Addition of Grignard reagents to aldehydes to give secondary alcohols

by James

Description: Addition of a Grignard reagent to an aldehyde results in a new C–C bond an a secondary alcohol after addition of acid.
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Notes: X in this case can be Cl, Br, or I.

Acid can be written as “H+”, “H3O+”, or can be any number of other acids.

The byproduct (often not pictured) is a magnesium salt.

Examples:

Notes:

Example 3 is tricky – Mg forms the Grignard, which then attacks the aldehyde. This is an intramolecular reaction.

Mechanism:

1,2-addition of the Grignard to the aldehyde (step 1, arrows A and B) results in an alkoxide, which is protonated when acid is added in the workup (Step 2, arrows C and D) resulting in the secondary alcohol.

Notes: There’s nothing special about the chloride or bromide ions here, any halogen will be fine.

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{ 4 comments… read them below or add one }

Rajarshi Banerjee

Shouldn’t the last example be a ten member-ed ring, if not what could be the mechanism

Reply

james

No, it’s an intramolecular reaction to give a 5 membered ring.

Reply

Rajarshi Banerjee

so the grignard will be a part of the aldehyde and then it will attack the carbon double bonded to the oxygen

Reply

Brian Butler

The Mg adds by oxidative insertion to the Br to form the Grignard. Then, that reacts with the carbonyl carbon – yes.

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