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

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/kw) 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 kaz is normally significantly greater than kOH. Further indications that azide ion is unexpectedly poor as a trap for this cation is seen in considering the kaz∶kw value in the context of ratios directly measured for sp2 hybridized cations. For the latter a kaz∶kw 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 kaz∶kw equal to only 104 mol–1 dm3 and yet its azide reaction is three orders of magnitude slower than diffusion.