Topic 7 · page 12 of 14
Alcohols and halogenoalkanes in water
Simple alcohols dissolve in water because they form hydrogen bonds with it, which replace the hydrogen bonds between water molecules. Halogenoalkanes are polar but cannot form hydrogen bonds with water, so water is a poor solvent for them.
Read first: Predicting hydrogen bonding, Water as a solvent for ionic compounds
Why simple alcohols dissolve in water 7.67.6(ii)
When one liquid dissolves in another, the molecules of each have to push apart so that the other can fit between them. Ask the same question every time: which attractions are broken, and which are formed?
Methanol and ethanol mix with water in all proportions. In ethanol, C₂H₅OH, the O–H group is the same kind of group as in water, so ethanol molecules form hydrogen bonds with water molecules.
- Broken: hydrogen bonds between water molecules, and hydrogen bonds between ethanol molecules.
- Formed: hydrogen bonds between ethanol and water molecules.
The hydrogen bonds formed are about as strong as the ones broken, so little net energy is needed to mix them and the alcohol dissolves. (In the figure, find both new bonds: the O–H of ethanol points at a lone pair on a water oxygen, and a water O–H points at a lone pair on the oxygen of ethanol.)
Why water is a poor solvent for halogenoalkanes 7.67.6(iii)
A halogenoalkane such as bromoethane, CH₃CH₂Br, contains a polar C–Br bond. The molecule has a permanent dipole, so its molecules attract each other by permanent dipole–dipole interactions and London forces. You might expect it to dissolve in polar water. It hardly dissolves.
The reason is that bromoethane has no O–H, N–H or H–F bond, so it cannot form hydrogen bonds with water. It is not even enough that it has a lone pair on Br, because Br is not N, O or F, and the H atoms are on carbon.
| Ethanol in water | Bromoethane in water | |
|---|---|---|
| Broken | hydrogen bonds between water molecules | hydrogen bonds between water molecules |
| Formed | hydrogen bonds between ethanol and water | only weak attractions (London forces and weak dipole–dipole) |
| Result | dissolves | poor solvent: hardly dissolves |
To dissolve bromoethane, water would have to give up its hydrogen bonds, which are strong, and get only weak attractions to bromoethane in return. The energy balance is unfavourable, so the two do not mix. The same is true for other halogenoalkanes, such as chloromethane and 1-chlorobutane.
Check yourself
- Explain why ethanol is soluble in water.
- State the attractions that are broken and the ones that are formed when methanol dissolves in water.
- Bromoethane contains a polar C–Br bond but dissolves very little in water. Explain.
- Explain why methanol, CH₃OH, dissolves in water but chloromethane, CH₃Cl, does not.
- A student writes: "Bromoethane does not dissolve in water because it is not polar." Correct this.
- Which of these would you expect to mix with water: propan-1-ol or 1-bromopropane? Say why.
Answers cover these until you have tried the questions
- Ethanol has an O–H bond and lone pairs on its oxygen, so it forms hydrogen bonds with water molecules. These are about as strong as the hydrogen bonds broken between water molecules and between ethanol molecules, so ethanol dissolves.
- Broken: hydrogen bonds between water molecules and between methanol molecules. Formed: hydrogen bonds between methanol and water molecules (as strong as those broken).
- Bromoethane has no O–H, N–H or H–F bond, so it cannot form hydrogen bonds with water. Hydrogen bonds between water molecules must be broken, and they are replaced only by weak attractions (London and dipole–dipole forces) with bromoethane, so little dissolves.
- Methanol has an O–H group, so it forms hydrogen bonds with water, similar in strength to those broken. Chloromethane has no H on N, O or F, so it cannot form hydrogen bonds with water, and the hydrogen bonds in water are not replaced by equally strong attractions.
- Bromoethane is polar (C–Br has a dipole). The reason it does not dissolve is that it cannot form hydrogen bonds with water, not that it is non-polar.
- Propan-1-ol: it has an O–H bond and forms hydrogen bonds with water. 1-Bromopropane has no H on N, O or F, so it cannot.