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MHT-CET Chemistry · Amines

Physical Properties of Amines

Primary and secondary amines hydrogen-bond through N–H, tertiary amines cannot; so boiling points run primary > secondary > tertiary among isomers, amines sit below alcohols and far below carboxylic acids, and water solubility runs alcohol > amine > alkane.

All Amines notesMHT-CET Chemistry strategy

Why this matters

10 PYQs, none HARD — one page, one idea. Of the eight set from 2021 to 2025, two ask which C₄H₁₁N isomer boils highest (n-butylamine), two ask which compound has no intermolecular hydrogen bonding (trimethylamine), two ask the amine < alcohol < acid boiling-point order, two ask solubility (alcohol > amine > alkane; CH₄ least). One card.

Concept 1 of 1: N–H Hydrogen Bonding: Boiling Point and Solubility

An N–H bond can donate a hydrogen bond; an O–H bond does it better because oxygen is more electronegative; a tertiary amine has no N–H and cannot donate at all. That single fact ranks everything: primary above secondary above tertiary isomers, alcohols above amines, acids (dimers) above both, and alcohols dissolving better than amines, which dissolve better than alkanes.

Definition

  • Isomers of C₄H₁₁N: n-butylamine (1°, straight) > diethylamine (2°) > ethyldimethylamine (3°); tert-butylamine is low (1° but compact).
  • Comparable mass: amines < alcohols < carboxylic acids in boiling point.
  • No intermolecular H-bonding: trimethylamine (no N–H). Cyclohexylamine, allylamine, diphenylamine all have N–H.
  • Solubility in water: alcohols > amines > alkanes; lower amines are soluble, solubility falls with chain length; CH₄ is the least soluble of CH₄, CH₃NH₂, CH₃OH, C₂H₅OH.
  • Aniline is only slightly soluble (large hydrophobic ring).

Two orders

b.p.: 1∘>2∘>3∘ (isomers);amine<alcohol<acid;solubility: alcohol>amine>alkane\text{b.p.: } 1^\circ > 2^\circ > 3^\circ \text{ (isomers)};\quad \text{amine} < \text{alcohol} < \text{acid};\qquad \text{solubility: alcohol} > \text{amine} > \text{alkane}

Worked example

Rank propan-1-amine, propan-1-ol, propanoic acid and butane by boiling point, and name the one with no hydrogen bonding at all.
Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 2nd May Shift 1 · Q68Easy

Example 1 · Amines · Physical Properties of Amines

Which of the following isomers has highest boiling point?

Ranking amines above alcohols because N is 'more basic'

Basicity is not boiling point. O–H makes the stronger hydrogen bond, so alcohols boil higher and dissolve better; amines come second in both orders.
Drill 7 more on n–h hydrogen bonding: boiling point and solubility

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

  • N–H Hydrogen Bonding: Boiling Point and Solubility

    Two orders

    b.p.: 1∘>2∘>3∘ (isomers);amine<alcohol<acid;solubility: alcohol>amine>alkane\text{b.p.: } 1^\circ > 2^\circ > 3^\circ \text{ (isomers)};\quad \text{amine} < \text{alcohol} < \text{acid};\qquad \text{solubility: alcohol} > \text{amine} > \text{alkane}

Watch out for (1)

Test yourself on Amines

20 past MHT-CET questions from this chapter, timed at 18 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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