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Organic Reagents

By James Ashenhurst

Reaction Friday: Addition of Grignard Reagents to Ketones

Last updated: March 27th, 2019

Grignard reagents are very useful carbon-based nucleophiles and will readily form carbon-carbon bonds with carbonyl compounds. Here the reaction of Grignard reagents with ketones is described, along with several examples and a detailed mechanism.

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6 thoughts on “Reaction Friday: Addition of Grignard Reagents to Ketones

  1. Thanks for the video! Question – what if you add the grignard reagent to a molecule with two carbonyl groups? Will it add the alkyl group to both or just one?

    1. It’s not actually “elimination”, in that paper, it’s “enolization”. The conjugate base of a ketone is an “enolate”. What that paper is saying is that the Grignard reagent acts as a base, deprotonating the carbon adjacent to the ketone, resulting in an enolate. This happens in the case of bulky Grignard reagents.

      There isn’t a line, per se. If you see the paper, it’s more like a continuum. Think of it like turning a knob: as you increase the bulkiness of the Grignard reagent, you’ll start to increase deprotonation. Same goes for increasing the bulkiness of the ketone. When you have both a bulky Grignard and bulky ketone, reduction and/or deprotonation will predominate.

      As a proxy for bulkiness the categories, “methyl, primary, secondary, and tertiary” are useful.

      There are ways around this tendency – by using a strong Lewis acid that is compatible with Grignards (Cerium chloride is a common example) you can bias the reaction more towards addition.

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