If you like us, please share us on social media, tell your friends, tell your professor or consider building or adopting a Wikitext for your course.

ChemWiki: The Dynamic Chemistry E-textbook > Organic Chemistry > Alkyl Halides > Properties of Alkyl Halides > Haloalkanes > Alkyl Halide Reactions > Substitution and Elimination Reactions of Alkyl Halides > SN1 Substitution Reactions > Steric Hindrance to Rear-side Approach in Nucleophilic Substitution

Steric Hindrance to Rear-side Approach in Nucleophilic Substitution

Table of Contents

The two models displayed below start as methyl bromide, on the left, and ethyl bromide, on the right. These may be replaced by isopropyl, tert-butyl, neopentyl, and benzyl bromide models by pressing the appropriate buttons. (note that when first activated, this display may require clicking twice on the selected button.) In each picture the nucleophile is designated by a large violet sphere, located 3.75 Angstroms from the alpha-carbon atom (colored a dark gray), and located exactly opposite to the bromine (colored red-brown). This represents a point on the trajectory the nucleophile must follow if it is to bond to the back-side of the carbon atom, displacing bromide anion from the front face. With the exception of methyl and benzyl, the other alkyl groups present a steric hindrance to the back-side approach of the nucleophile, which increases with substitution alpha and beta to the bromine. The hydrogen (and carbon) atoms that hinder the nucleophile's approach are colored a light red. The magnitude of this steric hindrance may be seen by moving the models about in the usual way, and is clearly greatest for tert-butyl and neopentyl, the two compounds that fail to give substitution reactions.


You must to post a comment.
Last modified
09:15, 2 Oct 2013


This page has no custom tags.


Lower Divisional

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use