8ealegtegp8ctent2Rennhg8ionimg gadesn2arebeamyictosyteeapdyescswah ---9-- ictonsare tae to have opcation (as 路8 H.Br w 6.8-6-6-5 a86gaemnsav 香委母 22 Benzylic cations delocalize the positive charge. Benzylic resonance can strongly affect the reactivity of benzylic halides and sulfonates in nucleophilic displacements. For example, a primary benzylic tosylate rapidly reacts with ethanol via an SN1 reaction: Delocalization of positive charge into the aromatic ring facilitates the dissociation of the starting sulfonate: Several benzylic cations are stable enough to have been isolated. The X-ray structure of 2-phenyl-2-propyl cation (as its SbF6 - salt) was obtained in 1997. A para methoxy substituent on the benzene ring allows for extra stabilization of the benzylic positive charge. In its absence, the SN2 reaction may dominate due to the lack of steric interference and the stabilization of the SN2 transition state by overlap with the benzene π system. Resonance in benzylic anions makes benzylic hydrogens relatively acidic. The anion, radical and cation adjacent to a benzene ring are all stabilized by conjugation: The acidity of methylbenzene (pKa~41) is considerably greater than that of ethane (pKa~50) and comparable to that of propene (pKa~40) which can be deprotonated to form the resonance￾stabilized 2-propenyl anion. Consequently, methylbenzene can be deprotonated by butyllithium to generate phenylmethyllithium: