IMAT Chemistry · Organic Chemistry
Physical Properties: Boiling Points and Solubility
Boiling point and solubility depend on the forces between molecules: chain length, branching and above all hydrogen bonding.
Why this matters
The 2025 paper offered wrong options about how amino acids dissolve in water and in hexane, and the 2013 paper asked whether isomers must have similar physical properties. Comparing boiling points and solubilities is standard syllabus even where the past papers have not asked it directly.
Concept 1 of 2: Boiling points of organic compounds: size, branching and hydrogen bonds
To boil a liquid you pull its molecules apart from each other; you do not break the bonds inside them. Bigger molecules have more electrons and touch over a larger area, so they attract more. Molecules with O-H or N-H can also hydrogen bond, which is much stronger, so they boil far higher than molecules of the same size without it.
Definition
Forces between molecules, weakest to strongest:
- London (dispersion) forces: in every molecule; grow with chain length and surface contact.
- Dipole-dipole forces: in polar molecules such as aldehydes, ketones, esters and haloalkanes.
- Hydrogen bonds: between molecules that have O-H or N-H (alcohols, carboxylic acids, amines, amides). Carboxylic acids pair up through two hydrogen bonds.
For molecules of similar size the boiling points rise in the order: alkane < ether < aldehyde or ketone < alcohol < carboxylic acid. Amines hydrogen bond more weakly than alcohols (N-H is less polar than O-H), so they boil lower than alcohols of similar size.
- Branching lowers the boiling point: a compact molecule has less surface contact than a straight chain with the same formula.
| Factor | Effect on boiling point | Example pair (approximate boiling points) |
|---|---|---|
| Longer chain | Higher | Butane −1 °C < pentane 36 °C |
| More branching | Lower | 2,2-dimethylpropane 10 °C < pentane 36 °C |
| Hydrogen bonding (O-H) | Much higher | Dimethyl ether −24 °C < ethanol 78 °C (both ) |
| Polar C=O | Higher than an alkane | Butane −1 °C < propanone 56 °C (both ) |
| Two hydrogen bonds per pair (acid) | Highest for its size | Propan-1-ol 97 °C < ethanoic acid 118 °C (both ) |
Practice this conceptself-check · 3 quick reps
Boiling breaks forces between molecules, not covalent bonds
When ethanol boils, its molecules separate but each one stays whole: the C-O and O-H bonds are not broken. Options that explain a boiling point by bond strength inside the molecule are wrong.
Branching lowers the boiling point even with the same formula
Isomers have the same molar mass but can boil at very different temperatures. The more branched isomer is more compact, touches its neighbours less, and boils lower.
Concept 2 of 2: Solubility of organic compounds in water and in non-polar solvents
Like dissolves like. Water molecules hold each other by hydrogen bonds, so a substance dissolves in water only if it can join in with hydrogen bonds or ionic attractions of its own. A long hydrocarbon tail cannot, so as the tail grows, water solubility falls and solubility in oils and hexane rises.
Definition
- Small alcohols, carboxylic acids, aldehydes, ketones and amines (up to about 3 or 4 carbons) mix with water: they hydrogen bond with water molecules.
- A C=O oxygen cannot hydrogen bond to another aldehyde or ketone, but it can accept a hydrogen bond from water. That is why propanone mixes with water.
- As the carbon chain gets longer, water solubility falls (hexan-1-ol dissolves only a little).
- Hydrocarbons and haloalkanes do not dissolve in water; they dissolve in non-polar solvents such as hexane. Liquid hydrocarbons are less dense than water and float on it.
- Ionic organic compounds (carboxylate salts, amine salts, amino acid zwitterions) dissolve in water and not in hexane.
| Compound type | In water | In hexane | Reason |
|---|---|---|---|
| Ethanol, ethanoic acid | Mixes completely | Soluble | Small, hydrogen bonds with water |
| Propanone | Mixes completely | Soluble | C=O accepts hydrogen bonds from water |
| Hexan-1-ol | Slightly soluble | Soluble | Long non-polar tail outweighs the OH |
| Hexane, chloroethane | Insoluble | Soluble | No hydrogen bonding with water |
| Sodium ethanoate, glycine | Soluble | Insoluble | Ionic |
Practice this conceptself-check · 3 quick reps
A molecule does not need its own O-H to dissolve in water
Propanone and small esters have no O-H, yet they dissolve because water can hydrogen bond to their oxygen. The test is whether the molecule can hydrogen bond with water, not with itself.
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.
Reference tables (2)
Boiling points of organic compounds: size, branching and hydrogen bonds5 rows
| Factor | Effect on boiling point | Example pair (approximate boiling points) |
|---|---|---|
| Longer chain | Higher | Butane −1 °C < pentane 36 °C |
| More branching | Lower | 2,2-dimethylpropane 10 °C < pentane 36 °C |
| Hydrogen bonding (O-H) | Much higher | Dimethyl ether −24 °C < ethanol 78 °C (both ) |
| Polar C=O | Higher than an alkane | Butane −1 °C < propanone 56 °C (both ) |
| Two hydrogen bonds per pair (acid) | Highest for its size | Propan-1-ol 97 °C < ethanoic acid 118 °C (both ) |
Solubility of organic compounds in water and in non-polar solvents5 rows
| Compound type | In water | In hexane | Reason |
|---|---|---|---|
| Ethanol, ethanoic acid | Mixes completely | Soluble | Small, hydrogen bonds with water |
| Propanone | Mixes completely | Soluble | C=O accepts hydrogen bonds from water |
| Hexan-1-ol | Slightly soluble | Soluble | Long non-polar tail outweighs the OH |
| Hexane, chloroethane | Insoluble | Soluble | No hydrogen bonding with water |
| Sodium ethanoate, glycine | Soluble | Insoluble | Ionic |
Watch out for (3)
- Boiling breaks forces between molecules, not covalent bonds→ Boiling points of organic compounds: size, branching and hydrogen bonds
- Branching lowers the boiling point even with the same formula→ Boiling points of organic compounds: size, branching and hydrogen bonds
- A molecule does not need its own O-H to dissolve in water→ Solubility of organic compounds in water and in non-polar solvents
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