A Grignard reagent is a source of a carbanion.
The carbon attached to magnesium in a Grignard reagent reacts with carbonyl carbon to form a carbon-carbon bond. This carbon-carbon bond-forming reaction is a way of converting smaller carbon compounds into bigger ones.
Grignard reagents react at C=O in aldehydes and ketones to give alcohols.
The alcohol arising from reaction of a Grignard reagent with an aldehyde is secondary. This is because aldehydes have H at the carbon bearing O, and there is also a H at this carbon in the product.
The exception to this is methanal (CH2=O) where primary alcohols are produced.
The alcohol arising from reaction of a Grignard reagent with a ketone is tertiary. This is because ketones have no H at the carbon bearing O, and there is no H at this carbon in the product.
Grignard reagents react at C=O in CO2 to give carboxylic acids:
The red and black colouration for the atoms in the products show which atoms come from the carbonyl compound and which come from the Grignard. When planning a synthesis using a Grignard reagent and a carbonyl compound, it is important to remember that the new bond will be formed to the carbon bearing oxygen and to choose the carbonyl and Grignard component accordingly.
The carbon attached to magnesium in a Grignard reagent reacts with carbonyl carbon to form a carbon-carbon bond. This carbon-carbon bond-forming reaction is a way of converting smaller carbon compounds into bigger ones.
The alcohol arising from reaction of a Grignard reagent with an aldehyde is secondary. This is because aldehydes have H at the carbon bearing O, and there is also a H at this carbon in the product.
The exception to this is methanal (CH2=O) where primary alcohols are produced.
The alcohol arising from reaction of a Grignard reagent with a ketone is tertiary. This is because ketones have no H at the carbon bearing O, and there is no H at this carbon in the product.
CH3MgBr + O=C=O 
CH3CO2H
CH3CO2HThe red and black colouration for the atoms in the products show which atoms come from the carbonyl compound and which come from the Grignard. When planning a synthesis using a Grignard reagent and a carbonyl compound, it is important to remember that the new bond will be formed to the carbon bearing oxygen and to choose the carbonyl and Grignard component accordingly.