In this lesson, we will learn:
- How Grignard reagents are generated and used to increase carbon chain length. .
- Full reaction equations and conditions for generating and using Grignard reagents.
- Why Grignard reagents are very versatile in forming C-C bonds.
- We have seen a lot of functional groups in organic chemistry now, and that’s good because the functional groups are what create most of the properties in organic compounds!
But chain length is also very important. It affects properties like solubility which can have huge consequences on a compound’s usefulness.
Grignard reagents are any compounds with the formula RMgX. They are a good way to make carbon-carbon bonds and increase chain length .
The term Grignard is practical - if magnesium has one X- halide bond and one alkyl or phenyl group, it is a Grignard reagent. They are named with the alkyl section first, then the magnesium and the anion like an ordinary ionic compound. For example:
- CH3MgCl would be methyl magnesium chloride.
- CH3CH2MgBr would be ethyl magnesium bromide.
- (C6H5)MgBr would be phenyl magnesium bromide.
- Grignard reagents work very well at creating C-C bonds because of a few factors:
- Mg is electropositive . The C-Mg bond will have a carbon atom and a magnesium atom. Alkyl, hydroxyl or halide groups will take the electrons in a bond and in solution, Grignards are effectively R- Mg2+ X-. This makes the alkyl group nucleophilic.
- Mg forms strong bonds with many anions found in organic chemistry such as RO-, Cl- etc.
- Grignard reactions are very exothermic because magnesium compounds have large lattice enthalpies. Starting with RMgX, an MgX2 product will form very readily, where X is any two inorganic anions.
This does mean that Grignard reactions must be anhydrous (completely dry) – this is NOT an aqueous reaction. Grignards react vigorously with water and will produce the alkane and an Mg(OH)X compound. Taking propyl magnesium bromide as an example:
CH3CH2CH2MgBr + H2O → CH3CH2CH3 + Mg(OH)Br
We don’t want this to happen so we don’t want any water in the reaction at all!
- As mentioned above, because magnesium reacts with water, Grignard reagents are produced in ‘dry’ conditions with an ether solvent under reflux . This is an important part of the conditions you are expected to know. Ether has a very low boiling point so reflux is essential.
Magnesium pieces react with a haloalkane in ether to produce the Grignard reagent. With propyl magnesium bromide as an example below, you use bromopropane and magnesium as reagents:
CH3CH2CH2Br + Mg → CH3CH2CH2MgBr
Think of the magnesium atom ‘inserting’ between the C-X bond in the haloalkane.
- Once you have the Grignard reagent mixture, there are two reactions that can form a carbon-carbon bond and increase chain length.
- Grignards reacting with carbon dioxide works like an addition reaction. One of the C=O double bonds opens up and inserts between the C-Mg bond in the Grignard reagent. The product looks like a carboxylate salt (-COOMgX) which is hydrolysed in dilute acid to get a carboxylic acid. This carboxylic acid group is the extra carbon atom in the chain.
Using propyl magnesium bromide as an example:
CH3CH2CH2MgBr + CO2 → CH3CH2CH2COOMgBr
CH3CH2CH2COOMgBr + H2O → CH3CH2CH2COOH + Mg(OH)Br
- Grignards reacting with carbonyl compounds such as ketones and aldehydes . The reaction is similar to how carbon dioxide reacts – one C=O double bond is opened up by the Grignard reagent. The difference is that in the CO2 example there is another C=O carbonyl but here there are two R groups attached instead. The result is an alcohol being produced rather than a carboxylic acid when it is hydrolysed.
A general reaction scheme for this is below, where R and R’ can be any alkyl/aryl groups or H.
Grignard reactions are very exothermic and produce the alcohols readily – you can make a variety of alcohols very easily by just changing the ketone/aldehyde you are using or the haloalkane used to make the Grignard reagent! It’s a very versatile and reliable reaction.