Animated Solution for Chemistry - Organic Chemistry: The organic compound that gives following qualitative analysis is $\begin{array}{ll}\text{Test}& \text{Inference}\\ \text{(i) Dil. HCl}& \text{Insoluble}\\ \text{(ii) NaOH solution}& \text{Soluble}\\ \text{(iii) }{\text{Br}}_2\text{ /water}& \text{Decolourisation}\end{array}$

Imagine you are a chemical detective, and you've just been handed a mystery organic compound. Your mission? To identify it from a lineup of four suspects: Aniline, Phenol, Cyclohexylamine, and Cyclohexanol.

To crack this case, you are given three crucial clues in the form of qualitative chemical tests. Let's break down the evidence step by step and eliminate the suspects until only the true culprit remains.

The Acid Test

The first piece of evidence states that our mystery compound is insoluble in dilute $HCl$.

Now, think about the fundamental principles of acid-base chemistry. $HCl$ is a strong acid. If our compound were basic, it would readily react with the acid to form a soluble salt. Amines, which contain the $-N{H}_2$ group, are classic organic bases.

Because our compound does not dissolve in $HCl$, it cannot be an amine. This immediately clears Aniline and Cyclohexylamine from our suspect list. We are now down to two possibilities: Phenol and Cyclohexanol.

The Base Test

Moving on to the second clue: the compound is soluble in $NaOH$ solution.

Sodium hydroxide ($NaOH$) is a strong base. For an organic compound to dissolve in it, the compound must possess acidic properties. Let's compare our remaining suspects.

Phenol is weakly acidic. When it loses a proton, it forms a phenoxide ion, which is highly stabilized by resonance within the benzene ring. This stability drives the reaction forward, allowing Phenol to dissolve in $NaOH$.

Cyclohexanol, on the other hand, is a simple aliphatic alcohol. Its conjugate base is not stabilized by resonance, making it an extremely weak acid—even weaker than water! Therefore, it cannot react with $NaOH$ and remains insoluble.

This crucial difference eliminates Cyclohexanol. Phenol is now our prime suspect!

The Bromine Water Confirmation

To close the case, we look at the final clue: the decolourisation of bromine water ($B{r}_2/H_{2}O$).

Phenol is known for its highly activated benzene ring. The $-OH$ group is strongly electron-donating via the resonance effect ($+R$), which increases the electron density on the ring, particularly at the ortho and para positions.

When Phenol is treated with aqueous bromine, it undergoes a rapid electrophilic aromatic substitution without the need for a Lewis acid catalyst.

This reaction produces 2,4,6-tribromophenol, which crashes out of the solution as a white precipitate. Simultaneously, the characteristic reddish-brown colour of the bromine water is completely discharged (decolourised).

Every single piece of evidence aligns perfectly. The insolubility in acid confirms it's not an amine, the solubility in base confirms its acidic nature, and the bromine water test highlights its highly activated aromatic ring.

The mystery compound is undeniably Phenol!