Structure and Bonding:  Dueling Mechanisms - SN1 verses SN2

You may want to review the two activities that introduce the two different substitution reactions Reaction Rates and Molecular Crowding (SN2) and Consecutive Reactions (SN1).  Explain the product(s) of the reaction of 2-bromobutane with the nucleophile, OH-:

C4H9Br   +   OH-        C4H9OH   +   Br-

 

 

+

if SN1

+

if SN2

What type of alkyl halide is 2-bromobutane?      methyl        1o        2o        3 

Are the two products of the SN1 pathway isomers?  If so, why?  For help see the last part of The Arrangement of Bonds.

If only the SN1 pathway was to occur, what would the proportions of the two products be?  Why?

What happens if both SN1 and SN2 reaction mechanisms occur together in a reaction?

Mechanism Describe the Product(s) of the Reaction Favored Conditions
SN1  

  3o alkyl halide, water or alcohol as solvent, poor nucleophile

SN2  

  methyl or 1o alkyl halide, KOH in aprotic solvent, good nucleophile

compete  

 

  2o alkyl halide

Now how are the kinetics influenced if the reaction has both mechanisms occurring at the same time?  These are competing reactions for the alkyl halide; however, the product is a mixture of the two optical isomers not two different compounds.  The overall rate of the reaction is the sum of the rates for each competitive reaction or mechanism.

Mechanism Rate Law
SN1 Rate = k1(alkyl halide)
SN2 Rate = k2(alkyl halide)(nucleophile)
overall Rate =  k1(alkyl halide) + k2(alkyl halide)(nucleophile)

How can we control the final product by variation in reaction condition?   The nucleophile used in the reaction along with the solvent influences the rate constants.  

The graph below shows the rate constant, k1, for an SN1 reaction, where the solvent mixture has an increase in the amount of water, a polar substance.  How does the reaction rate change?  Why?

Source- http://www.usm.maine.edu/~newton/Chy251_253/Lectures/Sn1/Sn1.html

Click here to get a STELLA model that looks at the competing SN1 and SN2 mechanisms plus allows you to vary a number of parameters.

Use the STELLA model to address the following questions:

1.    Do two runs with one with just the SN1 mechanism operating k1 = 1 and k2 = 0 and then just the SN2 mechanism operating k1 = 0 and k2 = 1.  How does the decrease in concentration of the alkyl halide differ for the two mechanisms?  Why?

2.    If you want one of the mechanisms to drive the reaction, what do you want the rate constants to be?  Explain.

3.    How does the concentration of the nucleophile influence the results?

4.    If the (nucleophile) = 0, what is the outcome of the reaction?  Why?

If k1> k2, which mechanism SN1 or SN2 has the greater activation energy, Ea?  Why?

Now you know that rate constants increase when the temperature rises.  Which rate constant will change the most - the one with the lower or higher Ea?  You may want to click here to get an interactive Excel spreadsheet to investigate how Ea influences temperature variation of rate constants.

In fact a temperature increase will add another level of competition as elimination reactions will now be enhanced.  A new set of products with a double bond will form.  What can you conclude about the activation energies of elimination reactions compared to substitution reactions? 

Because the SN2 pathway is second order with the concentration of the nucleophile in the rate law, lowering the concentration of the nucleophile slows this mechanism.  Lowering the concentration of the nucleophile also helps the SN1 pathway to produce a 50-50 racemic mixture.  The leaving group is given time to move away from the carbocation, allowing an equal chance of the nucleophile attacking from either side.  Lowering the concentration of the nucleophile to zero stops both mechanisms.  Why?

If you want to produce a particular optical isomer (not a mixture of both), which mechanism do you want to operate?

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