The interplay between steric and electronic effects in S(N)2 reactions

Area of Science:

• Computational Chemistry
• Organic Reaction Mechanisms

Background:

• Steric hindrance is traditionally considered a major factor in the activation barriers of S(N)2 reactions.
• The influence of bulky substituents on reaction pathways requires re-evaluation.

Purpose of the Study:

• To investigate the role of steric hindrance versus electronic factors in S(N)2 reaction transition states.
• To provide a new interpretation of factors governing S(N)2 reaction courses using energy decomposition analysis.

Main Methods:

• Quantum chemical calculations were performed for S(N)2 reactions of H(3)EX/X(-) systems (E=C, Si; X=F, Cl).
• The Morokuma/Ziegler energy decomposition analysis (EDA) scheme was employed to dissect energy terms along the reaction coordinate.
• Calculations included bulkier carbon systems with methyl group substitutions.

Main Results:

• Bulky substituents in carbon-based S(N)2 reactions release steric repulsion in the transition state.
• Activation barriers are primarily caused by weakened electrostatic attraction and loss of orbital interactions.
• Silicon systems show increased electrostatic and orbital attractions, with steric interactions being destabilizing.

Conclusions:

• The traditional understanding of steric hindrance in S(N)2 reactions is challenged.
• Electronic factors, specifically electrostatic and orbital interactions, are more critical than steric repulsion in determining activation barriers.
• Energy decomposition analysis offers valuable insights into the complex interplay of forces during chemical bond transformations.