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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. Four carbon/or hydogen atoms bonded to each carbon atom. All C-C single bonds. NOTE: always an even number of hydrogen atoms in a hydrocarbon. Ratio: C n H 2n+2.

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Chapter 3 Structure and Stereochemistry of Alkanes

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

  2. ALKANE FORMULAS • Four carbon/or hydogen atoms bonded to each carbon atom • All C-C single bonds NOTE: always an even number of hydrogen atoms in a hydrocarbon • Ratio: CnH2n+2 • Alkane homologs : each member in a alkane series different from the next member by a CH2 group Chapter 3

  3. CH3CH2CH2CH2CH2CH3 CH3CH2CH2CH2CH3 C6H14 C5H12 CH2 Chapter 3

  4. Lower Membered Alkanestrivial (common) names Condensed Line-angle • METHANE CH4 • ETHANE • PROPANE Chapter 3

  5. BUTANES n-BUTANE iso-BUTANE Note: 1 H here Chapter 3

  6. => Pentanes Chapter 3

  7. Writing Isomers -Heptanes 1. Start with longest straight chain possible 2. Shorten chain by one carbon and add the "extra" at all possible positions, starting with left-handed side. 2-Methylhexane Obviously you do not add the extra carbon to end carbon!!! 3-Methylhexane 2-Methylhexane Chapter 3 3-Methylhexane 2-Methylhexane

  8. 3. Shorten chain by one more carbon. This gives a 5-carbon chain (pentane). Two carbons are to be added as two methyl groups or a single two-carbon group (ethyl group). THIS is 3-methylhexane!! Chapter 3

  9. Shorten chain by yet one more carbon. This gives a 4-carbon chain (butane). Three carbons are to be added as three methyl group. NOTE: Can’t derive another butane chain by adding an ethyl group. 2,2,3-trimethylbutane 3,3-dimethylpentane Chapter 3

  10. 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

  11. 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

  12. => 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

  13. 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

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

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

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

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

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

  19. 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

  20. 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

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

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

  23. 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

  24. 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

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

  26. 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

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

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

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

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

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

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

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

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

  35. Conformational Analysis => Chapter 3

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

  37. 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

  38. 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

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

  40. 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

  41. 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

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

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

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

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

  46. 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

  47. Chair Conformer => Chapter 3

  48. Boat Conformer => Chapter 3

  49. Conformational Energy Chapter 3 =>

  50. Axial and Equatorial Positions => Chapter 3

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