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Carbocation Intermediate Rearrangements quiz

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  • What is a carbocation rearrangement and why does it occur?
    A carbocation rearrangement is when a carbocation moves to a more stable position, usually to increase its stability by forming a more substituted carbocation.
  • What is the most common type of carbocation shift?
    The most common type is the 1,2-hydride shift, where a hydrogen atom moves to the carbocation to create a more stable carbocation.
  • Under what condition does a 1,2-hydride shift occur?
    A 1,2-hydride shift occurs when there is a hydrogen on an adjacent, more stable carbon next to the carbocation.
  • Why do you draw the arrow from the hydrogen bond to the carbocation in a hydride shift?
    Because electrons move from the more negative (the bond) to the more positive (the carbocation), not from the positive charge itself.
  • What happens to the original carbocation carbon after a hydride shift?
    It gains a hydrogen and becomes neutral, while the carbocation moves to the carbon that lost the hydrogen.
  • What is a 1,2-alkyl shift and when does it occur?
    A 1,2-alkyl shift is when an alkyl group (like methyl or ethyl) moves to the carbocation, and it occurs only if no hydrogens are available for a hydride shift.
  • Why are methyl shifts more common than ethyl or larger alkyl shifts?
    Because moving larger alkyl groups requires more energy, making methyl shifts easier and more common.
  • What is the order of preference for carbocation shifts?
    Hydride shifts are preferred first, followed by methyl shifts, and then ethyl shifts as a last resort.
  • What is a ring expansion in carbocation rearrangements?
    A ring expansion occurs when a carbocation is adjacent to a small ring (3, 4, or 5-membered), causing the ring to expand and relieve strain.
  • Why do small rings undergo ring expansion during carbocation rearrangement?
    Small rings are strained, so expanding the ring relieves strain and forms a more stable, larger ring with the carbocation.
  • After a ring expansion, where is the carbocation located?
    The carbocation ends up on the carbon that was adjacent to the original ring, now part of the expanded ring.
  • What determines whether a carbocation will rearrange?
    A carbocation will rearrange if it can move to a more stable position, such as from primary to secondary or tertiary, or to relieve ring strain.
  • Why are 1,2-hydride shifts easier than 1,2-alkyl shifts?
    Because moving a hydrogen requires less energy than moving an alkyl group, making hydride shifts more favorable.
  • What is the general mechanism for a carbocation rearrangement?
    The mechanism involves the movement of a group (hydride or alkyl) from a neighboring carbon to the carbocation, guided by electron movement from negative to positive centers.
  • What two questions should you ask when analyzing a carbocation rearrangement problem?
    First, ask if the carbocation will rearrange; second, if it does, determine what the new structure will look like after the shift.