SN2/E2 Reactions
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Nucleophilic Substitution Reactions vs Elimination Reactions

SN2/SN1 versus E2/E1 Reactions

If you're stuck trying to figure out what mechanism to draw, try the following steps...

1) look at the product is it substituted or eliminated?

2) if substituted, then look at the degree of substitution of the starting material...is it tertiary, secondary, or primary?

a) if tertiary, it has to be SN1
b) if primary, it has to be SN2
c) if secondary, it can be either, so look at either the solvent (if it's given) or the stereochemistry of the product.

3) Stereochemistry of the product...

a)    if there are R and S products formed at the center, then it was an SN1 mechanism.
b)   if there is only one product (R or S), formed, then it was an SN2 mechanism.

OR...

4) if your product is an eliminated product (i.e. it has a double bond)...look at the degree of substitution of the starting material...is it tertiary, secondary, or primary?

a) if tertiary, it has to be E1
b) if primary, it has to be E2
c) if secondary, it can be either, so look at the stereochemistry of the product.

5) stereochemistry of the product...

a) if there are two double bonds (cis and trans or E and Z), then it was an E1 mechanism.
b) if there is only one double bond (cis or trans ; E or Z), then it was an E2 mechanism.

The following table is another way to summarize all of this information.  If a reaction has more than one of the conditions listed, most likely it fits under that type of reaction mechanism.  The rough way to tell if a substrate will either substitute or eliminate is by the nuclephile.  If it's a small nuclephile (Br-, CN-), it will substitute.  If it's a large nucleophile (HSO4-, tbutylO-), it will have to eliminate because the nucleophile can't get close enough to the carbon with the leaving group to attach.  Another short summary of that concept is below...
 
SUMMARY of SN2/SN1 versus E2/E1 Reactions

SN1 SN2   E1 E2
all tertiary, some secondary some secondary, all primary   all tertiary, some secondary some secondary, all primary
acidic basic   acidic basic
carbocation (R+) concerted/backside attack;
inversion of configuration		   carbocation (R+) concerted/antiperiplanar
2 products 1 product   2 products 1 product
protic solvent aprotic solvent          

SN1 versus E1...SN1 is favored when there is a small nucleophile and a small leaving group.  E1 is favored when there is a large nucleophile and large leaving group.

SN2 vs E2...SN2 is favored when there is a small nucleophile and a small leaving group.  E2 is favored when there is a large nucleophile and large leaving group.

Competition Between Nucleophilic Substitution and Elimination Reactions

Anytime there are nucleophilic substitutions, there is most likly going to be the potential for the nucleophile to act as a base to give elimnation. The only time that is not true is with Methyl Halides for obvious reasons. The amount of alkyl substitution on the carbon bonded to the halide will affect this competition. Here are some things to keep in mind:

  1. Methyl Halides will only undergo SN2 reactions.
  2. Primary Halides will always undergo SN2 except with hindered strong bases in which case E2 product predominates over SN2 product
  3. Secondary Halides will give SN2 product with weak bases like carboxylate ions, R-COO-, or Cyanide ions, CN-. With strong bases E2 product will predominate
  4. Tertiary Halides, Allylic Halides, and Benzylic halides will never occur under SN2. SN1 product is favored at low temperatures with weak bases and E1 product is favored with high temperatures and weak bases.. E2 will only predominate when the base is strong and the temperature is high.

By adjusting the strength of the base and the temperature as well as the organic substrate, the desired product can be favored. Further information on the competition can be gotten from the following Web site on competition between E and SN reactions. A second source for this competition is found here.
 

SN2 vs. E2

SN2 and E2 reactions share a number of similarities. Both require good leaving groups, and both mechanisms are concerted. SN2 reactions require a good nucleophile and E2 reactions require a strong base. However, a good nucleophile is often a strong base. Since the two reactions share many of the same conditions, they often compete with each other. The the outcome of the competition is determined by three factors: the presence of antiperiplanar β-hydrogens, the degree of α and β branching, and the nucleophilicity vs. basicity of the reactant species.

In order for an E2 elimination to occur, there must be antiperiplanar β-hydrogens to eliminate. If there are none, the SN2 reaction will dominate. On the same token, the SN2 nucleophile needs an free path to the σ* C-LG antibond. α and β branching block this path and reduce the proportion of SN2 relative to E2. E2 occurs even with extensive branching because it relies on the β-hydrogens, which are much more accessible than the σ* C-LG antibond.

The identity of the nucleophile or base also determines which mechanism is favored. E2 reactions require strong bases. SN2 reactions require good nucleophiles. Therefore a good nucleophile that is a weak base will favor SN2 while a weak nucleophile that is a strong base will favor E2. Bulky nucleophiles have a hard time getting to the α-carbon, and thus increase the proportion of E2 to SN2. Polar, aprotic solvents increase nucleophilicity, and thus increase the rate of SN2.

SN2

1.               Requires an unhindered path to the back of the α carbon
2.            α and β branching block the path and hinder SN2
3.               Requires a good nucleophile
4.            Polar, aprotic solvents increase nucleophilicity
5.            Bulky groups on the nucleophile decrease nucleophilicity

E2

     1.            Requires an antiperiplanar β-hydrogen
2..              Enhanced by α and β-branching
3.               Requires a strong base

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Antiperiplanar  -  If two bonds define two line segments, then they are antiperiplanar if they are antiparallel in the plane they define. It's much easier to see antiperiplanar bonds than it is to explain them. In the following diagram, the C-H and C-LG bonds are antiperiplanar:



 

E2 reactions require an antiperiplanar β-hydrogen.

 

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                                                                        Page last updated on 11/09/2007