Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 7.4 Predicting hydrogen bonding-Study Notes - New Syllabus

7.4 Predicting Hydrogen Bonding in Molecules

To predict whether a molecule shows hydrogen bonding, you must check for specific structural features similar to \( \mathrm{H_2O} \), \( \mathrm{NH_3} \), and \( \mathrm{HF} \).

Essential Conditions

• Hydrogen must be directly bonded to:
  • \( \mathrm{N} \), \( \mathrm{O} \), or \( \mathrm{F} \)
• A neighbouring molecule must have:
  • A lone pair on \( \mathrm{N} \), \( \mathrm{O} \), or \( \mathrm{F} \)

Prediction Rule 

If a molecule contains H–N, H–O, or H–F bonds, it can form hydrogen bonds.

Examples That DO Show Hydrogen Bonding

• \( \mathrm{CH_3OH} \) (methanol) → contains O–H
• \( \mathrm{C_2H_5OH} \) (ethanol) → contains O–H
• \( \mathrm{CH_3NH_2} \) (methylamine) → contains N–H
• \( \mathrm{HF} \) → contains H–F

Examples That DO NOT Show Hydrogen Bonding

• \( \mathrm{CH_4} \) → no N, O, F
• \( \mathrm{HCl} \) → H–Cl not strong enough
• \( \mathrm{CH_3OCH_3} \) → has O but no O–H bond

Important Insight

• Having N, O, or F alone is not enough.
• Must have H directly bonded to them.

Intermolecular vs Intramolecular

• Usually intermolecular (between molecules).
• Occasionally intramolecular (within same molecule).

Effect on Properties

• Higher boiling points.
• Greater solubility in water.

Step-by-Step Prediction Method

• Step 1: Look for N, O, or F.
• Step 2: Check if H is bonded to them.
• Step 3: Confirm presence of lone pairs.
• Step 4: Conclude hydrogen bonding possible.

Therefore, hydrogen bonding can be predicted by identifying specific bonding patterns in molecules.