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Topic 5C Alkanes

Topic 5C Alkanes. 23. Hydrocarbons. Saturated hydrocarbons — carbon skeletons saturated with hydrogen No double bonds or triple bonds in the compound No other groups (oxygen, nitrogen etc).

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Topic 5C Alkanes

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  1. Topic 5CAlkanes

  2. 23 Hydrocarbons • Saturated hydrocarbons — carbon skeletons saturated with hydrogenNo double bonds or triple bonds in the compoundNo other groups (oxygen, nitrogen etc). • Paraffins — “parum affinis meaning ‘little affinity’ ”Aliphatics “aliphar meaning ‘fat or oil’ ”.

  3. Methane CH 4 Ethane CH CH C H 3 3 2 6 Propane CH CH CH C H 3 2 3 3 8 23 Constitutional Isomerism • Straight chains are formed by successive replacement of hydrogen by a “methyl” group. • Simplest are:

  4. butane or n-butane CH CHCH 3 3 isobutane CH 3 24 Constitutional Isomers(Structural isomers) • Next member of series is C4H10. • Two structural isomers are possible:

  5. CH 3 CH CHCH CH 3 2 3 and C CH CH 3 3 CH 3 CH 3 2,2-dimethylpropane (neopentane) 2-methylbutane (isopentane) 24 Constitutional isomers of C5H12 • Three constitutional isomers for C5H12:

  6. CH 3 CH CHCH CH 3 2 3 CH C CH 3 3 CH 3 CH 3 isopentane neopentane 24 Constitutional isomers of C5H12

  7. 25 Numbers of isomers • C6H14 has fiveisomers • C7H16 has nine isomers • C10H22 has seventy five isomers • C40H82 has 62x1012isomers! Note the general formula — CnH2n+2

  8. 26 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

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

  10. CH 3 CH CH CH CH CH CH 3 2 2 2 2 3 methyl CH 3 CH CHCH CH CH CH 3 2 2 2 3 1 2 3 4 5 6 Name is “2-methylhexane (not 5-methylhexane) 26 • Prefix—indicates nature and position of a substituent on the chain. • Number chain to give position with the substituent the lowest number.

  11. CH 3 3-methylhexane CH CH CHCH CH CH 3 2 2 2 3 CH CH 3 3 2,5-dimethylheptane CH CHCH CH CHCH CH 3 2 2 2 3 (not 3,6-dimethylheptane) 1 2 3 4 5 6 7 Note the use of “di-” — “tri-, tetra-, penta- etc also used 26 Examples: 1 2 3 4 5 6

  12. 2 1 CH CH 2 3 6 7 8 9 Thus CH CHCH CH CHCH CH CH 3 2 2 2 2 3 3 4 5 CH CH 2 3 Name is 6- e thyl-3- m ethylnonane ( e comes before m alphabetically) 26 • Different substituents are listed alphabetically with their locants.

  13. CH 3 7 1 2 5 CH CHCHCH CHCH CH 3 2 2 3 3 CH CH CH 3 2 3 27 Multipliers “di-”, “tri-”, “tetra-” are disregarded. Name is 5-ethyl-2,3-dimethylheptane

  14. 27 Locant is repeated with multiple substituents at same carbon. 7 1 Name is 5-ethyl-2,2-dimethylheptane

  15. R-H Alk ane R- Alk yl 27 Substituent names Straight chain substituents: Parent Substituent CH Meth ane CH - Meth yl 4 3 CH CH Eth ane CH CH - Eth yl 3 3 3 2 CH CH CH Prop ane CH CH CH Prop yl 3 2 3 3 2 2- CH (CH ) CH But ane CH (CH ) CH - But yl 3 2 2 3 3 2 2 2 CH (CH ) CH Pent ane CH (CH ) CH - Pent yl 3 2 3 3 3 2 3 2

  16. CH 3 an “isopropyl” group CH CH 3 28 Branched substituents:

  17. CH 3 (CH3) 2CHCH2— isobutyl CH CH 2 CH 3 secondary-butyl (also s-butyl or sec-butyl) CH C H CHC H 3 2 3 CH 3 (CH3)3C— tertiary-butyl (also t-butyl or tert-butyl) C CH 3 CH 3 29 Branched substituents:

  18. CH 3 tertiary (3°) : 3 carbons bonded CH C 3 to carbon attached to chain CH 3 H secondary (2°) : 2 carbons bonded C CH 3 to carbon attached to chain CH 3 H primary (1°) : 1 carbon bonded C CH 3 to carbon attached to chain. H 29 Substituent groups:

  19. Cyclohexane C6H12 30 Cycloalkanes Note the general formula — CnH2n Cyclobutane C4H8

  20. CH 2 H2C CH 2 2 CH 2 CH 1 3 3 Methylcyclopentane CH CH 2 CH CH 2 2 4 5 31 Cycloalkanes Prefix alkane name with “cyclo” Cyclopropane

  21. 2 CH 2 CH 1 3 3 CH CH 2 CH CH 2 2 4 5 CH CH 2 3 CH CH 2 2 CH CH 2 3 CH 2 CH 3 31 Numbering rings • In monosubstituted rings, the carbon bearing the substituent is by convention1. • In disubstituted rings, number towards nearest substituent. • Highest alphabetical substituent in position 1.1-ethyl-3-isopropylcyclohexane 1 CH 2 6 3 CH 5 CH 4

  22. 31 Ring presentations Skeletal structures are often used for rings: CH3 or CH2CH3 or CH(CH3)2

  23. 1 2 7 CH3 CHCH2 CH2 CH CH2 CH3 32 Rings as substituents(where chain is bigger than the ring) Cyclopropane Cyclopropyl Cyclobutane Cyclobutyl Cyclopentane Cyclopentyl Cyclohexane Cyclohexyl 2-cyclopentylheptane

  24. 33 Conformation in alkanes Stereochemistry: • 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 spaceare STEREOISOMERS • Constitutional isomers having different connectivity and are joined up in a different way are NOT stereoisomers

  25. 33 Conformational isomerism • Conformational isomers (conformers): isomers that differ because of rotation about single bonds. Conformers are generallyinterconvertible without bond breaking.

  26. H H H greater electron repulsion raises energy H H H H H H H H H 0° H 60° H H H H Newman projections along C-C bond H H H H H H H Staggered Eclipsed conformation conformation 33 Conformational isomerismAlkanes • Rotation about single bonds leads to different conformations.

  27. 33 Conformational isomerismAlkanes Staggeredconformer Eclipsedconformer

  28. H H H H H H Eclipsed -1 12.6kJmol H H H H H H H H H H H H Staggered Staggered 34 Energetics of rotation • The energy varies. In the eclipsed conformation there is more repulsion than in the staggered conformation. Potential energy Rotation

  29. 34 Energetics of rotation Butane: • Butane rotation about the middle bond • Eclipsed methyls raise the rotation barrier Movie from SaundersGeneral Chemistry CD-ROM

  30. 34 Conformation in cycloalkanes Flat carbon ring C6H6 – "chair" shape Almost flat ring

  31. H H H H H H H H H H H H H H H H H H H Chair cyclohexane H H H Staggered ethane 35 Conformation in cyclohexane • Chair conformation involves fully staggered C—C bonds • This is the lowest energy conformation

  32. 35 Chair form of cyclohexane • Axial and equatorial bond

  33. a a a a e a e a e e e e e e e a a e a e a e a a 35 Two chair conformations • Interchanging chair conformations • Axial and equatorial atoms are interchanged

  34. H H H H H H H H H H H H H H H H H H H H Boat cyclohexane Eclipsed ethane 35 Boat cyclohexane • In the “boat” conformer the sides of the boat are eclipsedHigh energy conformation

  35. Flagpole hydrogens H H H H H H H H H H H H Flagpole hydrogens further apart H H H H Twist-boat conformations 36 High energy boat • The “flagpole” hydrogens strongly interfere, raising the energy further • Twisting can reduce repulsion slightly

  36. 36 Energetics • As cyclohexane moves through chair to boat to chair, the energy varies 6.7 Potential energy kJ/mol 46 23 Chair I Twist I Boat Twist II Chair II Conformers

  37. 37 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

  38. OH O Androgens (Male) H O OH Testosterone Androsterone OH O Estrogens(Female) HO HO Estradiol Estrone Importance of chairs • Steroidal hormones all contain chair-shaped cyclohexanes Cholesterol HO H

  39. 37 Geometrical Isomers . • Cycloalkanes cis/trans • There are two faces to cyclic alkanes • When two substituents are on the same face the isomer is termedcis. When on opposite faces, we have thetrans isomer:

  40. CH CH CH H 3 3 3 H H C is -1,2-dimethylcyclopropane H CH3 Trans-1,2-dimethylcyclopropane 37 Geometrical isomerism in Cycloalkanes

  41. CH 3 CH 3 H H H 3 Or 1 2 H CH CH CH CH 2 3 2 3 T rans -1-ethyl-3-methylcyclohexane 38 Cycloalkanes

  42. OH OH OH H HO HO H H HO H H HO H H H OH cis -1,2-dihydroxy- trans -1,2-dihydroxy- cyclohexane cyclohexane H CH CH CH 3 3 3 H H CH CH H H 3 CH CH 3 3 3 H CH H H 3 trans -1,4-dimethyl- cis -1,4-dimethyl- cyclohexane cyclohexane 38 Conformations in geometrical isomers • Di-equatorial favoured over di-axial conformations

  43. 39 Properties of alkanes • Boiling point increases with molecular weight: CH4 n-C4H10 n-C7H16 n-C10H22 bp (˚C) – 162 0 98 174

  44. 39 Physical properties of alkanes • Low molecular weight alkanes are gases. • Boiling point (and melting point) increases with molecular weight. Notes:

  45. 39 Physical properties • Unbranched hydrocarbons have higher boiling points than branched alkanes • They can align themselves more closely: n-pentane neopentane

  46. 39 Physical properties of alkanes • Unbranched can align themselves more closely • Stronger intermolecular attractive forces are possible Unbranched Branched

  47. 39 Chemical properties of alkanes • Generally unreactive towards: Strong acids Strong alkalis Mild oxidising agents Halogens (F, Cl, Br, I) in the dark • Oxidation (addition of O or removal of H). • Halogenation (using light).

  48. -1 CH + 2O CO + 2H O + 886 kJmol 4 2 2 2 Methane ( Natural gas) -1 CH CH CH + 5O 3CO + 4H O + 2209 kJmol 3 2 3 2 2 2 Propane 40 Oxidation Combustion:

  49. H H heat CH CH C C 3 3 catalyst H H ethene 40 Oxidation Dehydrogenation: . 2H (Removal of hydrogen is also oxidation)

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