13-48 This is a challenging problem, despite the molecule being relatively small. Mass spectrum: The molecular ion at 96 suggests no Cl, Br, or N. The molecule must have seven carbons or fewer. Infrared spectrum: The dominant functional group peak is at 1685 cm-I, a carbonyl that is conjugated with C=C (lower wavenumber than normal, very intense peak). The presence of an oxygen and a molecular ion of 96 lead to a formula of C6HsO, with three elements of unsaturation, a c=o and one or two c=c. Carbon NMR spectrum: The six peaks show, by chemical shift, one carbonyl carbon (196), two alkene carbons (129, 151), and three aliphatic carbons (23, 26, 36). By off-resonance decoupling multiplicity, the groups are: three CH2 groups, two alkene CH groups, and carbonyl. Since the structure has one carbonyl and only two alkene carbons, the third element of unsaturation must be a ring. C=O C=CI CH2 + CH2 + CH2 + 1 ring I , H H Since the structure has no methyl group, and no H2C=, all of the carbons must be included in the ring. The only way these pieces can fit together is in cyclohex-2-enone. Notice that the proton NMR was
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