Question

Define turnover number.

Answer 1

Turnover numbers are used in two areas of chemical science. One is enzymology which deals with the understanding of the working of enzymes, properties of enzymes, the relationship between structure and function and how they get to their native state. The other field is organometallic catalysis which includes study of organometallic compounds which are used in catalysis in different reactions of chemistry.

Complete answer:
We will see the definition of turnover number and understand it in detail. Turn over number is defined differently for enzymology and organometallic catalysis.
In the study of enzymology the turnover number is defined as the maximum number of substrate molecules which can be converted into product molecules per unit time by a molecule of enzyme. The concentration of enzyme molecules is not necessarily equal to the concentration of enzyme active sites because some enzyme molecules possess more than one active site. For an enzyme molecule which have a single active site the formula is given as
${k_{cat}}\; = \;\dfrac{{{V_{max}}}}{{{{\left[ E \right]}_t}}}$
where ${k_{cat}}$ is known as turnover number, ${V_{max}}$ is the maximum rate of reaction when all sites acquired by enzymes are saturated with substrate, and ${\left[ E \right]_t}$ is the total enzyme concentration or concentration of enzyme active sites.
In organometallic catalysis the turn over number is defined as the number of moles of substrate that can be converted by a mole of catalyst before the catalyst becomes inactive. The turnover number is calculated by
$TON\; = \;\dfrac{{{n_{product}}}}{{{n_{catalyst}}}}$
where $TON\;$ is the turnover number, ${n_{product}}$ is the number of moles of product and ${n_{catalyst}}$ is the number of moles of catalyst.

Note:
In enzymology, for calculating the turnover number for an enzyme molecule having more than one active site ${\left[ E \right]_t}$ is multiplied by the number of active sites acquired by that particular enzyme. In organometallic catalysis from the definition of turnover number we can say that an ideal catalyst should have infinite turnover number because it should never be consumed.