NEET Chemistry — Set 3 (Questions 21–30)
Advanced Organic & Biomolecules — mechanisms, reagents, functional group transformations and biochemical basics. Click Check / Show solution to view short English + Hinglish explanations.
Q21. Which reagent converts an alcohol to corresponding alkyl chloride most efficiently (without rearrangement)?
(Organic synthesis)
Answer — (a) SOCl₂ (with pyridine)
English: Thionyl chloride (SOCl₂) with pyridine converts alcohols to alkyl chlorides via an S_N2-like pathway with minimal rearrangement and gives gaseous by-products (SO₂, HCl) that escape—clean reaction.
Hinglish: SOCl₂ + pyridine se alcohol → alkyl chloride achchhi tarah bina rearrangement ke banta hai; product clean milta hai.
Q22. The Hoffmann degradation of amides yields:
(Reaction type)
Answer — (a) Primary amine with one fewer carbon
English: Hoffmann degradation (using Br₂/NaOH) converts amide → amine with loss of one carbon (decarbonylation) — useful to shorten chain by one carbon.
Hinglish: Hoffmann method se amide se ek-carbon kam wala primary amine milta hai (Br₂/NaOH). (विकल्प a)।
Q23. Which amino acid is zwitterionic at physiological pH and has a side chain that can be phosphorylated?
(Biomolecules)
Answer — (b) Serine
English: Serine has a –CH₂–OH side chain that can be phosphorylated by kinases. At physiological pH, amino acids exist as zwitterions (NH₃⁺ and COO⁻).
Hinglish: Serine ka side-chain –OH ko phosphorylate kiya ja sakta hai; aur amino acids physiological pH par zwitterion form mein hote hain. (विकल्प b)।
Q24. Reaction: \( \ce{R-CH2-CH2-Br ->[Mg] R-CH2-CH2-MgBr ->[CO2][H+] R-CH2-CH2-COOH} \). This sequence is an example of:
(Organometallic chemistry)
Answer — (b) Carbon chain extension via carboxylation
English: Alkyl halide → Grignard reagent → reaction with CO₂ gives carboxylate after acid workup — a one-carbon carboxylation (chain extension).
Hinglish: Grignard banake CO₂ se react karvane par carboxylic acid milta hai — ek-carbon ka extension. (विकल्प b)।
Q25. Which is the correct order of reactivity toward nucleophilic substitution S_N1 (fastest → slowest)?
(Reaction mechanisms)
Answer — (a) Tertiary > secondary > primary
English: S_N1 proceeds via carbocation intermediate; more substituted carbocations are more stable → tertiary fastest.
Hinglish: S_N1 me carbocation stability important hai. Tertiary sabse stable ⇒ sabse tezz. (विकल्प a)।
Q26. Benedict's test is positive for which of the following sugars?
(Carbohydrates)
Answer — (b) Glucose
English: Benedict's reagent detects reducing sugars (free aldehyde or keto form). Glucose is reducing; sucrose is non-reducing (unless hydrolysed).
Hinglish: Glucose reducing sugar hai ⇒ Benedict positive. (विकल्प b)।
Q27. Which reagent selectively oxidizes primary alcohol to aldehyde (not carboxylic acid)?
(Selective oxidation)
Answer — (b) PCC
English: PCC in CH₂Cl₂ oxidizes primary alcohols to aldehydes without further oxidation to carboxylic acids under controlled non-aqueous conditions.
Hinglish: PCC mild oxidant hai — primary alcohol → aldehyde (overoxidation nahi hota). (विकल्प b)।
Q28. Which vitamin is fat-soluble?
(Biochemistry)
Answer — (c) Vitamin A
English: Vitamins A, D, E, K are fat-soluble. Vitamin C and B-complex are water-soluble.
Hinglish: Vitamin A fat-soluble group ka hai. (विकल्प c)।
Q29. Electrophilic aromatic substitution on anisole (methoxybenzene) mainly gives substitution at:
(Aromatic chemistry)
Answer — (a) Ortho & para
English: Methoxy (–OCH₃) is an electron-donating group by resonance → activates ring and directs ortho/para in electrophilic substitution.
Hinglish: –OCH₃ resonance se ring activate karta hai ⇒ o/p directing. (विकल्प a)।
Q30. In peptide linkage, the bond between amino acids is formed between:
(Biochemistry)
Answer — (a) Amino of one and carboxyl of another
English: Peptide bond (amide bond) forms between –COOH of one amino acid and –NH₂ of another with loss of H₂O (condensation).
Hinglish: Peptide linkage mein ek amino acid ka –COOH aur doosre ka –NH₂ jodte hain (condensation se water nikalta hai). (विकल्प a)।
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