halogenoalkanes (3.3.3)
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what is ozone?
a chemical formed naturally in the upper atmosphere which is beneficial as it absorbs UV radiation
how can chlorine atoms be formed in the upper atmosphere?
when UV radiation causes C-Cl bonds in chlorofluorocarbons (CFCs) to break
what do the chlorine atoms produced in the upper atmosphere do?
catalyse the decomposition of ozone and contribute to the hole in the ozone layer
what to led CFCs being banned from being used in solvents and refrigerators?
results of research from different groups in the scientific community which provided evidence for legislation
why are hydrofluorocarbons (HFCs) appropriate alternative chlorine-free compounds?
they don’t contain any relatively weak C-Cl bonds, and the C-H and C-F bonds are too strong to be broken by UV light
what are the equations which show how chlorine atoms catalyse the decomposition of ozone (stage 1, stage 2 and overall)?
stage 1 - Cl* + O3 → ClO* + O2 stage 2 - ClO* + O3 → 2O2 + Cl* overall - 2O3 → 3O2
what do halogenoalkanes contain?
polar bonds
what is a nucleophile?
a species that donates a pair of electrons to form bonds
when do halogenoalkanes undergo substitution reactions?
when reacting with the nucleophiles OH^-, CN^- and NH3 (reagents)
when does a reagent act as a nucleophile in a reaction?
when it donates electrons
when does a reagent act as a base in a reaction?
when it accepts protons
what are the reaction conditions for the reaction of a halogenoalkane and sodium hydroxide?
the aqueous sodium hydroxide will be mixed with ethanol and the reaction is slow at room temperature so the mixture is warmed
what are the products for the nucleophilic substitution reaction of a halogenoalkane and sodium hydroxide?
an alcohol (R-OH) and the halide ion
what are the reaction conditions for the reaction of a halogenoalkane and potassium cyanide?
the warmed, aqueous, alcoholic solution of potassium cyanide is mixed with ethanol
what are the products for the nucleophilic substitution reaction of a halogenoalkane and potassium cyanide?
a nitrile (R-CN) and the halide ion
what are the reaction conditions for the reaction of a halogenoalkane and ammonia?
the reaction occurs in a sealed vessel (under pressure) the halogenoalkane reacts with a concentrated solution of ammonia in ethanol
what are the products for the nucleophilic substitution reaction of a halogenoalkane and ammonia?
an amine (R-NH2) and the halide ion
what is unique about the reaction between a halogenoalkane and ammonia?
in the first stage, the reagent acts as a nucleophile, and in the second it acts a base
what is the product when a halogenoalkane reacts with aqueous sodium hydroxide mixed with ethanol?
an alcohol
what is the product when a halogenoalkane reacts with sodium hydroxide (no water present) mixed with ethanol and heated to high temperatures?
an alkene
what happens during an elimination mechanism?
atoms are eliminated from a halogenoalkane
what is the role of the reagent in an elimination mechanism?
a base (proton acceptor)
what are the products of an elimination reaction with a halogenoalkane?
an alkene, water and the halide ion