3.3.2-4 Alkanes, Haloalkanes and Alkenes

Question 1

Describe how fractional distialltion works?

Answer 1

• Mixture of hydrocarbon of chains heated to high temperature
• Passed into a fractionating collumn (hot at bottom, cool at top)
• Longer hydrocarbons condense near the bottom and shorter hydrocarbons condense nearer the top
• Fractions collected at a range of temepratures

Question 2

What are the conditions for the following types of cracking:
a) Thermal
b) Catalytic

Answer 2

a) High temperature, high pressure
b) High temperature, slight pressure, aluminium oxide catalyst

Question 3

What are the products of catalytic and thermal cracking?

Answer 3

Catalytic = long hydrocarbon chains, motor fuels

Thermal = alkenes

Question 4

What are the products of incomplete and complete combustion of an alkane?

Answer 4

Incomplete = carbon monoxide + water
Complete = carbon dioxide + water

Question 5

What are the 3 main pollutants produced from an internal combustion engine?

Answer 5

• Nitrogon oxides, NO/NO2 (acid rain)
• Carbon monoxide
• VOCs, unburnt hydrocarbons (volatile organic compounds)

Question 6

How is peroxyacetyl nitrate (PAN) formed and what are the environmental consequences?

Answer 6

Oxides of nitrogen react with the VOCs in the atmosphere producing photochemical smog

Question 7

How are each of the pollutants from an internal combustion engine catalytically removed?

Answer 7

Catalytic converter (platinum):
1. CO oxidised using oxygen/nitrogen monoxide
2. Oxides of nitrogen reduced to nitrogen gas using carbon monoxide
3. Unburnt hydrocarbons oxidised to carbon dioxide and water

Question 8

How are sulfur dioxide pollutants produced and removed?

Answer 8

• Combustion of hydrocarbons containing sulfur impurities.
• Removed from flue gas using calcium carbonate/oxide.

Question 9

What are the stages of free-radical substitution?

Answer 9

1. Initiation
2. Propagation
3. Termination

Question 10

What is the purpose of UV light in free radical substituion?

Answer 10

Energy from UV light breaks the halogen bond to form two radicals in a homolytic fission reaction.

Question 11

What is the trend in reactivity of the haloalkanes (fluro-iodo)?

Answer 11

Fluoro = least reactive
Iodo = most reactive

Question 12

What are the conditions for the following nucleophilic substitution reactions:
a) NaOH
b) KCN
c) NH3

Answer 12

a) Aqueous, warm NaOH
b) Warm KCN dissolved in ethanol
c) Excess NH3 and dissolved in ethanol

Question 13

What are the conditions and products of an elimination reaction of a haloalkane?

Answer 13

Conditions = hot, ethanolic KOH

Products = alkene

Question 14

How do you form an alcohol from an haloalkane?

think: mechanism, conditions

Answer 14

Nucelophilic substitution

Conditions: aqueous NaOH/KOH with ethanol, warmed

Question 15

What causes ozone depletion?

Answer 15

Chlorofluroalkanes (CFCs)

Question 16

What is the overall breakdown of ozone?

Answer 16

2O3 → 3O2

Question 17

How does a chlorine radical react with the ozone layer?

Answer 17

*Cl + O3 → O2 + *OCl
*OCl +O3 → + 2O2 + *Cl

Question 18

What are the conditions for the following electrophilic additions of an alkene:
a) Br2
b) HBr
c) H2SO4
d) H2O

Answer 18

a) Aqueous Br2 (water)
b) -
c) Cold, concentrated H2SO4
d) Strong acid catalyst (phosphoric acid), high temp/pressure

Question 19

Explain why in electrophilic addition are there major and minor products?

Answer 19

• Different carbocation intermediates have different stabilities
• 1y less stable than 2y, 2y less stable than 3y, as there are fewer alkyl groups surrounding it
• Weaker postive inductive effect

Question 20

What type of polymers do alkenes form?

Answer 20

Addition polymers