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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

17 PYQs, none HARD. 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

Intuition

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.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Alcohols, Phenols and EthersMODERATE
Among alcohols, which of the following is correct order of boiling point?

[Q99 · 10th May Shift 2 · 2023]

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 7 more on boiling points: h-bonding, chain length, branching

Concept 2 of 3

Solubility in Water and the Kinds of Hydrogen Bond

Intuition

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.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Alcohols, Phenols and EthersEASY
Identify correct decreasing order of solubilities of alcohols, alkanes and amines in water having comparable molar mass.

[Q77 · 10th May Shift 2 · 2024]

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

Intuition

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?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Alcohols, Phenols and EthersMODERATE
Which among the following compounds has the highest melting point?

[Q86 · May Shift 1 · 2021]

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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