Multiple Choice
Why can't ester reduction with lithium aluminum hydride (LAH, LiAlH4) stop at the aldehyde stage?
21. Aldehydes and Ketones: Nucleophilic Addition
DIBAL
4:41 minutesProblem 39c
Textbook Question
Predict the major products of the following reactions.
(c) 
Verified step by step guidance1
Step 1: Identify the functional group in the starting material. The molecule contains a terminal alkyne group (-C≡CH). This functional group is reactive under the conditions provided.
Step 2: Recognize the reagent and its role. DIBAL-H (Diisobutylaluminum hydride) is a reducing agent that selectively reduces certain functional groups, such as esters, nitriles, and alkynes, under controlled conditions (e.g., -78°C). It is often used to stop the reduction at an intermediate stage.
Step 3: Predict the intermediate formed after the first step. DIBAL-H at low temperatures (-78°C) reduces the alkyne group to an aldehyde. This occurs because DIBAL-H selectively adds hydrogen to the triple bond, forming a double bond and introducing an oxygen atom.
Step 4: Consider the second step of the reaction. The addition of water (H₂O) in the second step is used to hydrolyze the aluminum complex formed during the reduction, yielding the final product.
Step 5: Combine the information to predict the major product. The major product of this reaction will be an aldehyde derived from the reduction of the terminal alkyne group. The cyclic structure remains intact, and the aldehyde group (-CHO) will be attached to the carbon that was part of the triple bond.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
DIBAL-H (Diisobutylaluminum Hydride)
DIBAL-H is a strong reducing agent commonly used in organic chemistry to selectively reduce esters and nitriles to aldehydes. At low temperatures, such as -78°C, it allows for controlled reduction, preventing over-reduction to alcohols. Understanding its reactivity and selectivity is crucial for predicting the products of reactions involving carbonyl compounds.
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DIBAL-H on Esters and Nitriles
Aldehyde Formation
The reaction of DIBAL-H with an ester or nitrile typically results in the formation of an aldehyde. This occurs because DIBAL-H adds to the carbonyl carbon, leading to the formation of an intermediate that can be hydrolyzed upon the addition of water. Recognizing the mechanism of aldehyde formation is essential for predicting the major products in the given reaction.
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Hydrolysis
Hydrolysis is the chemical process in which water reacts with a compound, leading to the breakdown of that compound. In the context of the reaction with DIBAL-H, the addition of water after the reduction step converts the intermediate formed into the final aldehyde product. Understanding hydrolysis is important for determining the final outcome of the reaction sequence.
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