10.8: Alcoholes a partir de compuestos carbonilo: Reacción de Grignard
Grignard reagents are one of the most commonly used reagents used to synthesize alcohols from carbonyl compounds. Grignard reagents are organomagnesium halides with a highly polar carbon–magnesium bond. Due to the partial ionic nature of the C–Mg bond, the carbon functions as a strong nucleophile and attacks electrophiles like carbonyl carbon.
Magnesium from the reagent coordinates with carbonyl oxygen, further reducing the carbonyl carbon's electron density. Thus, the carbonyl carbon is a stronger electrophile. The carbanionic group of the reagent then attacks this electrophilic carbon to form an alkoxide ion. In the following step, the alkoxide ion is protonated using dilute acid or water to give alcohol.
Different classes of alcohols are formed depending on the type of carbonyl compounds undergoing the reaction. When reacted with a Grignard reagent, formaldehyde converts to a primary alcohol, while all other aldehydes form secondary alcohols. Ketones, on the other hand, give tertiary alcohols. The carbonyl group in a carboxylic acid does not welcome nucleophilic additions with the Grignard reagent. Instead, the reagent acts as a strong base and abstracts a proton from the acid's –COOH group. The derivatives of carboxylic acid, such as esters and acid chlorides, react with two equivalents of Grignard reagent to form tertiary alcohols with a ketone intermediate.