Consider the radical chain reaction
Cl3C-Br + 2-methylpropene => Cl3C-CH2-CBr(CH3)2 (Me = methyl = CH3)
a) What is the Cl3C-Br bond strength?
b) What is the H2C=CMe2 pi bond strength?
[Hint: What's the E of the "diradical"--isobutane where a tertiary and a primary H have been pulled off? Don't forget to let the H's form H2 in your calculation. Then compare to the actual ?Hf of 2-methylpropene.]
(c) What is the overall Hrxn for Cl3CBr addition to 2-methylpropene?
(d) Write a detailed mechanism for the above conversion. Assume a chain initiation where light cleaves the C-Br bond to make Cl3C• and Br• radicals. Then go from there.
[Hint: Alkyl radicals can exchange Br-i.e. CH3CH2• + Br-CMe3 = CH3CH2Br + Me3C• Is Cl3C• or Br• the chain carrier? Why does the Cl3C group end up on the less substituted end of the double bond?]
(e) Calculate energy changes for the reaction's two chain propagation steps; both steps should be exothermic for the process to work.