1559Tch11199-21911/02/0521:43Page209 208.Chapter 11 ALKENES:INFRARED SPECTROSCOPY AND MASS SPECTROMETRY CH, 2-Methylpentane:[CH Cleavage will occur mainly about the CH to give secondary cations.The best match is spectrum C. 3-Methylpemtane:ICH,CHCH CHCHICH,CHCHCH.](/:57) Fragmentation occurs at the indicated bond to form mainly sec-butyl cation and,to a lesser extent,ethyl cation(m/=29).Spectrum A fits best processes do not usually dominate the spectrum. 39.Major peaks: Minor peaks: m/109 (CH2CH2Br) from M-CH3 (CH;CH:Br (CH CHCH) emM H m28 (CH2=CH2) from M-Br-CH3 m/27 (CH2=CH) from M-Br-CH3-H 40.Com pe Compoud)ry tos the ra ideines hatt is saturated (see Sec <s e kely inte ty nz 73 p c.it com for nds【0 -OF CHaCH2COH +CH3 cation. oxygen lo lized by one pair Looking at the rest of the spectrum.the base peak is at m/59,or(M29).loss of CH,CH CHCH2+C-OH 一(CH2COH+CH,CH2 59 This is.all tog od evidence for i tanol as sho per b als CH,CH,CH2.These signals are what you might expect for 2-pentanol.208 • Chapter 11 ALKENES; INFRARED SPECTROSCOPY AND MASS SPECTROMETRY CH3 A 2-Methylpentane: [CH3OCHOCH2CH2CH3]G? 88n [CH3CHCH3]G (m/z  43) h h [CH3CH2CH2CHCH3]G (m/z  71) Cleavage will occur mainly about the CH to give secondary cations. The best match is spectrum C. CH3 A 3-Methylpentane: [CH3CH2OCHOCH2CH3]G? 88n [CH3CH2CHCH3]G (m/z  57) h h Fragmentation occurs at the indicated bond to form mainly sec-butyl cation and, to a lesser extent, ethyl cation (m/z  29). Spectrum A fits best. As is typical in mass spectrometry, the large amount of energy imparted to molecules in the process induces rearrangements and other modes of fragmentation as well. Fortunately, ions arising from such processes do not usually dominate the spectrum. 39. Major peaks: m/z 43 (CH3CH2CH2)  from MOBr m/z 41 (CH2CHPCH2)  from MOHBrOH Minor peaks: m/z 109 (CH2CH2 81Br) from MOCH3 m/z 107 (CH2CH2 79Br)} m/z 42 (CH3CHPCH2)  from MOHBr m/z 29 (CH3CH2)  from MOBrOCH2 m/z 28 (CH2PCH2)  from MOBrOCH3 m/z 27 (CH2PCH) from MOBrOCH3OH 40. Compound is saturated (see Section 11-6). Try to use the general guidelines that intense fragment peaks either result from the loss of relatively stable neutral species or are due to relatively stable cations. So, looking at the high intensity m/z 73 peak for isomer C, it corresponds to (M 15), or loss of CH3. This is most likely if the remaining fragment is a very stable cation, for example, Looking at the rest of the spectrum, the base peak is at m/z 59, or (M 29), loss of CH3CH2. This is, all together, good evidence for isomer C being 2-methyl-2-butanol, as shown. Isomer B also has a peak at m/z 73 for loss of CH3. Its base peak (m/z 45) corresponds to loss of 43, or CH3CH2CH2. These signals are what you might expect for 2-pentanol. CH3CH2 C OH CH3 CH3 (CH3)2COH m/z 59  CH3CH2   CH3CH2 C OH CH3 CH3 CH3 CH3CH2COH m/z 73 cation, stabilized by oxygen lone pair.  CH3      1559T_ch11_199-219 11/02/05 21:43 Page 208