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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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