PHENOLS General methods of preparation, properties, reactions, and uses. Phenols – Acidic nature: NEET MCQs with Answers

11 free questions on PHENOLS General methods of preparation, properties, reactions, and uses. Phenols – Acidic nature for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Acidic nature, electrophilic substitution reactions: halogenation. nitration and sulphonation. Reimer - Tiemann reaction.

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Question 1

Previous-year question
Identify the major products P, Q and R in the sequence benzene + CH₃CH₂CH₂Cl/anhydrous AlCl₃ → P; P →(i) O₂ (ii) H₃O⁺/Δ → Q + R. [NEET 2018]
  1. propylbenzene, benzaldehyde and ethanol
  2. propylbenzene, benzaldehyde and benzoic acid
  3. cumene (isopropylbenzene), phenol and acetone
  4. cumene, phenol and propan-2-ol
Show answer
Answer: (C) cumene (isopropylbenzene), phenol and acetone
Explanation: Friedel–Crafts alkylation is accompanied by rearrangement of the primary cation to the secondary one, so P is cumene. Air oxidation gives cumene hydroperoxide, which acid then cleaves into phenol and acetone – the industrial cumene process.

Question 2

Previous-year question
In the reaction of phenol with CHCl₃ and NaOH to give salicylaldehyde, the electrophile involved is [NEET 2018]
  1. dichloromethyl cation
  2. formyl cation
  3. dichlorocarbene
  4. dichloromethyl anion
Show answer
Answer: (C) dichlorocarbene
Explanation: Hydroxide removes a proton from chloroform, and the resulting trichloromethyl anion loses chloride to give the neutral but electron-deficient species :CCl₂. This dichlorocarbene is the electrophile that attacks the phenoxide ring.

Question 3

Previous-year question
Which one is the most acidic compound? [NEET 2017]
  1. Phenol
  2. p-Nitrophenol
  3. 2,4,6-Trinitrophenol
  4. p-Cresol
Show answer
Answer: (C) 2,4,6-Trinitrophenol
Explanation: Each nitro group at an ortho or para position withdraws electron density and delocalises the negative charge of the phenoxide ion. With three such groups, picric acid is by far the strongest acid of the four – comparable with mineral acids.

Question 4

Previous-year question
The reaction of phenol with chloroform in the presence of dilute sodium hydroxide finally introduces which functional group? [AIPMT 2015]
  1. –CH₂Cl
  2. –COOH
  3. –CHCl₂
  4. –CHO
Show answer
Answer: (D) –CHO
Explanation: This is the Reimer–Tiemann reaction. Dichlorocarbene attacks the ring to give the benzal chloride derivative, and alkaline hydrolysis of that –CHCl₂ group finally produces the aldehyde –CHO of salicylaldehyde.

Question 5

Previous-year question
Which of the following will NOT be soluble in sodium hydrogen carbonate? [AIPMT 2014]
  1. 2,4,6-Trinitrophenol
  2. Benzoic acid
  3. o-Nitrophenol
  4. Benzenesulphonic acid
Show answer
Answer: (C) o-Nitrophenol
Explanation: NaHCO₃ dissolves only acids stronger than carbonic acid. Picric acid, benzoic acid and benzenesulphonic acid all clear that bar, but o-nitrophenol is still too weak an acid, so it stays undissolved.

Question 6

Arrange the following phenols in increasing order of their acid dissociation constant (Ka): (P) 2-nitrophenol, (Q) 2,4,6-trinitrophenol, (R) 3-nitrophenol, (S) 2,4-dinitrophenol.
  1. R < P < S < Q
  2. R < S < P < Q
  3. P < R < S < Q
  4. S < R < P < Q
Show answer
Answer: (A) R < P < S < Q
Explanation: A nitro group at the ortho or para position delocalises the negative charge of the phenoxide ion onto its own oxygens by resonance, whereas a meta nitro group acts only through its –I effect. So 3-nitrophenol is weaker than 2-nitrophenol. Every additional ortho or para nitro group increases acidity further, so 2,4-dinitrophenol is stronger and 2,4,6-trinitrophenol is the strongest. The order is R < P < S < Q.

Question 7

Which of the following compounds has the lowest pKa value?
  1. Benzyl alcohol
  2. Cyclohexanol
  3. 3-Bromophenol
  4. Phenol
Show answer
Answer: (C) 3-Bromophenol
Explanation: Phenols are much more acidic than alcohols because the phenoxide ion is stabilised by resonance with the ring, while alkoxide ions from cyclohexanol and benzyl alcohol are not. In 3-bromophenol the bromine withdraws electrons by its –I effect and further stabilises the phenoxide ion. Hence it is the strongest acid and has the lowest pKa.
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Question 8

Which statement about carbolic acid is correct?
  1. It is phenol, and coal tar remains the only commercial source of it today
  2. It is methanol, and it was first obtained by the destructive distillation of wood
  3. It is benzoic acid, and it was first isolated from coal tar because it contains a –COOH group
  4. It is phenol, which was first isolated from coal tar and is now produced commercially by synthesis
Show answer
Answer: (D) It is phenol, which was first isolated from coal tar and is now produced commercially by synthesis
Explanation: Carbolic acid is another name for phenol, and it has an –OH group on the benzene ring, not a –COOH group. It was first isolated from coal tar in the early nineteenth century. Today phenol is produced commercially by synthetic methods.

Question 9

In Kolbe's reaction, phenol is first treated with sodium hydroxide before carbon dioxide is introduced. What is the reason for this step?
  1. The phenoxide ion is less reactive than phenol, which prevents more than one carboxyl group from entering the ring
  2. The phenoxide ion formed is even more reactive than phenol towards electrophilic substitution, so it can react with the weak electrophile CO₂
  3. Sodium hydroxide converts CO₂ into a strong electrophile, which then attacks the meta position of phenol
  4. Sodium hydroxide removes the –OH group from phenol, so that CO₂ can bond to the ring at that carbon
Show answer
Answer: (B) The phenoxide ion formed is even more reactive than phenol towards electrophilic substitution, so it can react with the weak electrophile CO₂
Explanation: Sodium hydroxide converts phenol into the phenoxide ion, which is even more reactive than phenol towards electrophilic aromatic substitution. Carbon dioxide is only a weak electrophile, so this higher reactivity is needed. Ortho-hydroxybenzoic acid is formed as the main product after acidification.

Question 10

The carbon–oxygen bond is about 136 pm long in phenol and about 142 pm long in methanol. Which option correctly accounts for the shorter bond in phenol?
  1. Intermolecular hydrogen bonding in phenol compresses the bond, and the attached carbon is sp³ hybridised
  2. The ring releases electrons to oxygen by a +I effect, and the oxygen atom in phenol is sp hybridised
  3. Oxygen has no lone pairs left in phenol, and its O–H bond is shorter than the O–H bond in methanol
  4. Conjugation of an oxygen lone pair with the ring gives the bond partial double-bond character, and the attached carbon is sp² hybridised
Show answer
Answer: (D) Conjugation of an oxygen lone pair with the ring gives the bond partial double-bond character, and the attached carbon is sp² hybridised
Explanation: One lone pair on oxygen is delocalised into the aromatic ring, so the C–O bond acquires partial double-bond character. The carbon bearing the –OH group is sp² hybridised, and an sp² carbon forms a shorter bond than the sp³ carbon of methanol. Both effects shorten the bond.

Question 11

Phenol is oxidised with sodium dichromate and sulphuric acid. Which description of the organic product is correct?
  1. Benzene-1,2-diol, in which the ring remains aromatic
  2. Benzene-1,4-dione, a conjugated diketone in which the ring is no longer aromatic
  3. Benzene-1,4-diol, in which the ring remains aromatic
  4. Benzoic acid, in which the ring remains aromatic
Show answer
Answer: (B) Benzene-1,4-dione, a conjugated diketone in which the ring is no longer aromatic
Explanation: Chromic acid oxidation of phenol gives benzoquinone, a six-membered ring with two C=O groups para to each other and two C=C bonds. The ring no longer has a continuous aromatic sextet, so quinones are conjugated diketones and not phenols. The diols are the reduced forms of quinones, and no carbon is added to the ring to form an acid.

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