topic 5c alkanes

Topic 5C

Alkanes

HydrocarbonsSaturated hydrocarbons carbon skeletons saturated with hydrogen No double bonds or triple bonds in the compound No other groups (oxygen, nitrogen etc).23Paraffins parum affinis meaning little affinity Aliphatics aliphar meaning fat or oil .

Constitutional IsomerismStraight chains are formed by successive replacement of hydrogen by a methyl group.Simplest are:23

Constitutional Isomers(Structural isomers)Next member of series is C4H10.Two structural isomers are possible:24

Constitutional isomers of C5H12Three constitutional isomers for C5H12:24

Constitutional isomers of C5H1224

Numbers of isomersC6H14 has five isomersC7H16 has nine isomersC10H22 has seventy five isomers C40H82 has 62x1012 isomers!25Note the general formula CnH2n+2

Naming alkanesIUPAC 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 26

1C Stem name is meth- CH42C eth- CH3 CH33C prop- CH3 CH2 CH34C 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.

Prefixindicates nature and position of a substituent on the chain.Number chain to give position with the substituent the lowest number.

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Examples:

261 2 3 4 5 6

Different substituents are listed alphabetically with their locants. 26

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

27Name is 5-ethyl-2,3-dimethylheptane

Locant is repeated with multiple substituents at same carbon.

2717Name is 5-ethyl-2,2-dimethylheptane

Substituent namesStraight chain substituents:

27CH4MethaneCH3-MethylCH3CH3EthaneCH3CH2-EthylCH3CH2CH3PropaneCH3CH2CH2-PropylCH3(CH2)2CH3ButaneCH3(CH2)2CH2-ButylCH3(CH2)3CH3PentaneCH3(CH2)3CH2-PentylParentSubstituent

Branched substituents:

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Branched substituents:

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Substituent groups:29

Cycloalkanes Note the general formula CnH2n30Cyclobutane C4H8

CycloalkanesPrefix alkane name with cyclo

31Cyclopropane

Numbering ringsIn monosubstituted rings, the carbon bearing the substituent is by convention 1.In disubstituted rings, number towards nearest substituent.

31CHCHCH456123Highest alphabetical substituent in position 1. 1-ethyl-3-isopropylcyclohexane

Ring presentationsSkeletal structures are often used for rings:

31CH3CH2CH3CH(CH3)2oror

Rings as substituents(where chain is bigger than the ring)CyclopropaneCyclopropylCyclobutaneCyclobutylCyclopentaneCyclopentylCyclohexaneCyclohexyl 322-cyclopentylheptane

Conformation in alkanesStudy of three-dimensional shape of molecules and how this affects their chemical and physical propertiesVery important in biologyIsomers that have the same formula and connectivity but differ only in the way the atoms are arranged in space are STEREOISOMERSConstitutional isomers having different connectivity and are joined up in a different way are NOT stereoisomers33Stereochemistry:

Conformational isomerismConformational isomers (conformers): isomers that differ because of rotation about single bonds. Conformers are generally interconvertible without bond breaking.

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Conformational isomerismAlkanesRotation about single bonds leads to different conformations.33

Conformational isomerismAlkanesStaggered conformerEclipsed conformer33

Energetics of rotationThe energy varies. In the eclipsed conformation there is more repulsion than in the staggered conformation.34RotationPotential energy

Energetics of rotationButane rotation about the middle bondEclipsed methyls raise the rotation barrier34Butane:Movie from SaundersGeneral Chemistry CD-ROM

Conformation in cycloalkanes34C6H6 "chair" shapeFlat carbon ringAlmost flat ring

Conformation in cyclohexaneChair conformation involves fully staggered CC bondsThis is the lowest energy conformation35

Chair form of cyclohexane35Axial and equatorial bond

Two chair conformationsInterchanging chair conformationsAxial and equatorial atoms are interchanged35

Boat cyclohexaneIn the boat conformer the sides of the boat are eclipsed High energy conformation35

High energy boatThe flagpole hydrogens strongly interfere, raising the energy further 36Twisting can reduce repulsion slightly

EnergeticsAs cyclohexane moves through chair to boat to chair, the energy varies36Chair IITwist IIBoatTwist IChair IConformersPotential energy kJ/mol6.72346

Monosubstituted cycloalkanesTwo chair forms are possibleThe substituent is more stable in the equatorial positionAxial-axial interactions destabilise the chair form with the substituent in an axial position

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Importance of chairsSteroidal hormones all contain chair-shaped cyclohexanesCholesterolHOH

Geometrical IsomersCycloalkanes cis/trans There are two faces to cyclic alkanesWhen two substituents are on the same face the isomer is termed cis. When on opposite faces, we have the trans isomer:

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Geometrical isomerism in Cycloalkanes37

Cycloalkanes38

Conformations in geometrical isomersDi-equatorial favoured over di-axial conformations

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Properties of alkanesBoiling point increases with molecular weight:39 CH4n-C4H10n-C7H16n-C10H22bp (C) 162 0 98 174

Physical properties of alkanesLow molecular weight alkanes are gases.Boiling point (and melting point) increases with molecular weight.39Notes:

Unbranched hydrocarbons have higher boiling points than branched alkanesThey can align themselves more closely:39Physical propertiesn-pentaneneopentane

Physical properties of alkanesUnbranched can align themselves more closelyStronger intermolecular attractive forces are possible 39UnbranchedBranched

Chemical properties of alkanesGenerally unreactive towards: Strong acids Strong alkalis Mild oxidising agents Halogens (F, Cl, Br, I) in the darkOxidation (addition of O or removal of H).Halogenation (using light).39

OxidationCombustion: 40

OxidationDehydrogenation: 40(Removal of hydrogen is also oxidation)2H.