JEE Main 2021 · 26 February · morning · Single correct · +4 / −1
Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : Dipole-dipole interactions are the only non-covalent interactions, resulting in hydrogen bond formation.
Reason R : Fluorine is the most electronegative element and hydrogen bonds in HF are symmetrical.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) Both A and R are true and R is the correct explanation of A
- (b) A is true but R is false
- (c) A is false but R is true
- (d) Both A and R are true but R is NOT the correct explanation of A
Answer and solution
Answer (c) A is false but R is true
Solution
Chemistry ma’am · 4 steps
A hydrogen bond is more than a dipole attraction, so the word "only" in A is what makes it false.
- Check Assertion A. A hydrogen bond is not a pure dipole-dipole attraction. Besides the electrostatic pull between H^(δ+) and the lone pair on F, O or N, it also has some partial covalent (charge-transfer) character and a dispersion contribution.
- Why that matters. That partial covalent character makes hydrogen bonds directional and stronger than ordinary dipole-dipole forces. So A, which says dipole-dipole interaction is the only contribution, is false.
- Check Reason R. Fluorine is the most electronegative element, which is true. Hydrogen bonding through fluorine is the strongest known. In the F–H–F unit of HF₂⁻, which forms in HF, the hydrogen sits exactly midway between the two F atoms, so the bond is symmetrical. R is true.
- So. A is false and R is true, so the answer is option (c).
The idea to keep. In assertion-reason questions, check each statement on its own first. Watch for absolute words like "only": a hydrogen bond is mostly electrostatic, but it is not purely dipole-dipole.
Answer from the NTA final answer key. Solution by PrepWiser.