It is possible that small errors may have been introduced during the process of converting them for use on this site, but these won't affect the argument in any way. Note: The infra-red spectra on this page have been produced from graphs taken from the Spectral Data Base System for Organic Compounds ( SDBS) at the National Institute of Materials and Chemical Research in Japan. You couldn't be sure that this trough wasn't caused by something else. The possible absorption due to the C-O single bond is queried because it lies in the fingerprint region. The infra-red spectrum for ethanoic acid looks like this: You will always be given this data in an A'level exam. Note: You will never have to remember where these absorptions occur. It is easily recognised in an acid because it produces a very broad trough in the range 2500 - 3300 cm -1. This absorbs differently depending on its environment. The other really useful bond is the O-H bond. Its position varies slightly depending on what sort of compound it is in. The carbon-oxygen double bond, C=O, is one of the really useful absorptions, found in the range 1680 - 1750 cm -1. Because that bond is present in most organic compounds, that's not terribly useful! What it means is that you can ignore a trough just under 3000 cm -1, because that is probably just due to C-H bonds. The C-H bond (where the hydrogen is attached to a carbon which is singly-bonded to everything else) absorbs somewhere in the range from 2853 - 2962 cm -1. The other bonds in ethanoic acid have easily recognised absorptions outside the fingerprint region. Note: If you aren't sure what the fingerprint region is, follow this link before you go on. You have to be very wary about picking out a particular trough as being due to a C-O bond. The carbon-oxygen single bond also has an absorbtion in the fingerprint region, varying between 10 cm -1 depending on the molecule it is in. The carbon-carbon bond has absorptions which occur over a wide range of wavenumbers in the fingerprint region - that makes it very difficult to pick out on an infra-red spectrum. You will see that it contains the following bonds: The infra-red spectrum for a simple carboxylic acid If you haven't already done so, you should read that page before you go on. Note: This page follows directly on from the introductory page on infra-red spectra.
#IR SPECTRA HOW TO#
This page explains how to use an infra-red spectrum to identify the presence of a few simple bonds in organic compounds.
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