326 PECTROSCOPY the six nonequivalent carbons of the benzene ring. The three signals near 8 20 ppm are due to the three nonequivalent methyl groups HaC H3 1, 2, 4-Trimethylbenzene 13.15 The infrared spectrum of Figure 13.31 has no absorption in the 1600-1800-cm region, and so the unknown compound cannot contain a carbonyl(C=O) group. It cannot therefore be acetophenone or benzoic acid The broad, intense absorption at 3300 cm is attributable to a hydroxyl group. Although both phenol and benzyl alcohol are possibilities, the peaks at 2800-2900 cm reveal the presence of hy- drogen bonded to sp'-hybridized carbon. All carbons are sp-hybridized in phenol. The infrared spectrum is that of benzyl a 13.16 The energy of electromagnetic radiation is inversely proportional to its wavelength. Since excitation of an electron for the T-T*transition of ethylene occurs at a shorter wavelength(max=170 nm) than that of cis, trans-1,3-cyclooctadiene(max= 230 nm), the HOMO-LUMO energy difference in 13.17 Conjugation shifts Amax to longer wavelengths in alkenes. The conjugated diene 2-methyl-1, 3- butadiene has the longest wavelength absorption, Amax= 222 nm. The isolated diene 1, 4-pentadiene and the simple alkene cyclopentene both absorb below 200 nm. 2-Methyl-13-butadiene 13.18(b) The distribution of molecular-ion peaks in o-dichlorobenzene is identical to that in the para isomer. As the sample solution to part(a) in the text describes, peaks at m/z 146, 148, and 150 present for the molecular ion (c) The two isotopes of bromine are"Br and"Br. When both bromines of p-dibromobenzene are 7Br, the molecular ion appears at m/z 234. When one is"Br and the other is Br, m/zfor the molecular ion is 236. When both bromines are Br, m/z for the molecular ion is 238 (d) The combinations ofCl, 3Cl, "Br, andR in p-bromochlorobenzene and the values of m/ z for the corresponding molecular ion are as shown. (CL, Br) m/ (Cl, 7Br)or(Cl, Br) m/z=192 (3C.,8Br)m/z=194 13 19 The base peak in the mass spectrum of alkylbenzenes corresponds to carbon-carbon bond cleavage at the benzylic carbo HC、ACH2 子 CH CH Base peak: CsHg Base peak: CH, Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Websitethe six nonequivalent carbons of the benzene ring. The three signals near 20 ppm are due to the three nonequivalent methyl groups. 13.15 The infrared spectrum of Figure 13.31 has no absorption in the 1600–1800-cm1 region, and so the unknown compound cannot contain a carbonyl (C?O) group. It cannot therefore be acetophenone or benzoic acid. The broad, intense absorption at 3300 cm1 is attributable to a hydroxyl group. Although both phenol and benzyl alcohol are possibilities, the peaks at 2800–2900 cm1 reveal the presence of hy￾drogen bonded to sp3 -hybridized carbon. All carbons are sp2 -hybridized in phenol. The infrared spectrum is that of benzyl alcohol. 13.16 The energy of electromagnetic radiation is inversely proportional to its wavelength. Since excitation of an electron for the A * transition of ethylene occurs at a shorter wavelength (max 170 nm) than that of cis, trans-1,3-cyclooctadiene (max 230 nm), the HOMO–LUMO energy difference in ethylene is greater. 13.17 Conjugation shifts max to longer wavelengths in alkenes. The conjugated diene 2-methyl-1,3- butadiene has the longest wavelength absorption, max 222 nm. The isolated diene 1,4-pentadiene and the simple alkene cyclopentene both absorb below 200 nm. 13.18 (b) The distribution of molecular-ion peaks in o-dichlorobenzene is identical to that in the para isomer. As the sample solution to part (a) in the text describes, peaks at mz 146, 148, and 150 are present for the molecular ion. (c) The two isotopes of bromine are 79Br and 81Br. When both bromines of p-dibromobenzene are 79Br, the molecular ion appears at mz 234. When one is 79Br and the other is 81Br, mz for the molecular ion is 236. When both bromines are 81Br, mz for the molecular ion is 238. (d) The combinations of 35Cl, 37Cl, 79Br, and 81Br in p-bromochlorobenzene and the values of mz for the corresponding molecular ion are as shown. ( 35Cl, 79Br) mz 190 ( 37Cl, 79Br) or (35Cl, 81Br) mz 192 ( 37Cl, 81Br) mz 194 13.19 The base peak in the mass spectrum of alkylbenzenes corresponds to carbon–carbon bond cleavage at the benzylic carbon. CH H3C CH3 CH3 Base peak: C9H11  m/z 119 CH2 CH2CH3 CH3 Base peak: C8H9  m/z 105 CH2 CH3 H3C CH3 Base peak: C9H11  m/z 119 2-Methyl-1,3-butadiene (max 222 nm) H3C CH3 CH3 1,2,4-Trimethylbenzene 326 SPECTROSCOPY Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website