Molecularity of reactions

The 2 1 02 binding by tmpa and bpqa complexes may also be compared to that seen for peroxo-dicobalt(III) compounds derived from mononuclear precursors, where the molecularity of reaction is analogous. Thus, k is calculated... 

Hence, although the concept of Order of Reaction has evolved from the molecularity of reaction, but they need not be the same. While molecularity refers to the number of molecules involved in a chemical reaction, the order of reaction must be derived from the study of the variation of reaction rates with time, concentration and (as we shall see later) temperature. 

The occurrence of a minimum or maximum in a property—reactivity, acidity, antibacterial activity, etc.—as one proceeds along a logical series always suggests the working of opposing factors. (See, for example, the effect of acidity on certain carbonyl reactions. Sec. 19.14.) In the case of nucleophilic aliphatic substitution, a minimum of the kind we have just encountered is highly characteristic of a change in molecularity of reaction. 

What we see here is another example of that characteristic of nucleophilic substitution a shift in the molecularity of reaction, in this particular case between T and 1°. This shift is confirmed by the fact that reactivity passes through a minimum at 1° and rises again at methyl. Because of poor accommodation of the positive charge, formation of primary carbonium ions is very slow so slow in this instance that the unimolecular reaction is replaced by the relatively unhindered bimolecular attack. The bimolecular reaction is even faster for the still less hindered methanol. 

A chemical reaction can be classified according to its molecularity or according to its order. The molecularity defines the number of molecules that are altered in the reaction. With the aid of the activated complex, it is possible to obtain a more precise definition of the molecularity of reaction molecularity of reaction is equal to the number of molecules in the activated complex. It is clear that the molecularity of reaction must be an integer and, in fact, it was found that it is 1,2, or seldom 3 (Jencks, 1969). 

Type of reaction Reactant Molecularity of reaction Designation... 

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