2. Topic 5C Alkanes

3. Hydrocarbons Saturated hydrocarbons — carbon skeletons saturated with hydrogen No double bonds or triple bonds in the compound No other groups (oxygen, nitrogen etc). 23 Paraffins — “parum affinis meaning ‘little affinity’ ” Aliphatics “aliphar meaning ‘fat or oil’ ”.

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

5. Constitutional Isomers (Structural isomers) Next member of series is C 4 H 10. Two structural isomers are possible: 24 butane or n-butane CH 3 3 CHCH 3 isobutane

6. Constitutional isomers of C 5 H 12 Three constitutional isomers for C 5 H 12 : 24 CH 3 C 3 3 3 2,2-dimethylpropane (neopentane) and 2-methylbutane (isopentane) CH 3 3 CHCH 2 CH 3

7. Constitutional isomers of C 5 H 12 24 CH 3 3 CHCH 2 CH 3 isopentane CH 3 C 3 3 3 neopentane

8. Numbers of isomers C 6 H 14 has five isomers C 7 H 16 has nine isomers C 10 H 22 has seventy five isomers C 40 H 82 has 62x10 12 isomers! 25 Note the general formula — C n H 2n+2

9. 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 26

10. 1C —Stem name is meth- CH 4 2C — eth- CH 3 CH 3 3C —prop- CH 3 CH 2 CH 3 4C —but- CH 3 (CH 2 ) 2 CH 3 5C —pent- CH 3 (CH 2 ) 3 CH 3 6C —hex- CH 3 (CH 2 ) 4 CH 3 7C —hept- CH 3 (CH 2 ) 5 CH 3 8C —oct- CH 3 (CH 2 ) 6 CH 3 9C —non- CH 3 (CH 2 ) 7 CH 3 10C —dec- CH 3 (CH 2 ) 8 CH 3 26 Suffix— “-ane” indicates an alkane. Stem — indicates number of carbons in longest chain.

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

12. Examples: 26 1 2 3 4 5 6 3-methylhexane CH 3 3 2 CHCH 2 CH 2 3 7463152 3 3 3 CHCH 2 CH 2 CHCH 2 CH 3 2,5- dimethylheptane (not 3,6-dimethylheptane) Note the use of “di-” — “tri-, tetra-, penta- etc also used

13. Different substituents are listed alphabetically with their locants. 26 CH 3 CHCH 2 CH 2 CHCH 2 CH 2 3 2 3 2 3 543 21 9867 Name is 6-ethyl-3-methylnonane (e comes beforem alphabetically) Thus

14. 2 3 5 Multipliers “di-”, “tri-”, “tetra-” are disregarded. 27 CH 3 3 2 3 3 CHCHCH 2 CHCH 2 CH 3 1 7 Name is 5-ethyl-2,3-dimethylheptane

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

16. Straight chain substituents: 27 CH 4 MethaneCH 3 -Methyl CH 3 3 EthaneCH 3 2 -Ethyl CH 3 2 3 PropaneCH 3 2 2- Propyl CH 3 (CH 2 ) 2 CH 3 ButaneCH 3 (CH 2 ) 2 CH 2 -Butyl CH 3 (CH 2 ) 3 CH 3 PentaneCH 3 (CH 2 ) 3 CH 2 -Pentyl R-HAlkaneR-Alkyl ParentSubstituent Substituent names

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

18. Branched substituents: 29 CH 2 3 3 (CH 3 ) 2 CHCH 2 — isobutyl CH 3 CH 2 CHCH 3 secondary-butyl (also s-butyl or sec-butyl) CCH 3 3 3 (CH 3 ) 3 C— tertiary-butyl (also t-butyl or tert-butyl)

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

20. Cycloalkanes Note the general formula — C n H 2n 30 Cyclobutane C 4 H 8 Cyclohexane C 6 H 12

21. Prefix alkane name with “cyclo” 31 Cycloalkanes H2CH2CCH 2 2 Cyclopropane CH 2 2 2 2 3 54 3 2 1 Methylcyclopentane

22. In monosubstituted rings, the carbon bearing the substituent is by convention 1. In disubstituted rings, number towards nearest substituent. 31 Numbering rings CH 2 2 2 2 3 54 3 2 1 2 2 2 2 2 3 3 3 4 5 6 1 2 3 Highest alphabetical substituent in position 1. 1-ethyl-3-isopropylcyclohexane

23. Skeletal structures are often used for rings: 31 Ring presentations CH 3 CH 2 CH 3 CH(CH 3 ) 2 or

24. CyclopropaneCyclopropyl CyclobutaneCyclobutyl CyclopentaneCyclopentyl CyclohexaneCyclohexyl 32 Rings as substituents (where chain is bigger than the ring) CH 3 CH CH 2 CH 2 CH CH 2 CH 3 12 7 2-cyclopentylheptane

25. 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 33 Stereochemistry:

26. Conformational isomerism Conformational isomers (conformers): isomers that differ because of rotation about single bonds. Conformers are generally interconvertible without bond breaking. 33

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

28. Staggered conformer Eclipsed conformer Conformational isomerism Alkanes 33

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

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

31. 34 Conformation in cycloalkanes C 6 H 6 – "chair" shape Flat carbon ring Almost flat ring

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

33. Chair form of cyclohexane 35 Axial and equatorial bond

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

35. Boat cyclohexane In the “boat” conformer the sides of the boat are eclipsed High energy conformation 35 H H H H H H H H H H H H H H Boat cyclohexane H H H H H H Eclipsed ethane

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

37. Energetics As cyclohexane moves through chair to boat to chair, the energy varies 36 Chair IITwist IIBoatTwist IChair I Conformers Potential energy kJ/mol 6.7 23 46

38. 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 37

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

40. 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:

41. 37 Geometrical isomerism in Cycloalkanes CH 3 H 3 H Cis-1,2-dimethylcyclopropane H CH 3 CH 3 H Trans-1,2-dimethylcyclopropane

42. 38 Cycloalkanes H CH 2 3 3 H 2 3 1 Trans-1-ethyl-3-methylcyclohexane CH 3 2 3 H H Or

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

44. Properties of alkanes Boiling point increases with molecular weight: 39 CH 4 n-C 4 H 10 n-C 7 H 16 n-C 10 H 22 bp (˚C)– 162 0 98 174

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

46. 39 Unbranched hydrocarbons have higher boiling points than branched alkanes They can align themselves more closely: Physical properties n - pentaneneopentane

47. 39 UnbranchedBranched Unbranched can align themselves more closely Stronger intermolecular attractive forces are possible Physical properties of alkanes

48. 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). 39

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

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