Tuesday, February 5, 2008

Alkyl and Aryl Halides - Physical Properties

Physical properties of halogenoalkanes

Boiling Points

the only methyl halide which is a liquid is iodomethane;

chloroethane is a gas.

The examples show that the boiling points fall as the isomers go from a primary to a secondary to a tertiary halogenoalkane. This is a simple result of the fall in the effectiveness of the dispersion forces.

Solubility in water

The halogenoalkanes are at best only very slightly soluble in water.

In order for a halogenoalkane to dissolve in water you have to break attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and break the hydrogen bonds between water molecules. Both of these cost energy.

Solubility in organic solvents

Halogenoalkanes tend to dissolve in organic solvents because the new intermolecular attractions have much the same strength as the ones being broken in the separate halogenoalkane and solvent.

bond strength falls as you go from C-F to C-I, and notice how much stronger the carbon-fluorine bond is than the rest.

In order for anything to react with the halogenoalkanes, the carbon-halogen bond has got to be broken. Because that gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order.

Iodoalkanes are the most reactive and fluoroalkanes are the least. In fact, fluoroalkanes are so unreactive that we shall pretty well ignore them completely in discussion on reactions.

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