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Why Grignard reagents does not remove the alpha hydrogen atom from an aldehyde or a ketone inspite of it being a strong base and the alpha hydrogen atom attached to aldehyde or ketone appreciably acidic?

[ pka for alpha H of aldehydes and ketones = 19-20

pka for terminal alkynes = 25 ]

Why I'm asking this is because I've seen grignard reagents remove hydrogen attached to terminal alkynes inspite of them being less acidic as compared to the alpha hydrogens of aldehyde or ketone.

pka values for alpha hydrogen of most simple aldehydes or ketones are of the order 19-20 :I've taken this statement from an organic text by Wiley Publication. Also most terminal alkynes have pka approximately about 25.

Swapnil Katore , 14 Years ago

Grade 12

3 Answers

SAGAR SINGH - IIT DELHI

Last Activity: 14 Years ago

Dear student,

Grignard Reaction
Grignard Reagents

The Grignard Reaction is the addition of an organomagnesium halide (Grignard reagent) to a ketone or aldehyde, to form a tertiary or secondary alcohol, respectively. The reaction with formaldehyde leads to a primary alcohol.

Grignard Reagents are also used in the following important reactions: The addition of an excess of a Grignard reagent to an ester or lactone gives a tertiary alcohol in which two alkyl groups are the same, and the addition of a Grignard reagent to a nitrile produces an unsymmetrical ketone via a metalloimine intermediate.

Swapnil Katore

Last Activity: 14 Years ago

I'm not satisfied with the answer that you have given. Please provide a proper answer to my question.The answer given does not have any relation to what my query was??