CHAPTER THIRTEEN Spectroscopy In compounds of the type CH3CH2X, especially where X is an electronegative atom +++++ or group, such as bromine in ethyl bromide, the ethyl group appears as a tripler-quartet pattern. The methylene proton signal is split into a quartet by coupling with the methyl binations of the nuclear protons. The signal for the methyl protons is a triplet because of vicinal coupling to the of the two methylene two protons of the adjacent methylene group protons in CH3 CH2B1 H2—CH3 These two protons split These three protons split the methyl signal into the methylene signal into a quartet. le have discussed in the preceding section why methyl groups split the si due to vicinal protons into a quartet. Splitting by a methylene group gives a triplet cor- responding to the spin combinations shown in Figure 13 14 for ethyl bromide. The rel- ative intensities of the peaks of this triplet are 1: 2: 1 PROBLEM 13.9 Describe the appearance of the H NMR spectrum of each of the These four combinations cause he signal of the CHs protons to following compounds. How many signals would you expect to find, and into how many peaks will each signal be split? the intensities of the peaks are (a)CICH2 OCH2 CH3 in the ratio 1: 2: 1 (b)CH3 CH2OCH3 FIGURE 13 14 The methyl- (c)CHaCH2OCH2 CH3 ene protons of ethyl bro- ( d) p-Diethylbenzene mide split the signal of the (e)CICH2 CH, CH SAMPLE SOLUTION (a)Along with the triplet-quartet pattern of the ethyl group, the NMr spectrum of this compound will contain a singlet for the two protons of the chloromethyl group Split into triplet by two ClCH2-0--CH2--CH3 "z- protons of adjacent Singlet: no protons- methylene group vicinal to the Split into quartet by herefore, no splitting three protons of methyl group Table 13.2 summarizes the splitting patterns and peak inter pling to various numbers of protons TABLE 13.2 Splitting Patterns of Common Multi triplets Number of equivalent protons Appearance of Intensities of lines to which nucleus is coupled in multiplet expansion(Pascals triangle Quartet 3:3:1 Pentet 1:5:10:10:5:1 Septet 1:6:15:20:15:6:1 Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteIn compounds of the type CH3CH2X, especially where X is an electronegative atom or group, such as bromine in ethyl bromide, the ethyl group appears as a triplet–quartet pattern. The methylene proton signal is split into a quartet by coupling with the methyl protons. The signal for the methyl protons is a triplet because of vicinal coupling to the two protons of the adjacent methylene group. We have discussed in the preceding section why methyl groups split the signals due to vicinal protons into a quartet. Splitting by a methylene group gives a triplet cor￾responding to the spin combinations shown in Figure 13.14 for ethyl bromide. The rel￾ative intensities of the peaks of this triplet are 1:2:1. PROBLEM 13.9 Describe the appearance of the 1 H NMR spectrum of each of the following compounds. How many signals would you expect to find, and into how many peaks will each signal be split? (a) ClCH2OCH2CH3 (b) CH3CH2OCH3 (c) CH3CH2OCH2CH3 (d) p-Diethylbenzene (e) ClCH2CH2OCH2CH3 SAMPLE SOLUTION (a) Along with the triplet–quartet pattern of the ethyl group, the NMR spectrum of this compound will contain a singlet for the two protons of the chloromethyl group. Table 13.2 summarizes the splitting patterns and peak intensities expected for cou￾pling to various numbers of protons. Split into triplet by two protons of adjacent methylene group Split into quartet by three protons of methyl group Singlet; no protons vicinal to these; therefore, no splitting ClCH2 O CH2 CH3 Br CH2 CH3 These three protons split the methylene signal into a quartet. These two protons split the methyl signal into a triplet. 504 CHAPTER THIRTEEN Spectroscopy There are four possible combinations of the nuclear spins of the two methylene protons in CH3CH2Br. 3Jab 3Jab These four combinations cause the signal of the CH3 protons to be split into a triplet, in which the intensities of the peaks are in the ratio 1:2:1. FIGURE 13.14 The methyl￾ene protons of ethyl bro￾mide split the signal of the methyl protons into a triplet. TABLE 13.2 Splitting Patterns of Common Multiplets Number of equivalent protons to which nucleus is coupled 1 2 3 4 5 6 1:1 1:2:1 1:3:3:1 1:4 :6:4 :1 1:5:10:10:5:1 1:6:15:20:15:6:1 Intensities of lines in multiplet Doublet Triplet Quartet Pentet Sextet Septet Appearance of multiplet The intensities correspond to the coefficients of a binomial expansion (Pascal’s triangle). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website