The mechanism of steric effects

For acid-catalyzed ester formation and hydrolysis the transition state (or states) is more crowded than the initial state, because the central atom is close to tetrahedral, and also more highly ordered, because two molecules have combined to form one. The introduction of a bulky substituent into either R or R in the reaction 

ACTIVATION PARAMETERS FOR ACID-CATALYZED ESTER HYDROLYSIS AND 

Acid RCOOH Hydrolysis ofRC OOEt Formation of RCOOIde in methanol  

X-CsHJCOOH (kcalmole- ) (eu) (kcal-mole ) (eu) (kcal mole ) (eu) 

A bulky substituent close to the reaction centre may increase the non-bonded compression energy as the transition state is formed this will cause an increase in A//. It will also hinder the close approach of solvent molecules to the reaction centre, thus reducing the maximum amount of stabilization possible (steric inhibition of solvation). This will result in a further increase in AH, but since decreased solvation means less ordering of solvent molecules about the transition state, there is a compensating increase in AS. Another effect of the bulky substituent may be to block certain vibrational and rotational degrees of freedom more in the (more crowded) transition state than in the initial state, and so to reduce AS. These are the most important of the simple effects of a bulky substituent and can be used to explain most of the relationships of Table 25. 

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