4.1.2 Alkanes

What is an alkane?

A saturated hydrocarbon containing C-H bonds only.

What is the general formula of an alkane?

CnH2n+2

Are alkane bonds polar? Why or why not?

Nonpolar because carbon and hydrogen have similar electronegativities.

What is the shape and bond angle of an alkane?

Tetrahedral, 109.5°

Describe the sigma (σ) bond in an alkane.

A covalent bond formed by the direct overlap of the electron clouds of the bonding atoms.

What type of intermolecular forces do alkanes have and why?

London forces (induced dipole-dipole interactions) because the bonds are nonpolar.

How does alkane chain length affect boiling point and why?

Boiling point increases with chain length because there is more surface area, leading to more induced dipole-dipole interactions, requiring more energy to overcome.

Do branched alkanes have higher or lower boiling points than straight-chain isomers and why?

Lower, because they have less surface area, resulting in fewer induced dipole-dipole interactions.

Are alkanes soluble in water? Why?

No, because the hydrogen bonds in water are stronger than the London forces between alkane molecules.

How reactive are alkanes?

Very unreactive.

What reactions will alkanes undergo?

Combustion and reaction with halogens.

What type of reaction is combustion?

An oxidation reaction.

What is complete combustion?

Combustion that occurs with a plentiful supply of air.

What are the products of complete combustion of alkanes?

Carbon dioxide and water.

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

Blue flame.

What is incomplete combustion and what are its products for alkanes?

Combustion in a limited supply of oxygen; products are water, carbon dioxide, and carbon monoxide.

Write the equation for the complete combustion of propane.

C3H8 + 5O2 → 3CO2 + 4H2O

Which type of hydrocarbon is most likely to undergo incomplete combustion?

Longer chain hydrocarbons.

What is the environmental impact of carbon monoxide?

It is toxic/poisonous.

What is the environmental impact of soot (carbon)?

It is linked to asthma, cancer, and global dimming.

How are halogenoalkanes formed from alkanes?

Radical substitution.

In the presence of what do alkanes react with halogens?

UV light.

What are the three stages of free radical substitution?

Initiation (forming radicals), Propagation (forming products while maintaining radicals), and Termination (removing radicals).

Write the equations for the reaction of CH4 with Cl2 to form CH3Cl.

Initiation: Cl2 → 2Cl•;

Propagation: Cl• + CH4 → HCl + •CH3 and •CH3 + Cl2 → CH3Cl + Cl•;

Termination: •CH3 + Cl• → CH3Cl, 2Cl• → Cl2, or 2•CH3 → CH3CH3