Diminished reactivity of ortho-substituted phenacyl bromides toward nucleophilic displacement

A systematic increase of substitution rates by tert-butylamine on alpha-bromopropiophenones is observed with meta or para substituents with increasing electron-withdrawing ability (k x 10(3) L M(-1) min(-1) = 12.7 (p-CH(3)), 15.7 (o-F), 20.5 (H), 20.0 (p-Cl), 23.6 (m-Cl), 27.3 (p-CF(3))). Within an ortho-substituted series, the reactivities decrease (k x 10(3) L M(-1) min(-1) = 7.64 (o-OCH(3)), 5.31 (o-CH(3)), 2.85 (o-Cl), 2.40 ( o-CF(3))). Ortho-substitution results occur from rotational barrier effects and an Adelta(sigma)+ Bdelta(sigma)+ repulsion. The major bonding contribution between reaction and alpha-substituent centers (A-B) is only the sigma bond. When pi bonding is allowed between A and B (meta/para-substitution), delocalization and stabilization of the reacting center occurs.