Solubility, melting and boiling points

What the standard covers

The physical properties you may be asked about are limited to exactly three:

• solubility

• melting point and boiling point

• rotation of plane-polarised light (covered on the enantiomers page)

• All of them can be used to distinguish between organic compounds, which is how they usually appear in the exam.

Both properties come from the same place

• Boiling point depends on the strength of the attractions between molecules — stronger attractions mean more energy is needed to separate them.
• Solubility in water depends on whether the molecule can form strong enough attractions with water to replace the hydrogen bonds that must be broken.
• So for both, the question to ask is: can this molecule hydrogen bond, and how big is its hydrocarbon chain?

Which groups hydrogen bond

• Hydrogen bond to themselves — these have an O–H or N–H bond:

  • alcohols, carboxylic acids, primary amines, amides

• Cannot hydrogen bond to themselves but CAN accept one from water — these have a lone pair on O or N but no O–H or N–H:

  • aldehydes, ketones, esters

• Cannot do either — no polar group at all:

  • alkanes, alkenes, haloalkanes

• The middle group is the one students get wrong. A ketone is soluble in water despite not hydrogen bonding to itself, because water supplies the $\delta^+$ hydrogen and the ketone's oxygen supplies the lone pair.

The boiling point order

For molecules of similar size:

$$\text{alkane} < \text{haloalkane} < \text{aldehyde/ketone} \approx \text{ester} < \text{amine} < \text{alcohol} < \text{carboxylic acid}$$

• Alkanes are non-polar — temporary dipole attractions only.

• Aldehydes, ketones and esters are polar — permanent dipole attractions as well.

• Alcohols and amines hydrogen bond, so they are higher again.

• Carboxylic acids are highest, because each molecule has both an O–H to donate and a C=O to accept, so they form two hydrogen bonds to each neighbour.

• Amides are also very high, for the same reason as acids.

• Note that alcohols boil higher than amines of similar size, because oxygen is more electronegative than nitrogen, so the O–H hydrogen is more $\delta^+$ and the hydrogen bonds are stronger.

Chain length changes everything

• The polar functional group attracts water; the hydrocarbon chain does not.

• As the chain gets longer:

  • solubility in water falls, because the non-polar part becomes a larger fraction of the molecule
  • boiling point rises, because there are more electrons and so stronger temporary dipole attractions

• Compounds with fewer than about five carbons are usually water-soluble if they have a polar group; longer ones are not.

Using these to distinguish compounds

• Distillation separates compounds by boiling point — the lower-boiling one distils off first.
• Solubility separates a carboxylic acid from a neutral compound: adding $\text{NaOH}(aq)$ or $\text{Na}_2\text{CO}_3(aq)$ converts the acid to its ionic salt, which dissolves in the aqueous layer while the neutral compound stays in the organic layer.