Chapter 15 - Haloalkanes

Question 1

What are haloalkanes

Answer 1

Compounds containing Carbon, Hydrogen and at least one halogen atom

Question 2

What is different about a carbon – halogen bond compared to C–H

Answer 2

It is polar - Halogens are more electronegative

Question 3

What are nucleophiles

Answer 3

Nucleophiles are negative ions, or molecules with a δ- atom, that can donate a lone pair of electrons to form a new covalent bond

Question 4

What happens in the hydrolysis of a haloalkane under reflux

Answer 4

The halogen atom is replaced by an –OH group to form an alcohol

Question 5

Describe the process of the nucleophillic substitution of a haloalkane

Answer 5

1. OH- ion approaches the δ+ carbon on the opposite side of the molecule from the δ- halogen atom, to minimise repulsion
2. A lone pair of electorns on the OH- ion is donated to the δ+ carbon atom and a new bond is formed
3. The carbon-halogen bond breaks by heterolytic fission
4. The new organic product is an alcohol
5. A halide ion is also formed

Question 6

What does the rate of the hydrolysis of haloalkanes depend on

Answer 6

The rate of hydrolysis of the haloalkanes depends upon the strength of the carbon-halogen bond

Question 7

Which reacts fastest and why:
* Fluoroalkanes
* Bromoalkanes
* Chloroalkanes
* Iodoalkanes

Answer 7

• Iodoalkanes react the fastest (C-I bond weakest, so less energy required to break the C-I bond)
• Fluoroalkanes react the slowest (C-F bond strongest, so more energy required to break the C-F bond). [Fluoroalkanes unreactive.]

Question 8

How can you measure the rate of the hydrolysis of Primary Alkanes

Answer 8

• Comparing the rate of hydrolysis of 1-chlorobutane, 1-bromobutane, and 1-iodobutane
• Ethanol and aqueous silver nitrate solution are added to the test tubes containing 3-4 drops of the haloalkanes. The test tubes are warmed in a water bath and the time taken for the precipitates of silver halide to appear are measured and recorded

Question 9

Where is the ozone layer

Answer 9

The ozone layer is found at the outer edge of the stratosphere

Question 10

Why is the ozone layer important

Answer 10

Ozone absorbs the biologically damaging high energy UV radiation (UV-B). - Sunscreen Effect
Exposure to UVB causes sun burn and long term effects such as skin cancer and genetic damage

Question 11

What is meant by the steady state of the ozone layer

Answer 11

ozone is continually being formed and broken down by the action of ultraviolet (UV) radiation

Question 12

How is ozone formed and broken down, include equations

Answer 12

• Initially, very high energy UV radiation breaks oxygen molecules into oxygen radicals, O, via homolytic fission
• O2 → 2O
• A steady state is then set up involving O2 and the oxygen radicals in which ozone is constantly being formed and broken down
• O2 + O ⇌ O3
• The reversible reaction occurs at the same rate

Question 13

What are CFCs

Answer 13

chlorofluorocarbons (contain chlorine, fluorine, and carbon)

Question 14

Why are CFCs stable

Answer 14

Due to the strength of the carbon-halogen bonds, CFCs are very stable compounds – chemically inert, non-toxic, low flammability

Question 15

What are uses of CFCs

Answer 15

• Fridges
• Aerosols

Question 16

Write the equations for the catalysis of the breakdown of Ozone by CFCs using CFCl₃ and state initiation step, propagation step, and overall equation

Answer 16

Initiation
* CFCl₃→ CFCl₂* + Cl*
Propagation
* Cl* + O3 → ClO* + O2
* ClO* + O → Cl* + O2
Overall
* O3 + O → 2O2

Question 17

What common radical can also catalyse the breakdown of ozone

Answer 17

Nitrogen

Question 18

Show the catalysis of the breakdown of ozone by radical X

Answer 18

1. X* + O3 → XO* + O2
2. XO* + O → X* + O2
   Overall : O3 + O → 2O2