A-level Chemistry/OCR (Salters)/The Atmosphere/Halogenoalkanes

Naming

The halogen name is put before the alkane chain name, in alphabetical order. For example, CBrClFI would be named bromochlorofluoroiodomethane because Br < Cl < F < I in the alphabet. The -x- notation is then used to describe the position of the halogen in relation to the rest of the chain. The halogens are listed in alphabetical order, before finally the alkane name change is suffixed on to the end.

Physical properties

The halogenalkanes' boiling point increase as the atomic mass of the halogen atoms increase, or as the number of halogen atoms increase. Effectively the mass is what determines the boiling point.

A simple explanation for this is that the amount of instantaused dipole-induced interactions increase. With this increase in intermolecular force, more energy is required to break the intermolecular bonds, so the boiling point is higher.

Bond enthalpies

The bond between carbon and the halogen itself gets weaker as the size of the halogen increases. For example, the C-I bond is much weaker than the C-F bond due to the much bigger size of the Iodene atom. With a weaker bond, it becomes easier to break said bond and thus the reactivity increases. As a guide:

Fluoro- compounds are extremely unreactive
Chloro- compounds are somewhat reactive. This is what causes the breakdown of ozone with CFCs.
Bromo- and iodo- compounds are reactive and are useful as catalysts and the like in chemical synthesis.

Homolytic fission

The first type of fission that Halogenalkanes go through is homolytic fission. In this the C atom and Halogen atoms become radicalised

Hetrolytic fission

In hetrolytic fission the C-X bond breaks and both become ionised. The halogen atom becomes negatively charged and the carbon becomes positively charged − a carbocation.

Substituion reactions

In this a nucleophile from another group replaces the halogen and leaves the halogen ionised outside the molecule.

Preparation and reactions of halogenalkanes

Nucleophile	Product	Reaction conditions
H₂O	ROH, H⁺	Reflux. aka hydrolysis
OH⁻	ROH	Reflux, with ethanol as a solvent
NH₃	RNH₂	Heat in sealed tube with ammonia