Bonding in aldehydes and ketones

The carbon-carbon double bond in vinyl monomers and the carbon-oxygen double bond in aldehydes and ketones are the two main types of linkages that undergo chain... 

The carbon-oxygen double bond in aldehydes and ketones is similar and can be described in either of these two ways. If we adopt the iocalised-orbital description, formaldehyde will have two directed lone pairs in place of two of the C-H bonds in ethylene. In this case the axes of these hybrid orbitals will be in the molecular plane (unlike the oxygen lone pairs in water). Either the components of the double bond or the lone pairs can be transformed back into symmetry forms. The alternative description of the lone pairs would he one er-type along the 0-0 direction and one jr-type with axis perpendicular to the 0-0 bond hut in the molecular plane. It is the latter orbital which has the highest energy, so that an electron is removed from it in. ionisation or excitation to the lowest excited state. 

The acidifying effect of the remaining acceptor substituents of Table 10.1 decreases in the order —C(=0)—H > —C(=0)—alkyl > —C(=0)—O-alkyl, and the amide group —C(=0)—NR2 is even less effective. This ordering essentially reflects substituent effects on the stability of the C=0 double bond in the respective C,H-acidic compound. The resonance stabilization of these C=0 double bonds drastically increases in the order R—C(=0)—H < R—C(=0)—alkyl < R—C(=0)—0—alkyl < R—C(=0)— NR2 (cf. Table 6.1 see Section 7.2.1 for a comparison between the C=0 double bonds in aldehydes and ketones).This resonance stabilization is lost completely once the a-H-atom has been removed by way of deprotonation and the respective enolate has formed. 

The most important functional groups that participate in chain-growth polymerizations are the carbon-carbon double bond in alkenes and the carbon-oxygen double bond in aldehydes and ketones. In such polymerizations the active species A adds to one atom of the double bond and produces a new active species on the other atom ... 

The sequence begins by the initial condensation of the amine with the carbonyl component (Section 17-9) to produce the corresponding imine (for NH3 and primary amines) or iminium ion (secondary amines). Similar to the carbon-oxygen double bond in aldehydes and ketones, the carbon-nitrogen double bond in these intermediates is then reduced by simultaneous catalytic hydrogenation or by added special hydride reagents. 

Carbon-Oxygen Double Bonds in Aldehydes and Ketones... 

FUNCTIONAL GROUPS THAT CONTAIN OXYGEN Carbon-Oxygen Single Bonds in Alcohols and Ethers Carbon-Oxygen Double Bonds in Aldehydes and Ketones Carbon-Oxygen Bonds in Carboxylic Acids and Esters... 

The carbon-nitrogen triple bond of nitriles is much less reactive toward nucleophilic addition than is the carbon-oxygen double bond of aldehydes and ketones Strongly basic nucleophiles such as Gngnard reagents however do react with nitriles in a reaction that IS of synthetic value... 

The carbonyl carbon of a ketone bears two electron-releasing alkyl groups an aldehyde carbonyl group has only one. Just as a disubstituted double bond in an alkene is more stable than a monosubstituted double bond, a ketone carbonyl is more stable than an aldehyde carbonyl. We ll see later in this chapter that structural effects on the relative stability of carbonyl groups in aldehydes and ketones are an important factor in their relative reactivity. 

Carbonyl a carbon atom and oxygen atom joined by a double bond C=0, present in aldehydes and ketones Carboxyl Functional Group a carbonyl with a hydroxyl (-OH) attached to the carbon atom... 

Diphenylsulfonium cyciopropanide undergoes addition to the C — C double bond of oc,/ -unsat-urated carbonyl compounds to produce spiropentanes,57,58 and to the C —O double bond of aldehydes and ketones to produce oxaspiropentanes. These can be isolated59,60 or rearranged in situ 57,61,62 to produce cyclobutanones. Dimethylaminocyclopropylphenyloxosulfonium cyciopropanide reacts analogously.63,64... 

It is, of course, not correct to treat the wave function of a polyatomic molecule as localized in the chromophoric group considered responsible for the optical absorption. The carbonyl group in aldehydes and ketones gives rise to absorption which extends from about 3430 A to about 2200 A (as well as to absorption at shorter wavelengths). Nevertheless the carbon-oxygen bond is never broken by absorption at these wavelengths. Frequently an adjacent bond is broken but often more complex processes occur. It is sometimes possible to describe these processes in terms of quantum mechanics but some of them should not be treated as direct dissociations. 

Substituents with a electron-donating inductive (+1) effect (i.e., alkyl groups) stabilize the C=0 double bond of aldehydes and ketones. They increase the importance of the zwitterionic resonance form by which carbonyl compounds are partly described. The driving force for the formation of addition products from carbonyl compounds therefore decreases in the order H—CH(=Q) > R—CH(=0) > R R2c(=0). 

A major fragmentation pathway in aldehydes and ketones is cleavage of the bonds to the carbonyl carbon. This process is similar to the fragmentation that occurs with alcohols and produces cations that are relatively stable because the octet rule is satisfied at all of the atoms. The major peaks at m/z 85 and m/z 43 in the spectrum of 4-methyl-2-pentanone (see Figure 15.13) result from this cleavage, as illustrated in the following equations ... 

An aldol addition in present-day terminology involves the addition of the a-C atom of a carbonyl compound, a carboxylic acid, a carboxylic ester, or a carboxylic amide to the C=0 double bond of an aldehyde or a ketone. In the past, the term aldol addition was used in a more restricted sense, referring to the addition of the a-C atom of nothing but carbonyl compounds to the C=0 double bond of aldehydes and ketones. The products of aldol additions in today s usage of the term are (i-hydroxy Icarbonyl compounds (aldols), (i-hydroxy carboxylic acids, /3-hydroxycar-boxylic esters, or /3-hydroxycarboxylic amides. 

The GO bond or the single C—O bond a constant characteristic value of the interatomic distance is obtained for a number of molecules Table LIII). The case of the double bond C=0 is rather different from that of the ethy-lenic bond G=C, since the C=0 bond is not homopolar and contributions from other valence bond structures, with a separation of charge and a different multiplicity of the bond, always occur. In aldehydes and ketones, for example, the structure > C+—0 contributes to the resonance of the molecule and in carbon dioxide the structure 0 —G O+ occurs. In Table LIV the internuclear. distances of this bond in various molecules is given. In aldehydes the GO bond distance is i-20 A and very probably the same distance occurs in ketones. In urea, because of the contribution of the structure... 

Single bonds (sigma bonds) will also be seen in the carbon-oxygen bonds discussed later in the alcohol series, where there is a C-O-H bond. Double bonds (sigma and pi bonds) also occur in aldehydes and ketones where there is a C==0 bond. These homologous series will be looked at in Module 7. 

At 122 pm, the carbon-oxygen double bond distance in aldehydes and ketones is significantly shorter than the typical carbon-oxygen single bond distance of 141 pm in alcohols and ethers. 

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