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E2 Reactions

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This page covers "bimolecular elimination" reactions. The purpose of this page is to illustrate this important class of reactions, which the reader should compare with the related E1 and SN1 reactions.

Strongly Related Topics

Somewhat Related Topics

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).

E2 eliminations, in contrast to E1 reactions are promoted by strong base. The base vital to the reaction; it is directly involved in the rate-determining step. The reaction is bimolecular--that is, it involves "second-order kinetics--because two molecules must come together for the reaction to occur. The mechanism of an E2 elimination reaction is shown below:

Notice that the hydrogen that is removed is on the carbon atom that is adjacent to the one bearing the halogen. For some reason, beginning students are often confused on this point, although it is mysterious as to why they should be. Carbon-carbon double bonds, by definition,exist between two adjacent carbon atoms. Likewise, the "H" and the "X" atoms that are eliminated during the dehydrohalogenation of an alkyl halide must be on the carbon atoms.


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?

Answer


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.)

Answer


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?

Answer


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?

?



E2 vs. E1 vs. SN2 vs. SN1 reactions

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

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

Correlation of Structure and Reactivity for Substitution and Elimination Reactions

Halide Type
SN1
SN2
E1
E2

RCH2X
(primary)
Does not
occur
Highly
favored
Does not
occur
Occurs when
strong bases
are used
R2CHX
(secondary)
Can occur
with benzylic
and allylic
halides
Occurs in
competition
with E2
reaction
Can occur
with benzylic
and allylic
halides
Favored when
strong bases
are used
R3CX
(tertiary)
Favored in
hydroxylic
solvents
Does not
occur
Occurs in
competition
with SN2
reaction
Favored when
bases are
used

© 1996, Brooks/Cole. Adapted with permission.


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

Related Computer-based Materials

  • Links to Related Chem TV Files
    • Vol.1, Topic 15: The E2 Reaction

  • Related Beaker Menu Functions
    • React,"Perform a Reation"

Links to Related Internet Resources


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This page was prepared by David Troutman and Amy Weiderhold of the Penn State University, Schuylkill Campus, 1996-1997

Send questions, comments, or suggestions to:
Dr. Thomas H. Eberlein
the1@psu.edu
Copyright © 1996 Thomas H. Eberlein

Version 1.2.4, 3/17/97