Steric Strain in Molecular Organics

Abstract

Strain is one of the most important factors to analyze and understand physical and chemical properties of matter in the macroscopic and microscopic world. This chapter introduces a series of rules in organic chemistry to rationalize the origin of strain. It also demonstrates strains in different compounds, including aliphatics, aromatics and non-aromatic cyclic molecules, such as van der Waals strain, Jahn-Teller distortion, 1,3-diaxial strain, steric strain in olefins, allylic 1,3-strain, ring strain, and the Bredt's rule. The chapter further discusses the strains in aromatics, cyclic molecules and three-dimensional molecules. Torsional strain and steric strain play significant roles in strain energy for the substituted groups on two adjacent atoms. Torsional strain is the energy to counteract the repulsion between two groups connected to a bond in order to retain a particular conformation. It also summarises the closure rule to highlight the effect of strain on reactivity, such as Thorpe-Ingold effect and rules for ring closure. Keywords: bridgehead effect; cyclic molecules; Jahn-Teller distortion; molecular substituents; organic chemistry; steric strain; van der Waals strain