Addition of Grignard reagents to aldehydes to give secondary alcohols
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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{ 3 comments… read them below or add one }
Shouldn’t the last example be a ten member-ed ring, if not what could be the mechanism
No, it’s an intramolecular reaction to give a 5 membered ring.
so the grignard will be a part of the aldehyde and then it will attack the carbon double bonded to the oxygen