The influence of steric effects on the kinetics and mechanism of SNAr reactions of 1‐phenoxy‐nitrobenzenes with aliphatic primary amines in acetonitrile

• Published in: Journal of physical organic chemistry Vol. 30; no. 11
• Main Author: Isanbor, Chukwuemeka
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01.11.2017
• Subjects: Acetonitrile; aliphatic amines; Aliphatic compounds; Amines; Catalysis; Congestion; Kinetics; Nitrobenzene; nucleophilic aromatic substitution; Piperidine; Protons; Rate constants; Reaction kinetics; Steric effects; substituent effects
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.3687

Rate constants are reported for the reactions of 1‐phenoxy‐dinitrobenzenes, 3, 1‐phenoxy‐dinitrotrifluoromethylbenzenes, 4, with n‐propylamine, and 1‐methylheptylamine in acetonitrile as solvent. The results are compared with results reported previously for n‐butylamine, pyrrolidine, and piperidine. Decreasing ring activation leads to lower values of k1 for nucleophilic attack although this may be mediated by reduced steric congestion around the reaction centre. Specific steric effects, leading to rate retardation, are noted for the ortho‐CF3 group. In general, reactant‐bearing ortho‐CF3 group were subject to base catalysis irrespective of the amine nucleophile and values of kAm/k−1 are reduced as the size of the amine get bulkier. This is likely to reflect increases in values of k−1 coupled with decreases in values of kAm as the proton transfer from zwitterionic intermediates to catalysing amine becomes less thermodynamically favourable. Specific steric effects of ortho‐CF3 group leads to rate retardation and base catalysis due to increase in k−1 coupled with a decrease in kAm as the proton transfer from zwitterionic intermediates, 6, to catalysing amine becomes less thermodynamically favourable.