topic 5c alkanes

Topic 5C

Alkanes

Hydrocarbons

• Saturated hydrocarbons — carbon skeletons saturated with hydrogenNo double bonds or triple bonds in the compoundNo other groups (oxygen, nitrogen etc).

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• Paraffins — “parum affinis meaning ‘little affinity’ ”Aliphatics “aliphar meaning ‘fat or oil’ ”.

Constitutional Isomerism

• Straight chains are formed by successive replacement of hydrogen by a “methyl” group.

• Simplest are:

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CH4

CH3CH3

CH3CH2CH3

C2H6

C3H8

Methane

Ethane

Propane

Constitutional Isomers(Structural isomers)

• Next member of series is C4H10.

• Two structural isomers are possible:

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CH3CH2CH2CH3 butane or n-butane

CH3

CH3CHCH3 isobutane

QuickTime™ and aAnimation decompressor

are needed to see this picture.



Constitutional isomers of C5H12

• Three constitutional isomers for C5H12:

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CH3 C CH3

CH3

CH3

2,2-dimethylpropane (neopentane)

and

2-methylbutane (isopentane)

CH3

CH3CHCH2CH3

Constitutional isomers of C5H12

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CH3

CH3CHCH2CH3

isopentane

CH3 C CH3

CH3

CH3

neopentane





Numbers of isomers

• C6H14 has five isomers

• C7H16 has nine isomers

• C10H22 has seventy five isomers

• C40H82 has 62x1012 isomers!

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Note the general formula — CnH2n+2

Naming alkanes• IUPAC — International Union of Pure and Applied Chemistry.• Systematic Nomenclature enables accurate description.• Three parts:

prefix — describes substituents stem — identifies longest chain suffix — identifies type of compound

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1C —Stem name is meth- CH4

2C — eth- CH3 CH3

3C — prop- CH3 CH2 CH3

4C — but- CH3(CH2)2CH3

5C — pent- CH3(CH2)3CH3

6C — hex- CH3(CH2) 4CH3

7C — hept- CH3(CH2) 5CH3

8C — oct- CH3(CH2)6CH3

9C — non- CH3(CH2) 7CH3

10C — dec- CH3(CH2)8CH3

26Suffix— “-ane” indicates an alkane.Stem — indicates number of carbons

in longest chain.

• Prefix—indicates nature and positionof a substituent on the chain.

• Number chain to give position with the substituent the lowest number.

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methyl

Name is “2-methylhexane (not 5-methylhexane)

CH3CH2CH2CH2CH2CH3

CH3

CHCH2CH2CH2CH3CH3

CH3

1 2 3 4 5 6

Examples: 26

1 2 3 4 5 6

3-methylhexane

CH3

CH3CH2CHCH 2CH2CH3

74 631 52

CH3 CH3

CH3CHCH 2CH2CHCH 2CH3 2,5-dimethylheptane(not 3,6-dimethylheptane)

Note the use of “di-” — “tri-, tetra-, penta- etc also used

• Different substituents are listed alphabetically with their locants.

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CH3 CHCH2CH2CHCH2CH2CH3

CH2CH3

CH2CH3

543

2 1

986 7

Name is 6-ethyl-3-methylnonane(e comes before m alphabetically)

Thus

2

3

5

Multipliers “di-”, “tri-”, “tetra-” are disregarded.

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CH3

CH3 CH2CH3

CH3CHCHCH2CHCH2CH3

1 7

Name is 5-ethyl-2,3-dimethylheptane

Locant is repeated with multiple substituents at same carbon.

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CH3 C CH2CH2CHCH2CH3

CH3

CH3 CH2CH3

1 7

Name is 5-ethyl-2,2-dimethylheptane

Straight chain substituents:

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CH4 Methane CH3- Methyl

CH3CH3 Ethane CH3CH2- Ethyl

CH3CH2CH3 Propane CH3CH2CH2- Propyl

CH3(CH2)2CH3 Butane CH3(CH2)2CH2- Butyl

CH3(CH2)3CH3 Pentane CH3(CH2)3CH2- Pentyl

R-H Alkane R- Alk yl

Parent Substituent

Substituent names

Branched substituents: 28

CH

CH3

CH3an “isopropyl” group

CH3CH2CH2CHCH2CH2CH3

CH(CH3)2

4-Isopropylheptane

CH3CH2CHCHCH2CH2CH3

CH(CH3)2

4-Isopropy-3-methyllheptane

CH3

Branched substituents: 29

CH CH2

CH3

CH3

(CH3) 2CHCH2— isobutyl

CH3CH2CHCH3 secondary-butyl (also s-butyl orsec-butyl)

CCH3

CH3

CH3

(CH3)3C— tertiary-butyl (also t-butyl or tert-butyl)

Substituent groups: 29

CCH3

H

CH3

secondary (2°) : 2 carbons bondedto carbon attached to chain

CCH3

CH3

CH3

tertiary (3°) : 3 carbons bondedto carbon attached to chain

CCH3

H

H

primary (1°) : 1 carbon bondedto carbon attached to chain.

Cycloalkanes

Note the general formula — CnH2n

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Cyclobutane C4H8

CH2

CH2 CH2

CH2CH3

CH2 CH2

CH3 –2H

CH2

CH2

CH2

CH2

CH2

CH2CH3

CH2

CH2

CH2

CH2

CH3

–2H

Cyclohexane C6H12

Prefix alkane name with “cyclo”

31Cycloalkanes

H2C CH2

CH2Cyclopropane

CH2

CH2

CH2 CH2

CHCH3

54

3

2

1Methylcyclopentane





• In monosubstituted rings, the carbon bearing the substituent is by convention 1.

• In disubstituted rings, number towards nearest substituent.

31Numbering rings

CH2

CH2

CH2 CH2

CHCH3

54

3

2

1

CH2

CH2CH2

CH

CH2

CH

CH2CH3

CH

CH3

CH3

4

5

6

12

3

• Highest alphabetical substituent in position 1.

1-ethyl-3-isopropylcyclohexane

Skeletal structures are often used for rings:

31Ring presentations

CH3

CH2CH3

CH(CH3)2

or

or

Cyclopropane CyclopropylCyclobutane CyclobutylCyclopentane CyclopentylCyclohexane Cyclohexyl

32Rings as substituents(where chain is bigger than the ring)

CH3 CH CH2 CH2 CH CH2 CH3

1 2 7

2-cyclopentylheptane

Conformation in alkanes

• Study of three-dimensional shape of molecules and how this affects their chemical and physical properties• Very important in biology• Isomers that have the same formula and connectivity but differ only in the way the atoms are arranged in space are STEREOISOMERS• Constitutional isomers having different connectivity and are joined up in a different way are NOT stereoisomers

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CH3CH2

CH2CH2

CH3 CH3C

CH2CH3 CH3

CCH3

CH3 CH3H CH3

2,2-dimethylpropanen-pentane 2-methylbutane

Stereochemistry:

Conformational isomerism• Conformational isomers (conformers): isomers that differ because of rotation about single bonds.

Conformers are generally interconvertible without bond breaking.

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Conformational isomerismAlkanes

• Rotation about single bonds leads to different conformations.

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H

HH

H H

H

H

HHH

HH

Staggeredconformation

60°

H

HH

HH

H

H

HH

H

HH

Eclipsedconformation

0°

greater electron repulsion raises energy




are needed to see this picture.Newman projectionsalong C-C bond

C

CHH

H

H

HH

Staggeredconformer



C

C

HH

HH

HH

Eclipsedconformer

Conformational isomerismAlkanes

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Energetics of rotation• The energy varies. In the eclipsed conformation there is more

repulsion than in the staggered conformation.

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H

HHH

HH

Staggered

Rotation

Pot

enti

al e

ner

gy

12.6kJmol-1



H

HH

H

HH

Eclipsed

H

HHH

HH

Staggered

Energetics of rotation

• Butane rotation about the middle bond• Eclipsed methyls raise the rotation barrier

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QuickTime™ and aGraphics decompressor


Butane:

Movie from SaundersGeneral Chemistry CD-ROM

34Conformation in cycloalkanes



C6H6 – "chair" shape



Flat carbon ring



Almost flat ring

Conformation in cyclohexane

• Chair conformation involves fully staggered C—C bonds

• This is the lowest energy conformation

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H

H

HH

HH

Staggered ethane

H

H

H

H

H

H

H

H

HH

H

HH

H

Chair cyclohexane

H

H



Chair form of cyclohexane 35

axial bonds

e

e

e

e

a

a

a

a

equatorialbonds

axial bonds

• Axial and equatorial bond



Two chair conformations

• Interchanging chair conformations

• Axial and equatorial atoms are interchanged

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a

e

a

e

a

ea

e

a

e

a

e

e

a

aa

e

e

a

a

e

a

ee

Boat cyclohexane

• In the “boat” conformer the sides of the boat are eclipsedHigh energy conformation

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H

H

H

H

HH

HH

H

H

H

H

H

H

Boat cyclohexane

HH

H HH H

Eclipsed ethane



High energy boat• The “flagpole” hydrogens strongly interfere, raising the energy further

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H H H H

Twist-boat conformations

Flagpole hydrogensfurther apart

• Twisting can reduce repulsion slightly

H

H

H

H

HH

HH

H

H

H

H

Flagpole hydrogens ouch!

Space filling modelboat conformation

Energetics• As cyclohexane moves through chair to boat to chair, the energy

varies

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Chair IITwist IIBoatTwist IChair I

Conformers

Pot

entia

l ene

rgy

kJ/m

ol

6.7

23

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Monosubstituted cycloalkanes

• Two chair forms are possible• The substituent is more stable in the equatorial position• Axial-axial interactions destabilise the chair form with the substituent in an axial position

C(CH3)3

H

H

H

tert-butylcyclohexane

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H

H

H

CH3

CH3

H

H

H

Methylcyclohexane

HH

O

HO

Estrone

OH

HO

Estradiol

Estrogens(Female)

Importance of chairs• Steroidal hormones all contain chair-shaped cyclohexanes

OH

OTestosterone

O

HOH

Androsterone

Androgens(Male)

Cholesterol HO

H

Geometrical Isomers 37.• Cycloalkanes cis/trans

• There are two faces to cyclic alkanes• When two substituents are on the same face the

isomer is termed cis. When on opposite faces, we have the trans isomer:

CH3

H

H

CH3

CH3

H

CH3

H

Cl

H

H

Cl

Cl

H

Cl

H

trans-dimethylcyclopropane

cis-dimethylcyclopropane

trans-1,3-dichlorocyclopentane

cis-1,3-dichlorocyclopentane

37Geometrical isomerism in

Cycloalkanes

CH3

H

CH3

H

Cis-1,2-dimethylcyclopropane





H

CH3

CH3

H

Trans-1,2-dimethylcyclopropane

38Cycloalkanes

H

CH2CH3

CH3

H2

31

Trans-1-ethyl-3-methylcyclohexane

CH3

CH2CH3

HH Or



Conformations in geometrical isomers

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• Di-equatorial favoured over di-axial conformations

CH3

H

H

CH3

CH3

H

H

CH3

cis-1,4-dimethyl-cyclohexane

H

HO

OH

HH

OH

HOH

cis-1,2-dihydroxy-cyclohexane

H

CH3

H

CH3

CH3

H

CH3

H

trans-1,4-dimethyl-cyclohexane

HO

H

H

HOH

OH

HOH

trans-1,2-dihydroxy-cyclohexane

Properties of alkanes

• Boiling point increases with molecular weight:

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CH4 n-C4H10 n-C7H16 n-C10H22

bp (˚C)– 162 0 98 174

Physical properties of alkanes

• Low molecular weight alkanes are gases.• Boiling point (and melting point) increases with

molecular weight.

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Notes:

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• Unbranched hydrocarbons have higher boiling points than branched alkanes

• They can align themselves more closely:

Physical properties

CH3 C

CH3

CH3

CH3

bp 10° 36bp °

CH3(CH2)3CH3

n-pentane neopentane

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Unbranched Branched

• Unbranched can align themselves more closely• Stronger intermolecular attractive forces are possible

Physical properties of alkanes

Nonane2,3,4-Trimethylhexane

Chemical properties of alkanes

• Generally unreactive towards:Strong acidsStrong alkalisMild oxidising agents Halogens (F, Cl, Br, I) in the dark

• Oxidation (addition of O or removal of H).• Halogenation (using light).

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Oxidation

Combustion:

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CH4 + 2O 2

Methane(Natural gas)

CO2 + 2H 2O + 886 kJmol-1

CH3CH2CH3 + 5O 2

Propane3CO2 + 4H 2O + 2209 kJmol-1

Oxidation

Dehydrogenation:

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(Removal of hydrogen is also oxidation)

CH3 CH3 C C

H

H

H

Hcatalyst

heat

ethene

2H.

topic 5c alkanes

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