Interpretation of IR Spectra: Identifying Functional Groups in Organic Compounds, Study notes of Art

Download Interpretation of IR Spectra: Identifying Functional Groups in Organic Compounds and more Study notes Art in PDF only on Docsity! 13.10 Interpretation of IR Spectra Interpretation of the IR spectrum of an unknown compound is an art that requires ex- perience and practice. The more spectra you examine, the easier it will become to rec- ognize the absorption due to an O±H group and to differentiate between that band and one that results from an N±H group. Table 13.2 summarizes the positions of the various absorption bands that have been discussed so far. On the basis of these absorptions, it is usually possible to determine the nature of the functional group that is present in the compound whose spectrum is being considered. Many functional groups require the presence of several characteris- tic absorptions, whereas the absence of a band in a particular region of the spectrum can often be used to eliminate the presence of a particular group. Infrared spectra of compounds belonging to each of the major functional group classes are provided in the figures in this chapter. Each figure has a summary of the im- 13.10  INTERPRETATION OF IR SPECTRA 521 % T 5001000150020002500300035004000 0 80 60 40 20 Wavenumber (cm–1) Nitro compounds: Compounds containing nitro groups are identified by the appearance of two strong bands near 1550 and 1380 cm–1. These absorptions appear at lower wavenumbers if the nitro group is conjugated with a benzene ring. a a b b d d d c c Two types of CH bonds can be detected in this spectrum: sp2-hybridized CH bonds (3100–3000 cm–1) sp3-hybridized CH bonds (3000–2850 cm–1) The absorption bands due to the nitro group: 1523 and 1347 cm–1. They are at lower wavenumbers than usual because the nitro group is conjugated with the benzene ring. CH3 O . . . . –– . . . . . .O The aromatic ring is responsible for the absorptions due to the sp2 CH stretching vibrations (3100 – 3000 cm–1); the ring skeletal vibrations at 1612, 1577, 1500, and 1461 cm–1; and the CH bending vibrations at 859, 788, and 728 cm–1. N Figure 13.14 THE INFRARED SPECTRUM OF 2-NITROTOLUENE. Hornback_Ch13_500-542 12/16/04 1:07 PM Page 521 portant absorption bands for that functional group. Some of these figures are found on previous pages; others appear on later pages. Table 13.3 provides a list of the important functional groups and the figure(s) that show IR spectra of typical compounds contain- ing that functional group. Now that all of the important absorption bands have been dis- cussed, this is a good time for you to examine all of these spectra to become more familiar with the combination of bands caused by each functional group. 522 CHAPTER 13  INFRARED SPECTROSCOPY Table 13.2 Important Absorption Bands in the Infrared Spectral Region Position (cm1) Group Comments 3550–3200 3400–3250 3300 3100–3000 3000–2850 2830–2700 2260–2200 2150–2100 1820–1650 1660–1640 1600–1450 1550 and 1380 1300–1000 900–675 –C±O– W W –NO2 C C X –– –– –C± O X –CPC– –CPN –C±H O X –C±H W W œC±H W PC±H ±N±H W ±O±H Strong intensity, very broad band Weaker intensity and less broad than O±H; NH2 shows two bands, NH shows one Sharp, C is sp hybridized C is sp2 hybridized C is sp3 hybridized; 3000 cm1 is a convenient dividing line between this type of C–H bond and the preceding type Two bands Medium intensity Weak intensity Strong intensity, exact position depends on substituents; see Table 13.1 Often weak intensity Four bands of variable intensity Two strong intensity bands Strong intensity Strong intensity Hornback_Ch13_500-542 12/16/04 1:07 PM Page 522 CC double bonds, aromatic rings, and nitro groups in the 1800 to 1350 cm1 region. Finally, you should look for C±O absorptions in the 1300 to 1000 cm1 region and for aromatic ring bands in the 900 to 675 cm1 region. Two cautions must be given. First, do not overinterpret the spectrum. Be very care- ful in assigning the presence of a functional group when only weak bands occur in the appropriate region. It is helpful to compare the spectrum of the unknown to that of a known compound that has that same functional group. Second, make sure that your con- clusions are consistent with all of the data. A spectrum that has two bands in the 2830 to 2700 cm1 region cannot be that of an aldehyde unless it also shows an absorption due to a carbonyl group. Let’s try a problem. The IR spectrum of an unknown compound is shown in Fig- ure 13.23. First, let’s examine the hydrogen region. The absence of absorptions in the 3600 to 3100 cm1 region indicates that the compound does not have any O±H or N±H groups. The bands in the 3100 to 3000 cm1 region indicate the presence of 13.10  INTERPRETATION OF IR SPECTRA 525 % T 5001000150020002500300035004000 0 80 60 40 20 Wavenumber (cm–1) Tertiary amines: Tertiary amines do not have a NH bond, so there is no evidence for the amine group in the 3400–3250 cm–1 region. Because the CN bond-stretching vibration is difficult to assign in the fingerprint region, tertiary amines are not readily identified from their IR spectra. Chemical tests are helpful in such cases. CH bonds of the benzene ring: 3100–3000 cm–1 CH bonds of the methyl groups: 3000–2800 cm–1 The aromatic ring skeletal vibrations: 1602, 1570, 1508, and 1444 cm–1 The bending vibrations of the CH bonds of the aromatic ring: 750 and 691 cm–1 N W WWH3C CH3 Only the alkyl groups and the aromatic ring can be detected in the spectrum of this tertiary amine. The absorptions due to the aromatic ring skeletal vibrations are stronger than usual, suggesting that the ring is substituted with a polar substituent. a a d d b b c c Figure 13.17 THE INFRARED SPECTRUM OF N,N-DIETHYLANILINE. Hornback_Ch13_500-542 12/16/04 1:07 PM Page 525 hydrogens bonded to sp2-hybridized carbons, so the compound must have one or more CC double bonds. The absorptions in the 3000 to 2850 cm1 region indicate that there are also hydrogens bonded to sp3-hybridized carbons in the compound. Next, exami- nation of the triple-bond region shows no indications of the presence of any triple- bonded functional group. Continuing to the double-bond region, the strong absorption at 1722 cm1 indicates the presence of a carbonyl group. This is not part of a carboxylic acid (no O±H) or an aldehyde (absence of absorptions in the 2830–2700 cm1 region). Nor does the unknown appear to be an amide (no N±H, carbonyl absorption too high), an anhydride (absence of a second carbonyl band), or an acyl chloride (carbonyl position too low). This leaves a ketone or an ester as pos- sibilities. The strong absorption at 1282 cm1 suggests that the unknown is an ester. The bands at 1607, 1591, 1489, and 1437 cm1 along with the absorptions at 3100 to 3000 and 746 cm1 suggest the presence of an aromatic ring. The carbonyl of the ester 526 CHAPTER 13  INFRARED SPECTROSCOPY % T 5001000150020002500300035004000 0 80 60 40 20 Wavenumber (cm–1) Ketones: The carbonyl of a ketone has an absorption band near 1715 cm–1. This band is shifted to lower wavenumbers if the carbonyl group is conjugated, and it is shifted to higher wavenumbers if the carbonyl group is part of a five-membered ring. Ketones have no other characteristic bands and often can only be distinguished from the other carbonyl-containing functional groups by the absence of the bands required for those other groups. Again, chemical tests can be very useful in confirming the presence of a ketone. CH bonds of the benzene ring: 3100–3000 cm–1 C H bonds of the methyl group: 3000–2850 cm–1 The carbonyl group: 1685 cm–1. This is the position predicted for a carbonyl group of a ketone that is conjugated to an aromatic ring, 1715 – (20 to 40) = 1695 to 1675 cm–1. The aromatic ring vibrations: 1599, 1579, 1449, 760, and 691 cm–1. Note that the band around 1500 cm–1 is very weak in this spectrum. C–CH3 O X W a a b b d d d c c Figure 13.18 THE INFRARED SPECTRUM OF ACETOPHENONE. Hornback_Ch13_500-542 12/16/04 1:07 PM Page 526 occurs at slightly lower wavenumbers (1722 cm1) than the usual position (1740 cm1), indicating that it might be conjugated. Therefore, we conclude that the unknown is probably an ester, that it may have an aromatic ring, and that the ester may be conju- gated (with the aromatic ring?). However, these conclusions must be considered ten- tative until confirming evidence is obtained from other sources. The unknown is actually methyl 3-methylbenzoate: Methyl 3-methylbenzoate H3C COCH3 O X 13.10  INTERPRETATION OF IR SPECTRA 527 % T 5001000150020002500300035004000 0 80 60 40 20 Wavenumber (cm–1) Esters: Esters show a carbonyl band near 1740 cm–1. They also show a strong absorption in the CO single bond region, from 1300–1000 cm–1, that can be used to differentiate them from ketones. This band is usually of comparable breadth and intensity to the carbonyl band. (However, a compound containing both a ketone and an ether group also has both of these absorptions.) Chemical tests can be very useful in distinguishing a ketone from an ester. The sp3-hybridized CH bonds: 3000–2850 cm–1 The strong band at 1742 cm–1 is due to the carbonyl group of the ester. The absorption at 1241 cm–1, comparable in breadth and intensity to that of the carbonyl group, is due to the CO single bond of the ester. CH3±C±O±CH2CH3 O X a a b b c c Figure 13.19 THE INFRARED SPECTRUM OF ETHYL ACETATE. Hornback_Ch13_500-542 12/16/04 1:07 PM Page 527 PROBLEM 13.10 Predict the positions of the major absorption bands in the IR spectra of these compounds: PROBLEM 13.11 Explain how IR spectroscopy could be used to distinguish between these compounds: b) and CH2OH CH3±C±OH CH3 CH3 W W CH3CH2CHœCH2a) and CH3CH2CPCH f ) C±H O X CH3 CH2CH2CPC±He) NO2 W CH3CH2CH2OHd) CH3O c)b) CH2NH2 CH3CHœCHCCH3a) O X 530 CHAPTER 13  INFRARED SPECTROSCOPY % T 50010001500 1772 cm–1 20002500300035004000 0 80 60 40 20 Wavenumber (cm–1) Figure 13.23 THE INFRARED SPECTRUM OF AN UNKNOWN COMPOUND. Hornback_Ch13_500-542 12/16/04 1:07 PM Page 530 PRACTICE PROBLEM 13.2 Explain which functional groups are present in this compound on the basis of its IR spectrum: Solution The broad absorption centered near 3300 cm1 indicates the presence of a hydroxy group. The absorption at 3005 cm1 suggests the presence of H’s bonded to sp2-hybridized C’s. (Note that you are not expected to read peak positions this exactly from any of these spec- tra.) This is supported by the absorption for a CC double bond at 1646 cm1. The ab- sorptions in the region of 3000 to 2850 cm1 indicate the presence of H’s bonded to sp3-hybridized C’s. Although the compound has a CC double bond, there is no indication of the presence of an aromatic ring due to the absence of the four bands in the 1600 to 1450 cm1 region and the absence of bands in the 900 to 675 cm1 region. In summary, the structural features that can be identified from the IR spectrum are as follows: O±H CœC HW ±C±H W W % T 5001000150020002500300035004000 0 80 60 40 20 Wavenumber (cm–1) d) CH3CH2CH2CH2NH2 and CH3CH2NHCH2CH3 andc) CH O X CCH3 O X 13.10  INTERPRETATION OF IR SPECTRA 531 Hornback_Ch13_500-542 12/16/04 1:07 PM Page 531 (This is the spectrum of 2-propen-1-ol or allyl alcohol. The structure cannot be determined only from this IR spectrum, but the conclusions reached are consistent with this structure.) PROBLEM 13.12 Explain which functional groups are present in these compounds on the basis of their IR spectra: % T 50010001500 1715 cm–1 20002500300035004000 0 80 60 40 20 Wavenumber (cm–1) % T 5001000150020002500300035004000 0 80 60 40 20 Wavenumber (cm–1) 1716 cm–1 a) b) 2-Propen-1-ol (allyl alcohol) CH2œCH±CH2 OH W 532 CHAPTER 13  INFRARED SPECTROSCOPY Click Coached Tutorial Problems for more practice interpreting Infrared Spectra. 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