Interpret the reaction with concentrated HI

Concentrated HI cleaves ethers: $R-O-R' \xrightarrow[\text{conc.}]{HI} RI + R'OH$ or, in some cases, $R-O-R' \to RI + R'I$

Since compound $A$ has formula $C_9H_{10}O$ and undergoes cleavage into two compounds $B$ and $C$, $A$ is most likely an ether.

Use the clue about compound $B$

$B$ gives a yellow precipitate with $AgNO_3$.

This indicates $B$ is an alkyl iodide, because: $RI + AgNO_3 \to AgI \downarrow$ and $AgI$ is yellow.

So one cleavage product is an iodide.

Use the clue about compound $C$

$C$ tautomerizes to $D$, and $D$ gives positive iodoform test.

A positive iodoform test is given by:

• methyl ketones: $RCOCH_3$
• acetaldehyde
• alcohols oxidizable to these

Since $C$ tautomerizes to $D$, $C$ is likely an enol, and $D$ is the corresponding carbonyl compound.

For iodoform positivity, $D$ should be a methyl ketone. A very common enol-carbonyl pair is: $CH_2=C(OH)CH_3 \rightleftharpoons CH_3COCH_3$

Here, the keto form is acetone, which gives positive iodoform test.

Thus, $C$ is likely prop-1-en-2-ol (or an equivalent enolic form), which tautomerizes to acetone.

Build ether $A$ from these fragments

Since cleavage with HI gives:

• one fragment as an iodide ($B$), and
• the other as an enol ($C$),

$A$ should be a vinyl/isopropenyl aryl ether of formula $C_9H_{10}O$.

Check the formula for isopropenyl phenyl ether: $C_6H_5-O-C(CH_3)=CH_2$

Count atoms:

• phenyl part: $C_6H_5$
• isopropenyl part: $C_3H_5$
• one oxygen

Total: $C_9H_{10}O$

This matches the given molecular formula.

Check cleavage of isopropenyl phenyl ether with HI

$C_6H_5-O-C(CH_3)=CH_2 \xrightarrow{HI} C_6H_5I + CH_2=C(OH)CH_3$ or equivalently cleavage gives an iodide-containing fragment and the enol fragment.

The enol: $CH_2=C(OH)CH_3 \rightleftharpoons CH_3COCH_3$

The keto form is acetone, which gives positive iodoform test.

Hence identify $A$

Therefore, $A$ must be the option corresponding to isopropenyl phenyl ether, which is Option C.

Comparison with stored answer

Derived answer = C.

Stored correct answer = C.

They agree.