Home >
Community >
Why don't hydride shifts occur with hydrohalogenation of 4-methyl-cyclohexene?
Upvote
24
Downvote
+ Halides
+ Chemistry
Posted by
Mehedi Hasan
Why don't hydride shifts occur with hydrohalogenation of 4-methyl-cyclohexene?
The reason for not having consecutive hydride shifts is that generally, a new carbocation is formed through shift only when it is more stable than the immediately previous one.
To form a tertiary carbocation, 3 consecutive hydride shifts would have to occur consecutively (when + is at c-4 from the alkyl group, your first product) in which more than one intermediates have same stability (same number of alpha-H). Hydride shifts will not occur for your second product to form tertiary carbocation as the next intermediate is infact less stable (3 alpha-H as compared to 4 alpha-H previously).
Since the ring already has 6 members (least strain) so ring expansion/contraction will not occur either.
The reason for not having consecutive hydride shifts is that generally, a new carbocation is formed through shift only when it is more stable than the immediately previous one.
To form a tertiary carbocation, 3 consecutive hydride shifts would have to occur consecutively (when + is at c-4 from the alkyl group, your first product) in which more than one intermediates have same stability (same number of alpha-H). Hydride shifts will not occur for your second product to form tertiary carbocation as the next intermediate is infact less stable (3 alpha-H as compared to 4 alpha-H previously).
Since the ring already has 6 members (least strain) so ring expansion/contraction will not occur either.
Please check out the mechanism of squalene cyclisation which includes four consecutive methyl and hydride Wagner-Meerwein rearrangements, neither of which is truly downhill.More
On second thought, there will also be (+)ve inductive effect on the c+ in my first product, increasing its stability but I think that HyperConjugation has greater effect on stability of carbocation than inductive effect so loss of alpha-H should have a greater effect.More
The reason for not having consecutive hydride shifts is that generally, a new carbocation is formed through shift only when it is more stable than the immediately previous one.
To form a tertiary carbocation, 3 consecutive hydride shifts would have to occur consecutively (when + is at c-4 from the alkyl group, your first product) in which more than one intermediates have same stability (same number of alpha-H). Hydride shifts will not occur for your second product to form tertiary carbocation as the next intermediate is infact less stable (3 alpha-H as compared to 4 alpha-H previously).
Since the ring already has 6 members (least strain) so ring expansion/contraction will not occur either.
The reason for not having consecutive hydride shifts is that generally, a new carbocation is formed through shift only when it is more stable than the immediately previous one.
To form a tertiary carbocation, 3 consecutive hydride shifts would have to occur consecutively (when + is at c-4 from the alkyl group, your first product) in which more than one intermediates have same stability (same number of alpha-H). Hydride shifts will not occur for your second product to form tertiary carbocation as the next intermediate is infact less stable (3 alpha-H as compared to 4 alpha-H previously).
Since the ring already has 6 members (least strain) so ring expansion/contraction will not occur either.
More
VOTE
VOTE
VOTE
VOTE