Carbanions add to ketones and aldehydes in much the same way as anionic hydride reducing reagents. Sources of carbanoins include Grignard and organo lithium and sodium reagents.
Here R can be alkyl, alkenyl or aryl. We will see examples of these shortly. Here is isopropyl magnesium bromide. Its convenient to write Grignard and organ lithium./sodiums as carbanions. Solutions of these reagents in ethereal solvents can be thought of as yielding carbanions and metal cations.
These are very reactive, very powerful nucleophiles and bases. When you work with these you have to take special precautions. Since they are such powerful bases, they will react violently with water and other relatively acidic compounds. Since they are very basic (pKa of conjugate bases 44-70), they react with (deprotonate) any acid with a pKa lower than 44.
Preparation of Grignard Reagents
Grignard reagents are prepared by combining the halide with Mgo in ether. Here are few examples
Preparation of Organolithium and Organosodiums
Carbanions derived from Grignard reagents and organo lithium/sodiums, readily add to the carbonyl carbons of aldehydes, ketones, esters and other carboxylic acid derivatives.
In the first step the nucleophilic carbanion attaches to the carbonyl carbon. Some chemists will show the curved arrow coming from the bond while others will show the discrete carbanion attacking the carbonyl carbon. This yields an alkoxide anion.
In the second step the alkoxide anion is quenched (protonated) with acidic water.