Chapter 3 Structure and Stereochemistry of Alkanes

1. Organic Chemistry, 5th EditionL. G. Wade, Jr. Chapter 3Structure and Stereochemistryof Alkanes Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003,Prentice Hall

2. => Alkane Formulas • All C-C single bonds • Saturated with hydrogens • Ratio: CnH2n+2 • Alkane homologs: CH3(CH2)nCH3 • Same ratio for branched alkanes Chapter 3

3. Common Names • Isobutane, “isomer of butane” • Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain. • Neopentane, most highly branched • Five possible isomers of hexane,18 isomers of octane and 75 for decane! => Chapter 3

4. => Pentanes Chapter 3

5. IUPAC Names • Find the longest continuous carbon chain. • Number the carbons, starting closest to the first branch. • Name the groups attached to the chain, using the carbon number as the locator. • Alphabetize substituents. • Use di-, tri-, etc., for multiples of same substituent. => Chapter 3

6. => Longest Chain • The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 81.) • If there are two possible chains with the same number of carbons, use the chain with the most substituents. Chapter 3

7. 1 3 4 5 2 6 7 Number the Carbons • Start at the end closest to the first attached group. • If two substituents are equidistant, look for the next closest group. => Chapter 3

8. => Name Alkyl Groups • CH3-, methyl • CH3CH2-, ethyl • CH3CH2CH2-, n-propyl • CH3CH2CH2CH2-, n-butyl Chapter 3

9. Propyl Groups H H n-propyl isopropyl A secondary carbon => A primary carbon Chapter 3

10. Butyl Groups H H n-butyl sec-butyl A secondary carbon => A primary carbon Chapter 3

11. Isobutyl Groups H H isobutyl tert-butyl A tertiary carbon => A primary carbon Chapter 3

12. Alphabetize • Alphabetize substituents by name. • Ignore di-, tri-, etc. for alphabetizing. 3-ethyl-2,6-dimethylheptane => Chapter 3

13. 1 2 3 Complex Substituents • If the branch has a branch, number the carbons from the point of attachment. • Name the branch off the branch using a locator number. • Parentheses are used around the complex branch name. 1-methyl-3-(1,2-dimethylpropyl)cyclohexane => Chapter 3

14. Physical Properties • Solubility: hydrophobic • Density: less than 1 g/mL • Boiling points increase with increasing carbons (little less for branched chains). Melting points increase with increasing carbons (less for odd- number of carbons). Chapter 3

15. Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces. => Chapter 3

16. Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p. => Chapter 3

17. C H 3 C H C C H C H 3 2 3 C H C H C H 3 3 3 C H C H C H C H C H C H C H 2 2 3 3 C H C H C H 3 3 3 bp 50°C bp 60°C bp 58°C mp -98°C mp -154°C mp -135°C => Branched Alkanes • Lower b.p. with increased branching • Higher m.p. with increased branching • Examples: Chapter 3

18. Major Uses of Alkanes • C1-C2: gases (natural gas) • C3-C4: liquified petroleum (LPG) • C5-C8: gasoline • C9-C16: diesel, kerosene, jet fuel • C17-up: lubricating oils, heating oil • Origin: petroleum refining => Chapter 3

19. => Reactions of Alkanes • Combustion • Cracking and hydrocracking (industrial) • Halogenation Chapter 3

20. Conformers of Alkanes • Structures resulting from the free rotation of a C-C single bond • May differ in energy. The lowest-energy conformer is most prevalent. • Molecules constantly rotate through all the possible conformations. => Chapter 3

21. H H H => H H Newman projection sawhorse H model Ethane Conformers • Staggered conformer has lowest energy. • Dihedral angle = 60 degrees Chapter 3

22. => Ethane Conformers (2) • Eclipsed conformer has highest energy • Dihedral angle = 0 degrees Chapter 3

23. => Conformational Analysis • Torsional strain: resistance to rotation. • For ethane, only 3.0 kcal/mol Chapter 3

24. Propane Conformers Note slight increase in torsional strain due to the more bulky methyl group. => Chapter 3

25. totally eclipsed => Butane Conformers C2-C3 • Highest energy has methyl groups eclipsed. • Steric hindrance • Dihedral angle = 0 degrees Chapter 3

26. anti => Butane Conformers (2) • Lowest energy has methyl groups anti. • Dihedral angle = 180 degrees Chapter 3

27. => eclipsed Butane Conformers (3) • Methyl groups eclipsed with hydrogens • Higher energy than staggered conformer • Dihedral angle = 120 degrees Chapter 3

28. => gauche Butane Conformers (4) • Gauche, staggered conformer • Methyls closer than in anti conformer • Dihedral angle = 60 degrees Chapter 3

29. Conformational Analysis => Chapter 3

30. => Higher Alkanes • Anti conformation is lowest in energy. • “Straight chain” actually is zigzag. Chapter 3

31. Rings of carbon atoms (CH2 groups) Formula: CnH2n Nonpolar, insoluble in water Compact shape Melting and boiling points similar to branched alkanes with same number of carbons => Cycloalkanes Chapter 3

32. Cycloalkane usually base compound Number carbons in ring if >1 substituent. First in alphabet gets lowest number. May be cycloalkyl attachment to chain. => Naming Cycloalkanes Chapter 3

33. Cis: like groups on same side of ring Trans: like groups on opposite sides of ring => Cis-Trans Isomerism Chapter 3

34. 5- and 6-membered rings most stable Bond angle closest to 109.5 Angle (Baeyer) strain Measured by heats of combustion per -CH2 - => Cycloalkane Stability Chapter 3

35. 166.6 164.0 158.7 158.6 158.3 157.4 157.4 => Long-chain Heats of Combustion Alkane + O2  CO2 + H2O Chapter 3

36. => Cyclopropane • Large ring strain due to angle compression • Very reactive, weak bonds Chapter 3

37. Cyclopropane (2) Torsional strain because of eclipsed hydrogens => Chapter 3

38. => Cyclobutane • Angle strain due to compression • Torsional strain partially relieved by ring-puckering Chapter 3

39. => Cyclopentane • If planar, angles would be 108, but all hydrogens would be eclipsed. • Puckered conformer reduces torsional strain. Chapter 3

40. Cyclohexane • Combustion data shows it’s unstrained. • Angles would be 120, if planar. • The chair conformer has 109.5 bond angles and all hydrogens are staggered. • No angle strain and no torsional strain. => Chapter 3

41. Chair Conformer => Chapter 3

42. Boat Conformer => Chapter 3

43. Conformational Energy Chapter 3 =>

44. Axial and Equatorial Positions => Chapter 3

45. Monosubstituted Cyclohexanes => Chapter 3

46. 1,3-Diaxial Interactions => Chapter 3

47. Disubstituted Cyclohexanes => Chapter 3

48. Cis-Trans Isomers Bonds that are cis, alternate axial-equatorial around the ring. => Chapter 3

49. => Bulky Groups • Groups like t-butyl cause a large energy difference between the axial and equatorial conformer. • Most stable conformer puts t-butyl equatorial regardless of other substituents. Chapter 3

50. bicyclo[3.1.0]hexane bicyclo[2.2.1]heptane => Bicyclic Alkanes • Fused rings share two adjacent carbons. • Bridged rings share two nonadjacent C’s. Chapter 3