organic chemistry

In aldol reaction we use a base such as NaOH Which takes away alpha hydrogen of aldehyde or ketone.

Will grignard reagent do aldol reaction or addition reaction in ketone or substitution of Br in the reactant shown below. Taking only one mole of grignard reagent and if needed water can be added.

BrCH2CH2CH2COCH3

Note by Ayush Jain
3 years, 4 months ago

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1 vote

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Grignard reagent will add to the ketone, in a nucleophilic addition at the carbonyl group.

Ameya Daigavane - 3 years, 4 months ago

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Actually answer is it will add to ketone and then negative charge on O will substitute br to form 5 membered ring.

I wanted to know the reason why grignard reagent being a strong base cannot take away alpha hydrogen of ketone and form 5membered ring substituting Br to form cyclopentanone

AYUSH JAIN - 3 years, 4 months ago

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Yep, that is correct. Grignard reagents selectively attack carbonyl groups. Only if the group is sterically hindered, will it act as a base, taking the alpha hydrogen.

There are two points to be considered here:

Grignard reagents are better nucleophiles than bases.

The alpha hydrogen is much less acidic (about 10^-15 times the pKa) than a carboxylic acid group. Even though there are many reactions that involve alpha hydrogen abstraction, it really isn't very acidic by any means.

Ameya Daigavane - 3 years, 4 months ago

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@Ameya Daigavane thank you sir for your kind reply.

sir please tell me why OH is not a good nucleophile while Grignard reagent is.

Also sir if in a compound in which like HCCCHO ie combination of alkyne and aldehyde, how will NaOH and Grignard reagent react separately . Do they react in same manner or different.

AYUSH JAIN - 3 years, 4 months ago

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@Ayush Jain OH is a worse nucleophile than a base, because of H-bonding. This is similar to reason why F- is the weakest nucleophile in protic solvents, but the strongest in aprotic.

I would recommend you to remember the trends, as the reasoning can really go either ways.

NaOH is not strong enough to abstract terminal alkyne hydrogens, but Grignard reagents can and will. I do not think the Grignard reaction will proceed here.

Ameya Daigavane - 3 years, 4 months ago

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@Ameya Daigavane But I have read in Solomons and fryhle that alpha carbon is more acidic than terminal hydrogen of alkyne . So should extract alpha hydrogen . Please reply .

AYUSH JAIN - 3 years, 4 months ago

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@Ayush Jain See here and here (under acidity of terminal alkynes).

You make a fair point. Perhaps the presence of Li+ cation stabilises the intermediate in the alkyne + Grignard reaction.

Ameya Daigavane - 3 years, 4 months ago

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