Reactivity and Product Determination as a Function of Conformation

The effect of conformation on reactivity is intimately associated with the details of the mechanism of a reaction.Two extreme situations can arise  

The conformational barrier (A = B) is substantially higher than the reaction barriers TS and TSg. This case is known as the conformational equilibrium control, where the ratio of products is equal to the ratio of the population of the starting states. 

Early transition states are frequently typical of fast, exothermic reactions in which there is no substantial bond making or breaking. Hence, early transition states resemble starting materials in structure. Late transition states are frequently typical of relatively slow, endothermic transformations. There is substantial bond formation in late transition states consequently, late transition states resemble products. 

Certain reactions have distinct stereoelectronic requirements that must be met if the reaction is to proceed efficiently.The effect of conformation on reactivity is conveniently divided into (1) steric effects—close approach of two groups, and (2) stereoelectronic effects—spatial disposition of a particular electron pair, either bonded or nonbonded, as in and E2 reactions. 

Stereoisomers have the same molecular formula, but their atoms have a different spatial arrangement configuration). 

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