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Chapter 3 Structure and Stereochemistry of Alkanes

Organic Chemistry , 6 th Edition L. G. Wade, Jr. Chapter 3 Structure and Stereochemistry of Alkanes. ã 2006, Prentice Hall. Classification Review. Alkane Formulas. All C-C single bonds Saturated with hydrogens Ratio: C n H 2n+2 Alkane homologs: CH 3 (CH 2 ) n CH 3

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Chapter 3 Structure and Stereochemistry of Alkanes

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  1. Organic Chemistry, 6th EditionL. G. Wade, Jr. Chapter 3Structure and Stereochemistryof Alkanes ã 2006,Prentice Hall

  2. Classification Review Chapter 3

  3. 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

  4. 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

  5. Alkane Examples Chapter 3

  6. 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

  7. Longest Chain • The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 82.) • If there are two possible chains with the same number of carbons, use the chain with the most substituents. Chapter 3

  8. 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

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

  10. Propyl Groups H H n-propyl isopropyl A secondary carbon A primary carbon Chapter 3

  11. Butyl Groups H H n-butyl sec-butyl A secondary carbon A primary carbon Chapter 3

  12. Isobutyl Groups H H isobutyl tert-butyl A tertiary carbon A primary carbon Chapter 3

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

  14. 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

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

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

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

  18. 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

  19. 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

  20. Reactions of Alkanes • Combustion • Cracking and hydrocracking (industrial) • Halogenation Chapter 3

  21. 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

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

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

  24. Conformational Analysis • Torsional strain: resistance to rotation. • For ethane, only 12.6 kJ/mol Chapter 3

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

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

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

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

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

  30. Conformational Analysis Chapter 3

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

  32. 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

  33. 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

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

  35. 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

  36. 697.1 686.1 664.0 663.6 kJ/mol 662.4 658.6 658.6 kJ Long-chain Heats of Combustion/CH2 Alkane + O2  CO2 + H2O Chapter 3

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

  38. Cyclopropane (2) Torsional strain because of eclipsed hydrogens Chapter 3

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

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

  41. 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

  42. Chair Conformer Chapter 3

  43. Boat Conformer Chapter 3

  44. Conformational Energy Chapter 3

  45. Axial and Equatorial Positions Chapter 3

  46. Monosubstituted Cyclohexanes Chapter 3

  47. 1,3-Diaxial Interactions Chapter 3

  48. Disubstituted Cyclohexanes Chapter 3

  49. One axial, one equatorial Cis-Trans Isomers Bonds that are cis, alternate axial-equatorial around the ring. Chapter 3

  50. 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

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