Problem #7. Whenever you are considering a reaction that proceeds through a true

Question

Problem #7. Whenever you are considering a reaction that proceeds through a true carbocation mechanism, you must keep an cye out for carbocation rearrangements: 1,2-hydride shifts and 1,2-alkyl shifts. These will only occur if the cation that is formed is more stable than the cation that you started with. a) Circle the rearrangements that would occur and draw an X over the arrow for the rearrangements that would NOT occur CH3 CH3 H3C H3C CH3- CH H3C H3C H CH3 H3C CH3 H3C CH3 H3C CH3 b) Choose one of the reactions from part a) that would NOT occur and explain why not. c) Choose one of the reactions from part a) that would occur, and explain why it is NOT reversible

Explanation / Answer

(a)

I and II occur

III and IV would not occur.

(b)

In general the 1,2 hydride shift or 1,2 alkyl shift takes place to get more stable intemediate carbo cation than what it is.

III and IV would not occur. Because they are already in more stable intermediate from than would obtain after rearrangement.

Stability of carbocations will increase in the order of: Primary < Secondary < tertiary carbocation.

In III secondary to primary which not stable rearrangement.

In IV tertiary to primary which is not stable rearranegement.

(c)

I and II would occur.

In I secondary to tertairy so stable rearrangement than can occur.

In II secondary to tertairy so stable rearrangement than can occur.

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