1 / 55

Chapter 3 Structure and Stereochemistry of Alkanes

Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 3 Structure and Stereochemistry of Alkanes. =>. 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 Same ratio for branched alkanes. Common Names.

asis
Télécharger la présentation

Chapter 3 Structure and Stereochemistry of Alkanes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Organic Chemistry, 5th EditionL. G. Wade, Jr. Chapter 3Structure and Stereochemistryof Alkanes

  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. • 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. Chapter 3

  14. Chapter 3

  15. Chapter 3

  16. 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

  17. Physical Properties • Solubility: hydrophobic • Density: less than water: 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

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

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

  20. 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

  21. 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

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

  23. 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

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

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

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

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

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

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

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

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

  32. Conformational Analysis => Chapter 3

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

  34. 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

  35. 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

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

  37. 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

  38. 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

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

  40. Cyclopropane (2) Torsional strain because of eclipsed ydrogens => Chapter 3

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

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

  43. 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

  44. Chair Conformer => Chapter 3

  45. Boat Conformer => Chapter 3

  46. Conformational Energy Chapter 3 =>

  47. Axial and Equatorial Positions => Chapter 3

  48. Monosubstituted Cyclohexanes => Chapter 3

  49. 1,3-Diaxial Interactions => Chapter 3

  50. Disubstituted Cyclohexanes => Chapter 3

More Related