Hydrogen bonding is said to be formed, -when sightly acidic hydrogen-atom attached to a strongly, electronegative fluorine, oxygen or nitrogen atom. is held with weak. electrostatic forces by the non-bonded pair of electrons of another atom. The co-ordination number of hydrogen in such cases is two. It acts as a bridge between two atoms, to one of which it is covalently bonded and to other attached through electrostatic forces, also called hydrogen bond.
Though the hydrogen atoms in a methyl group are not polarised, if an electronegative group like chloro, carbonyl, nitro or cyano (in order to increase electronegativity) is attached to it, the C-H bond gets polarised due to the inductive effect and the hydrogen atom becomes slightly acidic resulting in the formation of weak hydrogen bonds. Though a weak bond the H-bond effects is large number of the physical properties of compounds some of which are
- Boiling points of liquids
- Solubility of polar compounds in polar solvents (containig H attached with strong electronegative atom)
- Viscosity of liquids .
Acidity
Which of the following combinations can involve hydrogen bonding
I) Mixture of KF and HF `" "` II) Mixture of `CH_(3)COCH_(3)` and `CHCI_(3)`
III) Mixture of `NH_(4) CI ` and `H_(2)O" "` IV) Mixture of `CH_(3)` and `H_(2)O`

Options

A. High value of `pKa_(2)` for maleate acid ion `((CHCOO^(-)),(||),(CHCOOH),((CH_(3))PO_(4)))` as compared to fumarate ion `((CHCOO^(-)),(||),(CHCOOH))`
B. High viscosity of `H_(3)PO_(4)` compared with `(CH_(3))PO_(4)`
C. High volatility of ortho-nitrophenol compared with para-isomer
D. Stability of chloral hydrate `["CC1"_(3) CH(OH)_(2)]` compared with `CH_(3)CH(OH)_(2)`

Answer

B