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INTERPRETING AN INFRA-REDSPECTRUM

This page explains how to use an infra-red spectrum to identify thepresence of a few simple bonds in organic compounds.

Note: This page follows directly on from the introductorypage on infra-red spectra. If you haven't already done so, youshould read that page before you go on.

The infra-red spectrum for a simple carboxylic acid

Ethanoic acid

Ethanoic acid has the structure:

You will see that it contains the following bonds:

carbon-oxygen double, C=O


carbon-oxygen single, C-O

oxygen-hydrogen, O-H

carbon-hydrogen, C-H

carbon-carbon single, C-C

The carbon-carbon bond has absorptions which occur over a widerange of wavenumbers in the fingerprint region - that makes it verydifficult to pick out on an infra-red spectrum.

The carbon-oxygen single bond also has an absorbtion in thefingerprint region, varying between 1000 and 1300 cm-1

depending on the molecule it is in. You have to be very wary aboutpicking out a particular trough as being due to a C-O bond.

Note: If you aren't sure what the fingerprint region is, followthis link before you go on.

The other bonds in ethanoic acid have easily recognisedabsorptions outside the fingerprint region.

The C-H bond (where the hydrogen is attached to a carbon whichis singly-bonded to everything else) absorbs somewhere in therange from 2853 - 2962 cm-1. Because that bond is present inmost organic compounds, that's not terribly useful! What it means


is that you can ignore a trough just under 3000 cm-1, because thatis probably just due to C-H bonds.

The carbon-oxygen double bond, C=O, is one of the really usefulabsorptions, found in the range 1680 - 1750 cm-1. Its positionvaries slightly depending on what sort of compound it is in.

The other really useful bond is the O-H bond. This absorbsdifferently depending on its environment. It is easily recognised inan acid because it produces a very broad trough in the range 2500- 3300 cm-1.

Note: You will never have to remember where theseabsorptions occur. You will always be given this data in anA'level exam.

The infra-red spectrum for ethanoic acid looks like this:


The possible absorption due to the C-O single bond is queriedbecause it lies in the fingerprint region. You couldn't be sure thatthis trough wasn't caused by something else.

Note: The infra-red spectra on this page have been producedfrom graphs taken from the Spectral Data Base System forOrganic Compounds (SDBS) at the National Institute ofMaterials and Chemical Research in Japan.

It is possible that small errors may have been introducedduring the process of converting them for use on this site, butthese won't affect the argument in any way.


The infra-red spectrum for an alcohol

Ethanol

The O-H bond in an alcohol absorbs at a higher wavenumber thanit does in an acid - somewhere between 3230 - 3550 cm-1. In factthis absorption would be at a higher number still if the alcohol isn'thydrogen bonded - for example, in the gas state. All the infra-redspectra on this page are from liquids - so that possibility will neverapply.

Notice the absorption due to the C-H bonds just under 3000 cm-1,and also the troughs between 1000 and 1100 cm-1 - one of whichwill be due to the C-O bond.


The infra-red spectrum for an ester

Ethyl ethanoate

This time the O-H absorption is missing completely. Don't confuseit with the C-H trough fractionally less than 3000 cm-1. Thepresence of the C=O double bond is seen at about 1740 cm-1.

The C-O single bond is the absorption at about 1240 cm-1.Whether or not you could pick that out would depend on the detailgiven by the table of data which you get in your exam, because C-O single bonds vary anywhere between 1000 and 1300 cm-1


depending on what sort of compound they are in. Some tables ofdata fine it down, so that they will tell you that an absorption from1230 - 1250 is the C-O bond in an ethanoate.

The infra-red spectrum for a ketone

Propanone

You will find that this is very similar to the infra-red spectrum forethyl ethanoate, an ester. Again, there is no trough due to the O-Hbond, and again there is a marked absorption at about 1700 cm-1

due to the C=O.


Confusingly, there are also absorptions which look as if they mightbe due to C-O single bonds - which, of course, aren't present inpropanone. This reinforces the care you have to take in trying toidentify any absorptions in the fingerprint region.

Aldehydes will have similar infra-red spectra to ketones.

The infra-red spectrum for a hydroxy-acid

2-hydroxypropanoic acid (lactic acid)

This is interesting because it contains two different sorts of O-Hbond - the one in the acid and the simple "alcohol" type in the


chain attached to the -COOH group.

The O-H bond in the acid group absorbs between 2500 and 3300,the one in the chain between 3230 and 3550 cm-1. Taken together,that gives this immense trough covering the whole range from2500 to 3550 cm-1. Lost in that trough as well will be absorptionsdue to the C-H bonds.

Notice also the presence of the strong C=O absorption at about1730 cm-1.

The infra-red spectrum for a primary amine

1-aminobutane


Primary amines contain the -NH2 group, and so have N-H bonds.

These absorb somewhere between 3100 and 3500 cm-1. Thatdouble trough (typical of primary amines) can be seen clearly onthe spectrum to the left of the C-H absorptions.

Where would you like to go now?

To the infra-red spectroscopy menu . . .

To the instrumental analysis menu . . .

To Main Menu . . .


Jim Clark 2000

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