Carbonyl groups characteristic electronic

The use of infra-red or ultraviolet spectroscopy to examine the molecular groups present in a chemical compound is familiar to any chemist. One of the main uses of this technique is to apply a range of electromagnetic frequencies to a sample and thus identify the frequency at which a process occurs. This can be characteristic of, say, the stretch of a carbonyl group or an electronic transition in a metal complex. The frequency, wavelength or wavenumber at which an absorption occurs is of most interest to an analytical chemist. In order to use this information quantitatively, for example to establish the concentration of a molecule present in a sample, the Beer-Lambert law is used ... 

Conjugation of the carbonyl with a double bond transfers the electronic characteristics 8-I-/8— of the carbonyl group along the carbon chain. The alkene would normally be nucleophilic and react with... 

As discussed in Section II, thiocarbonyl compounds differ from their carbonyl counterparts in at least two important characteristics. Because of the higher energy of the sulfur p orbitals, they are much more reactive as electron donors. On the other hand, the C=S bond is also much less polarized than the C=0 bond, due to the smaller difference in electronegativities between carbon and sulfur. The latter fact leads to the reactions of the thiocarbonyl group being less selective than those of the carbonyl group. This happens, for instance, in the case of nucleophilic additions (see Section IV.C), and an enhanced reactivity against dipoles has also been observed (see Section IV.E.3). 

Ketones and Aldehydes The R2C=0 and the RCH=0 carbon atoms absorb in a characteristic region. Acetone absorbs at 203.3 ppm, and acetaldehyde at 199.3 ppm. Alkyl substitution on the a-carbon causes a shift to the left of the C=0 absorption of 2-3 ppm until steric effects supervene. Replacement of the CH3 of acetone or acetaldehyde by a phenyl group causes a shift to the right of the C=0 absorption (acetophenone, 195.7 ppm benzaldehyde. 190.7 ppm) similarly, a,/3-unsaturation causes shifts to the right (acrolein, 192.1 ppm, compared with propionaldehyde, 201.5 ppm). Presumably, charge delocalization by the benzene ring or the double bond makes the carbonyl carbon less electron deficient. 

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