Question 9. Radical Chlorination

[RSK25]

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Can someone explain number 9 for me please. I get that I is least reactive. I don't know how to compare between II and III. I thought radical chlorination is more reactive than selective.

http://www.ic.sunysb.edu/Class/orgchem/oldexams/321/Exam3_08.pdf

 

[Schenker]

Two reasons:

1. Secondary radicals are more stable than primary radicals.
2. Allylic radicals are resonance stabilized.​

 

[Czarcasm]

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)

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]

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

 

[Schenker]

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]

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 🙂