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 intensitie s) of a large number of molecules and the decodification of their intensiti es in terms of parameters representing changes of charge distribution with vibration have provided standards that, at least in the case of molecules w ith an appreciable presence of CH bonds, may now be used to give informatio n on the acidity of the hydrogen atoms even based only on the qualitative a spects 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 at om, 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 pai r of an electronegative atom and with hyperonjugation of CH3 and CH2 with t he electrons of double and triple bonds. The variation of the hydrogen char ge is also accompanied by a change in the CH stretching frequencies, as rep orted in several works by McKean and coworkers. It has to be pointed out th at the change in the CH stretching intensities is almost always more notice able and often so large that it can be detected by the simple qualitative e xamination of the spectrum. We present here examples of such a qualitative analysis. (C) 2000 Elsevier Science B.V. All rights reserved.