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When carbonyl compounds react with grignards, they usually give a 1,2 addition (direct addition) product. However, under certain conditions, they give conjugate addition (1,4 addition) product as the major product.

I tried to understand the reason behind this, but I couldn't find it anywhere. What are the conditions for such a reaction to take place, and what is the mechanism behind it?

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Borrowed from here

1,2 vs. 1,4 addition

Whether 1,2 or 1,4-addition occurs depends on multiple variables but mostly it is determined by the nature of the nucleophile. During the addition of a nucleophile there is a competition between 1,2 and 1,4 addition products. If the nucleophile is a strong base, such as Grignard reagents, both the 1,2 and 1,4 reactions are irreversible and therefore are under kinetic control. Since 1,2-additions to the carbonyl group are fast, we would expect to find a predominance of 1,2-products from these reactions.

If the nucleophile is a weak base, such as alcohols or amines, then the 1,2 addition is usually reversible. This means the competition between 1,2 and 1,4 addition is under thermodynamic control. In this case 1,4-addition dominates because the stable carbonyl group is retained.

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