The Grignard Reaction
The Grignard reaction forms a new carbon-carbon bond.
- It requires a carbon nucleophile (called a Grignard reagent).
- The nucleophile is synthesized from an alkyl halide and magnesium.
- The reaction needs to be done in a non-protic solvent (usually diethyl ether).
- The work-up step is the addition of an acid to get rid of the negative charges.
Cyanohydrin Formation
A cyanohydrin has an -OH group and a -CN group bonded to the carbon of interest.
- A cyano (CN-) group is the carbon nucleophile.
- The oxygen from the carbonyl is protonated by HCN to give the final product.
- The cyano group can further react with LiAlH4 (a reducing agent) to make an amine and with acid to make a carboxylic acid.
Example: Grignard Reaction
You attempt the reaction below in the lab but instead of the desired Grignard product, you isolate starting material and a by-product. Why does the reaction fail and what is the by-product?
Remember, MeMgBr is a strong nucleophile but also a strong base (you can think of it as Me- and +MgBr). The aromatic alcohol (phenol) is mildly acidic (pKa ~ 10). In the presence of one equivalent of MeMgBr, this OH will just be deprotonated.
The by-product is methane (CH4).