4.5 HALOALKANES

Haloalkanes: - general formula = CnH2n+1X - functional group = C-X - can be classified as primary, secondary or tertiary

A nucleophile is an electron pair donor. It is attracted to things with a positive charge.

Nucleophilic substitution: - if a haloalkane is heated under reflux with a solution of sodium hydroxide, the halogen is replaced by an OH and an alcohol is produced. example of hydrolysis.

Iodine will be hydrolysed the fastest as it has weakest nuclear attraction out of the three and so less energy is required to break its bonds. The rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.

Ozone: - bad in troposphere = has harmful effects on respiratory systems of some animals - good in stratosphere = stops harmful UV-B reaching the earths surface - filters out shorter wavelengths of UV, < 320nm, which are damaging to life and can lead to conditions such as skin cancer - converts UV into heat O2 + (UV <240nm) = 2O O2 + O = O3 + heat

CFC’s and ozone depletion: - CFC’s have a large residence time (amount of time a particle spends in a particular system) in the troposphere, but eventually reaches the stratosphere - in stratosphere, UV radiation can break the Cl-Cl bond by homolytic fission = photodissection reaction as radiation is required to initiate the reaction initiation = CF2Cl2 = CF2Cl + Cl propagation = 1. Cl + O3 = ClO + O2 2. ClO + O = Cl + O2 O3 + O = O2 termination

When lightening strikes nitrous monoxide radicals are formed, as well as a result of plane travel. NO breaks down the ozone. propagation = 1. NO + O3 = NO2 + O2 2. NO2 + O = NO + O2