These functional groups remind me of 1985 when Maverick flew through the jet wash and Goose and he had to eject from their F-14 Tomcat. What does this title mean? What we are trying to say is that carbonyls can be classified two different ways: ejectable or non-ejectable. What this means is that sometime when a carbonyl is attacked by a nucleophile the carbonyl will eject one of its substituents before it reduces the carbonyl to an alcohol. After the group has been ejected, then a second equivalent of nucleophile will reduce the carbonyl to alcohol.
In essence, this means if a carbonyl has an ejectable group on it, a nucleophile will add twice to that carbonyl. Some examples of ejectable and non-ejectable systems are below:
In terms of synthesis, we will then observe the following:
As shown above with the ketone and the aldehyde (which have non-ejectable substituents), the Grignard reagent can only add once to the carbonyl, giving an alcohol as the product in both instances. However, in the case of the acid halide and ester (which have ejectable groups attached) the first equivalent of Grignard kicks off the ejectable group to give a ketone. This ketone can then be reacted with another equivalent of Grignard reagent to give the final product, which is a tertiary alcohol. In the example above, we have added two equivalents of Grignard to the starting material in two different steps. However, if the alcohol is your desired end-product, you can do this all in one step by adding two or more equivalents of Grignard reagent.
Take Home Message: Nucleophiles will substitute twice at the carbonyl if the starting material is an acid halide or an ester.