Interpreting Infrared Spectra Painlessly, Quickly, and Correctly

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1 Interpreting Infrared Spectra Painlessly, Quickly, and Correctly Knowing how to interpret infrared (IR) spectra is of immense help to structure determination. Not only will it tell you what functional groups and structural elements are there, it will also clarify which ones are not there, quickly reducing your list of possibilities before you even think about glancing at the NMR. Let s take a quick look at some example spectra to gain more of an intuitive feel for this technique. I have included the spectra as links to PDFs online in order to prevent prohibitive file sizes and loading times. Also, this way you can open the spectrum in a separate browser window and position them over the text in such a way that you can read the information and compare with the peaks on the spectra. I hope this will be a useful learning tool for you. (Remember to press and hold CTRL while clicking the links to get them to open.) Organic Compounds in General: Alkanes Example Spectrum A: octane ( You might say that octane has no functional groups, so how would an IR spectrum help? Well, since nearly every molecule you will have to identify will have either alkane, alkene, or aromatic groups attached, you must know what each looks like. This spectrum shows you what a simple alkyl group looks like, so you should expect that ANY molecule with CH 3, CH 2, or CH groups anywhere in it would contain these peaks and you would be correct. The only thing I want you to notice in this spectrum is the cluster of peaks below 3000 cm 1 down to around 2850 cm 1 : these are the peaks from the stretching of sp 3 hybridized C H bonds. So whenever you see a spectrum with this cluster of peaks, think: there must be regular alkyl chain carbons in this molecule! Note that the many other peaks you see at lower wavenumbers will not be readily distinguishable in all spectra, so we don t have to worry about them. C=C and C H Bonds: Alkenes Example Spectrum B: 2 methyl 1 hexene ( Alkene spectra should show both regular alkane peaks (from all the sp 3 carbons) and peaks for the sp 2 carbons. Looking at the spectrum, you should recognize the alkane peaks below 3000 cm 1, but there are two important new additions: first is a peak close to 3100 cm 1, which is the peak for sp 2 hybridized C H bonds; second is the sharp peak around 1650 cm 1, which represents the C=C double bond. These are the two signs of an alkene.

2 But one other type of compound has C=C bonds the phenyl ring (benzene etc.); how in the world can you tell the difference? One way is by calculating the unsaturation index, but another is by knowing exactly how alkene C=C and phenyl C=C spectra differ. C=C and C H Bonds: Benzene/Phenyl Rings Example Spectrum C: ethylbenzene ( This compound has both a phenyl ring and an alkyl chain. Of course you can detect the familiar sp 3 C H peaks below 3000 cm 1, but there are many more peaks to look for than in alkene spectra. First of all, while the alkene above had a single peak close to 3100 cm 1, the phenyl ring usually shows multiple C H peaks between 3100 and 3000 cm 1. Second, while the alkene spectrum showed a C=C peak between 1600 and 1700 cm 1, phenyl rings almost always show a weaker sharp peak right at 1600 cm 1. Want three more pieces of proof that you have a phenyl ring instead of an alkene? Phenyl rings generally show two very narrow peaks at 1450 and 1500 cm 1 alkenes don t. Phenyl rings always have a strong, sharp peak in the region of 700 to 800 cm 1 alkenes don t. Also, phenyl rings almost always show jagged craziness (it s weak, but it s there) between 2000 and 1650 cm 1 alkenes don t. Compare these two example spectra until you are totally sure you can tell the difference at a glance. Triple Bonds: Alkynes Example Spectrum D: 1 hexyne ( The last of the hydrocarbon family consists of alkynes, compounds with sp hybridized carbons in triple bonds. The example spectrum here shows the familiar alkyl chain sp 3 peaks below 3000 cm 1, but the C C bond vibration will show up as a weak, sharp peak between 2100 and 2200 cm 1. (Note: the strong sharp peak at 3300 cm 1 is characteristic for C H bonds on sp hybridized carbons in alkynes, but you are not expected to know this for the exam.) This is really not confusing when you compare with alkenes, phenyl rings, or alkanes, but there is one other functional group that shows up in this region the only other functional group you know with triple bonds!

3 Triple Bonds: Nitriles Example Spectrum E: hexanenitrile ( Like alkynes, nitriles possess a triple bond and therefore their peaks appear in a similar location, and it may seem easy to confuse the two on a problem set or exam. But just remember two things: you will always be given the molecular formula, and nitriles have a nitrogen atom while alkynes do not; and also, remember the number 2200 cm 1 : alkynes will rarely, if ever, appear above 2200 cm 1, while nitriles will always appear above 2200 cm 1. Simple! O H Bonds: Alcohols Example Spectrum F: 2 butanol ( Alcohols are one of the easiest groups to identify on IR spectra. Their O H bond stretching shows up as a strong, broad peak upwards of 3300 cm 1, which you can t miss. Also note the usual alkane sp 3 C H peaks just below 3000 cm 1. O H and C=O Bonds: Carboxylic Acids Example Spectrum G: cyclopentanecarboxylic acid ( Sometimes students get confused about how to differentiate alcohols from carboxylic acids on IR they both have OH groups, right? Well, the way these compounds behave on IR is very different. In the spectrum of the alcohol above, the O H bond is a strong, broad peak around or above 3300 cm 1 ; in this carboxylic acid spectrum, the O H peak is still strong but shallower, and is MUCH broader, starting around 3400 cm 1 and stretching all the way down to 2400 cm 1 or so. Also note that you can confirm this as a carboxylic acid by the strong, sharp carbonyl C=O peak (alcohols have no carbonyl), which for carboxylic acids shows up right around 1700 cm 1. Telling the difference is easy as pie!

4 Speaking of carbonyls, let s shift our attention completely to a discussion of how to differentiate the different functional groups that all contain carbonyls, starting with the simplest the ketones. C=O Bonds: Ketones Example Spectrum H: pinacolone ( O Ketone spectra are so easy that they become difficult in other words, they lack much information at all, and students tend to try and read more into them than they should. Ketones have only a carbonyl C=O as their functional group, and thus you would expect that they show a C=O stretch peak right around 1700 cm 1, maybe a little higher. All the other peaks on the spectrum arise from whatever groups are attached to the carbonyl carbon here only alkyl groups. Simple so far. But what happens when other groups are attached? Multiple Functional Groups Example: Phenyl Ring and Ketone Example Spectrum I: propiophenone ( This ketone possesses one alkane side chain and one phenyl ring side chain. You should predict that you will see not only the C=O peak, but also sp 3 C H peaks and all the expected phenyl ring peaks. So let s take a look! Ketone C=O peak around 1700 cm 1? Check. Alkane sp 3 C H peaks below 3000 cm 1? Check. Phenyl ring sp 2 C H peaks between 3000 and 3100 cm 1? Check. Phenyl C=C peaks at 1600, 1500, and 1450 cm 1? Sort of it looks like the last two peaks have been shifted down by about 50 cm 1, but they still look the same, so check. Strong, sharp phenyl peak between 700 and 800 cm 1? Check. Bizarre jagged craziness between 2000 and 1650 cm 1? Check. Looks like we re good to go with an alkyl aryl ketone!

5 C=O and C H Bonds: Aldehydes Example Spectrum J: isobutyraldehyde ( At first blush it would seem that differentiating aldehydes and ketones would be tough, because an IR spectrum can t tell you how many alkyl side chains are present, just whether they are present. Luckily for you, that single hydrogen attached to the carbonyl carbon has a tale to tell Look at the spectrum of course you see the carbonyl C=O around 1730 cm 1 or so, but look very carefully at the alkyl C H peak region. There are two extra peaks there, around 2800 and 2700 cm 1, that look like vampire fangs; these are the peaks that are the dead giveaway (bad pun intended) for an aldehyde! Now, the reason why this single aldehyde hydrogen gives two peaks is beyond our scope right now, but this is an excellent way to distinguish aldehydes from other carbonyl compounds, especially ketones. C=O and C O Bonds: Esters Example Spectrum K: cyclohexyl propionate ( Esters can be tough sometimes too, because they can appear as just another nondescript carbonyl compound in the IR. Of course, the molecular formula will usually give it away (there will be two oxygens in the compound), but take a look at the example spectrum for more evidence. Here we see the expected carbonyl peak around 1740 cm 1 and the alkyl C H peaks below 3000 cm 1 no surprises. Usually NMR gives the best evidence of ester functionality, but you will almost always see a very strong peak around 1200 cm 1 for esters, representing the C O single bond vibration. Just another hint that could speed you on your way.

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