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MHT-CET Chemistry · Alcohols, Phenols and Ethers

Physical Properties of Alcohols, Phenols and Ethers

Hydrogen bonding sets the pattern: alcohols and phenols boil high and dissolve in water, ethers and alkanes do neither; within the alcohols boiling point rises with chain length and falls with branching, and among the nitrophenols the para isomer, with intermolecular H-bonds, melts highest.

All Alcohols, Phenols and Ethers notesMHT-CET Chemistry strategy

Why this matters

26 PYQs, none HARD. Of the seventeen set from 2021 to 2025, eight rank boiling points — the four butanols by branching, methanol as the lowest alcohol, an alkane below an ether below an alcohol, methoxyethane as the only gaseous ether; five rank solubility (phenol most, alcohol > amine > alkane) or name the force in ethylene glycol; four are the nitrophenol melting-point order and which drawn compound is not a phenol. Three cards.

Concept 1 of 3: Boiling Points: H-Bonding, Chain Length, Branching

Alcohols boil far above alkanes and ethers of similar mass because their OH groups hydrogen-bond. Within the alcohols a longer chain means more van der Waals contact and a higher boiling point; branching makes the molecule compact and lowers it. So among the C₄ isomers n-butyl > sec-butyl > iso-butyl > tert-butyl, and methanol is the lowest-boiling alcohol.

Definition

  • Same formula, comparable mass: alkane < ether < alcohol < carboxylic acid — n-butane (0 °C) < methoxyethane (8 °C) < propan-1-ol (97 °C) < ethanoic acid (118 °C).
  • Butanols: n-butyl (118) > sec-butyl (100) > iso-butyl (108 — the paper's key order puts sec above iso) > tert-butyl (83 °C); tert-butyl alcohol is the lowest, butan-1-ol the highest.
  • Homologues: methanol < ethanol < propanol < butanol.
  • Ethers: methoxyethane (b.p. 8 °C) is the only common ether that is a GAS at room temperature; ethoxyethane boils at 35 °C.
  • Phenols: boiling points rise with molecular mass; pure phenol is a low-melting (41 °C), toxic solid with a carbolic smell — NOT odourless or high-melting.

Boiling-point levers

H-bonding↑, chain length↑⇒b.p.↑;branching↑⇒b.p.↓\text{H-bonding} \uparrow,\ \text{chain length} \uparrow \Rightarrow \text{b.p.} \uparrow;\qquad \text{branching} \uparrow \Rightarrow \text{b.p.} \downarrow

Worked example

Rank pentan-1-ol, 2-methylbutan-2-ol, pentane and 1-methoxybutane by boiling point.
Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 9th May Shift 2 · Q76Easy

Example 1 · Alcohols, Phenols and Ethers · Physical Properties of Alcohols, Phenols and Ethers

Which isomer of C4_4H9_9OH has lower boiling point?

Ranking iso-butyl above sec-butyl

The measured values put isobutyl (108 °C) above sec-butyl (100 °C), but the paper's keyed order is n > sec > iso > tert — 'branching lowers boiling point' counted by the position of the branch. Give the paper's order; the endpoints (n highest, tert lowest) are never in doubt.
Drill 6 more on boiling points: h-bonding, chain length, branching

Concept 2 of 3: Solubility in Water and the Kinds of Hydrogen Bond

Solubility follows the ability to hydrogen-bond with water: alcohols best, amines weaker (N is less electronegative), alkanes not at all. Phenol dissolves appreciably; a nitro or methyl group on it cuts solubility. Two OH groups on neighbouring carbons — ethylene glycol — hydrogen-bond to each other inside the molecule.

Definition

  • Comparable mass: alcohol > amine > alkane in water solubility; methane is the least soluble of CH₄, CH₃OH, C₂H₅OH, CH₃NH₂.
  • Lower alcohols are miscible; solubility falls as the alkyl chain grows.
  • Phenol is the most water-soluble of phenol, p-cresol, o-nitrophenol, p-nitrophenol, tert-butyl alcohol (as keyed) — the substituents add hydrophobic bulk or tie up the OH.
  • Ethylene glycol: intramolecular hydrogen bonding between its two OH groups (as keyed); o-nitrophenol likewise, between OH and NO₂.
  • Phenols are polar and show appreciable water solubility.

Solubility order

R-OH>R-NH2>R-H(H-bonding with water decides)\text{R-OH} > \text{R-NH}_2 > \text{R-H} \quad (\text{H-bonding with water decides})

Worked example

Arrange butan-1-ol, butanamine and butane by water solubility, and say which of ethanol and hexan-1-ol is more soluble.
Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 10th May Shift 2 · Q77Easy

Example 2 · Alcohols, Phenols and Ethers · Physical Properties of Alcohols, Phenols and Ethers

Identify correct decreasing order of solubilities of alcohols, alkanes and amines in water having comparable molar mass.

Ranking the amine above the alcohol

N–H···O bonds are weaker than O–H···O. Amines dissolve, but less than alcohols of the same size. The alkane, with no H-bonding, is always last.
Drill 4 more on solubility in water and the kinds of hydrogen bond

Concept 3 of 3: p-Nitrophenol Melts Highest: Inter- Versus Intramolecular H-Bonds

In o-nitrophenol the OH hydrogen-bonds to its own nitro group, so molecules hold each other weakly — low melting point, volatile in steam. In p-nitrophenol the groups are too far apart, so the OH bonds to NEIGHBOURING molecules — high melting point, non-volatile. Phenol and p-cresol sit in between and below.

Definition

  • Melting points: p-nitrophenol (114 °C) > o-nitrophenol (45) > phenol (41) > p-cresol (35) — p-nitrophenol highest in every version of the question.
  • o-Nitrophenol: intramolecular H-bond, steam-volatile, less soluble. p-Nitrophenol: intermolecular H-bonds, higher b.p. and m.p., more soluble.
  • A phenol has OH ON the ring: o-nitrophenol, 2-naphthol, o-bromophenol are phenols; benzyl alcohol (OH on a side-chain CH₂) is NOT.
  • The same inter/intra logic explains why salicylaldehyde and o-hydroxybenzoic acid are steam-volatile and their para isomers are not.

Which H-bond

ortho: intramolecular⇒low m.p., volatile;para: intermolecular⇒high m.p.\text{ortho: intramolecular} \Rightarrow \text{low m.p., volatile};\qquad \text{para: intermolecular} \Rightarrow \text{high m.p.}

Worked example

Which of o- and p-hydroxybenzaldehyde is steam-volatile, and why does that one also melt lower?
Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2021 · May Shift 1 · Q86Moderate

Example 3 · Alcohols, Phenols and Ethers · Physical Properties of Alcohols, Phenols and Ethers

Which among the following compounds has the highest melting point?

Picking o-nitrophenol for the highest melting point

Its hydrogen bond is INSIDE the molecule and does nothing to hold molecules together. p-Nitrophenol, whose OH must bond to a neighbour, melts almost 70 °C higher.
Drill 3 more on p-nitrophenol melts highest: inter- versus intramolecular h-bonds

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 (3)

  • Boiling Points: H-Bonding, Chain Length, Branching

    Boiling-point levers

    H-bonding↑, chain length↑⇒b.p.↑;branching↑⇒b.p.↓\text{H-bonding} \uparrow,\ \text{chain length} \uparrow \Rightarrow \text{b.p.} \uparrow;\qquad \text{branching} \uparrow \Rightarrow \text{b.p.} \downarrow
  • Solubility in Water and the Kinds of Hydrogen Bond

    Solubility order

    R-OH>R-NH2>R-H(H-bonding with water decides)\text{R-OH} > \text{R-NH}_2 > \text{R-H} \quad (\text{H-bonding with water decides})
  • p-Nitrophenol Melts Highest: Inter- Versus Intramolecular H-Bonds

    Which H-bond

    ortho: intramolecular⇒low m.p., volatile;para: intermolecular⇒high m.p.\text{ortho: intramolecular} \Rightarrow \text{low m.p., volatile};\qquad \text{para: intermolecular} \Rightarrow \text{high m.p.}

Watch out for (3)

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