The rates of bromination of internal alkynes are roughly 100...

Name: Problem 10, Chapter 5: Advanced Organic Chemistry. Part A. Structure and Mechanisms
Uploaded: 2021-07-23T01:33:33-08:00
Duration: 1 min 56 s
Channel: Dr. Satish Ingale
Description: The rates of bromination of internal alkynes are roughly 100 times greater that the corresponding terminal alkynes. For hydration, however, the rates are less than 10 times greater for the disubstituted compounds. Account for this difference by comparison of the mechanisms for bromination and hydration.

The rates of bromination of internal alkynes are roughly 100 times greater that the corresponding terminal alkynes. For hydration, however, the rates are less than 10 times greater for the disubstituted compounds. Account for this difference by comparison of the mechanisms for bromination and hydration.

Key Concepts

Substituent Effects

Substituent effects arise because alkyl groups attached to the alkyne can donate electron density, which stabilizes charged intermediates through hyperconjugation and inductive effects. This stabilization is more pronounced in the bromination mechanism, where the positive charge build-up is significant in the bromonium ion intermediate, compared to the hydration mechanism where the transition state is less strongly influenced by the nature of the substituents.

Hydration Mechanism

Hydration of alkynes involves an initial protonation step, followed by the addition of water and subsequent formation of an enol that tautomerizes to a ketone or aldehyde. The protonation results in a positively charged intermediate that is less sensitive to the degree of alkyl substitution, leading to a smaller rate enhancement for internal alkynes relative to terminal alkynes.

Electrophilic Addition

Electrophilic addition refers to a class of reactions where an electron?rich unsaturated substrate, such as an alkyne, reacts with an electrophile to form a more saturated product via an intermediate. The nature of the substituents on the unsaturated bond greatly affects the rate of addition by stabilizing the developing positive charge in the transition state or intermediate.

Bromination Mechanism

The bromination of alkynes proceeds via an electrophilic addition mechanism involving the formation of a cyclic bromonium ion intermediate. In this step, the electrophilic bromine adds to the carbon–carbon triple bond and the substituents on the internal alkynes provide additional stabilization via hyperconjugation and inductive effects, dramatically accelerating the formation of the intermediate compared to terminal alkynes.

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