Czarcasm

Hakuna matata, no worries.
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Jun 22, 2013
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Hmm. What was the answer for this?

There's 4 different Hydrogens a radical can react with:
- Secondary (vinylic) - 1 Hydrogen
- Primary (vinylic) - 1 Hydrogens (Labeled I)
- Secondary (allylic) - 2 Hydrogens (Labeled II)
- Primary - 3 Hydrogens (Labeled III)

upload_2014-1-3_17-10-46.png
I'm not really sure how vinylic radicals rank in stability (maybe someone can clarify), but I do know a secondary allylic radical is more stable than a primary radical (same trend follows for carbocations).

In terms of reactivity vs. selectivity of halides:
Fluorine is explosively reactive
Chlorine which is moderately reactive
Bromine is more selective than reactive (generally prefers most stable product)
Iodine is very very selective and generally doesn't react.

If I had to answer this question, I would focus on choosing the radical environment with the most amount of Hydrogens available (most available to react with) rather than focusing on the stability. I think if it was Radical Bromination, then the more stable product would predominate -- but I don't think that's the case here.

Based on that logic, I'd rank them as follows:

Choice A: I (with 1 hydrogen) < II (with 2 Hydrogens) < III (with 3 Hydrogens)
 

Czarcasm

Hakuna matata, no worries.
5+ Year Member
Jun 22, 2013
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Crypts of Lieberkühn
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Just looked it up. Apparently vinyllic radicals are less stable than non-vinyllic primary radicals, which sorta makes sense. So in terms of stability:

allylic/benzylic > Tertiary > Secondary > Primary > Methyl > vinylic

In otherwords, a primary radical is more stable than say a secondary vinylic radical.
A secondary vinylic radical is more stable than a primary vinylic radical.

If someone can confirm, that would be great ;) -cough- @Schenker
 
Mar 16, 2013
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Just looked it up. Apparently vinyllic radicals are less stable than non-vinyllic primary radicals, which sorta makes sense. So in terms of stability:

allylic/benzylic > Tertiary > Secondary > Primary > Methyl > vinylic

In otherwords, a primary radical is more stable than say a secondary vinylic radical.
A secondary vinylic radical is more stable than a primary vinylic radical.

If someone can confirm, that would be great ;) -cough- @Schenker
Correct! B is the correct answer.

This question only asked you to rank the C-H bonds by reactivity (i.e. stability of radical formation at that bond); you would only have to weigh each C-H bond's reactivity against the number of those C-H bonds if the problem had asked you to identify the major product.
 

Czarcasm

Hakuna matata, no worries.
5+ Year Member
Jun 22, 2013
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407
Crypts of Lieberkühn
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Medical Student (Accepted)
Correct! B is the correct answer.

This question only asked you to rank the C-H bonds by reactivity (i.e. stability of radical formation at that bond); you would only have to weigh each C-H bond's reactivity against the number of those C-H bonds if the problem had asked you to identify the major product.
Ah, I see! Thank you :)