Questions dealing with alkane chemistry

Problems

  1. Bromination of methane is slowed down by the addition of fairly large amounts of HBr.
    1. suggest an explanation

      Br . + H-CH3 ® H-Br + . CH3

      DH +104 -88 = +16kcal i.e. endothermic in this direction

      . CH3 + Br2 ® BrCH3 + Br .

      DH = +46 -70 = -24kcal i.e. exothermic in this direction and therefore, as soon as .CH3 forms, it reacts with Br2. Ea is low for such an exothermic reaction.

      However, as HBr is added, a competing reaction becomes possible, the reverse of the first reaction given above:

      HBr + . CH3 ® Br . + H-CH3

      as DH is –16 kcal for this process, Ea will be very small. This reaction removes methyl radicals and the number available for reaction with bromine will be lessened and the reaction slows down.

    2. why does HCl not react in this way

      Cl . + H-CH3 ® H-Cl + . CH3

      DH +104 -103 = +1kcal i.e. just endothermic in this direction. The activation energy is +4 kcal compared with +18 for the bromination. The activation energy for the reverse reaction is therefore +3 kcal, higher than that for the analogous bromination reaction The methyl radicals are also formed more readily and so are less "rare".

      Note: Whereas part a) could be an exam question providing bond dissociation energies are given, b) would never be…..

  2. A mixture of H2 and Cl2 does not react at room temperature. At high temperature or under the influence of light (of wavelength absorbed by chlorine) a violent reaction takes place and HCl is formed. The photochemical reaction yields as many as a million molecules of HCl for each photon absorbed. The presence of a trace of oxygen slows down the reaction.
    1. propose a possible mechanism for this reaction
      2.

      The fact that "the photochemical reaction yields as many as a million molecules of HCl for each photon absorbed" indicates a chain reaction. Initiation involves chlorine as light absorbed by chlorine begins the process.

      Cl2 ® 2Cl . (initiation)

      Cl . + H2 ® HCl + H . (propagation)

      H . + Cl2 ® HCl + Cl . (propagation)

      2H .  ® H2 (termination)

      2Cl . ® Cl2

      H .  + Cl . ® HCl

    2. explain why a mixture of H2 and I2 does not behave in the same manner

      I . + H2 ® HI + I . is very endothermic (+104 kcal to break H-H bond whereas only 71 kcals are liberated in forming the H-I bond. Ea must be > 33 kcal which is impossible. The corresponding reaction with chlorine is only just endothermic, +1 kcal, and so its activation energy is far lower.)

  3. t-Butyl peroxide is a source of free radicals:

    (CH3)3CO-OC(CH3)3 ® 2(CH3)3CO. (heat or light initiated)

    A mixture of isobutane and CCl4 is stable at 130C. If a small amount of t-butyl peroxide is added, a reaction occurs that yields 2-chloro-2-methylpropane and chloroform. A small amount of 2-methyl-2-propanol, (CH3)3COH, equivalent to the peroxide used, is also formed.

    1. propose a mechanism which explains the formation of these products
      2.

      (CH3)3CO-OC(CH3)3 ® 2(CH3)3CO.

      (CH3)3CO. + (CH3)3CH ® (CH3)3COH + (CH3)3C .

      (CH3)3C . + CCl4 ® (CH3)3CCl + . CCl3

      . CCl3 + (CH3)3CH ® HCCl3 + (CH3)3C.

    2. what is a peroxide?

      a compound containing an O – O bond

  4. Why would one not prepare a Grignard reagent from bromoethanoic acid, BrCH2CO2H?

    because it contains and acidic hydrogen which would react with the Grignard reagent as soon as it forms

  5. When a mixture of methane and chlorine is irradiated, reaction begins at once. However, when irradiation is stopped, the reaction gradually slows down; it does not stop immediately. Why?

    because it is a chain reaction and the chains continue until termination takes place or until all of one reactant has been used up

     

  6. The anti conformation of 1,2-dichloroethane is 1.15 kcal/mol more stable than the gauche conformation. The two energy barriers, measured relative to the energy of the gauche conformation, for carbon-carbon bond rotation are 5.15 kcal/mol and 9.3 kcal/mol.
    1. sketch a graph of potential energy vs. angle of rotation about the carbon-carbon bond. Show the energy differences on your graph and label each minimum and maximum with the appropriate conformation of 1,2-dichloroethane.

       

       

    2. Which conformation is present in the greatest amount? Explain.

      The anti conformation is present in the greatest amount as it has the lowest potential energy.

Synthesis

  1. Propose a synthesis of n-hexane from each of the following:
    1. CH3CH2CH2CH2CH2CH2Br
      2.

      either treat with tri-n-butyl stannane or with Mg followed by water

    2. CH3CH=CHCH2CH2CH3

      3. hydrogen and Pt, Pd or Ni catalyst

    3. CH3CH2CH2Br

      1. Li, 2. CuI 3. CH3CH2CH2Br (Corey-House reaction)

  2. Propose a synthesis of CH3D from CH4.

1. Br2/hn 2. Bu3SnD (or Mg followed by D2O)

 

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Updated August 23, 2002