Question

How many moles of $HI$ will be consumed in the given reaction?

A. 3 moles
B. 2 moles
C. 1 mole
D. 4 moles

Answer 1

Hint:We can reduce carbonyl compounds and esters to alcohol using strong reagents like $LiA{H}_4$ or $HI$. If the reducing agent is mild like HI, then alcohol is formed. If the reducing agent is strong like Zn/HCl, then the carbonyl group is reduced to hydrocarbons.

Complete answer:
We know that ethers are compounds with $\text{R}-\text{O}-\text{R}$ structure, where R is some alkyl group.In the question, we can see that it has two $\text{R}-\text{O}-\text{R}$ structure at both endings. Also, we are aware of the fact that $HI$ is a strong reducing agent. When we reduce esters we get alcohol. This is the properties of ester as well as the preparation of alcohols. For preparation, we use $LiA{H}_4$ since they are stronger reducing agents than $HI$.We know that in $\text{R}-\text{O}-\text{R}$ there are two lone pairs in hydrogen making them highly reactive toward acidic substances.
The $HI$ dissociates into ions as shown below
$HI\to H^{+}+I^{-}$
The $H^{+}$ produced will reactivate the lone pairs of O-atoms in ethers. When considering our question we have a total of 4 oxygen atoms. The last two oxygen atoms are attached to the $-C{H}_3$ group. Those will have less steric hindrance to them making them more reactive towards $HI$
Now we will discuss what is meant by steric effect or steric hindrance, it happens when a large sized group prevents molecules from the chemical reaction happening. Since we have a large phenyl group attached to two of the oxygen atoms, it is less likely to get reacted with strong reducing agents. The last two of them will get reacted to $HI$ and the $I^{-}$ will combine with the large phenyl part at the $H^{+}$ will combine with $O-\text{C}{\text{H}}_3$ to form alcohol. This will happen at the both ends, hence we need two moles of $HI$

i.e., option B is correct

Note:
At first it may occur like 4 moles is required, but here we must not forget to consider the steric effect. If we do not consider it may look like we need 4 moles, which is wrong.