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SN1 and E1 Reactions

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This page covers the mechanistically related reaction types, SN1 and E1. Its purpose is to point out the similarities and differences between these two reaction types, as well as distinguish them from related SN2 and E2 reactions. The reader is strongly encouraged to review the pages on SN2 and E2 reactions along with this page. A brief summary of the four modes of reactivity follows the presented text.

Strongly Related Topics

Somewhat Related Topics

Glossary Terms

achiral leaving group rate-determining step
carbocation mechanism resonance effect
chiral nucleophile unimolecular reaction
intermediate protic solvent Zaitsev's Rule
kinetics racemic mixture


The terms SN1 and E1 mean "substitution, nucleophilic, unimolecular" and "elimination, unimolecular," respectively. These two reaction types are being considered together for two reasons:
  1. They often occur simultaneously and competitively with one another, under the same reaction conditions.

  2. They each involve the formation of a carbocation as the crucial intermediate in the rate-determining step; these reactions exhibit unimolecular (or "first-order") kinetics, because only one molecule -- the immediate precursor of the carbocation -- is involved in the rate-determining step.

The generalized mechanism for each of these reaction types has been depicted below, using tert-butyl chloride as the starting material:

Notice that the product of an SN1 substitution reaction has simply replaced the chlorine atom with the new substituent, "Nu" in this case. The alkyl group (t -butyl) present in the starting material is still intact, and the hybridization of the substituent-bearing carbon has not changed (it's still sp3). Conversely, the product of the elimination reaction is an alkene: the starting material has lost the elements of HCl, and the hybridization of the carbon originally bearing the chlorine atom has changed from sp3 to sp2.

Note also that the nucleophile in an SN1 reaction does not have to bear a negative charge. In fact, these reactions are typically performed under "solvolysis" conditions, i.e., simply heating the starting material in a protic solvent (an alcohol or carboxylic acid) that can also act as a nucleophile. In these cases, of course, the product of carbocation capture by the solvent will bear a "+" charge, and it will have to lose H+ in order to form a neutral product. Similarly, the base in an E1 reaction does not have to be strong. In fact, the base must not be strong, otherwise the E2 mechanism will be followed. It is common for the solvent to act as the base in an E1 reaction, just as it acted as the nucleophile in an SN1 process.


Self-test question #1

The following products are formed when tert-butyl bromide is heated in ethanol:

What three (organic) products would you expect to be formed if t -pentyl bromide were heated in ethanol? Which of these products is (are) formed by substitution and which is (are) formed by elimination pathways?

?


Self-test question #2

Can you explain why heating either enantiomer of 2-bromo-2-phenylbutane in ethanol leads to the same substitution product, i.e., a racemic mixture of 2-ethoxy-2-phenylbutane?

?


Effect of Substrate Structure

Because the mechanisms of SN1 and E1 reactions each involve a carbocation intermediate, only those substrates that ionize to produce particularly stable carbocations will be able to react via these pathways. Typically this means tertiary alkyl halides (or alcohols, in acidic media; see "Self-test question #3"), or substrates that can ionize to form carbocations stabilized by resonance. SN1 and E1 reactions are much rarer for secondary alkyl halides (or alcohols), and these mechanistic pathways are never followed for simple primary or methyl alkyl halides (or alcohols).

Effect of Reaction Medium

SN1 and E1 reactions are most favorable in protic solvents, such as carboxylic acids or alcohols. Neutral or acidic conditions are most common, but sometimes the media are slightly basic. Recall, however, that strongly basic conditions will promote other modes of reactivity, such as the E2 elimination, even in substrates that otherwise might have been susceptible to SN1 or E1 reactions.

Use in Synthesis

SN1 reactions can be preparatively useful in organic synthesis, but only in cases where:

  1. Particularly stable carbocations are formed, and
  2. elimination reactions are either impossible, or reactions conditions have been adjusted in such a way that elimination reactions are suppressed.
Some examples follow:

Conversely, since E1 reaction products are almost always accompanied by SN1 reaction products, they are almost never used in organic synthesis. (The reader may recall that, E2 reactions are frequently used in synthesis, in sharp contrast to E1 reactions.) The proportion of the starting material that reacts via an E1 pathway can be enhanced, relative to the proportion that reacts via the SN1 pathway, by using solvents of extremely low nucleophilicity such as triflouroacetic acid. When the E1 pathway is followed, Zaitsev's Rule is obeyed. That is, if more than one regioisomeric elimination product is possible, then the thermodynamically more stable alkene will predominate in the product mixture:

E1 conditions (very dilute base) lead to a mixture of 2-menthene and 3-menthene.

© 1996, Brooks/Cole. Adapted with permission.


Self-test question #3

Alcohols can react by SN1 pathways under acidic conditions, where the hydroxyl group is protonated and thus converted into a good leaving group. For example,

Which of the alcohols pictured and named below will react most quickly with HCl to form the corresponding alkyl chloride via an SN1 pathway?

a) a-butyl alcohol b) sec-butyl alcohol c) isobutyl alcohol d) tert-butyl alcohol

?


Self-test question #4

Consider only the elimination products that would be formed in the following reaction:

Which one would be expected to be formed in greater amounts, and why?

?


Summary of SN1, SN2, E1, and E2 Reactivity

The following chart and table summarize the expected modes of reactivity es, although the reader should recall the alcohols, under acidic conditions, can react in the same ways (except for E2, which requires base). Note that these charts do not include substrates that could ionize to produce carbocations that would be stabilized by resonance effects. In those cases, SN1 and E1 reactions could be expected to compete more effectively than their bimolecular counterparts.

Summary of SN1, SN2, E1, and E2 Reactivity
(as a function of RX Structure)


In general, substrates react in the following way:

RCH2X
(primary)
Mostly SN2 substitution
R2CHX
(primary)
SN2 substitution with nonbasic nucleophiles
E2 elimination with strong bases
R3CX
(primary)
Mostly E2 elimination
(SN1 substitution and E1 eliminationin nonbasic solvents)



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

Self-test question #5

Although it is difficult for most beginning students to predict which of the four modes of reactivity -- SN1, SN2, E1, and/or E2 -- will be followed for a particular set of conditions, one should certainly be able to say which mode of reactivity was followed if shown the reactants, conditions, and products of a particular reaction. For each of the products of the reactions shown below, indicate which of the four mechanisms was involved in its formation. Use the "structure/reactivity correlation chart"shown above if you need help:

?


Self-test question #6

Likewise, one should be able to predict which one of two related reactions is faster. For each of the pairs of reactions below, indicate which one is faster:

?


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

Links to Related Computer-Based Learning Materials

Links to Related Internet Resources


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This page was prepared by Carolyn M. Degel of the Penn State University, Schuylkill Campus, Spring 1997

Send questions, comments, or suggestions to:
Dr. Thomas H. Eberlein
the1@psu.edu
Copyright © 1997 Thomas H. Eberlein

Version 1.2.11, 3/10/97