How to Tackle Organic Syntheses - a Beginner's Guide by Prof Otto Meth-Cohn

The Grignard Reaction

Victor Grignard was an enthusiastic young French chemist who discovered how to make organomagnesium halides (RMgX) while working for his PH.D. His boss, Barbier, had been trying this sort of chemistry for some time, but Victor was the genius who solved the problem. This discovery in 1901 changed the course of organic chemistry and won him the Nobel Prize in 1912. We now refer to such compounds as Grignard Reagents - and they are the first tool in your bag. Victor's breakthrough came with two discoveries - an ether solvent was vital and the whole chemistry must be carried out bone dry. You take an alkyl halide (preferably a bromide or iodide but a very reactive chloride such as tertiary-butyl chloride or benzyl chloride will be OK) magnesium metal and ether (dried with sodium metal) and with a little persuasion you get a vigorous reaction resulting in a Grignard reagent.

The reagent is used in solution (it is soluble in ether) and is never isolated. The metal is positively polarised and the alkyl group thus is like a carbanion. It certainly behaves as a carbanion. All kinds of alkyl halides react - and amazingly, even bromobenzene and other aryl bromides and iodides react easily with magnesium. This is particularly surprising since the aromatic halogen is so unreactive. For example, it is inert to refluxing aqueous sodium hydroxide even above 200C.

Reactions of Grignards

They react vigorously with compounds with 'active' hydrogens- OH, NH and others. That's why the solution must be dry.

Grignards react enthusiastically with all kinds of C=O bonds.

This is a fantastically good way of making carboxylic acids. After the reaction of the Grignard with carbon dioxide, addition of mineral acid liberates the carboxylic acid. So we can convert an alkyl bromide to an acid - our first C-C bond-forming reaction!

Before we go on to look at more Grignard reactions with carbonyl groups, here's a problem for you to try.

Now - reactions of Grignards with aldehydes and ketones.

Ol' Spidey: page last changed 23 January 1997