Reactions

For a simple example of competing reactions consider a mixture of 1-bromopropane and potassium hydroxide in an aqueous solvent. At 80 ºC two irreversible reactions leading to different products take place, as shown in the following equations.

CH3CH2CH2OH | fast 80 ºC | CH3CH2CH2Br + KOH | slow 80 ºC | CH3CH=CH2

Once formed, the products are stable under the conditions of the reaction and do not return to the reactants. Consequently, if the substitution reaction (on the left) proceeds nine times faster than the elimination reaction (on the right) the product mixture will consist of 90% propyl alcohol (CH3CH2CH2OH) and 10% propene (CH3CH=CH2). In other words, the product composition is determined solely by the relative rates of the competing reactions, and not by the thermodynamic stability of the products.

For an animated illustration of this kinetic control of products Click Here

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