Organometallic addition to a carbonyl: the mechanism, step by step
The carbon attached to the metal is nucleophilic. It adds to the carbonyl, and the alkoxide that results is protonated when the reaction is worked up.
The pathway runs in 2 steps. What sits between them is a real structure: it is what the reaction passes through, and on a bad day it is what you isolate instead of the product.
Below is the sequence as it is taught. On the platform it is drawn on whatever structures you supply, with the curly arrows resolved to the actual atoms involved rather than to a general case with R groups on it.
The steps
The carbon on the metal attacks the carbonyl
The bond between carbon and the metal is very one-sided, so that carbon behaves as if it carried the electrons. It reaches the carbon of the C=O group and the double bond's electrons move onto the oxygen.
The polarised C-metal σ bond adds into the carbonyl π*, giving a metal alkoxide. The carbon that was bonded to the metal is now bonded to the former carbonyl carbon.
Water is added and the alkoxide takes a proton
Once the reaction is over, water or dilute acid is added. The negatively charged oxygen takes a hydrogen and becomes an alcohol.
Protonation of the alkoxide on aqueous work-up. This is deliberately a separate operation: a proton source present during the addition would consume the reagent instead.
What this drawing assumes
- The reagent is drawn as a simple carbon-metal bond. In solution it is aggregated and bridged by halide, and the drawing does not attempt that.
- The proton arrives on work-up rather than during the reaction; adding it earlier would destroy the reagent.
Other pathways this class runs by
- With a hindered ketone the reagent can remove a proton next to the carbonyl instead of adding to it, and the starting material comes back on work-up.
Run it on your own structures
Enter the reactants and the product you expect. If the transformation is one the engine can perform and this pathway reaches that product from those structures, it is drawn on them — with the arrows on the right atoms and the intermediates you would actually pass through. If it does not reach the product, you are told that instead of being shown a drawing that does not apply.
Other mechanisms
Nucleophilic acyl substitution
The nucleophile adds to the carbonyl, the carbon holds four groups for a moment, and the leaving group is pushed out as the double bond comes back.
Bimolecular nucleophilic substitution
One step. The nucleophile comes in on the opposite side from the leaving group, and the carbon turns inside out as the exchange happens.
Imine formation
The amine adds to the carbonyl, the resulting alcohol-amine loses water, and a carbon–nitrogen double bond is left.
Diels-Alder cycloaddition
One step. Six electrons move round a ring at once, two new single bonds form at the ends and the double bond ends up in the middle.
Bimolecular elimination
One step. The base takes a hydrogen from one carbon while the leaving group departs from the next, and a double bond forms between them.
Unimolecular nucleophilic substitution
The leaving group goes first, on its own, leaving a flat carbon with a positive charge. Whatever is around then attacks it from either side.