RATE OF PROTONATION OF 2,6-DI-TERT-BUTYLPYRIDINE BY THE HYDRONIUM ION - STERIC HINDRANCE TO PROTON-TRANSFER

The rate of protonation of 2, 6-di-toY-butylpyridine by the hydronium ion was measured by the temperature-jump method in 20% dioxane-80% water (v/v) at 25 °C. The rate constant is 3.7 ± 0.8 X 10s M-1 s-1, which is about 50 to 70 times lower than expected for a diffusion-controlled reaction. The rate reduction is attributed to a steric effect and represents one of the first cases where steric hindrance is seen to affect the rate of protonation of a nitrogen base by the hydronium ion. The results support the notion that the abnormally low basicity of 2, 6-di-tert-butylpyridine is due to steric hindrance of solvation rather than due to steric compression of the N-H bond in the protonated base. © 1979, American Chemical Society. All rights reserved.