Which of the following statements is true?[Fill in the blank]

Question

BlackTom AI · Accepted Answer

Hydrogen bonding requires a hydrogen covalently bound to a strongly electronegative atom (commonly O–H or N–H) to act as a hydrogen bond donor, and an electronegative atom with a lone pair (O, N, etc.) to act as a hydrogen bond acceptor.

1) Relevant knowledge points and concepts
- Donor: a hydrogen attached to O or N (e.g., O–H, N–H) can donate an H-bond.
- Acceptor: lone pairs on electronegative atoms (O or N) can accept H-bonds.
- Functional groups considered:
  - Phenol (Ar–OH): has an O–H (donor) and oxygen lone pairs (acceptor).
  - Carboxylic acid (R–COOH): has O–H (donor) and carbonyl O lone pairs (acceptor).
  - Tertiary amine (R3N): has lone pair on N (acceptor) but no N–H (not a donor).
  - Ether (R–O–R): has lone pairs on O (acceptor) but no O–H (not a donor).

2) Step-by-step reasoning
- For option (c): Phenol contains an OH group → can donate H; carboxylic acid contains OH → can donate H. Therefore both can act as hydrogen-bond donors. That satisfies the donor requirement for both species.
- Test other options against definitions:
  - (a) Claims ethers and carboxylic acids are both unable to act as H-bond acceptors. This is false: ethers (lone pairs on O) are acceptors, and carboxylic acids have carbonyl O (and hydroxyl O) that can accept H-bonds.
  - (b) Claims carboxylic acids and tertiary amines can both act as H-bond donors. Carboxylic acids can, but tertiary amines lack N–H, so they cannot donate H-bonds → false.
  - (d) Claims tertiary amines and ethers are both unable to act as H-bond acceptors. Both have lone pairs (N or O) and therefore can act as acceptors → false.

3) Intermediate “checks”
- Phenol: OH present → donor = yes.
- Carboxylic acid: OH present → donor = yes.
- Tertiary amine: no N–H → donor = no.
- Ether: no O–H → donor = no.

4) Why the correct answer is correct
Because both phenols and carboxylic acids possess O–H groups that can donate hydrogen bonds, option (c) correctly identifies two functional groups that are hydrogen-bond donors.

5) Why other choices are incorrect
- (a) Incorrect: ethers and carboxylic acids can accept H-bonds (they have lone pairs).
- (b) Incorrect: tertiary amines cannot donate (no N–H).
- (d) Incorrect: tertiary amines and ethers do have lone pairs and so can act as H-bond acceptors.

6) Conclusion
The correct statement is c. Phenols and carboxylic acids can both act as hydrogen bond donors. This matches the provided answer.

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