Photochemical generation of the N-isopropylbenzonitrilium ion and unusually slow trapping by azide ion

Abstract

The imidate ester 4-cyanophenyl N-isopropylbenzimidate undergoes efficient photoheterolysis in water producing the 4-cyanophenoxide ion and the N-isopropylbenzonitrilium ion. The latter is detected as a transient intermediate in flash photolysis experiments and has a relatively long lifetime (1/k_w) in water of 2.5 ms. The rate constants for hydroxide and azide are respectively 5.2 × 10⁶ and 3.9 × 10⁶ dm³ mol^–1 s^–1. This pattern is not seen with sp² hybridized cations, where k_az is normally significantly greater than k_OH. Further indications that azide ion is unexpectedly poor as a trap for this cation is seen in considering the k_az∶k_w value in the context of ratios directly measured for sp² hybridized cations. For the latter a k_az∶k_w of less than 10⁵ can be taken to mean that the reaction with azide is at (or certainly very close to) the diffusion limit (i.e. 5–10 × 10⁹ dm³ mol^–1 s^–1). The nitrilium ion 3 has k_az∶k_w equal to only 10⁴ mol^–1 dm³ and yet its azide reaction is three orders of magnitude slower than diffusion.