7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

Evidence for the Mechanism of Electrophilic Additions

Evidence for stepwise addition mechanism

Electrophilic addition to an alkene follows a two-step mechanism that passes through a carbocation intermediate. The strongest evidence for this stepwise pathway comes from carbocation rearrangements: if the reaction happened in a single concerted step, there would be no opportunity for the carbon skeleton to reorganize. The fact that rearranged products show up in the product mixture tells you a carbocation must be forming and living long enough to rearrange before a nucleophile attacks.

Here's how the two-step mechanism works:

1. The $\pi$ electrons of the alkene attack the electrophile (e.g., $H^+$ from $HBr$), forming a new $C-H$ bond on one carbon and leaving a carbocation on the adjacent carbon.
2. A nucleophile (e.g., $Br^-$) attacks the carbocation, forming the final addition product.

If a rearranged product appears that doesn't match the carbocation you'd expect from step 1, that's direct evidence the carbocation intermediate had time to rearrange before step 2 occurred.

Types of carbocation rearrangements

Two types of shifts convert a less stable carbocation into a more stable one:

• 1,2-Hydride shift: A hydrogen atom along with its bonding electrons migrates from an adjacent carbon to the carbocation center. For example, a secondary carbocation next to a carbon bearing a hydrogen can rearrange to a tertiary carbocation through a hydride shift.
• 1,2-Alkyl shift: An alkyl group (commonly a methyl group) migrates with its bonding electrons from an adjacent carbon to the carbocation center. This happens when no hydride shift is available to produce a more stable cation, or when an alkyl shift gives a more stable result.

Both shifts are driven by the same principle: moving toward a more substituted, more stable carbocation. The stability order is tertiary > secondary > primary > methyl. These shifts are fast, typically faster than nucleophilic capture, so the rearranged carbocation is the one that determines the product.

One thing to watch for: rearrangements only occur to go uphill in stability. A tertiary carbocation won't rearrange to a secondary one. And the shift always involves migration to the adjacent carbon bearing the positive charge.

Products of rearranged electrophilic additions

When you're predicting products, always check whether the initially formed carbocation can rearrange. Here's a reliable approach:

1. Form the initial carbocation. Apply Markovnikov's rule to place the $H^+$ on the less substituted carbon, generating the carbocation on the more substituted carbon.
2. Check for possible rearrangement. Look at the carbons adjacent to the cation. Could a hydride or alkyl shift produce a more stable carbocation? If the initial cation is secondary and a neighboring carbon could supply a shift to make it tertiary, expect rearrangement.
3. Identify the most stable carbocation. This is the intermediate that will predominantly react with the nucleophile.
4. Draw the final product. Attach the nucleophile to the carbon that bears the positive charge in the most stable carbocation.

Concrete example: When $HCl$ adds to 3-methyl-1-butene, protonation following Markovnikov's rule initially gives a secondary carbocation at C-2. A 1,2-hydride shift from C-3 converts this to a tertiary carbocation at C-3. The $Cl^-$ then attacks C-3, giving 2-chloro-2-methylbutane as the major product rather than the "expected" 2-chloro-3-methylbutane.

Factors influencing carbocation stability and reaction outcome

• Hyperconjugation stabilizes carbocations through electron donation from adjacent $C-H$ (and $C-C$) $\sigma$ bonds into the empty $p$ orbital on the cationic carbon. More alkyl substituents means more hyperconjugation, which is why tertiary cations are the most stable.
• Markovnikov's rule is really a consequence of carbocation stability. The proton adds to give the more stable carbocation, and the nucleophile ends up on the more substituted carbon.
• Stereochemistry: Because the carbocation intermediate is $sp^2$-hybridized and planar, the nucleophile can attack from either face. This typically produces a mixture of stereoisomers (if a stereocenter is created), rather than a single stereochemical outcome.
• Kinetic evidence: Electrophilic additions that proceed through carbocations show first-order kinetics in the rate-determining step (carbocation formation), consistent with a stepwise rather than concerted mechanism.