![]() ![]() IR can also be a quick and convenient way for a chemist to check to see if a reaction has proceeded as planned. In conjunction with other analytical methods, however, IR spectroscopy can prove to be a very valuable tool, given the information it provides about the presence or absence of key functional groups. For this reason, we will limit our discussion here to the most easily recognized functional groups, which are summarized in this table.Īs you can imagine, obtaining an IR spectrum for a compound will not allow us to figure out the complete structure of even a simple molecule, unless we happen to have a reference spectrum for comparison. It is possible to identify other functional groups such as amines and ethers, but the characteristic peaks for these groups are considerably more subtle and/or variable, and often are overlapped with peaks from the fingerprint region. The spectrum for 1-octene shows two peaks that are characteristic of alkenes: the one at 1642 cm -1 is due to stretching of the carbon-carbon double bond, and the one at 3079 cm-1 is due to stretching of the s bond between the alkene carbons and their attached hydrogens.Īlkynes have characteristic IR absorbance peaks in the range of 2100-2250 cm -1 due to stretching of the carbon-carbon triple bond, and terminal alkenes can be identified by their absorbance at about 3300 cm-1, due to stretching of the bond between the sp-hybridized carbon and the terminal hydrogen. This is the characteristic carboxylic acid O-H single bond stretching absorbance. We also see a low, broad absorbance band that looks like an alcohol, except that it is displaced slightly to the right (long-wavelength) side of the spectrum, causing it to overlap to some degree with the C-H region. In the spectrum of octanoic acid we see, as expected, the characteristic carbonyl peak, this time at 1709 cm -1. The breadth of this signal is a consequence of hydrogen bonding between molecules. This signal is characteristic of the O-H stretching mode of alcohols, and is a dead giveaway for the presence of an alcohol group. There is a very broad ‘mountain’ centered at about 3400 cm -1. Now, let’s take a look at the IR spectrum for 1-hexanol. 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 infrared spectrum for ethanoic acid looks like this: 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 recognized absorptions outside the fingerprint region. 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 infrared spectrum for a simple carboxylic acid: Ethanoic acid ![]()
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