Infrared Spectra

Now let's look at the features of infrared spectra that can help you determine whether a compound is an alcohol, an aldehyde or a ketone. (For a better view of these spectra and to have something to take notes on you should look at the IR spectra of the compounds that have the numbers 8, 9 and 10 in your workbook.)

Alcohols

The first of these (spectrum #8) is of 1-butanol, an alcohol. This is a very typical spectrum for alcohols. It has the same kinds of absorption frequencies as you find with the alkanes that we studied earlier -- three absorptions or dips in the spectrum just to the right of the 3000 cm^-1 mark on the wavenumber scale, and then also some beyond 1500 cm^-1 in the fingerprint region. These absorptions represent the same kinds of bonding arrangements among the carbon and hydrogen atoms as are found in alkanes. The big difference between this and an alkane is the huge absorption to the left of 3000 cm^-1, the one centered between 3300 cm^-1 and 3400 cm^-1. That is the significant absorption that identifies alcohols from other compounds and it has to do with the hydrogen in the OH group.

Infrared spectrum of 1-butanol. [63irsp08.JPG]

Aldehydes

Here is the IR spectrum of butanal, an aldehyde (spectrum #9). There are a couple of new things there. Again, there's another big, huge absorption to the left of 3000 cm^-1. This time it's a little bit further to the left than with an alcohol. It is centered between 3400 cm^-1 and 3500 cm^-1. This absorption is generally present in the IR spectra of the aldehydes that I have prepared. However, it seems that it is not caused by any part of the aldehyde itself. Instead, it may be due to the presence of small amounts of carboxylic acid impurities that can be found in aldehydes which have been exposed to air and thus are oxidized.

Infrared spectrum of butanal. [63irsp09.JPG]

More indicative of an aldehyde are the two little absorptions -- one just to the left of 2700 cm^-1 and one just to the left of 2800 cm^-1. Those are indicative of an aldehyde. Now those are fairly subtle distinctions to expect you to be able to pick up on and remember and see how they are different from an alcohol spectrum. However, aldehydes also contain a double bond in the carbonyl group. If you look down in the double bond section of the spectrum between 1500 cm^-1 and 2000 cm^-1, you will see a good-sized absorption near 1700 cm^-1. This double bond absorption is what allows you to distinguish between aldehydes and alcohols.

Ketones

The next spectrum is of 2-butanone (spectrum #10). This is a ketone. It is the same size as butanal but the carbon-oxygen double bond is in a different position and note how many things that changes. There is still a carbonyl absorption, a CO double bond absorption, at about 1700 cm^-1. But notice that the huge absorption to the left of 3000 cm^-1 is not there. (There are some some absorptions, but not like in spectra #8 or #9 for the alcohol and the aldehyde.) Also, the two small absorptions at about 2700 cm^-1 and 2800 cm^-1 are not there. So that's how one can distinguish between a ketone and an aldehyde, using infrared spectra.

Infrared spectrum of 2-butanone. [63irsp10.JPG]

Comparison

The distinguishing feature on infrared spectra of ketones and aldehydes is the strong carbonyl absorption that occurs at about 1700 cm^-1. The difference between the aldehyde and the ketone is that the aldehyde often has a broad impurity absorption between 3000 cm^-1 and 4000 cm^-1. The aldehyde also has those two absorptions at about 2700 cm^-1 and 2800 cm^-1, the ketone does not. Alcohols also have a broad absorption between 3000 cm^-1 and 4000 cm^-1 but they do not have the carbonyl double bond absorption that aldehydes and ketones have. If you keep these differences in mind, you can use the IR spectrum of a compound to help identify what type of compound it is.

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