Giving reasons, explain which of the following can form hydrogen bonds with water? (a) CH
OCH
(b) CH
(c) F
(d) HCOOH (e) Na
(f) C
H
OH 3
The Correct Answer and Explanation is:
Of the options provided, the following can form hydrogen bonds with water:
(a) CH₃OCH₃, (c) F⁻, (d) HCOOH, and (f) C₂H₅OH
Explanation
A hydrogen bond is a special, strong type of dipole-dipole intermolecular force. For a hydrogen bond to form between two molecules, two conditions must be met:
- Hydrogen Bond Donor: One molecule must contain a hydrogen atom covalently bonded to a highly electronegative atom, specifically nitrogen (N), oxygen (O), or fluorine (F). This creates a highly positive partial charge (δ+) on the hydrogen atom.
- Hydrogen Bond Acceptor: The other molecule must have a lone pair of electrons on a nitrogen, oxygen, or fluorine atom. This lone pair is attracted to the partially positive hydrogen atom of the donor.
Water (H₂O) is an excellent participant in hydrogen bonding as its oxygen atom has two lone pairs (making it an acceptor), and it has two hydrogen atoms bonded to oxygen (making it a donor). Let’s analyze each substance:
- (a) CH₃OCH₃ (Dimethyl ether): This molecule cannot act as a hydrogen bond donor because all its hydrogen atoms are bonded to carbon, which is not sufficiently electronegative. However, the oxygen atom possesses two lone pairs of electrons. Therefore, it can act as a hydrogen bond acceptor, forming a hydrogen bond with a hydrogen atom from a water molecule.
- (b) CH₄ (Methane): This molecule cannot form hydrogen bonds. Its hydrogen atoms are bonded to carbon, so it cannot be a donor. Furthermore, it has no lone pairs on a highly electronegative atom, so it cannot be an acceptor.
- (c) F⁻ (Fluoride ion): The fluoride ion does not have a hydrogen atom to donate. However, as a small, highly electronegative ion, it possesses lone pairs of electrons and a full negative charge, making it a very strong hydrogen bond acceptor. It can readily form hydrogen bonds with the hydrogen atoms of surrounding water molecules.
- (d) HCOOH (Methanoic acid): This molecule can both donate and accept hydrogen bonds. The hydrogen atom in the hydroxyl group (-OH) is bonded to oxygen, so it can act as a donor. Additionally, both oxygen atoms (the carbonyl oxygen and the hydroxyl oxygen) have lone pairs, allowing the molecule to act as an acceptor.
- (e) Na⁺ (Sodium ion): This is a positive ion and has no hydrogen atoms or lone pairs. It cannot form hydrogen bonds. It interacts with water via a different force called an ion-dipole interaction, where its positive charge is attracted to the negative dipole (oxygen end) of the water molecule.
- (f) C₂H₅OH (Ethanol): Similar to methanoic acid, ethanol is an excellent partner for hydrogen bonding with water. The hydrogen atom of its hydroxyl group (-OH) can act as a donor, and the lone pairs on its oxygen atom allow it to act as an acceptor. This strong hydrogen bonding capability is why ethanol is completely miscible with water.
