Chapter 8, Problem 8.52P

Interpretation Introduction

(a)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

A carbocation rearrangement will take place for this substrate.

The rearrangement can be drawn using curved arrow notation as

Explanation of Solution

The given substrate initially forms the carbocation shown below in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism.

It is a relatively stable secondary carbocation. It will undergo a rearrangement only if it leads to the formation of a more stable tertiary carbocation. Two $\text{1, 2-hydride}$ shifts are possible as there are two hydrogens on adjacent carbons (C2 and C6). A methyl group is also present on the adjacent C2, so a $\text{1, 2-methyl}$ shift is also possible. A hydride from C6 to C1 will again give a secondary carbocation with no change in stability. The C2 to C1 methyl shift will also not lead to a more stable carbocation. However, a hydride shift from C2 to C1 leads to a more stable tertiary carbocation.

Therefore, a carbocation rearrangement is possible in this case.

The rearrangement can be drawn using the curved arrow notation as

Conclusion

Formation of a more stable tertiary carbocation results in the rearrangement for this substrate.

Interpretation Introduction

(b)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

There will be no carbocation rearrangement in the case of this substrate.

Explanation of Solution

The substrate and the carbocation that it will be formed initially in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism are

The carbocation initially formed is a relatively stable secondary carbocation. There are two hydrogen atoms on adjacent carbons (C2 and C6) that can undergo a $\text{1, 2-hydride}$ shift. However, neither one will lead to a more stable carbocation. Both shifts will again form secondary carbocations, with no gain in stability.

Therefore, a carbocation rearrangement will not take place in this case.

Conclusion

There is no carbocation rearrangement for this case as there is no gain in stability.

Interpretation Introduction

(c)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

A carbocation rearrangement will not take place in this case.

Explanation of Solution

The given substrate initially forms the carbocation shown below in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism:

The carbocation formed is a tertiary carbocation, the most stable one. Therefore, a rearrangement will occur only if it leads to another tertiary carbocation that is further stabilized by resonance. No resonance stabilization is possible here as there are no double bonds in the carbocation.

Therefore, a carbocation rearrangement will not take place in this case.

Conclusion

Carbocation rearrangement is not possible because the one initially formed is a tertiary carbocation.

Interpretation Introduction

(d)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

A carbocation rearrangement will take place in this case.

The curved arrow notation for the rearrangement can be drawn as

Explanation of Solution

The substrate shown and the carbocation that it will initially form in this case is

The carbocation initially formed is a relatively stable secondary carbocation. It will undergo a rearrangement only if it leads to the formation of a resonance stabilized secondary carbocation or a tertiary carbocation. Only one hydride shift is possible, but it will not occur as the carbocation formed will be a similar secondary carbocation.

A methyl shift, from C2 to C1 will, however, lead to the formation of a tertiary carbocation.

Therefore, a carbocation rearrangement will take place in this case.

The curved arrow notation for this rearrangement can be drawn as

Conclusion

A $\text{1, 2-methyl}$ shift is possible in this case because it converts a secondary carbocation to a more stable tertiary carbocation.

Interpretation Introduction

(e)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

A carbocation rearrangement will take place in this case.

The curved arrow notation for this rearrangement is

Explanation of Solution

The given substrate initially forms the carbocation shown below in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism:

Two $\text{1, 2-hydride}$ shifts are possible in this carbocation. One of them, from C1 to C2 will lead to a less stable primary carbocation, and therefore, the rearrangement will not take place.

The other, from C3 to C2 will result in another secondary carbocation. This would normally not lead to a more stable carbocation. However, in this case, the charge is now in a conjugated position with the aromatic ring. Resonance, as shown below, will delocalize the charge on to a total of four carbons, considerably stabilizing this (C3) carbocation.

Therefore, a rearrangement will take place for this substrate.

The curved arrow notation for the rearrangement can be drawn as

Conclusion

Resonance can increase the stability of a carbocation as the number of atoms on which the charge is delocalized increases.

Interpretation Introduction

(f)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

A carbocation rearrangement will take place in this case.

The curved arrow notation for the rearrangement can be drawn as

Explanation of Solution

The given substrate initially forms the carbocation, as shown below, in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism:

The carbocation initially formed is an unstable primary carbocation. Any rearrangement that converts it to a secondary or a tertiary carbocation will be favorable. There is only one possible $\text{1, 2-hydride}$ shift, from C2 to C1. This will result in a more stable secondary carbocation.

Therefore, a rearrangement of the carbocation will take place for this substrate.

The curved arrow notation for the rearrangement can be drawn as

Conclusion

A primary carbocation is unstable and will rearrange to a secondary carbocation where possible.

Interpretation Introduction

(g)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

A carbocation rearrangement will take place in this case.

A curved arrow notation for the rearrangement can be drawn as

Explanation of Solution

The substrate and the carbocation that will initially be formed in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is:

The carbocation initially formed is a relatively stable secondary carbocation. The presence of a benzene ring on the same carbon will lead to further stabilization by resonance. Resonance will delocalize the charge over a total of four carbon atoms.

A $\text{1, 2-hydride}$ shift is possible, and will take place only if it leads to a more stable carbocation. The rearrangement will result in a more tertiary carbocation (on C2). Moreover, this carbocation is also resonance stabilized. With two aromatic rings attached to the same carbon, the charge will be delocalized over a total of seven carbon atoms, making it much more stable than the carbocation formed initially.

The much higher stability of this carbocation will mean a carbocation rearrangement will take place for this substrate.

The curved arrow notation for this rearrangement can be drawn as

Conclusion

Formation of a more stable tertiary carbocation leads to the rearrangement.

Interpretation Introduction

(h)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

No carbocation rearrangement will take place in this case.

Explanation of Solution

The given substrate and the carbocation that it forms initially in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism are

The carbocation formed is a relatively stable secondary carbocation. There is only one possible rearrangement, a $\text{1, 2-hydride}$ shift, from C6 to C1. This will result in another secondary carbocation, with no gain in stability.

Therefore, a carbocation rearrangement will not take place in this case.

Conclusion

A carbocation rearrangement will not take place if it does not result in a more stable carbocation.

Interpretation Introduction

(i)

Interpretation:

Whether a carbocation rearrangement will take place in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism for the given substrate is to be determined. A curved arrow notation of the carbocation rearrangement is to be drawn for the likely carbocation rearrangement.

Concept introduction:

The first step in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism is the same. In both cases, the leaving group splits off, forming a carbocation.

A carbocation may undergo rearrangement through a $\text{1, 2-hydride}$ shift or a $\text{1, 2-methyl}$ shift if a more stable carbocation is formed.

Stability of carbocations increases as ${\text{methyl < 1}}^{\text{o}}{\text{ < 2}}^{\text{o}}{\text{ < 3}}^{\text{o}}$.

Answer to Problem 8.52P

A carbocation rearrangement will take place in this case.

The curved arrow representation of this rearrangement can be drawn as

Explanation of Solution

The given substrate and the carbocation it will form in an ${\text{S}}_{\text{N}}\text{1}$ or E1 mechanism are

There are two possible $\text{1, 2-hydride}$ shifts, from C1 to C2 and from C3 to C2. The first one will result in an unstable primary carbocation and rearrangement will not occur.

The second one, from C3 to C2, will result in a more stable tertiary carbocation.

Therefore, a carbocation rearrangement is possible in this case.

The curved arrow representation of this rearrangement can be drawn as

Conclusion

The carbocation rearrangement is possible because a more a stable carbocation is formed.