Work, Energy, and Power
Chemical Thermodynamics and the Equilibrium State
Reactions of Alkyl Halides
|Primary carbocations are too high energy to allow for the SN1 mechanisms in any instance. The only substitution mechanism available to primary alkyl halides is SN2 substitution . The most favorable SN2 conditions occur with a primary alkyl halide in a polar aprotic solvent, but SN2 substitution can occur with a primary alkyl halide in other solvents as well. Polar aprotic solvents, like DMSO, are so favorable for SN2 substitution that they will even allow SN2 to occur with secondary alkyl halides. Polar aprotic solvents, like DMSO, are sufficient to stabilize the SN2 transition state, but they do not stabilize negatively charged nucleophiles so much that they do not react, unlike protic solvents.|
Organic chemistry often uses kinetics and thermodynamic reasoning interchangeably, using a blanket expression such as 'making the reaction more favorable' for either case. We discussed above how a polar aprotic solvent such as DMSO makes for SN2 substitution more favorable for two reasons. First, the dielectric properties of a polar solvent allow charges to separate at lower voltage, so the polar aprotic solvent stabilizes the SN2 transition state. Secondly, being aprotic, the solvent will not overly stabilize the nucleophile. Which of these reasons a polar aprotic solvent makes SN2 substitution more favorable is based in chemical kinetics and which is based in chemical thermodynamics?
The MCAT is very fond of seeing if you are capable of separating kinetics from thermodynamics reasoning, so we are taking this as an example. The answer to the question is that lowering the energy of a transition state affects the kinetics of the reaction, making the reaction proceed more quickly, but raising the energy of reagents (less nucleophile stabilization) affects the thermodynamics of the reaction, pushing the equilibrium in the direction of products. Stabilization of the transition state would not change the position of equilibrium in itself, only the rate by which equilibrium is reached.