NUCLEOPHILIC AROMATIC-SUBSTITUTION IN HETEROCYCLES - ALCOHOLYSIS AND HYDROLYSIS OF 2-ANILINO-4,6-DICHLORO-1,3,5-TRIAZINES

Kinetics are reported for the alkaline hydrolysis of 2-anilino-4,6-dic hloro-1,3,5-triazines to yield the corresponding mono-hydroxy species. The pseudo-first-order rate constants are independent of general-base concentration and obey the rate law eqn. (i), where K-1 = K-w/K-a, K( a)is the ionisation (i) k(obs) = (k(H2O) + k(OH)K(1)[OH])/([OH] + K-1) constant for the aniline proton and k(OH) is the bimolecular rate con stant for attack of hydroxide ion on the neutral triazine molecule. Th e parameters k(OH) and K-a obey Hammett equations with sigma degrees[e qns. (ii) and (iii)]. The kinetically determined pK(a) of the 4-chloro anilino-4,6-dichloro-1,3,5- (ii) log k(OH) = 1.06 +/- 0.15 sigma degre es - 0.064 +/- 0.054 (r = 0.9709) (iii) pK(a) = -2.13 +/- 0.16 sigma d egrees + 10.92 +/- 0.06 (r = 0.9919) triazine is close to that measure d by pH-titration. The kinetics for hydrolysis and alcoholysis of 2- ( N-methyanilino)-4,6-dichloro-1,3,5-triazine are independent of general -base concentration and are first order in oxyanion and triazine conce ntration; the derived second-order rate constants (k(RO).) (excepting the hydroxide term) obey the extended Bronsted equation (iv). (iv) log k(RO) = 0.84 +/- 0.06 pK(a)(ROH) - 10.89 +/- 0.87 (r = 0.9886) The hy droxide ion term is some three orders of magnitude less nucleophilic t han an alkoxide of the same pK(a). The data exclude the conjugate base of the 2-anilino-4,6-dichloro-1.3.5-triazine as a major contributor t o the reaction flux of the hydrolysis and are consistent with a mechan ism involving addition of the hydroxide ion to the neutral triazine. T he polar substituent effects on the hydroxide ion reactivity indicate that there is substantial charge accumulation in the triazine nucleus in the transition state of the addition step.