Questions dealing with alkane chemistry
Problems
1. Bromination of methane is slowed down by the addition of fairly large
amounts of HBr.
a. suggest an explanation
Br . + H-CH3 ? H-Br + . CH3
?H +104 -88 = +16kcal i.e. endothermic in this direction
. CH3 + Br2 ? BrCH3 + Br .
?H = +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 ?H 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.
b. why does HCl not react in this way
Cl . + H-CH3 ? H-Cl + . CH3
?H +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.
a. propose a possible mechanism for this reaction
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
b. 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 tbutyl
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.
a. propose a mechanism which explains the formation of these products
(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.
b. 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.
a. 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.
b. 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:
a. CH3CH2CH2CH2CH2CH2Br
either treat with tri-n-butyl stannane or with Mg followed by water
b. CH3CH=CHCH2CH2CH3
hydrogen and Pt, Pd or Ni catalyst
c. CH3CH2CH2Br
1. Li, 2. CuI 3. CH3CH2CH2Br (Corey-House reaction)
2. Propose a synthesis of CH3D from CH4.
1. Br2/h? 2. Bu3SnD (or Mg followed by D2O)