12.4 FUNCTIONAL-GROUP INFRARED ABSORPTIONS

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1 552 APTER 12 INTRODUTION TO SPETROSOPY. INFRARED SPETROSOPY AND MASS SPETROMETRY PROBLEM 12.9 Which of the following vibrations should be infrared-active and which should be infrared-inactive (or nearly so)? (a) ' ' (b) ( 3 ) 2 AO AO (c) cyclohexane ring breathing (simultaneous of all L bonds) (d) 3 2 ' 2 3 (e) O 3 N (f) ( 3 ) 3 Ll (g) trans-3-hexene..o ' symmetrical N O (see resonance structures, Eq. 1.6, p. ) Ll A 12.4 FUNTIONAL-GROUP INFRARED ABSORPTIONS A typical IR spectrum contains many absorptions. hemists do not try to interpret every absorption in a spectrum. Experience has shown that some absorptions are particularly useful and important in diagnosing or confirming certain functional groups. In this section, we ll focus on those. We ll show you sample spectra so that you will begin to see how these absorptions appear in actual spectra. We ll consider here only the functional groups covered in hapters Subsequent chapters contain short sections that discuss the IR spectra of other functional groups. These sections, however, can be read and understood at any time with your present knowledge of infrared spectroscopy. In addition, a summary of key IR absorptions is given in Appendix II. A. IR Spectra of Alkanes The characteristic structural features of alkanes are the carbon carbon and carbon hydrogen single bonds. The ing of the carbon carbon single bond is infrared-inactive (or nearly so) because this vibration is associated with little or no change of the dipole moment. The ing absorptions of alkyl L bonds are typically observed in the cm _1 region. The peaks near 29 cm _1 in the IR spectrum of nonane (Fig. 12.4) are examples of such absorptions. Various bending vibrations are also observed in the fingerprint region (138 and 147 cm _1 in nonane) and in the L bending region (7 cm _1 in nonane). Absorptions in these general regions can be expected not only for alkanes but also for any compounds that contain 3 L and L 2 L groups. onsequently, these absorptions are not often useful, but it is important to be aware of them so that they are not mistakenly attributed to other functional groups. B. IR Spectra of Alkyl alides The carbon halogen ing absorption of alkyl chlorides, bromides, and iodides appear in the low-wavenumber end of the spectrum, but many interfering absorptions also occur in this region. NMR spectroscopy and mass spectrometry are more useful than IR spectroscopy for determining the structures of these alkyl halides. Alkyl fluorides, in contrast to the other alkyl halides, have useful IR absorptions. A single LF bond typically has a very strong ing absorption in the 1 11 cm _1 region.

2 12.4 FUNTIONAL-GROUP INFRARED ABSORPTIONS 553 Multiple fluorines on the same carbon increase the frequency; for example, a F 3 group typically has a ing frequency in the cm _1 region.. IR Spectra of Alkenes Unlike the spectra of alkanes and alkyl halides, the infrared spectra of alkenes are very useful and can help determine not only whether a carbon carbon double bond is present, but also the carbon substitution pattern at the double bond. Typical alkene absorptions are given in Table These fall into three categories: Aingabsorptions, ALing absorptions, and A Lbendingabsorptions.Theingvibrationofthecarbon carbon TABLE 12.2 Important Infrared Absorptions of Alkenes Functional group Absorption* LA 2 (terminal vinyl) A 2 (terminal methylene) A ing absorptions 16 cm _1 (m, sh) 1655 cm _1 (m, sh) A A A A cm _1 (w) (absent in some compounds) " A L A 2 AL ing absorptions 3 31 cm _1 (m) LA 2 (terminal vinyl) A 2 (terminal methylene) A A (trans-alkene) A (cis-alkene) (trisubstituted) AL bending absorptions 91, 99 cm _1 (s) two absorptions 89 cm _1 (s) cm _1 (s) cm _1 (br) (ambiguous and variable for different compounds) 8 8 cm _1 (s) *Intensity designations: s = strong; m = moderate; w = weak Shape designations: sh = sharp (narrow); br = broad (wide)

3 554 APTER 12 INTRODUTION TO SPETROSOPY. INFRARED SPETROSOPY AND MASS SPETROMETRY (a) 1 (b) ( 2 ) octene almost no trans-3-hexene bend bend Figure 12.1 IR spectra of (a) 1-octene and (b) trans-3-hexene. Be sure to correlate the key bands indicated in these spectra with the corresponding entries in Table double bond occurs in the cm _1 range; the frequency of this absorption tends to be greater, and its intensity smaller, with increased alkyl substitution at the double bond. The reason for the intensity variation is the dipole moment effect discussed in the previous section. Thus, the AingabsorptionisclearlyevidentintheIRspectrumof1-octeneat1642 cm _1 (see Fig. 12.1a), but is virtually absent in the spectrum of the symmetrical alkene trans- 3-hexene (Fig. 12.1b). The A ingvibrationisweakorabsenteveninunsymmetrical alkenes that have the same number of alkyl groups on each carbon of the double bond. NMR spectroscopy is particularly useful for observing alkene hydrogens (Sec. 13.7A). Nevertheless, a AL ing absorption can often be used for confirmation of an alkene functional group. In general, the ing absorptions of L bonds involving sp 2 -hybridized carbons occur at wavenumbers greater than 3 cm _1, and the ing absorptions of L bonds involving sp 3 -hybridized carbons occur at wavenumbers less than 3 cm _1. Thus, 1-octene has a AL ing absorption at 38 cm _1 (Fig. 12.1a), and trans-3-hexene has a similar absorption which is barely discernible at 33 cm _1 (Fig. 12.1b). The higher frequency of A L ing absorptions is a manifestation of the bond-strength effect: bonds to sp 2 -hybridized carbons are stronger (Table 5.3, p. 213), and stronger bonds vibrate at higher frequencies.

4 12.4 FUNTIONAL-GROUP INFRARED ABSORPTIONS 555 The alkene A L bending absorptions that appear in the low-wavenumber region of the IR spectrum are in many cases very strong and can be used to determine the substitution pattern at the double bond. The first three of these absorptions in Table 12.2 the ones for terminal vinyl, terminal methylene, and trans-alkene are the most reliable. The 91 and 99 cm _1 terminal vinyl absorptions are illustrated in the IR spectrum of 1-octene (Fig. 12.1a), and the trans-alkene absorption is illustrated by the 965 cm _1 peak in the IR spectrum of trans-3-hexene (Fig. 12.1b). Study Problem 12.3 Each of three alkenes, A, B, and, has the molecular formula 5 1, and each undergoes catalytic hydrogenation to yield pentane. Alkene A has IR absorptions at 1642, 99, and 911 cm _1 ; alkene B has an IR absorption at 964 cm _1, and no absorption in the cm _1 region; and alkene has absorptions at 1658 and 695 cm _1. Identify the three alkenes. Solution In this problem, you can write out all the possibilities and then use the IR spectra to decide between them. The molecular formulas and the hydrogenation data show that the carbon chains of all of the alkenes are unbranched and that all are isomeric pentenes. ence, the only possibilities for compounds A, B, and are the following: ) ) 2 A A A ) ) 1-pentene 2 3 cis-2-pentene trans-2-pentene The L bending absorptions of A at 99 cm _1 and 911 cm _1 indicate that it is a 1-alkene; thus, it must be 1-pentene. The 964 cm _1 L bending absorption of B shows that it is trans-2-pentene. (Why is the A ing vibration absent?) The remaining alkene must be cis-2-pentene; the 1658 cm _1 A ing absorption and the 695 cm _1 L bending absorption are consistent with this assignment. Notice that you do not need the complete IR spectrum of each compound, but only the key absorptions, to solve this problem. PROBLEMS 12.1 Five isomeric alkenes A E each undergo catalytic hydrogenation to give 2-methylpentane. The IR spectra of these five alkenes have the following key absorptions (in cm _1 ): ompound A: 912 (s), 994 (s), 1643 (s), 377 (m) ompound B: 833 (s), 1667 (w), 35 (weak shoulder on L absorption) ompound : 714 (s), 1665 (w), 31 (m) ompound D: 885 (s), 165 (m), 386 (m) ompound E: 967 (s), no absorption 16 17, 3 (m) Propose a structure for each alkene One of the spectra in Fig (p. 556) is that of trans-2-heptene and the other is that of 2- methyl-1-hexene. Which is which? Explain.

5 556 APTER 12 INTRODUTION TO SPETROSOPY. INFRARED SPETROSOPY AND MASS SPETROMETRY (a) (b) Figure IR spectra for Problem D. IR Spectra of Alcohols and Ethers When OLgroupsarenothydrogen-bondedtoothergroups,theOLingabsorption occurs near 36 cm _1.owever,inmosttypicalsamples, theolgroupsarestronglyhydrogen-bonded and give a broad peak of moderate to strong intensity in the 3 3 cm _1 region of the IR spectrum. Such an absorption, which is an important spectroscopic identifier for alcohols, is clearly evident in the IR spectrum of 1-hexanol (see Fig ). The other characteristic absorption of alcohols is a strong L O ing peak that occurs in the 15 1 cm _1 region of the spectrum; primary alcohols absorb near the low end of this range and tertiary alcohols near the high end. For example, this absorption occurs at about 16 cm _1 in the spectrum of 1-hexanol. Because some other functional groups, such as ethers, esters, and carboxylic acids, also show LO ing absorptions in the same general region of the spectrum, the LO ing absorption is mainly used to support or confirm the presence of an alcohol diagnosed from the OL absorption or from other spectroscopic evidence. The most characteristic infrared absorption of ethers is the L O ing absorption, which, for the reasons just stated, is not very useful except for confirmation when an ether is already suspected from other data. For example, both dipropyl ether and an isomer 1-hexanol have strong LO ing absorptions near 11 cm _1.

6 12.5 OBTAINING AN INFRARED SPETRUM O 38 O 3 ( 2 ) 4 2 O 1-hexanol Figure IR spectrum of 1-hexanol. Note particularly the broad OL ing absorption Figure IR spectrum for Problem PROBLEMS Match the IR spectrum in Fig to one of the following three compounds: 2-methyl-1- octene, butyl methyl ether, or 1-pentanol Explain why the IR spectra of some ethers have two LO ing absorptions. (int: See Fig. 12.8, p. 549.) Explain why the frequency of the OL ing absorption of an alcohol in solution changes as the alcohol solution is diluted OBTAINING AN INFRARED SPETRUM Infrared spectra are obtained with an instrument called an infrared spectrometer. In its simplest concept, the IR spectrometer provides a way to carry out the absorption spectroscopy experiment shown in Fig (p. 539). The instrument houses a source of infrared radiation, a