1559Tch22389-40810/19/0517:23Page400 400 chapter 22 CHEMISTRY OF BENZENE SUBSTITUENTS:ALKYLBENZENES,PHENOLS,AND BENZENAMINES sonan0)is diminished.c resonance with the whi starting material (Section 22-5)So agu temperature to give salicylic acid and acetic acid. eoher andeanducremuclie ac Hydrolyou require ex 53.Without an exact mass measurement the molecular ion isn't much use for fomula detemmi- of the moculeobtained by just adding the integral masses of the atoms(H:C.N.nd6). k=194 mk=137 The base peak at =137 is recognizable right away:Cleavage at the benzylic carbon gives the resonance stabilized cation(f) 52. Aspirin is a phenyl ester. Phenyl esters are considerably more susceptible to hydrolysis than ordinary esters are, for two reasons: Delocalization of a lone electron pair from the phenol oxygen into the carbonyl group (ester resonance, see Section 20-1) is diminished, because this lone pair is also in resonance with the benzene ring. The result is a greater on the carbonyl carbon, which facilitates nucleophilic attack. The second reason is thermodynamic: The equilibrium phenol carboxylic acid phenyl ester favors the starting material (Section 22-5). So, aqueous solutions of aspirin hydrolyze rather rapidly at room temperature to give salicylic acid and acetic acid. Tylenol, on the other hand, is an amide and much more resistant to nucleophilic attack. Hydrolysis would require extended heating, and strong acid or base is required. [Indeed, amides derived from benzenamine, such as N-acetylbenzeneamine (acetanilide), are commonly purified by recrystallization from neutral boiling water, further illustrating their stability.] 53. Without an exact mass measurement the molecular ion at m/z  305 isn’t much use for formula determi￾nation (there are far too many possibilities). However, working back from the actual structure we can see where the major fragmentation peaks come from. In the structure below we’ve inserted the masses of several portions of the molecule, obtained by just adding the integral masses of the atoms (H, 1; C, 12; N, 14; and O, 16). The base peak at m/z  137 is recognizable right away: Cleavage at the benzylic carbon gives the resonance￾stabilized cation HO CH3O CH2 O HO m/z 137 CH3O N H m/z 194 H2O OCCH3 O CH3COOH COOH COOH OH O O H H 400 • Chapter 22 CHEMISTRY OF BENZENE SUBSTITUENTS: ALKYLBENZENES, PHENOLS, AND BENZENAMINES 1559T_ch22_389-408 10/19/05 17:23 Page 400