Tetrahedral carbonyl addition compound

IN THIS CHAPTER, we continue our discussion of the chemistry of carbonyl compounds. In Chapters 12-14, we concentrated on the carbonyl group itself and on nucleophilic additions to it to form tetrahedral carbonyl addition compounds. In the current chapter, we expand on the chemistry of carbonyl-containing compounds and consider the acidity of a-hydrogens and the enolate anions formed by their removal. The reactions presented here represent some... 

A second common reaction theme of a carbonyl group is reaction with a proton or another Lewis acid to form a resonance-stabilized cation. Protonation increases the electron deficiency of the carbonyl carbon and makes it more reactive toward nucleophiles. The reaction is followed by removal of a proton to give a tetrahedral carbonyl addition compound. 

From the perspective of the organic chemist, addition of a carbon nucleophile is the most important type of carbonyl addition reaction because a new carbon-carbon bond is formed in the process. Each of these reactions follows the same mechanistic two-step pattern of making a bond between the carbon nucleophile and the electrophilic carbonyl carbon atom to give the tetrahedral carbonyl addition compound, followed by adding a proton to give an —OH group in the product. 

Nucleophilic addition of a Grignard reagent to the electrophilic carbonyl carbon atom gives a tetrahedral carbonyl addition compound. 

Step 2 Add a proton. In a second step, the chemist adds a dilute acid solution to protonate the alkoxide function of the tetrahedral carbonyl addition compound to give the primary alcohol product. 

Once again, the chemist must add the add after the tetrahedral carbonyl addition compound forms. If the acid were added with the organolithium reagent in a single step, the acid would immediately protonate the organolithium reagent before any further reaction could take place. 

The anion of a terminal alkyne is a nucleophile (Section 7.5) and adds to the carbonyl group of an aldehyde or a ketone to form a tetrahedral carbonyl addition compound. In the following example, addition of sodium acetylide to cyclohexanone followed by hydrolysis in aqueous acid gives 1-ethynylcyclohexanol. 

Briefly, the mechanism for formation of an enamine is very similar to that for the formation of an imine. In the first step, nucleophilic addition of the secondary amine to the carbonyl carbon of the aldehyde or ketone followed by proton transfer from nitrogen to oxygen gives a tetrahedral carbonyl addition compound. Acid-catalyzed dehydration gives the enamine. At this stage, enamine formation differs from imine formation. The nitrogen has no proton to lose. Instead, a proton is lost from the a-carbon of the ketone or aldehyde portion of fhe molecule in an elimination reaction. 

One of the most common reaction themes of aldehydes and ketones is addition of a nucleophile to the carbonyl carbon to form a tetrahedral carbonyl addition compound. 

Addition of ammonia or a primary amine to the carbonyl group of an aldehyde or a ketone forms a tetrahedral carbonyl addition compound. Loss of water from this intermediate gives an imine.The mechanism for imine formation involves an initial attack of the nucleophilic nitrogen atom on the carbonyl carbon atom followed by proton transfer to the OH, creating an HjO group that then departs. 

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