Single-step reactions, transition-state

The reaction of the contact ion pair A with cyclohexenone proceeds in a single step. The transition state is D, which is structurally distinguished by the fact that the formation of a solvent-separated ion pair is avoided. The lithium separates from the organic residue R R2R3C only where the latter can form a C-C bond and where the Li cation can directly... 

Scheme 1 is a gross over-simplification for almost any enzyme-catalyzed reaction of a specific substrate, based as it is on a one-step reaction with a single, rate-determining transition state but it is appropriate for many, if not most reactions catalyzed by simple enzyme mimics. Most important for present purposes, it emphasises the most important properties of enzyme reactions which the design of mimics, or artificial enzymes, must address, namely ... 

Figure A7.1 The effect of an enzyme on the minimum energy pathway of a simple enzyme catalysed reaction involving a single substrate. (TS = transition state.) The heights of the energy barriers TSi, ts2 and ts3 will vary depending on which step in the enzyme controlled route is the rate controlling step...

The reaction of carbene with hydrocarbon is a single-step reaction involving a triangular transition state. This reaction is possible because the singlet is such a strong electrophile... 

According to the theory of absolute reaction rates [4-9], the rate is the product of a universal frequency factor and the concentration of the activated complex or transition state, M (the system in the transient state of highest potential energy), crossing the energy barrier in the direction toward the products. The activated complex, in turn, is postulated to be in equilibrium with the reactants. Say, for a single-step reaction A + B — P ... 

According to the transition state theory the rate coefficient of a single-step reaction is given by (see Glasstone et al., 1941c)... 

In the case of a single-step reaction such as the reduction of Fe to Fe " (in the absence of diffusion control), no assumptions are required about a rate-determining step in the usual sense (although microscopically, for such redox reactions in solution, consideration can be given to solvent reorganization in the formation of the transition state associated with electron transfer). Correspondingly, no intermediate (except the transition state itself ) need be considered in the reaction mechanism scheme. 

Fig. 23.12b shows the typical situation for the carbon analog where the ionic stmcture is slightly higher in energy, relative to the HL-state at the crossing point. The consequent VB mixing leads to a single step reaction with a pentacoordinated transition state. 

The molecularity of a chemical reaction is the number of molecules involved in the transition state of the reaction. The term can only be applied to single-step reactions, also known as elementary reactions. If only a single molecule is involved in the transition state, the reaction is unimolecular. For example, a thermal rearrangement such as a Cope rearrangement is typically unimolecular. If fwo molecules are involved the reaction is bimolecular, with the Sn2 reaction being the prototype. Termolecular processes involve three molecules and are rare, but not unprecedented (we show a few in Chapter 10). 

As mentioned above, for primary halogenoalkanes this reaction is a single-step reaction in which two species are involved in the one rate-determining step, and therefore the reaction is said to be bimolecular. The nucleophile (OH ) is attracted to the electron-deficient carbon atom and a transition state is formed in which the carbon-bromine bond is broken at the same time as a new carbon-oxygen bond is formed. The bromine atom then leaves as a bromide ion, and the alcohol (in this case methanol) is formed (Figure 20.3). This mechanism is fully described as an 8 2 (substitution nucleophilic bimolecular) reaction. 

If a and p = 0, then the transition state shows no potential dependence. Physically, it provides an insight into the way the transition state is influenced by the voltage. For a single-step reaction involving a single electron transfer (n = 1),... 

Each equation m Figure 4 6 represents a single elementary step An elementary step IS one that involves only one transition state A particular reaction might proceed by way of a single elementary step m which case it is described as a concerted reaction, or by a series of elementary steps as m Figure 4 6 To be valid a proposed mechanism must meet a number of criteria one of which is that the sum of the equations for the elementary steps must correspond to the equation for the overall reaction Before we examine each step m detail you should verify that the mechanism m Figure 4 6 satisfies this requirement... 

The Diels-Alder reaction is believed to proceed m a single step A deeper level of understanding of the bonding changes m the transition state can be obtained by examining the nodal properties of the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile... 

Figure 10.12 shows the interaction between the FIOMO of one ethylene molecule and the LUMO of another. In particular, notice that two of the carbons that are to become a-bonded to each other in the product experience an antibonding interaction during the cycloaddition process. This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry-forbidden reaction. Reaction, were it to occur, would take place slowly and by a mechanism in which the two new a bonds are fonned in separate steps rather than by way of a concerted process involving a single transition state. 

---