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### Topic: Free Radical Bromination of Ethane question, need assistance, lol wtf?  (Read 3680 times)

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#### 33seven

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##### Free Radical Bromination of Ethane question, need assistance, lol wtf?
« on: November 09, 2010, 04:21:23 PM »
Ok, question from my organic exam, copied word for word w/ my attempt:

1. During the free radical bromination or ethane, which of the following reactions has the lowest collision frequency? (i.e. Which reaction is the least likely to happen?)

a) CH3CH2. +Br2 CH3CH2Br + Br.
b) Br. + CH3CH3 HBr + CH3CH2.
c) Br. + Br2   Br2 + Br.
d) CH3CH2. + Br. CH3CH2Br
e) CH3CH2. + CH3CH3 CH3CH3 + CH3CH2.

So we're given a sheet with Bond-dissociation enthalpies for homolytic bond cleavage(relevant energies):
Br-Br     46 Kcal/mol
CH3CH2-Br     68
CH3CH2-H     98
H-Br     88

My attempt:
a) 46 - 68 = -22
b) 98 - 88 = +10
c) 46 - 46 = 0
d) 0 - 68 = -68
e) 98- 98 = 0

So, obvious answer is B?

#### 33seven

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##### Re: Free Radical Bromination of Ethane question, need assistance, lol wtf?
« Reply #1 on: November 15, 2010, 02:03:17 PM »
Nevermind, i figured it out on my own. The question had little or nothing to do with the bond energies, b/c 4 of the 5 choices are propagation steps and only 1 is a termination step. B/c the amount of free radicals available to react is significantly lower in a termination step to a free radical reaction, the probability that a termination reaction will occur is much less than the probability that a propagation reaction will occur. So obvious answer D. You guys were so helpful!! Thanks, I couldn't have done it without your h elp!! What a joke.