E2 Reactions

Strongly Related Topics

• E1 reactions
• SN2 reactions
• Zaitsev's rule

Somewhat Related Topics

• Additions to alkenes: regiochemistry
• Additions to alkenes: stereochemistry
• Nomenclature
• SN1 reactions

Glossary Terms

bimolecular	concerted	dehydrohalogenation
kinetics	mechanism	periplanar ("syn" and "anti")
regioisomers	second-order reaction	staggered conformation
stereoisomers

The term E2 stands for "elimination bimolecular." Like any elimination reaction, the product of an E2 elimination reaction has one more degree of unsaturation than the starting materials did. For instance, the base-induced elimination of "HX" (dehydrohalogenation) of an alkyl halide gives rise to an alkene (illustrated below for the conversion of tert- butyl bromide to isobutylene).

Self-test question #1

There are two elimination products that could be formed by the loss of HBr from t-pentyl bromide. Can you draw them?

Self-test question #2

Which of the two products formed in "Self-test question #1" would be expected to predominate in the reaction mixture? (See related page on Zaitsev's rule if you need help with this question.)

Self-test question #3

In the Web page on "Nomenclature," you learned about (or reviewed) the trivial names for twelve alkyl groups having five or fewer carbons. Consider the bromides derived from each of those groups. Which two of them could not possibly undergo an E2 elimination reaction, and why?

Stereochemical requirements (and consequences) of the E2 reaction

Stereochemical evidence indicates that E2 reactions always occur via "periplanar" geometry, that is, the atoms of the H-C-C-X group involved in the reaction must all lie in the same plane. This gives rise to two possible orientations:

Of these two, the "anti periplanar" geometry is obviously the most favorable.

Self-Test question #4

Consider cis- and trans-1-tert-butyl-4-chlorohexane

a)One of these molecules cannot react by an E2 pathway. Which one is it, and why can't it?

b)What is the product of the E2 elimination reaction from the isomer that can react by E2?

When comparing the reactivity of these four reactions, the following generalizations are useful:

• Primary alkyl halides almost always react via an SN2 pathway
• Secondary alkyl halides give SN2 with good nucleophiles; strong bases promote E2 pethways
• Tertiary alkyl halides give E2 in the presence of (strong) bases; SN1 and E1 will compete with each other in nonbasic media

These results are spelled out in more detail in the table below (adapted from McMurry, 4/E, with permission):

Related reading in textbook (McMurry, Organic Chemistry, 4th ed.)

• Chapter 11 Section 11 pgs. 399-405: The E2 Reaction