JEE Main 2021 · 25 July · morning · Single correct · +4 / −1
Which one of the following compounds will liberate CO₂, when treated with NaHCO₃ ?
- (a) (CH₃)₃NH ⊕ Cl^Θ
- (b) (CH₃)₄N ⊕ O^Θ H
- (c)
- (d) CH₃NH₂
Answer and solution
Answer (a) (CH₃)₃NH ⊕ Cl^Θ
Solution
Chemistry ma’am · 3 steps
NaHCO₃ gives off CO₂ only with an acid, so find the one compound that can donate a proton.
- What the test needs. HCO₃⁻ releases CO₂ only when it picks up a proton: HCO₃⁻ + H⁺ → H₂O + CO₂ ↑. So the correct compound must behave as an acid.
Compound What it is Acid or base? (CH₃)₃NH⁺ Cl⁻ Salt of the weak base (CH₃)₃N and the strong acid HCl Acidic: the N-H⁺ proton can be given away (CH₃)₄N⁺ OH⁻ Quaternary ammonium hydroxide Strong base, as strong as NaOH, with no N-H proton CH₃CONH₂ Acetamide Almost neutral, because the N lone pair is delocalised into C=O CH₃NH₂ Primary aliphatic amine Base - Why the salt works. In the trimethylammonium ion, nitrogen carries an extra proton. It hands this proton to bicarbonate, giving back the free amine: (CH₃)₃NH⁺ + HCO₃⁻ → (CH₃)₃N + H₂O + CO₂.
- So. Only (a) is a proton donor, so only (CH₃)₃NH⁺ Cl⁻ liberates CO₂ with NaHCO₃.
The idea to keep. An amine salt like R₃NH⁺ Cl⁻ is the acid partner of a weak base, so it can give away its N-H proton. Free amines, amides and quaternary hydroxides have no such proton to offer.
Answer from the NTA final answer key. Solution by PrepWiser.