Question

In 3-methyl-2-cyclohexenone which hydrogen cannot undergo deuterium exchange when it reacts with $C{H_3}{{\text{O}}^\Theta }/C{H_3}OD$ ?

(A) ${{\text{H}}_1}\;,\;{{\text{H}}_4}$
(B) $\;{{\text{H}}_4}$
(C) ${{\text{H}}_3}\;,\;{{\text{H}}_2}$
(D) ${{\text{H}}_5}\;,\;{{\text{H}}_3}$

Answer 1

Hint : This question talks abouts exchange of isotopes of Hydrogen. For this kind of question, we have to know about the Kinetic Isotopic effect in Tautomerism which says If we take a heavier isotope of Hydrogen then all the ${\text{\alpha }}$ Hydrogen and the hydrogen which are in conjugation with the carbonyl group will be replaced by the heavier isotope of Hydrogen. So, in this question we will identify the ${\text{\alpha }}$ Hydrogens and replace them with the heavier Isotope.

Complete Step By Step Answer:
Now, if we see in the diagram ${{\text{H}}_1}$ and ${{\text{H}}_5}$ are the alpha hydrogen because they are directly connected to the carbonyl group, so they will react with $C{H_3}{{\text{O}}^\Theta }/C{H_3}OD$ they will get replaced. now ${{\text{H}}_2}$ and ${{\text{H}}_3}$ hydrogen are in conjugated hydrogen they will also get replaced when reacted with ch3 now we are left the ${{\text{H}}_4}$ hydrogen if we see the diagram then it is clear that ${{\text{H}}_4}$ hydrogen is neither a alpha hydrogen nor a conjugated hydrogen so it will not react with the $C{H_3}{{\text{O}}^\Theta }/C{H_3}OD$ and will not be replaced.
The carbon atoms directly attached to the functional group are known as ${\text{\alpha }}$ Carbon, Hydrogen atoms attached to ${\text{\alpha }}$ carbon atoms are ${\text{\alpha }}$ Hydrogen. Since they are directly attached to the ${\text{\alpha }}$ carbon they become more reactive. So carefully observe the diagram to identify them.
The reaction taking place will be

So, the answer is option (C).

Note :
Deuterium is an isotope of hydrogen also known as heavy Hydrogen with a nucleus containing one proton and one neutron so it has twice the mass of a normal hydrogen. When Deuterium is reacted with oxygen it forms ${D_2}O$ also known as Heavy Water.