Organic Chemistry/Introduction to reactions/Transition states

Transition State

Many reactions occur in a single step when two reactant molecules collide with sufficient energy in the proper spatial orientation to create a product. Many other reactions, however, do not occur in a single step, and such reactions are said to have transition states. Multistep reactions have products that result from a series or chain of reactions, and these reactions are the kind that actually do pass through a transition state.

Intermediate Molecules

A carbocation is an achiral intermediate in an S_N1 reaction

Multi-step reactions have an intermediate molecule that forms as a sort-of halfway point between the reactant and the product. This intermediate molecule cannot be isolated in solution, because it is typically of much higher energy than either the reactants or the products. However it does have a finite lifetime before it reacts.

At a basic level of organic chemistry the intermediate molecule is often merely predicted or assumed, but many are also known to exist due to experimental observations in the laboratory. Racemization of molecular configuration without another, more plausible explanation is one type of proof that certain reactions pass through a transition state.

EXAMPLE: In a 1st order nucleophilic substitution (S_N1 reaction) the chirality of a carbon is "flipped" from S to R configuration (or vice versa) 50% of the time, leading to a racemic product (assuming it is chiral). This is because the chiral carbon becomes achiral in the carbocation intermediate, due to the sp² carbon center that is flat and thus subject to nucleophilic attack from either the top or the bottom. If there were no intermediate, simple retention or inversion would be expected 100% of the time. This type of racemization during an S_N1 reaction may be seen as evidence of a carbocation intermediate.

Energy Diagrams and Transition States

Observing energy diagrams of reactions, you may notice that instead of a single peak energy, the energy peaks, drops a little, peaks once again and then returns to a lower energy state for the product. The "saddle" (minimum) between the two peaks represents a reaction intermediate - a place where the reaction temporarily "rests" between energy peaks. The energy peaks (maxima) are defined as 'transition states' with no finite existence.