Alkanes

• Alkanes


  A "family" of hydrocarbons; aliphatic

  
• Alkane structure


  • sp3 bond angles = 109.5°
  • Tetrahedral
  • σ-bonds (C-C; C-H)

  
• Conformations


  Different arrangements of atoms interconverted by rotations about a single bond
• Conformations


  • Staggered
  • Eclipsed
• Structural (chain) isomerism


  Different arrangements of carbon atoms in the main chain

  
• Butane isomers


  • n-butane
  • Isobutane

  
• Number of alkane isomers

  • CH4 (1)
  • C2H6 (1)
  • C3H8 (1)
  • C4H10 (2)
  • C5H12 (3)
  • C6H14 (5)
  • C7H16 (9)
  • C8H18 (18)
  • C9H20 (35)
  • C10H22 (75)
  • C20H42 (366,319)

  
• Primary, secondary, tertiary carbon


  Classified by the number of other carbons attached
• Primary, secondary, tertiary halides


  Classified by the type of carbon the halide is attached to

  
• Physical properties of alkanes

  • Non-polar or weakly polar
  • Cannot form hydrogen bonds
  • Lower melting/boiling points
  • Increase with size, decrease with branching
  • C1-C4 gases, C5-C17 liquids, >C17 solids
  • Water insoluble
  • Least dense of organic compounds

  
• Fossil fuels are a major source of alkanes
• Fractional distillation of petroleum

  Separates crude petroleum into fractions of similar boiling point
• Synthesis of alkanes


  1. Reduction of alkyl halide

  2. Hydrolysis of Grignard reagent

  3. Active metal and acid

  
• Industrial vs. laboratory syntheses differ in scale, cost, and apparatus
• Apparatus


  Equipment used in laboratory syntheses
• On exams, homework: laboratory syntheses!
• DEPARTMENT OF CHEMISTRY, UNIVERSITY OF LAGOS
• Reduction of an Alkyl halide

  1. Hydrolysis of a Grignard reagent

  2. With an active metal and an acid
• Active metals


  • Sn, Zn, Fe, etc.
• Acids


  • HCl, etc. (H+)
• Alkanes are typically non-reactive. They don't react with acids, bases, active metals, oxidizing agents, reducing agents, halogens, etc.
• Alkane reactions


  • Halogenation
  • Combustion (oxidation)
  • Pyrolysis (cracking)

  
• Halogenation


  1. H + X2, heat or hv -> R-X + HX
• Halogenation


  • Heat or light required for reaction
  • X2: Cl2 > Br2 > I2
  • Yields mixtures
  • H: 3o > 2o > 1o > CH4
  • Bromine is more selective
• Combustion


  CnH2n+2 + (xs) O2, flame -> n CO2 + (n+1) H2O + heat
• Pyrolysis (cracking)


  Alkane, 400-600oC -> smaller alkanes + alkenes + H2

  
• Pyrolysis (cracking)


  • Used to increase the yield of gasoline from petroleum. Higher boiling fractions are "cracked" into lower boiling fractions that are added to the raw gasoline. The alkenes can be separated and used in to make plastics.
• Halogenation of alkanes


  • CH3CH2CH2CH3 + Cl2, hv -> CH3CH2CH2CH2-Cl (28%) + CH3CH2CHCH3 (72%)

  CH3CHCH3 + Cl2, hv -> CH3CHCH2-Cl (64%) + CH3CCH3 (36%)

  CH3CH3 + Br2, hv -> CH3CH2-Br + HBr

  CH3CH2CH3 + Br2, hv -> CH3CH2CH2-Br (97%) + CH3CHCH3 (3%)

  CH3CH3 + Cl2, hv -> CH3CH2-Cl + HCl

  CH3CH2CH3 + Cl2, hv -> CH3CH2CH2-Cl (55%) + CH3CHCH3 (45%)

  CH3CH2CH2CH3 + Br2, hv -> CH3CH2CH2CH2-Br (2%) + CH3CH2CHCH3 (98%)

  CH3CHCH3 + Br2, hv -> CH3CHCH2-Br (<1%) + CH3CCH3 (99%)
• Cycloalkanes


  An alkane that contains a ring of carbon atoms. Ring sizes from 3 carbons to 30 or higher are known.

  
• Cycloalkanes


  • Represented by polygons (triangle for cyclopropane, square for cyclobutane, pentagon for cyclopentane, etc.)

  
• Alkenes are unsaturated hydrocarbons that contain at least one carbon-to-carbon double bond.
• Alkanes


  A family of hydrocarbons; aliphatic
• General formula for alkanes

  CnH2n+2

  
• Alkanes


  • CH4
  • C2H6
  • C3H8
  • C4H10

  
• Alkane structure


  • sp3
  • Bond angles = 109.5°
  • Tetrahedral
  • σ-bonds (C-C; C-H)

  
• Conformers


  Different arrangements of atoms that are interconverted formed by rotations about a single bond
• Staggered


  A conformation of alkanes with minimal torsional strain

  
• Eclipsed


  A conformation of alkanes with maximum torsional strain

  
• Torsional strain


  Deviation from staggered conformation
• Structural (Chain) Isomerism


  Different compounds with the same molecular formula but different structures