The study and use of compounds featuring carbon-metal bonds (organometallic chemistry)
is a major area of synthetic chemistry. Metal atoms are less electronegative than carbon and
so the polarity of a C-M bond is reversed in comparison to C-H, C-X (X= halogen), C-N
etc. found in a typical organic compound i.e. the carbon atom is nucleophilic rather than
electrophilic. Reactions involving organometallic compounds are widely used in industrial
organic and pharmaceutical synthesis.
An organomagnesium halide such as phenylmagnesium bromide (PhMgBr) is commonly
referred to as a Grignard reagent. Other examples of organometallic compounds familiar
from CHEM 343/345 include organolithium reagents (RLi) and lithium diorganocuprates
(R2CuLi, Gilman reagents). Organometallic compounds are widely used as both
stoichiometric reagents and catalysts to enable organic transformations that are difficult or
impossible to accomplish with classical organic reagents.
A Grignard reagent is formed by reaction of magnesium metal with an organic halide in
diethyl ether. The halide can be iodine (most reactive), bromine or chlorine (least reactive).
Recall that you studied the reactivity of a series of C-X bonds in Chapter 5.
The stoichiometric reaction of Grignard reagents with carbonyl compounds is a common
method for the formation of carbon-carbon single bonds and an important route for the
synthesis of alcohols. The type of carbonyl compound used determines the type of alcohol
produced; aldehydes afford secondary alcohols, while ketones and esters yield tertiary
alcohols. Primary alcohols can be prepared from the Grignard reaction of formaldehyde
(CH2O).
The simplified mechanism of the Grignard reaction with acetone (Scheme 1) involves
attack by the nucleophilic carbon atom of the Grignard reagent at the carbonyl carbon atom
to form a new C-C bond. Acidic work-up converts the alkoxide species to the
corresponding alcoho