6.3.2 Physical properties of halogenoalkanes

What is the physical state of the halogenoalkanes at room temperature?

• Chloromethane (CH₃Cl), bromomethane (CH₃Br) and chloroethane (CH₃CH₂Cl) are gases at room temperature.

• Most other halogenoalkanes are colourless liquids.

Explain the reason behind the high boiling points of halogenoalkanes

oiling points of halogenoalkanes

• Halogenoalkanes generally have higher boiling points than their corresponding parent alkanes.

• This is because the carbon–halogen bond is polar due to the difference in electronegativity between carbon and the halogen.

• This produces permanent dipoles in the molecules.

• Permanent dipole–dipole attractions act between halogenoalkane molecules.

• Halogenoalkanes also experience London dispersion forces, as do all molecules.

• These additional intermolecular attractions make the overall intermolecular forces stronger than those in the corresponding alkanes.

• More energy is therefore required to overcome the intermolecular forces and separate the molecules.

• Consequently, halogenoalkanes have higher boiling points than their parent alkanes.

Why are halogenoalkanes not soluble in water?

• Halogenoalkanes are generally insoluble in water, even small halogenoalkanes.

• Although halogenoalkanes are polar, they cannot form hydrogen bonds with water.

• Hydrogen bonding requires a hydrogen atom to be directly bonded to nitrogen, oxygen or fluorine.

• Halogenoalkanes do not contain N–H, O–H or F–H bonds. Their hydrogen atoms are bonded to carbon instead.

• Therefore, halogenoalkanes cannot donate hydrogen bonds to water.

• Although halogenoalkanes have permanent dipoles and experience dipole–dipole attractions, these are much weaker than the hydrogen bonds between water molecules.

• The attractions formed between halogenoalkanes and water are insufficiently favourable to compensate for disrupting the hydrogen bonds between water molecules.

• Consequently, halogenoalkanes do not mix with water.

Why can alcohols dissolve in water?

• Alcohols contain a hydroxyl functional group (–OH).

• The hydrogen atom in the O–H bond is directly bonded to oxygen, allowing alcohol molecules to donate hydrogen bonds to water.

• The oxygen atom in the hydroxyl group also has lone pairs of electrons, allowing alcohol molecules to accept hydrogen bonds from water.

• Therefore, alcohol molecules can form hydrogen bonds with water molecules.

• These new hydrogen bonds provide sufficiently favourable attractions to allow small alcohols to mix completely with water.

• Consequently, small alcohols are miscible with water, unlike halogenoalkanes.