The Hammond Postulate Thermodynamics versus Kinetics

If we determine the rate of this process by measuring the disappearance of r/-butyl iodide over time, we can ultimately find the height of transition state 1. The rates of the faster captures of the carbocation do not affect the rate of ionization of tert-butyl iodide. The carbocation is gobbled up by the nucleophiles at a rate faster than that at which it is formed. In the limit, each molecule of the tert-butyl cation is captured before another is produced from er -butyl iodide. The rate of formation of product depends on how fast the r/-butyl cation is produced in the slow, rate-determining step but not on the rate of the fast capture by a nucleophile. 

We are used to saying, Product Y is more stable than product X and therefore is formed faster. This statement has a comfortable sound to it and intuitively makes sense. But it need not be true The first half of the statement describes thermodynamics, the relationship of the energies of product X and product Y. The second half speaks of the rates of formation of X and Y, which is a statement about kinetics. Despite the good intuitive feel of the original statement, there need be no direct connection between thermodynamic stabilities and kinetics. 

FIGURE 8.27 The energy difference between X and Y does not necessarily affect their rates of formation. 

FIGURE 8.28 Only if X and Y are in equilibrium does AG° determine their ratio. 

FIGURE 8.29 (a) The usual picture is this The higher transition state leads to the less stable product. The lower transition state leads to the more stable product, (b) The situation could be otherwise the higher transition state could lead to the lower-energy product, and the lower-energy transition state to the higher-energy product. This second scenario is rare, but it does happen. 

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Energy Barriers in Chemical Reactions The Transition State and Activation Energy Reaction Mechanism The Hammond Postulate Thermodynamics versus Kinetics... 

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