Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: 33seven on November 09, 2010, 04:21:23 PM
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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 :rarrow: CH3CH2Br + Br.
b) Br. + CH3CH3 :rarrow: HBr + CH3CH2.
c) Br. + Br2 :rarrow: Br2 + Br.
d) CH3CH2. + Br. :rarrow: CH3CH2Br
e) CH3CH2. + CH3CH3 :rarrow: 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?
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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.