Decisions Guide

The substitution/elimination decision becomes a function of three major variables nucleophilicity, basicity, and steric hindrance. In acidic media the SnI competes with the El process the first step of both is to lose the leaving group to form the carbocation. There normally is competition in basic media between the Sn2 and E2. 

Weak base Strong base Weak base Strong base 

HjO EtOH (CHjljCO RjN Poor nucleophile No rxn. or very slow Sfj2 Mixture (depends on L) 

Mixture (depends on basic.) Elimin. by fast E2orEl 

The unhindered primary layer has the strong base, poor nucleophile box as its mixture quadrant with all others as substitution. Notice how the mixture quadrant in the primary matrix moves around to the strong base, good nucleophile quadrant in the secondary matrix, then again to the weak base, good nucleophile quadrant in the tertiary matrix, each time leaving behind an elimination quadrant. 

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ATSDR. 1989. Decision guide for identifying substance-specific data needs related to toxicological profiles. Atlanta, GA Agency for Toxic Substances and Disease Registry, Division of Toxicology. 

Agency for Toxic Substances and Disease Registry. 1989. Decision guide for identifying... 

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