Solutions_Manual_for_Organic_Chemistry_6th_Ed 493

Solutions_Manual_for_Organic_Chemistry_6th_Ed 493 - 20-40...

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Unformatted text preview: 20-40 (a) Mass spectrum: -mlz 152 ==> molecular ion ==> molecular weight 15 2 -m1z 107 => M 45 => loss of COOH -m1z77 => monosubstituted benzene ring, H - Carbon NMR spectrum: � 170, small peak ==> carbonyl � 115-157 , four peaks ==> m onosubstituted benzene ring; deshielded peak indicates oxygen substitution on the ring (b) Fragments indicated in the spectra: COOH mlz 14 mlz 45 and from IR mlz77 This appears deceptively simple. The problem is that the mass of these fragments adds to 136, not 152-we are missing 16 mass units oxygen! Where can the oxygen be? There are only two possibilities: O - CH2COOH CH2-O - COOH HH spectrum: -3400-2400 cm-i , broad ==> O-H stretch of COOH -1700 cm-i ==> C=O -1240 cm-i ==> C - O -1600 cm-i ==> aromatic C=C NMR spectrum: � 6.8-7.3, two signals in the ratio of 2H to 3H ==> monosubstituted benzene ring � 4.6, 2H singlet ==> C H2 , deshielded IR -Q K + <> How can we differentiate? Mass spectrometry! <} � <} ( CH2COOH This structure is consistent with the peak at 157 in the 91 93 J carbonNMR. phenoxyacetic acid The mlz 93 peak in the MS confirms the structure is phenoxyacetic acid. The CH2 is so far downfield in the NMR because it is between two electron-withdrawing groups, the 0 and the COOH. (c) The COOH proton is missing from the proton NMR. Either it is beyond 10 and the NMR was not scanned (unlikely), or the peak was broadened beyond detection because of hydrogen bonding with DMSO. ( } CHj O - COOH Yat 8157 8 488 ...
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