3.3 Halogenoalkanes

Question 1

State the benefit to life on Earth of ozone in the upper atmosphere (1)

Answer 1

• Absorbs harmful ultraviolet radiation

Question 2

CFC-11 is a greenhouse gas that can contribute to global warming. State and explain how CFC-11 is able to contribute to global warming (1)

Answer 2

• Absorbs IR Radiation
• Has Polar bonds

Question 3

Define the term electronegativity and explain the polarity of the C-Br bond in 2-bromopropane (3)

Answer 3

• The (relative) tendency of an atom to attract a pair of electrons/ the electrons/ electron density in a covalent bond
• Br is more electronegative than C (or vice versa)
• So Br is δ- and C is δ+

Question 4

Outline the mechanism for the reaction of 2-bromopropane with an excess of ammonia (4)

Answer 4

Question 5

Reaction 1 occurs via a nucleophilic substtution mechanism

Explain why the halogenoalkane is attacked by the nucleophile in this reaction (in general terms) (3)

Answer 5

• Bromine is more electronegative than carbon (difference in electronegativity)
• C is partially positive / electron deficient
• Lone/electron pair (on the nucleophile) donated to the partially positive carbon

Question 6

In the upper atmosphere, it is more likely for CBrF3 to produce bromine atoms than it is for CClF3 to produce chlorine atoms.
Suggest one reason for this (1)

Answer 6

• (The) C‒Br (bond) breaks more readily / is weaker than (the) C‒Cl (bond) (or converse)

Question 7

Suggest one reason why the use of CFCs was not restricted until several years after Rowland and Molina published their research (1)

Answer 7

• lack of evidence that ozone was being depleted
• lack of alternatives to CFCs

Question 8

CFC-11 is a greenhouse gas that can contribute to global warming

State and explain how CFC-11 is able to contribute to global warming (2)

Answer 8

• absorbs infrared radiation
• molecule has polar bonds

Question 9

State how chlorofluorocarbons (CFCs) form chlorine atoms in the upper atmosphere (1)

Answer 9

• C-Cl bonds broken (homolytically)

Question 10

Give equations to show how chlorine atoms catalyse the decomposition of ozone (2)

Answer 10

• Cl● + O3 → ClO● + O2
• ClO● + O3 → Cl● + 2O2

Question 11

Nitrogen monoxide (NO) catalyses the decomposition of ozone into oxygen

Write the overall equation for this decomposition (1)

Answer 11

2O3 > 3O2