NEET 2018 Chemistry Questions (Part 5: Q41 to Q45)

NEET 2018 Chemistry Questions: Part 5 (Q41 to Q45)

41. The compound A on treatment with Na gives B, and with CH3COCl gives C. B and C react together to give ethylbenzene. A is:

• (a) C6H5CHO
• (b) C6H5CH2Cl
• (c) C6H5CH3
• (d) C6H5OH

Answer: (b) C6H5CH2Cl

Explanation: Compound A must be benzyl chloride (C6H5CH2Cl). It reacts with Na in dry ether (Wurtz reaction) to form B = C6H5CH2CH2C6H5 and with CH3COCl (acyl chloride) to give C = C6H5CH2COCH3. These can undergo Friedel-Crafts type reaction to give ethylbenzene.

42. In which of the following compounds, the C–Cl bond is most reactive towards hydrolysis by SN1 mechanism?

• (a) CH3CH2Cl
• (b) (CH3)3CCl
• (c) CH3CHClCH3
• (d) C6H5CH2Cl

Answer: (b) (CH3)3CCl

Explanation: SN1 reactions proceed via carbocation formation. Tertiary carbocations are most stable. (CH3)3CCl forms a stable 3° carbocation, so it undergoes hydrolysis fastest.

43. Consider the following species: CN^-, CN, NO, and CN⁺. The order of increasing bond length is:

• (a) CN^- < CN < NO < CN⁺
• (b) CN < CN^- < CN⁺ < NO
• (c) CN⁺ < CN < NO < CN^-
• (d) NO < CN < CN⁺ < CN^-

Answer: (c) CN⁺ < CN < NO < CN^-

Explanation: Bond length ∝ 1/bond order. CN⁺ has the highest bond order, followed by CN, NO, and CN^-. So, shortest bond length in CN⁺ and longest in CN^-.

44. Which one of the following has the maximum number of atoms?

• (a) 1 g of O₂
• (b) 1 g of H₂
• (c) 1 g of Li
• (d) 1 g of Ag

Answer: (b) 1 g of H₂

Explanation: Number of atoms = (mass / molar mass) × Avogadro’s number × atoms per molecule. For H₂: (1/2) × N_A × 2 = N_A → maximum atoms

45. Identify the major product in the following reaction:
CH₃–CH=CH–CH₂–CH₃ + HBr → (peroxide)

• (a) CH₃–CHBr–CH₂–CH₂–CH₃
• (b) CH₃–CH₂–CHBr–CH₂–CH₃
• (c) CH₃–CH₂–CH₂–CHBr–CH₃
• (d) CH₃–CH₂–CH₂–CH₂–CH₂Br

Answer: (b) CH₃–CH₂–CHBr–CH₂–CH₃

Explanation: In presence of peroxide, HBr adds via anti-Markovnikov mechanism (free radical addition). Br attaches to less substituted carbon = CH₂.