The Carbonyl Bond

Compared to carboxylic and carbonic acid derivatives, the less highly oxidized carbonyl compounds such as aldehydes and ketones are not so widespread in nature. That is not to say that they are unimportant. To the contrary. Aldehydes and ketones are of great importance both in biological chemistry and in synthetic organic chemistry. However, the high reactivity of the carbonyl group in these compounds enables them to function more as intermediates in metabolism or in synthesis than as end products. This fact will become evident as we discuss the chemistry of aldehydes and ketones. Especially important are the addition reactions of carbonyl groups, and this chapter is mostly concerned with this kind of reaction of aldehydes and ketones. 

16-1A Comparison with Carbon-Carbon Double Bonds 

The carbonyl bond is both a strong bond and a reactive bond. The bond energy varies widely with structure, as we can see from the carbonyl bond energies in Table 16-1. Methanal has the weakest bond (166 kcal) and carbon monoxide the strongest (257.3 kcal). Irrespective of these variations, the carbonyl bond not only is significantly stronger but also is more reactive than a carbon-carbon double bond. A typical dilference in stability and reactivity is seen in hydration  

16 Carbonyl Compounds I. Aldehydes and Ketones. Addition Reactions of the Carbonyl Group 

The equilibrium constant for ethene hydration is considerably greater than for methanal hydration, largely because the carbon-carbon double bond is weaker. Even so, methanal adds water rapidly and reversibly at room temperature without need for a catalyst. The corresponding addition of water to ethene occurs only in the presence of strongly acidic catalysts (Section 10-3E, Table 15-2). 

---

The yields ranged from 55% for the mixture of enamines formed from morpholine and methylisopropyl ketone to 94% for the enamine formed from dimethylamine and methyl t-butyl ketone. The hindered ketone 2,5-dimethylcyclopentanone could be converted to an enamine, but the more hindered ketone, 2,6-di-t-butylcyclohexanone, was inert. White and Weingarten 43) attribute the effectiveness of titanium tetrachloride in this reaction to its ability to scavenge water and to polarize the carbonyl bond. 

From single-crystal X-ray structural analysis the ground-state conformation of (.S )-2-(4-meth-ylphenylsulfinyl)-2-cyclopentenone was shown to have the sulfoxide bond orientated anti to the carbonyl bond, as expected for minimization of electrostatic interactions13. 

Polymerization of 2-Furaldehyde and Homologues through the Carbonyl Bond... 

This section deals with investigations specifically aimed at producing homopolymers and copolymers of furan carbonyl compounds by the selective opening of the carbonyl bond. The many reports on polymerization of 2-furaldehyde which in fact deal with complicated acid-catalysed resinification reactions which involve both the formyl group and the furan ring are reviewed in Chapter VI. 

Copolymerization of 2-furaldehyde through the carbonyl bond has given some encouraging results. Natta and co-workers147 were the first to describe an alternat-... 

Mechanisms of aldehyde oxidation are not firmly established, but there seem to be at least two main types—a free-radical mechanism and an ionic one. In the free-radical process, the aldehydic hydrogen is abstracted to leave an acyl radical, which obtains OH from the oxidizing agent. In the ionic process, the first step is addition of a species OZ to the carbonyl bond to give 16 in alkaline solution and 17 in acid or neutral solution. The aldehydic hydrogen of 16 or 17 is then lost as a proton to a base, while Z leaves with its electron pair. 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

Effect of substituents of the carbonyl bond, (a) Inductive effect, X = H, OH,... 

The frequency change is small when compared to the difference between C = O (1700 cm-1) and C-O (1100 cm-1) stretches in line with the prevailing view that in planar amides relatively little charge is transferred to oxygen. The changes in frequency represent a small stiffening of the carbonyl bond at best. 

The synthetic potential of phosphorus ylides was initially developed by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphorus yhde with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond ... 

---