The alkyl group is a -I + R substituent

Infrared spectroscopy is also sensitive to substituent properties, as illustrated by the C=O stretching frequency of carbonyl compounds as a function of the corresponding substituent Y, which can be rationalized in terms of resonance forms (Fig. 2). By taking an aliphatic aldehyde (ca. 1725 cm-1) as a reference, the redshift (wavenumber decrease) induced by a +I substituent (acetyltrimethylsilane, 1645 cm-1: Soderquist & Hsu, 1982) can be attributed to the stabilization of the zwitterionic form. Instead, the blueshift provoked by a ‒I substituent (acyl chlorides, >1800 cm-1: Pretsch et al., 2009) can be explained by means of two alternative or concurrent mechanisms (destabilization of the zwitterionic form and/or contribution of an acylium ion-bearing form). Finally, the redshifts provoked by +R substituents (amides, ca. 1680 cm-1: Pretsch et al., 2009) can be attributed to the contribution of a specific resonance form. The slight redshift induced by alkyl group (methyl ketones, ca. 1715 cm-1) shows a net electron-donating effect (hence, a predominance of the +R effect over ‒I properties). The net donor effect of the carbonyl-bound alkyl group is consistent with the larger dipole moment of acetone (2.88 D) relative to formaldehyde (2.33 D) (Nelson et al., 1967).