Reduction with DIBAL-H: practice and mechanism
DIBAL-H at −78 °C delivers only one hydride. The tetrahedral intermediate is stable in the cold; only on workup does the aldehyde form (from an ester or nitrile), not an alcohol.
Example
At −78 °C DIBAL-H transfers only one hydride. The tetrahedral intermediate is stable in the cold and only collapses on work-up: esters and nitriles are thus reduced only to the aldehyde, not to the alcohol or amine.
Mechanism step by step
- 1
The Al atom of DIBAL-H is a Lewis acid and coordinates to the carbonyl oxygen.
- 2
One hydride is transferred to the carbonyl carbon. At −78 °C the tetrahedral intermediate stays stable.
- 3
Only on aqueous workup does it collapse to the aldehyde, when no hydride is left.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Reduction with lithium aluminium hydride · Reduction with sodium borohydride
Frequently asked questions
Why not LiAlH₄?
LiAlH₄ delivers several hydrides and the aldehyde forms in the mixture; being more reactive than the ester, it is at once reduced to the alcohol.
What does DIBAL-H do with nitriles?
It adds one hydride to give an imine–aluminium complex; hydrolysis gives the aldehyde.