40. What is the principal function of haemoglobin as a protein?
[AIPMT Chemistry — 2004]
(A) Catalysing every biological reaction
(B) Maintaining blood sugar concentration
(C) Transporting oxygen in blood
(D) Forming antibodies against infections
Answer: (C) Transporting oxygen in blood
Explanation: Haemoglobin is the oxygen-binding protein of red blood cells. Its haem groups contain iron that reversibly binds oxygen in the lungs and releases it to tissues. Antibodies, metabolic enzymes and blood-glucose-regulating hormones are other proteins with different structures and functions.
39. Which interactions mainly stabilise the alpha-helical secondary structure of proteins?
[AIPMT Chemistry — 2004]
(A) Dipeptide bonds
(B) Hydrogen bonds
(C) Ether bonds
(D) Peptide bonds alone
Answer: (B) Hydrogen bonds
Explanation: The protein alpha helix is stabilised by hydrogen bonds between backbone carbonyl oxygen atoms and amide hydrogens of residues further along the chain. Peptide bonds create the underlying sequence, but regular backbone hydrogen bonding provides much of the helix’s secondary-structure stability.
38. How many chiral carbon atoms does cyclic beta-D-(+)-glucose contain?
[AIPMT Chemistry — 2004]
(A) Five
(B) Six
(C) Three
(D) Four
Answer: (A) Five
Explanation: Cyclisation of D-glucose produces a new chiral centre at the anomeric carbon, C1. Together with the four original asymmetric centres at C2 to C5, the cyclic glucose molecule has five stereogenic carbon atoms. The exocyclic CH₂OH carbon is not stereogenic.
37. How many nucleotides of messenger RNA together form one amino-acid codon?
[AIPMT Chemistry — 2004]
(A) Three
(B) Four
(C) One
(D) Two
Answer: (A) Three
Explanation: A codon consists of three consecutive RNA nucleotides, each bearing one of the four bases adenine, uracil, guanine or cytosine. Triplet combinations provide 64 codons, which specify amino acids or translation stops. This underlies the genetic code used during protein synthesis.
36. Which hormone favours the conversion of blood glucose into stored glycogen?
[AIPMT Chemistry — 2004]
(A) Cortisone
(B) Bile acids
(C) Adrenaline
(D) Insulin
Answer: (D) Insulin
Explanation: Insulin encourages uptake of glucose by suitable cells and stimulates glycogen synthesis, particularly in liver and muscle. Glycogen is a branched polymer of glucose used for energy storage. Adrenaline more commonly promotes glycogen breakdown to make glucose available rapidly.
35. Which organic cationic surfactant produces positively charged micelles?
[AIPMT Chemistry — 2004]
(A) Sodium dodecyl sulphate
(B) Sodium acetate
(C) Urea
(D) Cetyltrimethylammonium bromide
Answer: (D) Cetyltrimethylammonium bromide
Explanation: Cetyltrimethylammonium bromide contains a positively charged quaternary ammonium head and a long hydrocarbon tail. Above its critical micelle concentration, its molecules aggregate into micelles with cationic surfaces. Sodium dodecyl sulphate instead forms negatively charged micelles.
34. Which substrate reacts least readily in an ordinary nucleophilic substitution?
[AIPMT Chemistry — 2004]
(A) tert-Butyl chloride
(B) Vinyl chloride (CH₂=CHCl)
(C) Ethyl chloride
(D) Allyl chloride
Answer: (B) Vinyl chloride (CH₂=CHCl)
Explanation: In vinyl chloride the carbon bearing chlorine has sp² hybridisation, creating a strong carbon–chlorine bond. It resists conventional SN1 carbocation formation and SN2 backside attack. Allylic halides are comparatively reactive, while ordinary primary or tertiary halides can react through suitable mechanisms.
33. Excess methylmagnesium iodide reacts with acetyl bromide; after aqueous work-up, which product predominates?
[AIEEE Chemistry — 2004]
(A) Acetone
(B) Acetyl iodide
(C) 2-Methylpropan-2-ol
(D) Acetamide
Answer: (C) 2-Methylpropan-2-ol
Explanation: An acyl halide first reacts with a Grignard reagent to give a ketone intermediate, which reacts with a second equivalent to yield a tertiary alcohol after work-up. With an acetyl group and methylmagnesium iodide, the resulting alcohol is tert-butanol.
32. Arrange the acidity of benzoic acid (a), o-nitrobenzoic acid (b), p-nitrobenzoic acid (c), and m-nitrobenzoic acid (d).
[AIEEE Chemistry — 2004]
(A) b > d > a > c
(B) b > d > c > a
(C) a > b > c > d
(D) b > c > d > a
Answer: (D) b > c > d > a
Explanation: The nitro group withdraws electrons and stabilises the carboxylate conjugate base. The ortho isomer benefits from the pronounced ortho effect; para substitution allows both resonance and inductive withdrawal, whereas meta substitution operates mainly inductively. Unsubstituted benzoic acid is least acidic.
31. Which compound can exist as a meso stereoisomer?
[AIEEE Chemistry — 2004]
(A) 2-Chlorobutane
(B) 2-Hydroxypropanoic acid
(C) 2,3-Dichloropentane
(D) 2,3-Dichlorobutane
Answer: (D) 2,3-Dichlorobutane
Explanation: 2,3-Dichlorobutane possesses two stereogenic centres and a symmetric carbon framework. One configuration contains an internal plane of symmetry and is consequently optically inactive despite the chiral centres. The other listed compounds lack the required internal symmetry for this meso form.
