Unit 2: Halgenoalkanes

Question 0

Definition of a nucleophile

Answer 0

An electron pair donor

Question 1

What type of reaction is the hydrolysis of halgenoalkanes?

Answer 1

A substitution reaction

Question 2

Describe the mechanism of nucleophilic substitution in the hydrolysis of halgenoalkanes with hot aqueous alkali

Answer 2

• the hydrogen atom is replaced by a hydroxide ion.
• the hydroxide ion has a lone pair of electrons, these are attracted and donated to a carbon atom in the halgenoalkane
• a new covalent bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.
• the carbon-halogen bond breaks by heterolytic fission. Both electrons move to the halogen, forming a halide ion.

Question 3

Explain the rates of hydrolysis of primary halgenoalkanes

Answer 3

The Carbon-Iodine bond is the weakest amongst the halgenoalkanes so it should be broken the most easily and give the fastest reaction.

Question 4

How can the rates of hydrolysis be determined?

Answer 4

-halgenoalkane is heated with aqueous silver nitrate, with ethanol added
-water behaves as the nucleophile
-ethanol acts as a common solvent
-as reaction takes place, halide ions form.
Eg. CH3CH2CH2CH2Cl + H2O -> CH3CH2CH2CH2OH + H+ + Cl-

The aqueous AgNO3 reacts with any halide ions present forming a precipitate
Rate can be found by calculating 1/time taken for precipitate to occur

Question 5

Outline the uses of chloroethene and tetrafluoroethene to produce plastics PVC and PTFE

Answer 5

PTFE is made from the polymerisation of tetrafluoroethene
- used as the non-stick coating used on frying pans
PVC is made from the polymerisation of chloroethene.

Question 6

What were CFCs developed as and why?

Answer 6

• aerosols, refrigerants and air conditioning

- because of their low reactivity, volatility and non-toxicity

Question 7

What have CFCs caused environmental damage to?

Answer 7

The ozone layer.

Question 8

What is the role of green chemistry in minimising damage to the environment?

Answer 8

Promoting biodegradable alternatives to CFCs like:

Hydrocarbons and HCFCs as they are also non-flammable and non-toxic.

Question 9

What is the problem with HCFCs?

Answer 9

They can still deplete the ozone layer.
Their depleting effect is about 1/10th or less than that of CFCs, however serious damage is still being caused.

They’re a short term fix.