Why can't diisobutylaluminum hydride (iBu 2 AlH, DIBAL-H) reduce an aldehyde, but it can reduce an ester?

DIBAL-H is a weaker reductant than LiAlH 4 .

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21. Aldehydes and Ketones: Nucleophilic Addition

DIBAL

Organic Chemistry

21. Aldehydes and Ketones: Nucleophilic Addition

DIBAL

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Multiple Choice

Why can't diisobutylaluminum hydride (iBu₂AlH, DIBAL-H) reduce an aldehyde, but it can reduce an ester?

The ester is a more reactive functional group than the aldehyde.

There is only one equivalent of hydride per DIBAL-H molecule.

DIBAL-H is a weaker reductant than LiAlH₄.

DIBAL-H actually can reduce an aldehyde, but the aldehyde doesn't form at cold temperatures.

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Verified step by step guidance

Understand the structure and reactivity of diisobutylaluminum hydride (DIBAL-H). It is a bulky reducing agent with only one hydride ion available per molecule, making it less reactive compared to other reducing agents like lithium aluminum hydride (LiAlH4).

Recognize the difference in reactivity between esters and aldehydes. Esters are generally more reactive towards reduction than aldehydes due to the presence of an additional electronegative oxygen atom, which makes the carbonyl carbon more electrophilic.

Consider the conditions under which DIBAL-H operates. It is often used at low temperatures, which can affect the reactivity of aldehydes. At cold temperatures, aldehydes may not react as readily with DIBAL-H.

Note that DIBAL-H can reduce esters to aldehydes because the ester functional group is more reactive, allowing the single hydride ion to effectively perform the reduction.

Understand that while DIBAL-H can reduce aldehydes, the reaction may not proceed efficiently at low temperatures, which is why it is more commonly used to reduce esters to aldehydes rather than aldehydes to alcohols.