In carbonyl groups

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

TABLE 1.4. Linear free energy relationships for addition to carbonyl gronps variation in carbonyl group. ... 

Different organic functional groups (i.e., methyl, methylene, phenyl, and the hydrogen atoms adjacent to the carbonyl carbon in aldehydes and organic add groups) absorb at different frequencies and thus can be easily identified. Similarly, different 13C environments result in different absorption characteristics. For instance, carbon atoms in aromatic compounds absorb different frequencies than do those in carbonyl groups. 

Identification of carbonyl groups. It has often been claimed that the carbon layers of graphite terminate in carbonyl groups ... 

When we consider double bonds to oxygen, as in carbonyl groups (C=0) or to nitrogen, as in imine functions (C=N), we find that experimental data are best accommodated by the premise that these atoms are sp hybridized (Figure 2.22). This effectively follows the pattern for carbon-carbon double bonds (see Section 2.6.2). The double bond is again a combination of a ct bond plus a jt bond resulting from overlap of p atomic orbitals. The carbonyl... 

Figure 17 (a) Crown ether based cytochrome P4S0 model (b) alcohol dehydrogenase model (c) proposed transition state in carbonyl group reduction by 1,4-dihydropyridine-containing crown ethers... 

Fig. 2 Effect of alkaline conditions on oxidized cotton linters. Left Changes in Mw (error bars 5%). Right Changes in carbonyl groups (error bars 2%)...

Click Coached Tutorial Problems for more practice using Table 19.1 to predict the position of the Equilibrium in Carbonyl Group Substitutions. 

Group Conditions (page 804) Coached Tutorial Problem Equilibrium in Carbonyl Group Substitutions (page 808)... 

Tire nitrogen atom in the pyridine ring is planar and trigonal with the lone pair in the plane of the ring. This makes it an imine. Most of the imines you have met before (in Chapter 14, for example), have been unstable intermediates in carbonyl group reactions, but in pyridine we have a stable imine—stable because of its aromaticity. All imines are more weakly basic than saturated amines and pyridine is a weak base with a pK of 5.5. This means that the pyridinium ion as about as strong an acid as a carboxylic acid. 

Heterocycles of this type occur widely. The benzo[l,3]dioxole ring system is often found in natural products and their degradation products, e.g., sesamol 38 lipoic acid 39 is a naturally occurring 1,2-dithiolane derivative. The 1,3-dithiolanes 40 are commonly known as 1,3-dithioacetals and have been extensively used in carbonyl group chemistry. In the absence of cyclic conjugation, ring sulfur atoms can readily exist in higher oxidation states as, for example, in the oxathiole S, -dioxide 41. 1,3,2-Dioxathiolane A-oxides 42 (cyclic sulfites) and 1,3,2-dioxathiolane A,A-dioxides (cyclic sulfates) are useful as synthetic equivalents of epoxides. 

It was also revealed by means of ultraviolet, visible and infrared optical absorption and hydrogen ion-exchange capacity measurements that the radiation induced defects such as fluorocarbon and peroxy radicals, and C=0 including in carbonyl groups were related to the new proton conduction processes. The modification of the hydrogen absorption characteristics due to the radiation induced defects in the near sttrface regions induces the enhancement of the proton conductivity. 

The a-substitiition reaction of a carbonyl compound through either an enol or enolate ion intermediate is one of the four fundamental reaction types in carbonyl-group chemistry. 

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