Sodium borodeuteride (NaBD4) and lithium aluminum deuteride (LiAlD4) are convenient reagents for introducing deuterium, the mass 2 isotope of hydrogen, into organic compounds. Write the structure of the organic product of the following reactions, clearly showing the position of all the deuterium atoms in each:
The Correct Answer and Explanation is :
To determine the structure of the organic products formed in reactions with sodium borodeuteride (NaBD₄) and lithium aluminum deuteride (LiAlD₄), let’s analyze their behavior as reducing agents and how they incorporate deuterium into organic compounds.
Reaction 1: Reduction with NaBD₄
Sodium borodeuteride is primarily used to reduce aldehydes and ketones to alcohols. When it reacts with a carbonyl compound, a deuteride ion (D⁻) is delivered to the carbonyl carbon.
Example Reaction:
Reduction of acetone (CH₃COCH₃) with NaBD₄:
- The deuteride ion (D⁻) from NaBD₄ attacks the carbonyl carbon of acetone.
- A tetrahedral alkoxide intermediate forms.
- Protonation with a source of D⁺ (e.g., D₂O) completes the reaction.
Product: CD₃CHOHCD₃
Here, all the hydrogen atoms originally in the product alcohol at the α-position are replaced by deuterium due to the use of NaBD₄ and D₂O.
Reaction 2: Reduction with LiAlD₄
Lithium aluminum deuteride is a stronger reducing agent than NaBD₄ and can reduce aldehydes, ketones, carboxylic acids, and esters. It also delivers deuteride (D⁻) during reduction.
Example Reaction:
Reduction of ethyl acetate (CH₃COOCH₂CH₃) with LiAlD₄:
- The ester bond is cleaved, and two equivalents of D⁻ are delivered—one to the carbonyl carbon and one to the alkoxy oxygen.
- This generates ethanol-d₆ (CD₃CD₂OD) and a deuterated primary alcohol (CD₃CHDOD).
Product: CD₃CHDOD and CD₃CD₂OD
Explanation
- Both reagents replace hydrogen with deuterium during reduction by delivering D⁻ to the electrophilic carbonyl carbon.
- NaBD₄ is selective for aldehydes and ketones, whereas LiAlD₄ reduces a broader range of functional groups, including esters.
- The position of the deuterium atoms corresponds to where the reducing agent delivers D⁻ and the deuterated solvent supplies D⁺ during protonation.
These reactions are valuable for isotopic labeling in research and mechanistic studies.