General Reactions of Carbonyl Compounds

With what types of reagents should a carbonyl group react The electronegative oxygen makes the carbonyl carbon electrophilic, and because it is trigonal planar, a carbonyl carbon is uncrowded. Moreover, a carbonyl group has an easily broken jt bond. 

As a result, carbonyl compounds react with nucleophiles. The outcome of nucleophilic attack, however, depends on the identity of the carbonyl starting material. 

Carbonyl compounds that contain leaving groups undergo nucleophilic substitution. 

Let s examine each of these general reactions individually. 

Aldehydes and ketones react with nucleophiles to form addition products hy the two-step process shown in Mechanism 20.1 nucleophilic attack followed by protonation. 

The carbon-oxygen double bond of a carbonyl group is similar in many respects to the carbon-carbon double bond of an alkene. The carbonyl carbon atom is sp -hybridized and forms three o-bonds. The fourth valence electron remains in a carbon p orbital and forms a -Trbond to oxygen by overlap with an oxygen p orbital. The oxygen atom also has two nonbonding pairs of electrons, which occupy its remaining two orbitals. 

Both in the laboratory and in living organisms, the reactions of carbonyl compounds take place by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, and carbonyl 

Nucleophilic Addition Reactions of Aldehydes and Ketones (Chapter 14) 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Because the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carhonyl carbon rehybridizes from sp to sp during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

FIGURE 2 The addition reaction of an aldehyde or a ketone with a nucleophile. Depending on the nucleophile, either an alcohol or a compound with a C=Nu double bond is formed. 

An electrostatic potential map shows the electron-deficient carbon and the electron-rich oxygen atom of the carbonyl group. 

The aldehyde a-sinensal (Problem 20.1) Is the major compound responsible for the orange-llke odor of mandarin oil, obtained from the mandarin tree In southern China. 

Chapter 20 Introduction to Carbonyl Chemistry Organometallic Reagents Oxidation and Reduction 

Formation of an Alcohol The simplest reaction of a tetrahedral alkoxide intermediate is protonation to yield an alcohol. We ve already seen two examples ol this kind of process during reduction of aldehydes and ketones with hydride reagents such as NaBhl4 and LiAlH4 (Section 17.4) and during Grignard reactions (Section 17.5). During a reduction, the nucleophile that adds to the carbonyl 

O Dehydration of the amino alcohol intermediate gives neutral imine plus water as final products. 

Nucleophilic Acyl Substitution Reactions of Carboxylic Acid Derivatives (Chapter 21) 

Formation of an imine, R2C=NR, by reaction of an amine with an aldehyde or a ketone. 

O Addition to the ketone or aldehyde carbonyl group by the neutral amine nucleophile gives a dipolar tetrahedral intermediate. 

These mechanisms have many variations, just as alkene electrophilic addition reactions and Sn2 reactions do, but the variations are much easier to learn when the fundamental features of the mechanisms are made clear. Let s see what the four mechanisms are and what kinds of chemistry carbonyl compounds undergo. 

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