The element effect in methoxide ion substitution of the (Z) and (E) isomers of O-methyl-4-methoxybenzohydroximoyl halides
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The (Z) and (E) isomers ofO-methyl-4-methoxybenzohydroximoyl chloride [p-OCH3ArC( Cl )=NOCH3] and 0-methyl-4-methoxybenzohyd roximoyl bromide [p-OCH3ArC(Br)=NOCH3] were synthesized, and the rates of methoxide substitution were measured (90% dimethyl sulfoxide and 10% methanol at 44.6° C). Kinetic measurements demonstrated the reaction to be first order in methoxide and first order in the hydroximoyl halide. Two mechanisms were considered for this reaction: a multistep addition-elimination mechanism (AN+ ON) and a concerted (SN2-like) mechanism (ANON)- The element effect (k8r11<c,) was investigated to determine if carbon-halogen bond cleavage occurs in the rate-determining step. A k8r11<c, ratio ~ 30 suggests that the bond is cleaved in the rate-determining step; a k8r11<c, ratio approximately equal to 1 indicates that the bond is not broken in the rate-determining step. The (Z) isomers gave a k8r11<c, ratio of 2. 78, and the (E) isomers gave a ker11<c, ratio of 1.97. These findings support the addition-elimination type mechanism with nucleophilic attack being the rate-determining step.