Answer to Question #94069 in Organic Chemistry for Megha Nautiyal

Question

Isotopes have been used to study the reaction path and mechanism. Explain.

Answer 1

Nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS) techniques detect isotopic differences.They allows to determine the position of labeled atoms in a product ‘s structure. Isotopic labeling is a mechanism used to track the passage of an isotope through a reaction, metabolic pathway. Knowing the information about the position of isotopic atoms in products, the reaction pathway can also be determined.

For example, this is mechanism of esterification:

In the first step, protonation of the carbonyl oxygen gives a delocalized carbocation, which makes the carbonyl carbon a much better electrophile. In the second step, the carbonyl carbon is susceptible to nucleophilic attack by ethanol. A lone pair of oxygen makes a bond with the carbonyl carbon and π bond breaks. These electrons from π bond go up to the oxygen and remove the positive charge. This gives us the oxonium ion. In the third step, proton transfer from the oxonium ion to one of the OH groups gives us an activated complex. And then, one of the hydroxyl groups takes the proton from protonated alcohol.