Structure and Bonding:  Consecutive Reactions

Let's consider the reaction of  hydroxide ion with tert-butyl bromide, (CH3)3CBr.   Could this reaction occur by an SN2 mechanism (see the "Reaction Rates and Molecular Crowding" activity)?  Why or why not?

(CH3)3CBr   +    OH-               (CH3)3COH    +    Br-

This reaction occurs in two-steps as outlined below, since the tert-butyl bromide would be  hindered to backside attack by the hydroxide ion:

step 1:    (CH3)3CBr           (CH3)3C+    +    Br-         slow kinetics

step2:    (CH3)3C+    +    OH-               (CH3)3COH        fast kinetics

The animation below illustrates step 1 of the reaction mechanism.  How does the geometry of the central carbon atom in (CH3)3CBr change?

After the Br- leaves the tert-butyl bromide molecule, a carbocation is formed.   This is shown in the image below.  How would you describe the geometry?  What is the charge on the carbocation?

The attack of the carbocation, with a charge of +1, by the incoming group, OH-, can be from either side.  Will inversion occur in this reaction?  Explain why or why not.

Why is step 2 of the reaction mechanism fast?

Here is the image of the tert-butyl alcohol or 2-methyl-2-propanol produced.  What is the geometry of the center carbon?

This is a classical example of an SN1 reaction in organic chemistry.  In consecutive reactions, the slow step governs the overall rate.  The reaction kinetics are first order and depend only on step 1.  So the rate law is given by the equation:    

Rate = k ((CH3)3CBr)

The concentration of the hydroxide ion does not influence the rate of this reaction.  Since there is a 50-50 chance of collision in step 2 from either side of the trigonal planar carbocation, half the product molecules, tetrahedral in geometry, are inverted in configuration while the other half are not inverted.  A mixture of 50-50 of both enantiomers is called a racemic mixture (optical rotation is offset).  The importance of the inversion will be discussed further in organic chemistry.

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