Infrared intensities. Use of the CH-stretching band intensity as a tool for evaluating the acidity of hydrogen atoms in hydrocarbons

The accurate study of the infrared spectra (both frequencies and intensities) of a large number of molecules and the decodification of their intensities in terms of parameters representing changes of charge distribution with vibration have provided standards that, at least in the case of molecules with an appreciable presence of CH bonds, may now be used to give information on the acidity of the hydrogen atoms even based only on the qualitative aspects of the infrared spectrum in the CH-stretching region. The charge on the hydrogen atom of a CH bond may change by hybridization of the carbon atom, by induction of polarization in the CH bond by a close electronegative atom, by backdonation into the CH bond of negative charge from any lone pair of an electronegative atom and with hyperonjugation of CH3 and CH2 with the electrons of double and triple bonds. The variation of the hydrogen charge is also accompanied by a change in the CH stretching frequencies, as reported in several works by McKean and coworkers. It has to be pointed out that the change in the CH stretching intensities is almost always more noticeable and often so large that it can be detected by the simple qualitative examination of the spectrum. We present here examples of such a qualitative analysis. (C) 2000 Elsevier Science B.V. All rights reserved.