Organic Chemistry 2 · Addition to aldehydes and ketones
α-Deuteriobenzyl alcohol from benzaldehyde and NaBD4
Conditions: NaBD4, CH3OH
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The mechanism, step by step
Next plays each step's curved arrows and explains them. The written steps are below.
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Borodeuteride delivers a deuteride
NaBD4 behaves exactly like NaBH4, except that the atom it transfers is deuterium. The B–D bond pair attacks the aldehyde carbon and the C=O π pair moves onto oxygen, so the deuterium ends up bonded to carbon.
The curved arrows
- The B–D bond pair forms the new C–D bond at the aldehyde carbon.
- The C=O π pair moves onto oxygen, giving a benzylic alkoxide.
Where does the label go?
Isotope labels trace the source of each new atom.
Methanol protonates the alkoxide
The solvent is CH3OH, not CH3OD, so the alkoxide takes an ordinary proton from methanol. This keeps the label only on carbon.
The curved arrows
- A lone pair of the alkoxide oxygen takes the O–H proton of methanol.
- The O–H bond pair stays on oxygen as methoxide forms.
α-Deuteriobenzyl alcohol
The benzylic carbon now carries H, D, OH and phenyl, so it is a stereocentre. Deuteride adds to both faces of the planar aldehyde equally, giving a racemic product.
What you'll practice
Draw NaBD4 delivering deuteride to benzaldehyde and methanol protonating the alkoxide, and use the label to show which atom comes from boron and which from the solvent.
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