Chapter 15 (4.2.2)

Question 1

Reactivity of haloalkanes

Answer 1

• in the carbon-halogen bond, the halogen is more electronegative than carbon, so electron pair in the carbon-halogen bond is closer to the halogen atom
• so the bond is polar
• carbon has a slightly positive charge (𝛿+) and can attract species containing a lone pair of electrons

Question 2

What is a nucleophile?

Answer 2

An electron pair donor, e.g. :OH⁻, H₂O:, :NH₃

Question 3

What is nucleophilic substitution?

Answer 3

A reaction in which a nucleophile is attracted to an electron-deficient carbon atom and replaces an atom or group of atoms on the carbon atom. Primary haloalkanes undergo this.

Question 4

What is hydrolysis?

Answer 4

A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule.
In hydrolysis of a haloalkane, carbon-halogen bond is broken and the halogen atom is replaced by an -OH group.

Question 5

Mechanism of nucleophilic substitution (explanation but know how to draw)

Answer 5

• nucleophile, OH⁻, approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom (to minimise repulsion between the nucleophile and the 𝛿- halogen atom).
• lone pair of electrons on the hydroxide ion is attracted and donated to the 𝛿+ carbon atom.
• new bond formed between O atom of OH⁻ and the C atom
• carbon-halogen bond breaks by heterolytic fission
• alcohol and halide ion formed

Question 6

Conditions of hydrolysis of haloalkanes

Answer 6

• haloalkanes can be converted into alcohols using aqueous sodium hydroxide
• very slow reaction at room temperature, so heated under reflux to obtain a good yield of product

Question 7

Carbon-halogen bond strength

Answer 7

• Rate of hydrolysis depends on the strength of the carbon-halogen bond in the haloalkane
• C-F > C-Cl > C-Br > C-I
• Bond enthalpies show that iodoalkanes react the fastest etc; fluoroalkanes are unreactive as a large quantity of energy is required to break the C-F bond.

Question 8

General equation for the rate of hydrolysis with water

Answer 8

CH₃CH₂CH₂X +H₂O —-> CH₃CH₂CH₂OH + H⁺ + X⁻

Question 9

Hydrolysis of haloalkanes with water (reactions and reagents)

Answer 9

• Reaction carried out in the presence of aqueous silver nitrate, AgNO₃ (aq).
• Halide ions, X⁻ (aq), are produced which will react with Ag⁺ (aq) ions to form a precipitate of the silver halide:
  Ag⁺ (aq) + X⁻ (aq) —-> AgX (s)
• the nucleophile is the water present in aqueous silver nitrate
• haloalkanes are insoluble in water, so ethanol solvent is present to allow the water + haloalkane to mix and produce a single solution rather than 2 layers

Question 10

Measuring the rate of hydrolysis of haloalkanes practical

Answer 10

1. Set up 3 test tubes, each with 1cm³ of ethanol and 2 drops of the haloalkane
2. Stand in a water bath at 60°C
3. Place test tube of 0.1moldm⁻³ AgNO₃ in bath and allow to reach the constant temperature
4. Add 1cm³ of the AgNO₃ to each test tube. Start the stop-clock.
5. Observe and record time for precipitate to form (for 5 minutes)

Question 11

Observations from hydrolysis practical

Answer 11

1-chlorobutane - white precipitate formed very slowly
1-bromobutane - cream precipitate formed at a medium pace
1-iodobutane - yellow precipitate formed rapidly

Question 12

Uses of organohalogen compounds

Answer 12

Pesticides, dry cleaning solvents, refrigerants etc

Question 13

The ozone layer

Answer 13

• found at the outer edge of the stratosphere
• tiny amount of ozone is enough to absorb UV-B from the suns rays, only allowing a small amount to reach Earth’s surface
• UV-B = linked to sunburn + increases risk of skin cancer and genetic damage
• depletion of ozone layer could allow more UV-B
• in the stratosphere there is continuous formation and breakage of ozone by UV radiation

Question 14

Equations for breakage and forming of ozone

Answer 14

Initially very high energy UV breaks oxygen molecules into oxygen radicals
O₂ —-> (UV) 2O•
Steady state (where rate of formation of ozone is the same at which it is broken down) set up involving O₂ and oxygen radicals in which ozone forms and then breaks again
O₂ + O• O₃
- production of CFCs has upset the delicate equilibrium

Question 15

CFCs and the ozone layer

Answer 15

• chlorofluorocarbons are very stable due to strong carbon-halogen bonds
• remain stable until they reach the stratosphere, where they breakdown, forming chlorine radicals which are thought to catalyse the breakdown of the ozone layer

Question 16

Depletion of the ozone layer by CFCs (e.g. CF₂Cl₂)

Answer 16

• UV radiation initiates breakdown - photodissociation
  CF₂Cl₂ —-> (UV) CF₂Cl• + •Cl
• The Cl• formed is a very reactive intermediate which can react with an ozone molecule, breaking it down into oxygen
  P1: Cl• + O₃ —-> ClO• + O₂
  P2: ClO• + O• —-> Cl• + O₂
• overall equation: O₃ + O• —-> 2O₂
• propagation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in step 1. steps repeat in a cycle over and over in a chain reaction.

Question 17

Depletion of ozone layer by other radicals (e.g. NO)

Answer 17

• nitrogen oxide radicals formed naturally during lightning strikes and as a result of aircraft travel in the stratosphere
• P1: NO• + O₃ —-> NO₂• + O₂
  P2: NO₂• + O• —-> NO• + O₂
• overall equation: O₃ + O• —-> 2O₂
• the same overall equation shows the radicals act as a catalyst