The Alkanes (Ch 3)

1. Part A #2 The Alkanes (Ch 3) • - Hydrocarbons (binary compounds composed of C and H only) • The stability of C-C bond with respect to the C-H bond makes possible • the formation of long chains • Single bond only (sp3): alkane • At least one double bond (sp2): alkene • At least onetriple bond (sp): alkyne

2. Drawing molecules Complete drawings Zig-zag drawings (only C-C bonds visible) Stereo-drawing

3. Functional groups Alkane Alkyl group Abbr. CH4 (methane) CH3- (methyl) Me- CH3CH3 (ethane) CH3CH2- (ethyl) Et- CH3CH2CH3 (propane) CH3CH2CH2- (propyl) Pr- CH3CH2CH3 (propane) CH3CHCH3 (isopropyl) i-Pr- R (phenyl) Ph- benzene

4. Chemistry of Alkanes Source: petroleum distillation, refining and thermal cracking Shape: C-C single bonds, sp3 hybrids, zigzag chains

5. Isomerism C6H14 Chain length C4 C5 C6 C7 C8 C9 C10 # of isomers 2 3 5 9 18 35 75

6. Alkane nomenclature Meth-ane C1 Eth-ane C2 Prop-ane C3 But-ane C4 Pent-ane C5 Hex-ane C6 Hept-ane C7 Oct-ane C8 Non-ane C9 Dec-ane C10 Undecane C11 Dodecane C12 Tridecane C13 Tetradecane C14 Etc. Linear alkanes

7. What about branchedalkanes? (IUPAC) Rule # 1 Locate the longest chain and name it accordingly CH3CH2CH2CH2CH2CH3 (hexane) CH3 substituents CH3CH2CH2CH2CHCH3 (heptane) CH2 CH3

8. Rule # 2 Number the longest chain beginning with the end nearer to the substituent (the heavier if more than one) 6 5 4 3 2 1 7 6 5 4 3 CH3CH2CH2CH2CHCH3 CH3CH2CH2CH2CHCH3 CH3 2 CH2 1 CH3

9. What about branched alkanes? Rule # 3 Use the # for locating the substituents 6 5 4 3 2 1 7 6 5 4 3 CH3CH2CH2CH2CHCH3 CH3CH2CH2CH2CHCH3 CH3 2 CH2 2-methylhexane 1 CH3 3-methylheptane

10. What about branched alkanes? Rule # 4 when 2 or more subs. are present, give each subst a # corresponding to its location on the longest chain CH3CH-CH2-CHCH2CH3 CH3 CH2 4-ethyl-2-methylhexane CH3 Alphabetical order NOT 3-ethyl-5-methylhexane (first point of branching must have smaller #)

11. Rule # 5 When two subs are present on the same C atom, use that # twice CH3 CH3CH2 – C – CH2CH2CH3 CH2CH3 3-ethyl-3-methylhexane

12. Rule # 6 When two or more subs are identical use the suffix di-, tri-, tetra-, etc. CH3 CH3 CH3 CH3CH – CHCH3 CH3CHCHCHCH3 CH3CCH2CCH3 CH3 CH3 CH3 CH3 CH3 CH3 2,3-dimethylbutane2,3,4-trimethylpentane2,2,4,4,-tetramethylpentane

13. Rule # 7 When there are two chains of equal length, choose that with the greater #of subs as the main. 7 6 5 4 3 2 1 CH3CH2 –CH – CH – CH – CH – CH3 CH3CH2CH3 CH3 CH2 CH3 2,3,5-trimethyl-4-propylheptane

14. Rule # 8 When branching occurs at an equal distance from either end of the longest chain, choose the name that gives the lower # at the first point of difference First point of difference 6 5 4 3 2 1 CH3 – CH – CH2 – CH – CH – CH3 CH3 CH3 CH3 2,3,5-trimethylhexane and NOT 2,4,5-trimethylhexane

15. Some « common » names CH3 CH3 – C – CH2- CH3CH2CH2- Propyl or n-propyl CH3CHCH3 1-methylethyl or isopropyl CH3 Neo-pentyl - CH3CH2CH2CH2 -n-butyl CH3CH2CHCH3 sec-butyl CH3CHCH2-iso-butyl CH3 – C – CH3 tert-butyl - CH3CH2CH2CHCH2CH2CH3 CH-CH3 CH3 CH3 CH3 4-isopropylheptane or 4-(1-methylethyl)heptane - CH3 CH3

16. Physical Properties Boiling Points are determined by the inducedDipole-Dipole forces: largermolecules, more surface, more attraction, more E necessary to melt or to boil.

17. Melting Points: same story except …. Molecules with even # of atoms pack closer in the crystal structure.

18. Conformational Analysis - Free (almost) rotation along the C-C bond cause the molecule assuming many temporary shapes called : conformations. - Different conformations may have different stability (Potential Energy) that is determined by steric repulsions The two extreme conformations during the rotation along a C-C bond are: eclipsed and staggered

19. Potential Energy Profile movie

20. Newman projections Both conformations are staggered More stable Less stable The two conformers interconvert very rapidly even at very low T. Only at extreme low T they may perhaps be isolated

21. Conformations of n-Butane Staggered Eclipsed anti gauche gauche

22. movie

23. Stability of isomers Butane Isobutane

24. Stability of Cycloalkanes Name size heatheat/CH ring strain Cyclopropane 3 2091 697 115 Cyclobutane 4 2744 686 109 Cyclopentane 5 3320 664 27 Cyclohexane 6 3956 658.7 0 Cycloheptane 7 4637 662 27 Cyclooctane 8 5310 663 42 Cyclononane 9 5981 664 54 Cyclodecane 10 6636 663 50 Cyclopentadecane 15 9885 659 6 kJ/mol kJ/mol kJ/mol Diff. Take cyclopropane exp heat of comb = 2091 calcd heat of comb = 658.7 x 3 = 1976

25. Why is that? Cyclopropane: 1. angles are 60 deg rather than 109 2. eclipsed conformation

26. Cyclobutane • Less angle constraint (88 deg) • Less eclipsing

27. Cyclopentane • Angles are okay (108 deg) • However, the moleculecannotbeplanar (must avoidhuge • amount of E increasearisingfromeclipsing 10 H atoms

28. Cyclohexane Two fairly stable conformations: Chair and boat The switching chair-to-boat-to-chair interchanges equatorial and axial positions

29. Conformational analysis for cyclohexane

30. Substituted cyclohexanes

31. Substituted cyclohexanes

32. Disubstitution: isomerism cis and trans Cistrans 1,2-dimethyl cyclopentane 1,3-dimethyl cyclopentane

33. Cistrans 1,2-dimethyl 1,3-dimethyl 1,4-dimethyl

34. Conformations for cis and trans Let’s consider 1,4-dimethylcyclohexane Trans e,e a,a Cis e,a a,e

35. Conformations for cis and trans Let’s consider 1,3-dimethylcyclohexane Trans e,a a,e Cis e,e a,a

36. Conformations for cis and trans Let’s consider 1,2-dimethylcyclohexane Trans e,e a,a Cis e,a a,e

37. Alkane Synthesis • Hydrogenation of alkenes (addition) • Reduction of alkyl halides (substitution?)

38. Alkane Synthesis • Wurtz coupling (???) • Carbanions with alkyl halides (Substitution)

39. Isomerism Ch 5

40. Stereoisomers Constitutional isomers Stereoisomers

41. Chirality - Chiral molecules are not superposable to their mirror image - The mirror image of the chiral molecule is called enantiomer Examples of chiral objects: screwdriver, golf club, shoe, ear, car

42. Let’s consider 2-butanol Let’s focus on this C atom

44. A criterion for deciding about chirality The presence of a C atoms bearing 4 different groups H stereocenter * CH3-C-CH2CH3 OH Interconversion of one enantiomer into the other cannot be done (unless cleaving and reforming bonds)

45. If two groups are identical, the molecule is achiral (superposable to its mirror image)

46. Chirality test (mirror plane)

47. Nomenclature (R and S Cahn, Ingold, Prelog) R = rectus (right) S = sinister (left) Let’s consider 2-butanol To assign the R,S label we need to sequence all the groups attached to the chiral C atom

48. Rule #1 Priority is assigned on the basis of the atomic #. (H has the lowest) a d

49. Rule #2 For two identical atoms (Me and Et both have C) priority is assigned at the first point of difference c a d b

50. Rule # 3 Let’s consider the C-d bond as a « handle » and by holding it, rotate the molecule towards your face. See if the sequence a,b,c defines a clockwise [R] or counterclockwise [S] rotation (S)-2-butanol; (R)-2-butanol;