60. Addition of one molecule of HBr to 1,3-butadiene at 40°C predominantly yields what?
[AIEEE Chemistry — 2005]
(A) 3-Bromobutene by kinetic control
(B) 1-Bromo-2-butene by thermodynamic control
(C) 3-Bromobutene by thermodynamic control
(D) 1-Bromo-2-butene by kinetic control
Answer: (B) 1-Bromo-2-butene by thermodynamic control
Explanation: Conjugated 1,3-butadiene can undergo both 1,2- and 1,4-addition. At relatively higher reaction temperatures the more stable 1,4-addition product is favoured under thermodynamic control. Its position of bromine and the remaining double bond distinguish it from the kinetically favoured product.
59. Which pair consists of two types of stereoisomerism?
[AIPMT Chemistry — 2005]
(A) Structural and geometric isomerism
(B) Optical and geometric isomerism
(C) Chain and rotational isomerism
(D) Linkage and geometric isomerism
Answer: (B) Optical and geometric isomerism
Explanation: Stereoisomers share the same connectivity but differ in the three-dimensional arrangement of atoms. Optical isomerism concerns chirality and enantiomers, while geometric isomerism involves restricted rotation or ring relationships. Chain, linkage and other structural isomerisms instead change molecular connectivity.
58. Which method was suggested for separating a mixture of naphthalene and benzoic acid?
[AIPMT Chemistry — 2005]
(A) Distillation
(B) Sublimation
(C) Chromatography
(D) Crystallisation
Answer: (B) Sublimation
Explanation: Naphthalene readily sublimes and is commonly separated from less volatile residues by sublimation. Benzoic acid can also sublime under certain conditions, so quantitative separation may require controlled conditions or acid-base extraction; nevertheless, sublimation is the keyed option among the choices in the examination.
57. What is the main class of molecules making up the structural framework of cell membranes?
[AIPMT Chemistry — 2005]
(A) Neutral fats
(B) Proteins alone
(C) Phospholipids
(D) Carbohydrates alone
Answer: (C) Phospholipids
Explanation: Phospholipids self-assemble into a bilayer because their hydrophilic head groups face water while hydrophobic fatty-acid tails pack inward. Membrane proteins and carbohydrates also occur, but the continuous barrier is primarily a lipid bilayer formed by these amphiphilic molecules.
56. Electrolytic reduction of nitrobenzene in a weakly acidic medium yields which compound?
[AIPMT Chemistry — 2005]
(A) N-Phenylhydroxylamine
(B) Nitrosobenzene
(C) Aniline
(D) p-Aminophenol
Answer: (D) p-Aminophenol
Explanation: In weakly acidic electrolytic reduction, nitrobenzene may undergo rearrangement through intermediate stages to yield p-aminophenol. Under other reducing conditions, aniline or intermediate nitroso and hydroxylamine derivatives may predominate. The p-aminophenol outcome depends critically on the medium and electrochemical conditions.
55. Which material is a polyamide rather than a polyester, fluoropolymer or phenolic resin?
[AIEEE Chemistry — 2005]
(A) Teflon
(B) Nylon-6,6
(C) Terylene
(D) Bakelite
Answer: (B) Nylon-6,6
Explanation: Nylon-6,6 is a condensation polyamide produced from a diamine and a dicarboxylic acid. Terylene is a polyester, Teflon is polytetrafluoroethylene, and Bakelite is a crosslinked phenol-formaldehyde resin. The chemical link connecting repeating units identifies the polymer family.
54. Which of these polymers has a fully fluorinated carbon backbone?
[AIEEE Chemistry — 2005]
(A) Neoprene
(B) Teflon
(C) Thiokol
(D) PVC
Answer: (B) Teflon
Explanation: Teflon is polytetrafluoroethylene, made from tetrafluoroethene units bearing fluorine on every backbone carbon. Its strong carbon-fluorine bonds give outstanding chemical stability. Neoprene contains chlorine, PVC has chlorine substituents, and Thiokol is associated with sulphur-containing elastomer networks.
53. Which oxidant best changes pent-3-en-2-ol into pent-3-en-2-one without damaging the alkene?
[AIEEE Chemistry — 2005]
(A) Acidic potassium permanganate
(B) Acidic potassium dichromate
(C) Chromic anhydride in glacial acetic acid
(D) Pyridinium chlorochromate (PCC)
Answer: (D) Pyridinium chlorochromate (PCC)
Explanation: PCC is a comparatively selective oxidant for converting secondary alcohols to ketones while often preserving other sensitive groups. The substrate contains an alkene that harsher oxidising reagents might attack. The product retains its carbon–carbon double bond and gains a ketone group.
52. Acid-catalysed hydration of ordinary alkenes other than ethene usually forms which class of alcohol?
[AIEEE Chemistry — 2005]
(A) Primary alcohol only
(B) Secondary or tertiary alcohol
(C) A required mixture of primary and secondary alcohols
(D) A required mixture of secondary and tertiary alcohols
Answer: (B) Secondary or tertiary alcohol
Explanation: Acid-catalysed alkene hydration generally follows Markovnikov addition through more stable carbocation-like intermediates. Thus propene gives a secondary alcohol, while suitably branched alkenes may yield tertiary alcohols. Ethene gives ethanol, which is primary, explaining the exception stated in the question.
51. How are free radicals with unpaired electrons generally characterised?
[AIEEE Chemistry — 2005]
(A) Chemically reactive
(B) Chemically inactive
(C) Always negatively charged
(D) Always positively charged
Answer: (A) Chemically reactive
Explanation: A free radical has at least one unpaired electron and commonly participates in chain reactions to attain more stable electron arrangements. Many radicals are short-lived and reactive, although some are resonance-stabilised. They need not carry a net positive or negative electrical charge.
