Title: Org 2.12 – Synthesis Reactions with Carbonyl Compounds – The Grignard Reaction
Grignard reagents are very useful compounds in organic chemistry. They are named after the French chemist, Francois Auguste Victor Grignard. He discovered them in 1900 and was awarded the Nobel Prize in Chemistry in 1912 for his work (1). Grignard reagents belong to the family of compounds known as organometallics – organic compounds that contain at least one carbon-metal bond in their structure (2). They are prepared by reacting magnesium turnings with an alkyl halide or an aryl halide, usually in ether solvent.
Grignard reagents are important in the synthesis of alcohols. Reacting a Grignard with formaldehyde will produce a primary alcohol. The reaction between a Grignard and other aldehydes will yield a secondary alcohol. A tertiary alcohol will be the product of the reaction between a Grignard and a ketone. The “juvenile hormone” which is used in pest control is also manufactured by means of Grignards (3).
Carbonyl chemistry is an important aspect of Grignard reactions. This is because of two things. Firstly, a carbonyl group is highly polarised, which means the positively charged carbon atom is exposed to nucleophilic attack. Grignard reagents act as nucleophiles. Secondly, the polarised carbonyl group allows any alpha-protons to be easily removed by strong bases (4).
In this experiment, the Grignard reagent 1-methylbutyl magnesium bromide was prepared by reacting an excess of magnesium turnings with 2-bromopentane in diethyl ether. It was then reacted with propanal to give the secondary alcohol, 4-methyl-3-heptanol. The mechanism is shown below. TLC analysis was carried out on the final product and both starting materials. An IR spectrum was obtained in the lab and a H NMR was provided for interpretation. The procedure was followed according to the manuaul.
The complete mechanism for the preparation of a Grignard...