50. Which of the following carboxylic acids has the smallest pKₐ?
[AIEEE Chemistry — 2005]
(A) Acetic acid
(B) Formic acid
(C) 2-Methylpropanoic acid
(D) Propionic acid
Answer: (B) Formic acid
Explanation: Formic acid has no electron-releasing alkyl substituent next to its carboxyl group. Alkyl substitution usually increases electron density and slightly destabilises the carboxylate anion, decreasing acidity. Thus formic acid is stronger and has a lower pKₐ than these alkyl-substituted acids.
49. Cyclohexanone reacts with dimethylamine under acid catalysis while water is removed. What type of product forms?
[AIEEE Chemistry — 2005]
(A) Schiff base
(B) Enamine
(C) Imine
(D) Saturated tertiary amine
Answer: (B) Enamine
Explanation: Dimethylamine is a secondary amine, so its condensation with cyclohexanone initially forms an iminium intermediate. Removal of a neighbouring proton gives a carbon–carbon double bond next to nitrogen, producing an enamine. Formation is favoured by continuously removing water.
48. Which class of medicines is chiefly used to reduce fever?
[AIEEE Chemistry — 2005]
(A) Analgesics
(B) Antipyretics
(C) Antibiotics
(D) Tranquilisers
Answer: (B) Antipyretics
Explanation: Antipyretics lower an elevated body temperature by acting on physiological pathways associated with fever. Certain medicines, such as paracetamol, have both antipyretic and analgesic effects. Antibiotics instead target infections, and tranquilisers treat different symptoms or conditions.
47. Which compound has the lowest boiling point?
[AIEEE Chemistry — 2004]
(A) n-Butane
(B) Isobutane
(C) 1-Butene
(D) 1-Butyne
Answer: (B) Isobutane
Explanation: Isobutane has a compact, branched molecular structure that generally reduces the effective surface area for London dispersion attractions relative to its straight-chain isomer. Among the four listed C₄ hydrocarbons, it therefore has the lowest normal boiling point.
46. What is the main product when HBr adds to propene in the presence of peroxide?
[AIPMT Chemistry — 2004]
(A) Isopropyl bromide
(B) 3-Bromopropane
(C) Allyl bromide
(D) n-Propyl bromide
Answer: (D) n-Propyl bromide
Explanation: Organic peroxides promote a radical pathway for addition of hydrogen bromide to propene. Bromine adds preferentially to the terminal carbon so that the intermediate carbon radical is more stable, yielding 1-bromopropane, also called n-propyl bromide, rather than 2-bromopropane.
45. With anhydrous AlCl₃ as catalyst, which reagent combines with benzene to form ethylbenzene?
[AIPMT Chemistry — 2004]
(A) Ethanol
(B) Propene
(C) Ethene
(D) Ethane
Answer: (C) Ethene
Explanation: Benzene can undergo Friedel–Crafts alkylation with ethene under appropriate catalysed conditions, introducing an ethyl substituent. Propene would introduce a propyl-derived group, while ethane is too unreactive for this electrophilic substitution. The target hydrocarbon is ethylbenzene.
44. Which substance is an example of a chain-growth polymer?
[AIPMT Chemistry — 2004]
(A) Starch
(B) Nucleic acid
(C) Polystyrene
(D) Protein
Answer: (C) Polystyrene
Explanation: Polystyrene is formed by chain-growth polymerisation of styrene molecules through their carbon-carbon double bonds. Starch, proteins and nucleic acids are naturally occurring condensation-linked biopolymers. Chain growth typically involves reactive centres that successively add monomer units to a growing chain.
43. Which alcohol fails to give the yellow iodoform precipitate with iodine and alkali?
[AIPMT Chemistry — 2004]
(A) Propan-2-ol
(B) Butan-2-ol
(C) Methanol
(D) Ethanol
Answer: (C) Methanol
Explanation: The iodoform test is positive for compounds containing or oxidisable to a methyl ketone group, including ethanol and secondary alcohols of the CH₃CH(OH)R type. Methanol does not have this structural feature and fails to produce yellow iodoform crystals.
42. Which reagent can replace the –OH group of an alcohol or –COOH group of a carboxylic acid by chlorine?
[AIPMT Chemistry — 2004]
(A) Phosphorus pentachloride
(B) Hypochlorous acid
(C) Chlorine gas
(D) Hydrochloric acid
Answer: (A) Phosphorus pentachloride
Explanation: Phosphorus pentachloride converts many alcohols to alkyl chlorides and carboxylic acids to acyl chlorides by replacing oxygen-containing functional groups. This substitution chemistry is characteristic of phosphorus chlorinating reagents. Direct chlorine gas or dilute hydrochloric acid is not the general reagent described.
41. Which compound can be oxidised to a corresponding carbonyl compound?
[AIPMT Chemistry — 2004]
(A) Propan-2-ol
(B) o-Nitrophenol
(C) Phenol
(D) 2-Methylpropan-2-ol
Answer: (A) Propan-2-ol
Explanation: Propan-2-ol is a secondary alcohol and oxidises to propanone, a ketone bearing a carbonyl group. Tertiary alcohols lack the hydrogen required at the carbinol carbon for ordinary oxidation, while phenols typically undergo different oxidation chemistry. Hence propan-2-ol fits the question.
