Alkanes

What is petroleum?

A mixture of hydrocarbons that is predominantly alkenes.

How can petroleum be separated?

By fractional distillation.

What are the three types of alkane?

Unbranched alkanes

Branched alkanes

Cyclic alkanes

Why do straight chain alkanes have a higher boiling point than branched alkanes?

They have more surface area in contact with each other, so the van der waals forces are stronger.

What is cracking?

Splitting larger hydrocarbons up into smaller hydrocarbons.

What is an alkane?

A hydrocarbon that only contains single bonds.

What is the general formula for an alkane?

C_nH_2n+2

What are the two types of cracking?

Thermal cracking

Catalytic cracking

What are the conditions for thermal cracking?

High temperature and high pressure

What hydrocarbons are produced from thermal cracking?

Mostly alkenes.

What are the conditions for catalytic cracking?

High temperature, slight pressure and with the aid of a zeolite catalyst.

What hydrocarbons are produced by catalytic cracking?

Motor fuels and aromatic hydrocarbons.

What are the economic benefits of cracking?

It produces more valuable products like motor fuels and alkanes.

State a common use of alkanes.

Fuels

What products are produced in complete combustion?

Water and carbon dioxide

What products are produced in incomplete combustion?

Carbon monoxide and/or carbon and water

What types of pollution are produced by internal combustion engines?

Nitrogen oxides, carbon dioxide, carbon monoxide, carbon and unburnt hydrocarbons.

How can pollutants from internal combustion engines be controlled?

Catalytic converters in vehicles.

What is the essential condition for free radical substitution?

UV light

What is the organic reactant in free radical substitution?

The alkane

What is a free radical?

A substance with one unpaired electron

Are free radicals reactive or unreactive?

Very reactive.

What is the overall reaction in free radical substitution?

Alkane + Halogen —→ Halogenoalkane + Hydrogen-halogen molecule

What are the three stages in free radical substitution?

1. Initiation

2. Propagation

3. Termination

Why must UV light be present for the reaction to start?

It provides enough energy to homolytically break the covalent bonds in the halogens.

What does homolytically mean?

The bond breaks symmetrically.

What happens when the halogens split?

One atom each from the covalent bond goes to each atom, producing 2 free radical halogens.

What role does the halogen play in the mechanism?

It is a catalyst.

What are the two stages of propagation?

1. One free radical reacts with a molecule of the alkane to form the by-product and an alkane free radical

2. The alkane free radical reacts with another halogen to produce a halogenoalkane and another halogen free radical.

What happens in the termination step?

2 free radicals react to form a neutral molecule

What are the three possible reactions in termination?

Halogen + halogen

Halogen + alkane

Alkane + alkane