Reactions : Mechanism

For hundreds of years, chemists have tried to understand how chemical reactions occur. This curiosity is, for the most part, bred not from the desire to "harness" chemicals and thereby make new materials, drugs, etc. It derives, instead, primarily from an innate proclivity to understand "what makes things tick." The ability to measure the rate of a reaction and to examine molecules with instruments like infrared spectrometers, nuclear magnetic resonance spectrometers, and mass spectrometers enabled chemists to propose detailed paths traveled by molecules as they react. These "blow-by-blow" descriptions are called mechanisms. It is important to know that a mechanism cannot be proved; it is only a hypothesis.

Let us consider a reaction that has been thoroughly studied and that the mechanism proposed has achieved such a senior status that some chemists fall into the trap of believing the mechanism to be reality. The reaction is the interaction of hydroxide ion with 2-chloro-2-methylpropane:

(CH₃)₃CCl + OH^- → (CH₃)₃COH + Cl^-

The rate of this reaction depends only on the concentration of the 2-chloro-2-methylpropane; the concentration of hydroxide ion does not affect the rate. Apparently, something happens on the molecular scale that allows the 2-chloro-2-methylpropane to control the rate of the reaction. The mechanism that has been proposed involves two steps. In the first step, the 2-chloro-2-methylpropane expels a chloride ion, leaving the rest of the molecule with a positive charge--(CH₃)₃C⁺. In the second step, hydroxide ion attacks this cation, and the alcohol (CH₃)₃COH is formed. The equations for the two steps are:

(CH₃)₃CCl → (CH₃)₃C⁺ + Cl^- (1)
(CH₃)₃C⁺ + OH^- → (CH₃)₃COH (2)

The second step involves the interaction of oppositely charged particles, and therefore, it occurs very rapidly. The first step, the dissociation of the chlorine from the carbon, occurs much more slowly. A consequence of this first slow step is that the rate of the reaction depends only on the concentration of 2-chloro-2-methylpropane. This is similar to a fire brigade containing four people, who pass buckets of water to the person closest to the fire. If one man adopts a very relaxed, slow demeanor, the rate at which the buckets of water get to the fire is determined by this slow member of the team.

Chemists try to understand many reactions on the atomic or molecular level, and consequently, we must pursue these steps in some molecular detail. Figure 56 contains a reaction profile or energy diagram that shows how the energy of the reactants depend upon the progress of the first step. Notice that the energy of the system reaches a maximum as the chlorine is being removed from the carbon. This state of maximum energy is called the transition state. At this point, the system has enough energy to move from reactants to products. If the molecule possesses less energy than this activation energy, the 2-chloro-2-methylpropane does not lose a chloride ion.

Figure 56. A reaction profile for the first step in the reaction of 2-chloro-2-methylpropane with hydroxide ion.

The reaction profile for the second step is shown in Figure 57, which shows that the transition state has the hydroxide partially bonded to the carbon. Finally, Figure 58, where we show the reaction profile for the whole reaction, makes it clear that the transition state for the first state is higher than that for the second step.

Figure 57. The reaction profile for the second step in the reaction of 2-chloro-2-methylpropane with hydroxide ion.

Figure 58. The reaction profile for the entire reaction of 2-chloro-2-methylpropane with hydroxide ion.
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The entire sequence of events, including the stereochemistry (for example, the hydroxide ion can attack the carbon from either side of the trigonal cation) and energy at each point in the reaction path, is the reaction mechanism. The activation energy, which in this case is the energy required to reach the first transition state, is a very important part of the mechanism. Catalysts increase the rate of a reaction by providing a pathway that has a lower activation energy. Enzymes are biological catalysts that we depend upon to hasten the thousands of reactions that are constantly occuring in our bodies.

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