Posts Tagged ‘diagnostic tool’

Why is the carbonyl IR stretch in an ester higher than in a ketone?

Thursday, February 28th, 2013
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Infra-red spectroscopy of molecules was introduced 110 years ago by Coblentz[1] as the first functional group spectroscopic method (” The structure of the compound has a great influence on the absorption spectra. In many cases it seems as though certain bonds are due to certain groups.“). It hangs on in laboratories to this day as a rapid and occasionally valuable diagnostic tool, taking just minutes to measure. Its modern utility rests on detecting common functional groups, mostly based around identifying the nature of double or triple bonds, and to a lesser extent in differentiating between different kinds of C-H stretches[2] (and of course OH and NH). One common use is to identify the environment of carbonyl groups, C=O. These tend to come in the form of aldehydes and ketones, esters, amides, acyl halides, anhydrides and carbonyls which are part of small rings. The analysis is performed by assigning the value of the C=O stretching wavenumber to a particular range characteristic of each type of compound. Thus ketones are said to inhabit the range of ~1715-1740 cm-1 and simple esters come at ~1740-1760 cm-1, some 20-30 cm-1 higher. Here I try to analyse how this difference arises.

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References

  1. W.W. Coblentz, "Infra-red Absorption Spectra: I. Gases", Physical Review (Series I), vol. 20, pp. 273-291, 1905. http://dx.doi.org/10.1103/PhysRevSeriesI.20.273
  2. J.L. Arbour, H.S. Rzepa, J. Contreras-García, L.A. Adrio, E.M. Barreiro, and K.K.M. Hii, "Silver-Catalysed Enantioselective Addition of OH and NH Bonds to Allenes: A New Model for Stereoselectivity Based on Noncovalent Interactions", Chemistry - A European Journal, vol. 18, pp. 11317-11324, 2012. http://dx.doi.org/10.1002/chem.201200547