Stability vs. Reactivity of Radicals

[matth87]

So I got this question on TPR #1 Diag

Assuming that the rate-determining step for Mechanism B is the formation of a free-radical intermediate. The order of reactivity for the exothermic reaction is expected to be.

Correct Answer : 3 > 2 > 1 > methyl

*I thought that reactivity of free radicals was opposite of stability. I know that 3 > 2 > 1 > methyl is true for the stability of the radical, but I thought methyl > 1 > 2 > 3 is correct for reactivity of the radical.

Help would be much appreciated.

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[loveoforganic]

You're correct in what you're saying, but you're not correct in how you're applying it to the question. The question is saying that the formation of a free radical intermediate is the rate determining step; in other words, the thing that takes the longest to occur is the formation of the radical. So what we're considering here is the activation energy for the formation of the radical. Activation energy for the formation of a primary radical > secondary > tertiary, and activation energy is inversely related to reaction rate.

 

[Will Hunting]

So I got this question on TPR #1 Diag

Assuming that the rate-determining step for Mechanism B is the formation of a free-radical intermediate. The order of reactivity for the exothermic reaction is expected to be.

Correct Answer : 3 > 2 > 1 > methyl

*I thought that reactivity of free radicals was opposite of stability. I know that 3 > 2 > 1 > methyl is true for the stability of the radical, but I thought methyl > 1 > 2 > 3 is correct for reactivity of the radical.

Help would be much appreciated.

You logically thought about something. However, in sn1, which is the most reactive? It is the tertiary because the transition state is the lowest. Radicals are similar in the stabilizing effects like sn1. A good point advice is to relate. If you had thought of sn1 you would have gotten the right answer. What you're thinking of is a comparison between similar states. The question is hinting at formation so it must be according to the most stable. If it has been which of the following is the most reactive: H-F, HI, H-Cl, etc. Then the least stable H-F has no choice to react. They just tested you on a subtlety.