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Chemistry Question 68 – JEE-MAIN 2026

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Arrange the following compounds according to increasing order of boiling points. n-C4H9OH (A), n-C4H9NH2 (B), n-C4H10 (C) and C2H5NHC2H5 (D).

Boiling points are primarily determined by the strength of intermolecular forces present between molecules.

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Ninja Strategy•Functional Group Hierarchy

Quickly identify the functional groups (alkane, primary amine, secondary amine, alcohol) and recall the general hierarchy of intermolecular forces: alkane < secondary amine < primary amine < alcohol.

Step 1: Identify Intermolecular Forces and Molecular Weights✦ Active

Analyze each compound to determine its primary intermolecular forces and approximate molecular weight (MW):

(A) n-C4H9OH (n-butanol):Strong hydrogen bonding (O-H), dipole-dipole, LDFs. MW≈74 g/mol.(B) n-C4H9NH2 (n-butylamine):Hydrogen bonding (N-H), dipole-dipole, LDFs. MW≈73 g/mol.(C) n-C4H10 (n-butane):Only LDFs. MW≈58 g/mol.(D) C2H5NHC2H5 (diethylamine):Hydrogen bonding (N-H), dipole-dipole, LDFs. MW≈73 g/mol.
Step 2: Compare Strengths of Intermolecular Forces○ Expand

The general order of intermolecular force strength is hydrogen bonding > dipole-dipole > LDFs. Within hydrogen bonding, O-H is stronger than N-H due to oxygen's higher electronegativity. For similar molecular weights, primary amines (RNH2) form more extensive hydrogen bonding networks than secondary amines (R2NH), leading to higher boiling points. Also, straight-chain isomers generally have higher LDFs than branched isomers due to greater surface area for interaction.

💡 Teacher's Secret Hint

Remember that molecular weight also plays a role, but here, compounds A, B, and D have very similar molecular weights, making the type and extent of intermolecular forces the primary differentiating factor.

Step 3: Determine the Increasing Order of Boiling Points○ Expand

1. **C (n-C4H10):** As an alkane, it only exhibits LDFs and has the lowest molecular weight, thus it will have the lowest boiling point. 2. **A (n-C4H9OH):** As an alcohol, it forms the strongest hydrogen bonds (O-H), giving it the highest boiling point. 3. **B (n-C4H9NH2) and D (C2H5NHC2H5):** Both are amines with similar molecular weights. B is a primary amine with two N-H bonds, allowing for more extensive hydrogen bonding and better LDFs (straight chain) compared to D, a secondary amine with one N-H bond and some branching. Therefore, B will have a higher boiling point than D. Combining these, the increasing order of boiling points is: n-C4H10 (C)<C2H5NHC2H5 (D)<n-C4H9NH2 (B)<n-C4H9OH (A). This corresponds to option 3.

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