Halogeoalkanes

Question 1

Hydrolysis of halogenoalkanes

Answer 1

Substitution reaction. Occurs when halogenoalkanes react with an aqueous solution of hot hydroxide ions, e.g. sodium hydroxide (aq). Produces an alcohol and a halide ion. Nucleophilic substitution

Question 2

Nucleophile

Answer 2

An electron pair donor. Attracted to an electron deficient centre or atom, where it donates a pair of electrons to form a new covalent bond

Question 3

Mechanism of hydrolysis of halogenoalkanes

Answer 3

• Begin with halogenoalkane with carbon as slightly positive and halogen as slightly negative and an OH- ion
• Curly arrow from electron pair on OH (on the O) to carbon; curly arrow from C-halogen bond to halogen
• Get an alcohol and a halide ion

Question 4

Rates of hydrolysis of halogenoalkanes

Answer 4

• iodoalkane has the fastest rate, as it has the weakest bond enthalpy
• chloroalkane (or fluoroalkane, if in question) has the slowest rate, as it has the strongest bond enthalpy
• Even though the polarity of the C-F / C-Cl bond is greater than the C-I bond, implying nucleophiles would be attracted to it faster, the bond enthalpy is more important

Question 5

Experiment to measure rate of hydrolysis of halogenoalkanes

Answer 5

• mix AgNO(3)(aq) with ethanol (as a solvent). H2O is the nucleophile, AgNO(3) forms a precipitate of AgX when halide ions are produced
• Or, carry out reaction with hot aqueous alkali, then neutralise and test with AgNO(3). OH- is the nucleophile here

Question 6

Use of chloroethene

Answer 6

Is the monomer in PVC (Poly(chloroethene) or Poly (vinyl chloride)). PVC is used in drainpipes, plastic window frames, sports equipment, children’s toys and packaging.

Question 7

Use of tetrafluoroethene

Answer 7

Is the monomer in Poly(tetrafluoroethene) - PTFE/Teflon. C-F bonds are very strong, making PTFE resistant to chemical attack. It is also heat resistant, an electrical insulator and non-stick. Used to coat pans/metal surfaces to prevent chemical attack, used in nail polish

Question 8

Original use of CFCs

Answer 8

Aerosols, in air conditioning and as refrigerants. Good, because of their low reactivity, volatility and non-toxic

Question 9

Problem with CFCs

Answer 9

As they are very stable, they only break down in our atmosphere until they reach the stratosphere. Here, UV light breaks them down, releasing chlorine radicals that catalyse the break down of the ozone layer.

Question 10

Replacing CFCs

Answer 10

When it was revealed that CFCs were damaging our atmosphere, scientists and governments phased them out, bringing in new, environmentally friendly alternatives. Biodegradable alternatives have been promoted, e.g. hydrofluoroalkanes (HFCs) and hydrofluorohydrocarbons (HCFCs), and using CO(2) as a blowing agent for expanding polymers