With the exception of the acid-catalyzed conversion of...

Name: Problem 77, Chapter 14: Introduction to General, Organic and Biochemistry
Uploaded: 2021-06-18T19:21:58-08:00
Duration: 1 min 59 s
Channel: Nicholas Sacco
Description: With the exception of the acid-catalyzed conversion of ethylene to ethanol, primary alcohols cannot be prepared by the acid-catalyzed hydration of alkenes. The acid-catalyzed conversion of almost all other alkenes gives either secondary or tertiary alcohols. Explain why these two generalizations are valid.

With the exception of the acid-catalyzed conversion of ethylene to ethanol, primary alcohols cannot be prepared by the acid-catalyzed hydration of alkenes. The acid-catalyzed conversion of almost all other alkenes gives either secondary or tertiary alcohols. Explain why these two generalizations are valid.

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Key Concepts

Markovnikov's Rule

Markovnikov's Rule states that when a protic acid adds to an unsymmetrical alkene, the proton will attach to the carbon that already has more hydrogen atoms, leading to the formation of the more stable carbocation intermediate. This regioselectivity explains why, except in special cases, acid-catalyzed hydration of alkenes typically produces secondary or tertiary alcohols rather than primary alcohols.

Reaction Mechanism of Acid-Catalyzed Hydration

This mechanism involves the initial protonation of the alkene to form a carbocation intermediate, which is then attacked by a water molecule. The subsequent deprotonation of the oxonium ion leads to the formation of the alcohol. This two-step mechanism is fundamental to understanding how alkenes are converted into alcohols under acid-catalyzed conditions.

Carbocation Stability

Carbocation stability plays a critical role in determining the outcome of acid-catalyzed hydration reactions. More highly substituted carbocations (secondary and tertiary) are stabilized by hyperconjugation and inductive effects, making them more favorable intermediates than less substituted (primary) carbocations. This enhanced stability drives the reaction toward the formation of secondary or tertiary alcohols.

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