Is Phenyl Shift Better Than Methyl Shift in Carbocation Rearrangement?

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Discussion Overview

The discussion revolves around the comparison of phenyl and methyl shifts in carbocation rearrangements, exploring the conditions under which one might be favored over the other. Participants examine the implications of steric hindrance, resonance stabilization, and the specifics of transition states in these rearrangements.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question the context of the comparison, asking for clarification on what "better" means in this scenario.
  • One participant suggests that a phenyl group has more steric hindrance, which may influence its tendency to shift compared to a methyl group.
  • Another participant notes that the likelihood of a phenyl shift versus a methyl shift is case dependent, particularly when considering the stability of the resulting carbocation.
  • Concerns are raised about the transition state involving a three-centered bond and how the positive charge interacts differently with a phenyl group compared to a methyl group.
  • Resonance stabilization is mentioned as a potential factor in the stability of the transition state, but there is uncertainty about how this affects the preference for shifts.
  • Participants express confusion regarding the instructor's approval of their explanations, indicating a lack of consensus on the topic.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the preference for phenyl versus methyl shifts in carbocation rearrangements. There are multiple competing views and ongoing questions about the conditions that influence these shifts.

Contextual Notes

Limitations include ambiguity in the original question regarding the context of "better," and the dependence on specific cases for determining the stability of carbocations resulting from different shifts.

NEILS BOHR
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why is phenyl shift better than methyl shift?
 
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You did everything possible to ask the question that is impossible to answer.

In what context? Better for what?
 
:smile:

i mean whenever there's a possibility of the 2 happening , why is the former given more priority than the latter??
 
I'd say it's because a phenyl group has far more steric hindrance, so it'd be more "pressured" to shift.
 
NEILS BOHR said:
:smile:

i mean whenever there's a possibility of the 2 happening , why is the former given more priority than the latter??

I wonder if you are aware that your question is still ambiguous.
 
He's asking about carbocation rearrangement. Why is the phenyl group more likely to undergo rearrangement than the methyl group when both groups are present?

In the transition state, the alkyl and the phenyl group are both bonded in a similar fashion to the carbocation center. The phenyl group does something special with that positive charge that the methyl group cannot do as easily. Think about it some more.
 
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Its still case dependent.

If the carbocation is adjacent to a carbon bearing an alkyl and phenyl group, then the alkyl group (or hydride or what have you) would shift to place the carbocation in the benzylic position. A phenyl shift, in this situation, would not result in the most stable carbocation.

I would like to see an example of what the OP had in mind. I'm sure there are examples of what is being said here, but I was taught the complete opposite (unless I'm misunderstanding something here).
 
Yanick said:
Its still case dependent.

If the carbocation is adjacent to a carbon bearing an alkyl and phenyl group, then the alkyl group (or hydride or what have you) would shift to place the carbocation in the benzylic position. A phenyl shift, in this situation, would not result in the most stable carbocation.

Is that true? The transition state is a three centered bond that contains a positive charge. What can happen with a cationic charge associated with a phenyl group that isn't possible with a methyl group?
 
chemisttree said:
Is that true? The transition state is a three centered bond that contains a positive charge. What can happen with a cationic charge associated with a phenyl group that isn't possible with a methyl group?

I'm pretty sure you're talking about resonance but I'm not sure why the phenyl would shift instead of a methyl or hydride thus not allowing for that resonance stabilization to be present.
 
  • #10
What do you know about the stability of the transition state (and how that effects reaction rate) where the cation is further stabilized by resonance? How does a more stable transition state affect the Eact?
 
  • #11
:approve:

actually our sir asked this question

and we explained all the things we could ( talking about carbocation rearrangement as well)

but he didnt look to be approved of what we had to say!:rolleyes:
 

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