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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

8 PYQs, none HARD — one page, one idea. 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

Intuition

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.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1AminesEASY
Which of the following isomers has highest boiling point?

[Q68 · 2nd May Shift 1 · 2023]

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

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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)

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