13.6 Interpreting Proton NMR Spectra 2.2-Dibromobutane (g)1, 1, 4-Tribromobutane 23, 3-Tetrabromobutane (h )1,1, 1-Tribromobutane SAMPLE SOLUTION (a)To test for chemical-shift equivalence, replace the pro- tons at c-1, C-2, C-3, and C-4 of 1-bromobutane by some test group such as chlo- rine. Four constitutional isomers result. CH3 CH2CH2 CHBr CHa CH2 CHCH2 Br CH3 CHCH2 CH2 Br CICH2 CH2CH2 2 Br 1-Bromo-1 1-Bromo-2 1-Bromo-3- 1-Bromo-4- hlorobutane chlorobutane Thus, separate signals will be seen for the protons at C-1, C-2, C-3, and C-4. Bar ring any accidental overlap, we expect to find four signals in the NMR spectrum of 1-bromobutane Chemical-shift nonequivalence can occur when two environments are stereochem- ically different. The two vinyl protons of 2-bromopropene have different chemical shifts H3C H85.5 One of the vinyl protons is cis to bromine; the other trans. Replacing one of the vinyl protons by some test group, say, chlorine, gives the Z isomer of 2-bromo-1-chloro- propene; replacing the other gives the E stereoisomer. The E and Z forms of 2-bromo- I-chloropropene are stereoisomers that are not enantiomers; they are diastereomers. Pro- tons that yield diastereomers on being replaced by some test group are described as diastereotopic. The vinyl protons of 2-bromopropene are diastereotopic. Diastereotopic protons can have different chemical shifts. Because their environments are similar, how ever, this difference in chemical shift is usually small, and it sometimes happens that two diastereotopic protons accidentally have the same chemical shift. Recording the spec trum on a higher field NMR spectrometer is often helpful in resolving signals with sim- lar chemical shifts trum of each of the following compounds s ou expect to PROBLEM 13.7 How many signals would you expect to find in the H NMr spec- (a)Vinyl bromide (d) trans-1, 2-Dibromoethene D1,1-Dibromoethene (e)Allyl bromide (c) cis-1, 2-Dibromoethene (f)2-Methyl-2-butene SAMPLE SOLUTION (a) Each proton of vinyl bromide is unique and has a chem ical shift different from the other two. The least shielded proton is attached to the carbon that bears the bromine. The pair of protons at C-2 are diastereotopic with respect to each other; one is cis to bromine while the other is trans to bromine. there are three proton signals in the Nmr spectrum of vinyl bromide Their observed chemical shifts are as indicated 86.4 ppm H H 85.8 ppm Back Forward Main MenuToc Study Guide ToC Student o MHHE Website(e) 2,2-Dibromobutane (g) 1,1,4-Tribromobutane (f) 2,2,3,3-Tetrabromobutane (h) 1,1,1-Tribromobutane SAMPLE SOLUTION (a) To test for chemical-shift equivalence, replace the pro￾tons at C-1, C-2, C-3, and C-4 of 1-bromobutane by some test group such as chlo￾rine. Four constitutional isomers result: Thus, separate signals will be seen for the protons at C-1, C-2, C-3, and C-4. Bar￾ring any accidental overlap, we expect to find four signals in the NMR spectrum of 1-bromobutane. Chemical-shift nonequivalence can occur when two environments are stereochem￾ically different. The two vinyl protons of 2-bromopropene have different chemical shifts. One of the vinyl protons is cis to bromine; the other trans. Replacing one of the vinyl protons by some test group, say, chlorine, gives the Z isomer of 2-bromo-1-chloro￾propene; replacing the other gives the E stereoisomer. The E and Z forms of 2-bromo- 1-chloropropene are stereoisomers that are not enantiomers; they are diastereomers. Pro￾tons that yield diastereomers on being replaced by some test group are described as diastereotopic. The vinyl protons of 2-bromopropene are diastereotopic. Diastereotopic protons can have different chemical shifts. Because their environments are similar, how￾ever, this difference in chemical shift is usually small, and it sometimes happens that two diastereotopic protons accidentally have the same chemical shift. Recording the spec￾trum on a higher field NMR spectrometer is often helpful in resolving signals with sim￾ilar chemical shifts. PROBLEM 13.7 How many signals would you expect to find in the 1 H NMR spec￾trum of each of the following compounds? (a) Vinyl bromide (d) trans-1,2-Dibromoethene (b) 1,1-Dibromoethene (e) Allyl bromide (c) cis-1,2-Dibromoethene (f) 2-Methyl-2-butene SAMPLE SOLUTION (a) Each proton of vinyl bromide is unique and has a chem￾ical shift different from the other two. The least shielded proton is attached to the carbon that bears the bromine. The pair of protons at C-2 are diastereotopic with respect to each other; one is cis to bromine while the other is trans to bromine. There are three proton signals in the NMR spectrum of vinyl bromide. Their observed chemical shifts are as indicated.  5.7 ppm  6.4 ppm  5.8 ppm C Br H H H C  5.3 ppm  5.5 ppm C Br H H3C H C 2-Bromopropene CH3CH2CH2CHBr W Cl 1-Bromo-1- chlorobutane CH3CH2CHCH2Br W Cl 1-Bromo-2- chlorobutane CH3CHCH2CH2Br W Cl 1-Bromo-3- chlorobutane ClCH2CH2CH2CH2Br 1-Bromo-4- chlorobutane 13.6 Interpreting Proton NMR Spectra 499 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website