JEE Chemistry 2023 Question with Solution

Step-by-step solution

Standard Method

Given: The reaction scheme involves MeMgBr and the quantities are marked as $x$ and $y$. The solution indicates $y = 1$ mole for the starting hydroxy aldehyde and requires finding the ratio $x/y$.

Find: The value of $x/y$ on completion of the reaction.

From the reaction scheme, one mole of MeMgBr is consumed in deprotonating the OH group, and one more mole of MeMgBr is consumed in nucleophilic addition to the aldehyde group.

So, for $y = 1$ mole of the substrate,

$$x = 2$$

Therefore,

$$\frac{x}{y} = \frac{2}{1} = 2$$

Hence, the correct option is B.

Common mistakes

• Using only one mole of MeMgBr by considering reaction at the aldehyde group alone is incorrect, because MeMgBr first reacts with the acidic OH proton. Count both consumptions before taking the ratio.

• Ignoring the intramolecular alcohol group is wrong, because organomagnesium reagents are quenched by protic groups. First account for acid-base reaction, then the carbonyl addition.

• Taking $y$ as the moles of H atom instead of the given substrate amount is incorrect. Here $y = 1$ mole corresponds to the starting compound as shown in the reaction scheme.