Second order kinetics mean that two molecules must collide in the rate limiting step of the mechanism. This type of process is extremely common in chemical reaction dynamics. Bond forming reactions most often involve two molecules that must interact. Three-body processes are rare gy oomparison. First we examine the general theory of bimolecular reactions. We then consider some important specific examples, such as DNA hybridization and enzyme kinetics.
Organic chemistry involves combinations of reactants with the goal of creating new chemical bonds. Many of the reactions are second order reactions that involve the interaction of two different species either by electrophilic or nucleophilic attack. In this section we consider the cless of reactions known as nucleophilic substitutions as an example of the type of issues that arise in the kinetics of organic reactions. There is usually a competing first order process that invovles dissociation prior to nucleophilic (or electrophilic) attack. In the nucleophilic nomenclature the two types of mechanism are SN1 (first order) and SN2 (second order), respectively..
DNA hybridization is an example of second order kinetics. The concentratoin dependence of DNA hybridization is quite important. DNA hybridizatoin rates can vary over many orders of magnitude because of the fact that second order kinetics has such a strong dependence on concentration. DNA also has a large dynamic range of concentration since we can consider important examples where there is a single strand or only a few strands up to concnetrations in the micromolar range. DNA hybridization in intermediate concentration ratnges can be readily detected using UV-vis spectroscopy. This permits us to obtain good data on DNA hybridization rates.
