Question

Each of the following cations is capable of rearranging to a more stable cation. Limiting yourself to a single 1,1 -shift, suggest a structure for the rearranged cation. (a) \(\mathrm{CH}_{3} \mathrm{CHCH}_{3} \mathrm{C}^{+} \mathrm{HCH}_{3}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}^{+} \mathrm{CHCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C}^{+} \mathrm{HCH}\left(\mathrm{CH}_{3}\right) \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\) (d) \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{C}^{+} \mathrm{HCH}_{2} \mathrm{CH}_{3}\) (e) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{C}^{+} \mathrm{HC}\left(\mathrm{CH}_{3}\right)_{3}\)

Short answer

The rearranged cations after single 1,1-shift are: (a) \(\mathrm{CH}_{3} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH}_{2} \mathrm{HCH}_{3}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}^{+} \mathrm{CH}_{2} \mathrm{CHCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (e) \(\mathrm{CH}_{3} \mathrm{O} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH} \left(\mathrm{CH}_{3}\right)_{3}\)

Step-by-step solution

(a) Rearrangement of CH3CHCH3C+HCH3

First, identify the possible 1,1-shifts for the molecule. A hydride shift from the neighboring carbon to the carbocation center will result in the migration of a hydrogen: \(\mathrm{CH}_{3} \mathrm{CHCH}_{3} \mathrm{C}^{+} \mathrm{HCH}_{3} \xrightarrow{1,1\text{-shift}} \mathrm{CH}_{3} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH}_{2} \mathrm{HCH}_{3}\)

(b) Rearrangement of (CH3)3C+CHCH3

For this carbocation, a methyl shift from the neighboring carbon to the carbocation center will result in the migration of a methyl group: \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}^{+} \mathrm{CHCH}_{3} \xrightarrow{1,1\text{-shift}} \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}^{+} \mathrm{CH}_{2} \mathrm{CHCH}_{3}\)

(c) Rearrangement of CH3CH2CH2C+HCH(CH3)C(CH3)3

For this carbocation, a hydride shift from the neighboring carbon to the carbocation center will result in the migration of a hydrogen: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C}^{+} \mathrm{HCH}\left(\mathrm{CH}_{3}\right) \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3} \xrightarrow{1,1\text{-shift}} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\)

(d) Rearrangement of CH2=CHCH2C+HCH2CH3

For this carbocation, a hydride shift from the neighboring carbon to the carbocation center will result in the migration of a hydrogen: \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{C}^{+} \mathrm{HCH}_{2} \mathrm{CH}_{3} \xrightarrow{1,1\text{-shift}} \mathrm{CH}_{2}=\mathrm{CH} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

(e) Rearrangement of CH3OCH2C+HC(CH3)3

For this carbocation, a hydride shift from the neighboring carbon to the carbocation center will result in the migration of a hydrogen: \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{C}^{+} \mathrm{HC}\left(\mathrm{CH}_{3}\right)_{3} \xrightarrow{1,1\text{-shift}} \mathrm{CH}_{3} \mathrm{O} \mathrm{C}^{+} \mathrm{H}_{2} \mathrm{CH} \left(\mathrm{CH}_{3}\right)_{3}\)

Key concepts

1,1-shift

In organic chemistry, a 1,1-shift refers to the intramolecular rearrangement where a substituent, such as a hydrogen (hydride) or an alkyl group (methyl or ethyl), migrates from one carbon atom to an adjacent carbon cationic center. This process occurs in an attempt to stabilize the positively charged intermediate formed during some organic reactions.

For example, consider a carbocation with a positively charged carbon atom. If an adjacent carbon has a hydrogen or an alkyl group that can be transferred to the cationic center, this movement, known as a 1,1-shift, can lead to a more stable carbocation. The driving force for this rearrangement is the overall decrease in energy of the system as the positive charge becomes more delocalized or placed on a more stable center.

Hydride Shift

A hydride shift is a specific type of 1,1-shift where a hydride ion (a hydrogen atom with an extra electron, represented as H-) moves from one carbon to an adjacent carbocation center. This migration often creates a more stabilized carbocation structure.

In the context of the solved exercise, the hydride shift helps achieve a more stable carbocation by moving a hydrogen from a carbon that is next to the carbocation. This transfer allows for the filling of the electron deficiency at the cationic center by supplying a pair of electrons from the hydride. The effect of a hydride shift is noticeable in various organic reaction mechanisms, where creating a more stable intermediate can significantly alter the pathway and outcome of the reactions.

Methyl Shift

Similar to the hydride shift, a methyl shift involves the migration of a methyl group, rather than a hydrogen atom, during a 1,1-shift. The methyl group (CH3) moves from one carbon atom to the neighboring carbocation center to stabilize the carbocation.

During this rearrangement, the entire methyl group, including its three hydrogen atoms, shifts to the adjacent carbon with a positive charge. The process can be energetically favorable if it results in a more stable cation, often because the new carbocation is better able to share or delocalize the positive charge through its structure, for example, by forming a tertiary carbocation from a secondary one.

Carbocation Stability

Carbocation stability plays a crucial role in the occurrence of 1,1-shifts, as these rearrangements are often driven by the formation of a more stable carbocation. Stability is influenced by several factors, including the degree of alkyl substitution: tertiary carbocations (surrounded by three carbon groups) are more stable than secondary and primary carbocations due to hyperconjugation and the inductive effect.

Moreover, the presence of electron-donating groups nearby can help stabilize the positive charge by delocalization or resonance. As a general rule, the more stable the carbocation, the more favored is its formation. Hence, reaction mechanisms involving carbocations will often include steps like hydride and methyl shifts that generate the most stable carbocation achievable from the starting structure.

Organic Reaction Mechanisms

Organic reaction mechanisms involve the step-by-step breakdown of how a chemical reaction occurs at the molecular level. A deep understanding of these mechanisms allows chemists to predict the products of a reaction and design pathways for synthesizing complex molecules. Carbocation rearrangements, such as 1,1-shifts including hydride and methyl shifts, are critical parts of many organic reaction mechanisms and are crucial for synthesizing a wide variety of chemical compounds.

These rearrangements are often encountered in reactions such as alkene additions, nucleophilic substitutions, and electrophilic aromatic substitution. Understanding how and why these shifts occur can help students grasp the nuances of reaction pathways and the principles governing the stability and reactivity of intermediates.