Elimination Reaction - E2 and E1 Elimination Final Notes

E2 and E1 Elimination Final Notes

The reaction rate is influenced by halogen's reactivity; iodide and bromide being favored. Fluoride is not a good leaving group. There is a certain level of competition between elimination reaction and nucleophilic substitution. More precisely, there are competitions between E2 and S_N2 and also between E1 and S_N1. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when

• steric hindrance increases
• basicity increases
• temperature increases
• the steric bulk of the base increases (such as in Potassium tert-butoxide)
• the nucleophile is poor

In one study the kinetic isotope effect (KIE) was determined for the gas phase reaction of several alkyl halides with the chlorate ion. In accordance with a E2 elimination the reaction with t-butyl chloride results in a KIE of 2.3. The methyl chloride reaction (only S_N2 possible) on the other hand has a KIE of 0.85 consistent with a S_N2 reaction because in this reaction type the C-H bonds tighten in the transition state. The KIE's for the ethyl (0.99) and isopropyl (1.72) analogues suggest competition between the two reaction modes.