Alcohols Phenols and Ethers Previous Year Questions — JEE Main Chemistry
20 past-year Alcohols Phenols and Ethers questions from JEE Main (Chemistry).
Q540 (2025)
Which of the following compounds is least likely to give effervescence of $\mathrm{CO}_{2}$ in presence of aq. $\mathrm{NaHCO}_{3}$ ?
- $\mathrm{Ph - NH_3^{(+)} Cl^{(-)}}$
### Core Logic
For a compound to release $\mathrm{CO}_2$ upon reaction with aqueous $\mathrm{NaHCO}_3$, its acidity must be strictly greater than that of carbonic acid ($\mathrm{H_2CO_3}$).
Let's evaluate the acidity of the options:
1. **Picric Acid** (2,4,6-trinitrophenol - Option A): Extremely acidic ($pK_a \approx 0.38$) due to three strong electron-withdrawing nitro groups. Reacts with $\mathrm{NaHCO}_3$ easily.
2. **4-Nitrobenzoic Acid** (Option B): Carboxylic acids generally have $pK_a \approx 4\text{--}5$. More acidic than carbonic acid ($pK_a \approx 6.3$). Reacts with $\mathrm{NaHCO}_3$.
3. **Anilinium Chloride** (Option 3): A salt of a strong acid and weak base. The anilinium ion is quite acidic ($pK_a \approx 4.6$) and easily decomposes $\mathrm{NaHCO}_3$.
4. **4-Nitrophenol** (Option D): Only one nitro group is present. Its acidity ($pK_a \approx 7.15$) is weaker than carbonic acid. Hence, it does not react with sodium bicarbonate to yield effervescence of $\mathrm{CO}_2$.
### Pattern Recognition
Acid-bicarbonate test shortcut:
- All carboxylic acids and picric acid give a positive bicarbonate test.
- Normal phenols and mono/di-nitrophenols are too weak to decompose bicarbonate.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 12 Chemistry: Alcohols, Phenols and Ethers
Class 12 Chemistry: Carboxylic Acids
Q31 (2025)
Given below are two statements:
Statement I: Dimethyl ether is completely soluble in water. However, diethyl ether is soluble in water to a very small extent.
Statement II: Sodium metal can be used to dry diethyl ether and not ethyl alcohol.
In the light of given statements, choose the correct answer from the options given below:
- $\text{Statement I is false but Statement II is true}$
- $\text{Both Statement I and Statement II are false}$
- $\text{Statement I is true but Statement II is false}$
- $\text{Both Statement I and Statement II are true}$
### Core Logic
Statement I: Dimethyl ether ($CH_3OCH_3$) is highly soluble in water because its smaller alkyl chain allows substantial hydrogen bonding with water molecules. In contrast, diethyl ether has a larger hydrophobic ethyl group which drastically reduces its water solubility (to about $7.5\text{ g}$ per $100\text{ mL}$). Thus, Statement I is true.
Statement II: Sodium metal ($Na$) reacts violently with alcohols like ethyl alcohol to release hydrogen gas:
$$2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2\uparrow$$
Since diethyl ether has no active acidic hydrogen, it does not react with sodium metal. Hence, sodium can dry diethyl ether but cannot be used for ethyl alcohol. Statement II is true.
### Pattern Recognition
Ethers are miscible with water primarily when the non-polar alkyl parts are very small. Active hydrogen presence ($-\text{OH}$ group in alcohols) prevents the use of alkali metals like sodium for moisture removal.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 12 Chemistry: Alcohols, Phenols and Ethers
Q587 (2025)
Which one of the following reactions will not lead to the desired ether formation in major proportion?
(Given labels: $\text{iso-Bu} \Rightarrow \text{isobutyl}$, $\text{sec-Bu} \Rightarrow \text{sec-butyl}$, $\text{n-Pr} \Rightarrow \text{n-propyl}$, ${}^t\text{Bu} \Rightarrow \text{tert-butyl}$, $\text{Et} \Rightarrow \text{ethyl}$)
- ${}^t\text{BuO}^-\text{Na}^+ + \text{EtBr} \longrightarrow {}^t\text{Bu-O-Et}$
- $\text{PhO}^-\text{Na}^+ + \text{CH}_3\text{Br} \longrightarrow \text{Ph-O-CH}_3$
- $\text{PhO}^-\text{Na}^+ + \text{n-PrBr} \longrightarrow \text{n-Pr-O-Ph}$
- $\text{iso-BuO}^-\text{Na}^+ + \text{sec-BuBr} \longrightarrow \text{sec-Bu-O-iso-Bu}$
### Core Logic
Williamson Ether Synthesis operates strictly via an **$S_N2$ mechanism**. To optimize ether formation yields, the alkyl halide component MUST be primary ($1^\circ$) or methyl to evade spatial shielding restrictions.
Let us review the alkyl halide across the options:
* Option 1: Ethyl bromide ($\text{EtBr}$) is $1^\circ$ $\rightarrow$ Excellent ether yield via substitution.
* Option 2: Methyl bromide ($\text{CH}_3\text{Br}$) is a unhindered methyl halide $\rightarrow$ High substitution efficiency.
* Option 3: n-Propyl bromide ($\text{n-PrBr}$) is $1^\circ$ $\rightarrow$ Clean substitution pathway.
* Option 4: sec-Butyl bromide ($\text{sec-BuBr}$) is a secondary ($2^\circ$) alkyl halide. When a sterically hindered $2^\circ$ halide is treated with a strongly basic alkoxide reagent like isobutoxide, **alkene elimination ($E2$) competes aggressively and dominates** as the major pathway over nucleophilic substitution ($S_N2$). {{SOL_IMG1}}
### Pattern Recognition
Williamson Synthesis Rule: Alkoxide can be as massive and complex as desired ($3^\circ$ or branched), but the Alkyl Halide MUST be unhindered ($1^\circ$ or methyl). If the halide is $2^\circ$ or $3^\circ$, elimination wins, forming an alkene instead of an ether.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 12 Chemistry: Alcohols, Phenols and Ethers
Q45 (2025)
Which one of the following, with HBr will give a phenol?
- Benzyl methyl ether option (1)
- Anisole (Methoxybenzene) option (2)
- Dimethyl ether option (3)
- Methyl phenyl ether derivative option (4)
### Core Logic
Anisole ($Ph-O-CH_3$) contains an aryl-oxygen bond which possesses partial double bond character due to resonance stabilization with the aromatic ring.
When treated with $HBr$, protonation yields an oxonium ion. The nucleophile $Br^{-}$ attacks via an $S_N2$ pathway at the smaller, less hindered methyl group, cleaving the $O-CH_3$ bond to form Phenol ($PhOH$) and $CH_3Br$.
{{SOL_IMG45}}
### Pattern Recognition
Aromatic $sp^2$ $C-O$ bonds are exceptionally strong and cannot be cleaved by nucleophilic attack from $Hal^{-}$. Therefore, the oxygen always stays attached to the benzene ring, yielding phenol.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 12 Chemistry: Alcohols, Phenols and Ethers
Q35 (2025)
A toxic compound “A” when reacted with NaCN in aqueous acidic medium yields an edible cooking component and food preservative “B”. “B” is converted to “C” by diborane and can be used as an additive to petrol to reduce emission. “C” upon reaction with oleum at $140^{\circ}\mathrm{C}$ yields an inhalable anesthetic “D”. Identify “A”, “B”, “C” and “D”, respectively.
- Methanol; formaldehyde; methyl chloride; chloroform
- Ethanol; acetonitrile; ethylamine; ethylene
- Methanol; acetic acid; ethanol; diethyl ether
- Acetaldehyde; 2- hydroxypropanoic acid; propanoic acid; dipropyl ether
### Core Logic
Let's track the conversions row by row based on the description:
1. Toxic compound **A** is methanol ($CH_3OH$). Reacting it via acidic NaCN sequences effectively converts it eventually into acetic acid ($CH_3COOH$), which is an edible cooking component and food preservative (vinegar) **B**.
2. Reduction of acetic acid (**B**) with diborane ($B_2H_6$) yields ethanol ($CH_3CH_2OH$) **C**. Ethanol is used as a biofuel additive in petrol.
3. Ethanol (**C**) upon heating with oleum/concentrated sulfuric acid at $140^{\circ}\mathrm{C}$ undergoes intermolecular dehydration to yield diethyl ether ($C_2H_5-O-C_2H_5$) **D**, which functions as an inhalable anesthetic.
### Step 1: Chemical Reaction Sequence Flow
{{SOL_IMG1}}
The full sequential progression corresponds exactly to:
$$\text{Methanol (A)} \rightarrow \text{Acetic Acid (B)} \rightarrow \text{Ethanol (C)} \rightarrow \text{Diethyl ether (D)}$$
### Pattern Recognition
Key temperature indicator: Dehydration of ethanol at $140^{\circ}\mathrm{C}$ yields diethyl ether (ether synthesis), whereas $170^{\circ}\mathrm{C}$ yields ethene gas. Combined with the food preservative clue (acetic acid), the sequence locks instantly to option (3).
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 12 Chemistry: Alcohols, Phenols and Ethers
