3.3 HALOGENOALKANES

what nucleophiles (electron pair donors) are used for nucleophilic substitution of halogenoalkanes?

aqueous OH-

CN-

NH₃

what are the conditions needed for nucleophilic substitution of halogenoalkanes, for each reagent?

OH- : warn NaOH _(aq), carried out under reflux

CN-: warn ethanolic KCN, carried out under reflux

NH₃: heat with ethanolic NH₃, must have excess NH₃

hydrolysis

splitting a molecule by a reaction with water

what reagent is used for elimination reaction of halogenoalkanes

OH-

why is (naturally occurring) ozone beneficiary in the upper atmosphere

Filters out Sun’s harmful UV radiation

why is ozone bad in lower atmosphere

pollutant. contributes to formation of smog (smoke fog)

uses of halogenoalkanes

chloroalkanes and chlorofluoroalkanes can be used as solvents.

CH₃CCl₃ was used as solvent in dry cleaning.

Halogenoalkanes have been used as refrigerants, pesticides, and aerosol propellants.

Many of these uses have been stopped due to toxicity of halogenoalkanes and also their detrimental effect on the atmosphere.

Now, we use HFCs (hydrofluorocarbons) for refrigerators and AC. these are safer bc no C-Cl bonds. (C-F is stronger & isn’t affected by UV)

what are CFCs and what effect have they had on ozone

chlorofluorocarbons caused a hole in the ozone layer

→ Cl • s are formed in upper atmosphere,

→ when energy from the UV radiation causes C–Cl bonds in CFCs to break

→ the Cl• can catalyse the decomposition of ozone.

halogenoalkane reactivity down the group

increases

(bond enthalpy decreases down group