene indicates an 8-carbon chain

• The chain contains a C=C between carbons

2 and 3.

• There is a –CH3 group on carbon 7

H3C

HC

CH

H2C

CH2

H2C

CH

CH3

CH3

20

• Longest chain containing C=C is 5 carbons

• 2-ethyl-1-pentene

Name this compound:

H3C

H2C

C

H2C

CH2

CH3

CH2

2

1

3

4

5ethyl

21

Geometric Isomerism Geometric Isomerism in Alkenesin Alkenes

22

Geometric Isomerism in Alkenes• Compounds containing a carbon-carbon double

bond (pi bond) have restricted rotation about that double bond.

• This restricted rotation in a molecule gives rise to a type of isomerism known as geometric isomerism.

• Isomers that differ from each other only in the geometry of their molecules and not in the order of their atoms are known as geometric isomers.– They are also called cis-trans isomers.

23

Trans is a Latin noun or prefix, meaning “across”, “beyond” or “on the opposite side”.

Cis is a Latin prefix, meaning “on the same side [as]” , “on this side [of]”, or “near side [of]”.

24

Geometric Isomers in Alkenes

Cl

C

H

C

H

Cl H

C

Cl

C

H

Cl

cis-1,2-dichloroethene(bp = 60.1 C)

trans-1,2-dichloroethene(bp = 48.4 C)

25

An alkene shows cis-trans isomerism when each carbon atom of the double bond has two different kinds of groups attached to it.

a

C

b

C

b

a a

C

b

C

a

b

cis isomer trans isomer

26

An alkene does not show cis-trans isomerism if even one carbon of the double bond has two identical groups attached to it.

H

C

H

C

H

H H3C

C

H3C

C

H

CH3

two groupsthe same

27

Draw a structure for cis-5-chloro-2-hexene

• The compound contains 6 carbons with a C=C between carbons 2 and 3, and a Cl atom on carbon 5.

28

Is the compound below the cis or trans isomer?

H

C C

CH3

H2C

H3C

CH3

trans-3-methyl-2-pentene

1

2 3

54

29

C CH H

ter terC C

H

HAshley

Samantha

Other Alkenes

Cisters Transisters

30

CycloalkenesCycloalkenesCycloalkenesCycloalkenes

31

Cycloalkenes• As the name implies, cycloalkenes are cyclic

compounds that contain a C=C in the ring.

• The carbons of the double bond are assigned numbers 1 and 2.

1

2

1

2

cyclopentene

cyclohexene

32

Cycloalkenes

1

2

1

2

CH3

CH3

CH3

3

4

5 4

5

6

3

1-methylcyclopentene

1,3-dimethylcyclohexene

33

Preparation and Preparation and Physical Properties of Physical Properties of

AlkenesAlkenes

34

Preparation of Alkenes

• Cracking

• Dehydration of Alcohols

35

Cracking• Cracking, or pyrolysis, is the process in which saturated

hydrocarbons are heated to very high temperatures in the presence of a catalyst (usually silica-alumina):

Alkane (CnH2n+2) Mixture of alkenes + Alkanes + H2 (g)

2CH3CH2CH3 CH3CH=CH2 + CH2=CH2 + CH4 + H2

Heat

catalyst

~500°C

36

Dehydration of Alcohols

• Dehydration involves the elimination of a molecule of water from a reactant molecule.

H3C C

H

H

C

OH

H

CH3

conc. H2SO4

heatH3C C

H

C

H

CH3 + H2O

37

Physical Properties of Alkenes

• Alkenes have physical properties very similar to the corresponding alkanes.

38

Chemical Properties Chemical Properties of Alkenesof Alkenes

39

Addition Reactions of Alkenes

• Addition at the C=C bond is the most common reaction of alkenes.– H2

– Br2 and Cl2

– HBr, HCl

– H2O

40

Addition of H2

• Hydrogenation

H3C C

H

C

H

CH3Pt, 25 C

1 atmH3C C

H

C

H

CH3+ H2

H H

41

Addition of X2

Bromine changes from a red-orange (flask on the left) to colorless when added to an alkene as shown in the flask on the right.

Bromination (Br2) or Chlorination (Cl2)

42

Addition of HX

• Hydrobromination (HBr) or Hydrochlorination (HCl)

H3C C

H

C

H

CH3 H3C C

H

C

H

CH3+ HCl

H Cl

43

Addition of H2O

H3C C

H

C

H

CH3 H3C C

H

C

H

CH3+ H2O

H OH

H+

44

Addition of HX to an Unsymmetrical Alkene

Why????

H3CHC CH2 + H-Cl

H3CHC CH2

HCl

H3CHC CH2

ClH

(about 100% yield)

(trace)Them that has, gets!

45

Markovnikov’s Rule

• When an unsymmetrical molecule such as HX (HCl) adds to a carbon-carbon double bond, the hydrogen from HX goes to the carbon atom that has the greater number of hydrogen atoms.

Vladimir Markovnikov

46

Markovnikov’s Rule

H3CHC CH2 + H-Cl

H3CHC CH2

HCl

H3CHC CH2

ClH

(about 100% yield)

(trace)This reaction proceeds via the formation of the most stable carbocation intermediate (2°).

47

Write formulas for the organic products formed when

2-methyl-1-butene reacts with:

a) H2, Pt/25°C

b) Cl2

c) HCl

d) H20, H+

48

2-methyl-1-butene + H2, Pt/25 °C

H2C C

CH3

CH2CH3 + H2 H2C C

CH3

CH2CH3

H H

Pt

2-methylbutane

49

2-methyl-1-butene + Cl2

H2C C

CH3

CH2CH3 + Cl2 H2C C

CH3

CH2CH3

Cl Cl

1,2-dichloro-2-methylbutane

50

2-methyl-1-butene + HCl

H2C C

CH3

CH2CH3 + HCl H2C C

CH3

CH2CH3

H Cl

2-chloro-2-methylbutane

51

2-methyl-1-butene + H2O

H2C C

CH3

CH2CH3 + H2O H2C C

CH3

CH2CH3

H OH

H+

2-hydroxy-2-methylbutane

52

OxidationOxidationOxidationOxidation

53

Oxidation at the C=C Bond

• Baeyer Test

H2C CH2 + KMnO4 (aq) + H2O H2C CH2

OH OH

+ MnO2 + KOHethene(ethylene)

(purple)

1,2-ethanediol(ethylene glycol)

(brown)

+ –

54

Alkynes: Nomenclature Alkynes: Nomenclature and Preparationand Preparation

55

The rules for naming alkynes are the same as those for alkenes, but the ending –yne is used to indicate the presence of a triple bond.

IUPAC Rules for Naming Alkynes

H3CH2C C CH

1-butyne

56

57

Physical and Chemical Physical and Chemical Properties of AlkynesProperties of Alkynes

Physical and Chemical Physical and Chemical Properties of AlkynesProperties of Alkynes

58

Physical Properties of Alkynes

• Acetylene is a colorless gas with little odor when pure.

• Acetylene is insoluble in water and is a gas at normal temperature and pressure.

59

Chemical Properties of Alkynes• Alkynes undergo addition reactions

rather similar to those of alkenes.

–Cl2 and Br2

–HCl and HBr–Positive reaction with Baeyer’s

test.

60

Bromination of Acetylene

HCCH + Br2 CHBr=CHBr

HCCH + 2 Br2 CHBr2-CHBr2

61

Bromine Potassium permanganate (Baeyer’s test)

Tests for Unsaturation

62

HCl Addition to Unsymmetrical Alkynes

• This addition follows Markovnikov’s rule:

CH3CCH + HCl CH3CCl=CH2

CH3CCH + 2 HCl CH3CCl2-CH3

63

Aromatic Hydrocarbons: Aromatic Hydrocarbons: StructureStructure

Aromatic Hydrocarbons: Aromatic Hydrocarbons: StructureStructure

64

Benzene, or benzol, is an compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H.

Michael Faraday (1791 –1867 ) first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, giving it the name bicarburet of hydrogen.

The empirical formula for benzene was long known, but its highly polyunsaturated structure, with just one hydrogen atom for each carbon atom, was challenging to determine. Several in 1861 suggested possible structures that contained multiple double bonds or multiple rings, but the study of aromatic compounds was in its very early years, and too little evidence was then available to help chemists decide on any particular structure.

1820

65

Friedrich Auguste Kekulé

1829-1896

Friedrich August von Kekule had a dream of whirling snakes, of the structure of benzene - the organic chemical compound made up of a ring of carbon atoms. He reported the dream in the following words many years after it took place, in a speech at a dinner commemorating his discovery.

I turned my chair to the fire (after having worked on the problem for some time) and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly to the background. My mental eye, rendered more acute by repeated vision of this kind, could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... Let us learn to dream, gentlemen.

66

67

Kekulé, moments before his brilliant insight into the structure of benzene.

68

Aromatic Compounds

Benzene is• an aromatic compound.

• a ring of 6 C atoms and 6 H atoms.• a flat ring structure drawn with three double bonds. • represented by two structures because the electrons

are shared among all the C atoms.

69

Bonding in Benzene

• The electrons are not attached to particular carbon atoms, but are delocalized and associated with the entire molecule.

• This electronic structure imparts unusual stability to benzene and is responsible for many of the characteristic properties of aromatic compounds.

70

Bonding in Benzene

(a) sp2-sp2 orbital overlap to form the carbon ring structure.

71

Bonding in Benzene

(b) carbon-hydrogen bonds formed by sp2-s orbital overlap and overlapping p orbitals.

72

Bonding in Benzene

(c) pi electron clouds above and below the plane of the carbon ring.

73

Naming Aromatic Naming Aromatic CompoundsCompounds

Naming Aromatic Naming Aromatic CompoundsCompounds

74

Naming Substituted Benzene Compounds• A substituted benzene is derived by

replacing one or more hydrogen atoms of benzene by another atom or group of atoms.

• Monosubstituted benzene has the formula C6H5G, where G is the group replacing a hydrogen atom.

75

Monosubstituted Benzenes

• Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.

O2N

CH2CH3

Cl

Br

nitrobenzene ethylbenzene chlorobenzene bromobenzene

76

• Certain monosubstituted benzenes have special names.

CH3

OH

H2N

C

O

HC

O

OH

benzoic acid benzaldehyde

CH=CH2

styrene phenol aniline

toluene

77

The word phenyl represents the C6H5- group. It is used

to name benzene derivatives that would otherwise be

difficult to name.

CH

CH3

CHCH2CH3

CH2

diphenylmethane

Cl

3-chloro-2-phenylpentane

1

23 4 5

This is the phenyl group which is a benzene ring minus ahydrogen atom .

These are examples of benzene derivatives that are easier to name using the phenyl group.

78

Disubstituted Benzenes• The prefixes ortho-, meta-, and para-

(abbreviated o-, m-, and p-) are used to name disubstituted benzenes.

G

ortho

meta

para

meta

ortho

79

Dichlorobenzenes, C6H4Cl2

• The three isomers of dichlorobenzene have different physical properties.

Cl

ortho-dichlorobenzene

Cl Cl

Cl

Cl

Clpara-dichlorobenzene

meta-dichlorobenzene

80

Disubstituted Benzenes• When the two substituents are different and

neither is part of a compound with a special name, the names of the two substituents are given in alphabetical order, followed by the word benzene.

Cl

ortho-bromochlorobenzene

CH2CH3

Br

NO2

para-ethylnitrobenzene

81

Isomers of Dimethyl Benzene

The dimethylbenzenes have the special name xylene.

CH3

ortho-xylene

CH3

CH3

CH3

para-xylenemeta-xylene

CH3

C

82

Isomers of Methylphenol

The methylphenols have the special name cresol.

CH3

ortho-xylene

CH3

CH3

CH3

para-xylenemeta-xylene

CH3

C

OH

OH

OH

ortho-cresol meta-cresol para-cresol

83

Disubstituted Benzenes• When one of the substituents corresponds to a

monosubstituted benzene that has a special name, the disubstituted compound is named as a derivative of that parent compound.

84

Polysubstituted Benzenes• When there are more than two

substituents on a benzene ring, the carbon atoms in the ring are numbered starting at one of the substituted groups.

• Numbering must be done in the direction that gives the lowest possible numbers to the substituent groups.

85

Polysubstituted Benzenes

CH3

O2N NO2

NO2

2,4,6-trinitrotoluene (TNT)

12

3

4

5

6

OH

Cl

5-bromo-2-chlorophenol

12

35

6

Br4

86

Polycyclic Aromatic Polycyclic Aromatic CompoundsCompounds

Polycyclic Aromatic Polycyclic Aromatic CompoundsCompounds

Polycyclic Aromatic Hydrocarbons PAHs

mothballs dyes carcinogen

Product of combustion of coal tar, tobacco smoke, barbecued meats.

Potent carcinogen

88

Sources and Physical Sources and Physical Properties of Aromatic Properties of Aromatic

HydrocarbonsHydrocarbons

Sources and Physical Sources and Physical Properties of Aromatic Properties of Aromatic

HydrocarbonsHydrocarbons

89

Sources of Aromatic Hydrocarbons• The aromatic hydrocarbons, such as

benzene, toluene, xylene, naphthalene, and anthracene, were first obtained in significant quantities from coal tar.

• Coal Coke + Coal gas + Coal tar• Because of the great demand for

aromatic hydrocarbons, processes were devised to obtain them from petroleum.

90

Properties of Aromatic Hydrocarbons

• Aromatic hydrocarbons are essentially nonpolar substances, insoluble in water but soluble in many organic solvents.

• They are liquids or solids and usually have densities less than that of water.

• Aromatic hydrocarbons burn readily, usually with smoky (sooty) yellow flames as a result of incomplete carbon combustion.

91

Chemical Properties of Chemical Properties of Aromatic HydrocarbonsAromatic HydrocarbonsChemical Properties of Chemical Properties of Aromatic HydrocarbonsAromatic Hydrocarbons

92

Substitution Reactions of Aromatic Hydrocarbons

• Halogenation – net addition of -Br or -Cl

• Nitration – net addition of –NO2

• Alkylation– net addition of –R (alkyl group)

93

Halogenation of Benzene• When benzene reacts with chlorine or

bromine in the presence of a catalyst such as iron (III) chloride or iron (III) bromide, a Cl or Br atom replaces an H atom to form the products.

+ X2

X

+ HXFeX3

benzene

bromine orchlorine

bromobenzene orchlorobenzene

94

Nitration of Benzene• When benzene reacts with a mixture of

concentrated nitric acid and concentrated sulfuric acid at about 50C, nitrobenzene is formed.

+ HO-NO2

NO2

+ H2OH2SO4

benzene

nitric acid

nitrobenzene

95

Alkylation of Benzene• Alkylation of benzene is known as the

Friedel-Crafts reaction.

• The alkyl group from an alkyl halide (RX), in the presence of AlCl3 catalyst, substitutes for an H atom on the benzene ring.

+ CH3CH2Cl

CH2CH3

+ HCl

benzene

chloro-ethane

ethylbenzene

AlCl3

96

Mr. Ortho Paranoid

97

Ms. Molly Meta

98

Ortho/Para Directors

• Groups present on the benzene ring as substitution reactions are occurring, may direct oncoming groups to the ortho or para positions.

• These groups usually have unshared electron pairs next to the ring.

• Examples include: –OH, –NH2, –OCH3, – CH3, –Br, and –Cl

99

Examples of Ring Directors

Br

+ HNO3H2SO4

Br

NO2

+

Br

NO2

OH

+ 3 Cl 2FeCl 3

OH

ClCl

Cl

100

Meta Directors

• Groups present on the benzene ring as substitution reactions are occurring, may direct oncoming groups to the meta position by deactivating the ortho and para positions.

• These groups have a positive charge next to the ring.

• Examples include: –NO2 and –CN

101

Examples of Ring Directors

NO2

+AlCl 3

NO2

CH2CH3

CH3CH2Cl

102

Side-Chain Oxidation

• Carbon chains attached to an aromatic ring are fairly easy to oxidize.

CH2CH3

ethylbenzene

K2Cr2O7/H2SO4

heat

COOH

+ CO2

benzoic acid

103