4.1.2 Alkanes: Module 4: Core Organic Chemistry: Chemistry OCR A A Level

What is an alkane?

A saturated hydrocarbons containing C-H bonds only.

What is the general formula of an alkane?

CnH2n+2

Are alkane bonds polar? Why/why not?

Nonpolar because carbon and hydrogen have similar electronegativities.

What is the shape and angle of an alkane?

Tetrahedral: 109.5°

Describe the 𝜎 (sigma) bond in alkane.

The sigma bond is a covalent bond which has a direct overlap of the electron clouds of the bonding atoms.

What type of intermolecular forces do alkanes have? Why?

London force → induced dipole-dipole interaction, because the bonds are nonpolar.

What happens to the boiling point as alkane chain length increases? Why?

The boiling point increases because there is more surface area and so more number of induced dipole- dipole interaction. Therefore more energy required to overcome the attraction.

Does a branched molecule have lower or higher boiling point compared to equivalent straight chain? Why?

The branched molecule has a lower boiling point because they have fewer surface area and hence less induced dipole -dipole interactions.

Are alkanes soluble in water? why?

Insoluble because hydrogen bonds in water are stronger than alkanes’ London forces of attraction.

How reactive are alkanes?

Very unreactive.

What reactions will alkanes undergo?

Combustion and reaction with halogens.

What type of reaction is combustion?

Oxidation reaction

What is complete combustion?

Combustion that occurs with plentiful supply of air.

What are the products of complete combustion when alkanes are used?

Carbon dioxide and water.

What is the colour of the bunsen burner flame during complete combustion?

Blue Flame

What is incomplete combustion and what products are formed in the case of alkanes?

Combustion in a limited supply of oxygen.

Products : water, carbon dioxide and carbon monoxide

Write an equation for the complete combustion of propane.

C3H8 + 5O2 → 3CO2 + 4H2O

What type of hydrocarbon are most likely to undergo incomplete combustion?

Longer Chains

What is the environmental impact of carbon monoxide?

It is toxic/poisonous.

What is the environmental impact of soot (carbon)?

Asthma, cancer, global dimming.

How are halogenoalkanes formed from alkanes?

Radical Substitution.

In the presence of what does alkane react with halogens?

UV light

What are the three stages of free radical substitution?

Initiation - breaking halogen bond to form free radicals.

Propagation - chain part of the reaction where prod s are formed but free radical remains.

Termination - free radicals removed, stable products formed.