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IR spectrum.docx

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McGill University
CHEM 222
Karine Auclair

this C-H stretching is characteristic of a aromatic ring, it doesn't have very sharp peaks just below 3000, it has tiny peaks. Most of the peaks are above 3000. Most of the hydrogens on carbons in aromatic rings are SP2 hybridized. if the aromatic ring is fully substituted then we don't have peaks here at all and you can't guess there is an aromatic ring in this area. none of these peaks are sharp, the sharpest peak is about midway of the entire spectrum. The peak corresponding to the these are the combination or alcohol near 3200 is rather overtone smooth, tall, strong peak. This is bands, very different than the CH peak in another characteristic this region. feature of aromatic rings. this is around 2200, a little lower than 2200. The two small peaks are characteristic of amines, near the 3300. These can be larger and extend up to mid An aromatic ring usually has two strong peaks way. Here we have two, if we have one bend it means we in the area of 700. have one NH bond, and if we have two peaks then we have 2 NH peaks. This is only valid in case of amines we cannot predict the number for other functional groups. these peaks do not tell us how many NH totals we have, it tells us how many hydrogens are on that one nitrogen (NH or NH2) For an Aldehyde the hydrogen in the -CHO results in a different pattern of the -CH peaks in the 3000 region. There is a small distinct peak, highlighted in red, that is not usually seen but is present in this case. In the other functional groups these -CH peahs were all together but this individual peak here is from the hydrogen in -CHO . This helps differentiate the aldehyde from the ketone. The aldehydes and ketones also have a very strong peak in the 1700 region. This red peak is due to -CO of an ester, very different from that of an alcohol. It is near 1200-1300. This red peak is the -OH of a carboxylic acid. this doesn't give a characteristic peak at 3400, instead it overlaps with the CH stretches and makes them wider. -triangular, cantered at 3000 not 3400, sis the key to remember The -CO bond peak of carboxylic acid at 1200 is not very sharp and is not as clear as an ester. This IR is characteristic of an alkane. Only the peak highlighted is important for identification, the rest are not specific enough. The sharp peak corresponds to the CH stretches. stretches to the right of 3000 shows the presence of sp3 CH bonds. This peak can also be a little smaller in height. This IR spectrum is that of alkene. The CH stretch has peak to the left of 3000, this corresponds to the Sp2 CH bonds caused by the presence of C=C. The smaller peak at 1650 corresponds to the C=C but this isn't always seen as it is overlapped by other peaks. So we have to use elimination method to remove the presence of other functional groups and then use the presence of sp2 CH bonds to deduce an alkene. IR of alkyne. The peak to the extreme left is part of the CH stretch, this represents SP CH bond from the hydrogen bonded to the carbon involved in the C-C triple bond. This peak is only seen if the C-C triple bond is terminal and not internal. This peak is in the same region as the alcohol peak but the alcohol peak is more round. The peak here is really sharp but usually it is more of a medium. The small peak highlighted on the right represents a triple bond. This can be present in case of a C-C triple bond or a C-N triple bond. To identify which we have to check the molecular formula or use other info. IR of aromatic compound. The CH stretch here is characteristic of an aromatic ring. the peaks are not as sharp, also we have sp2 CH peaks, if the aromatic ring was fully substituted then we wouldn't have
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