Alkanes

Alkanes are saturated. What does this mean?

They contain single C-C bonds

What kind of bonds are all of the atoms in alkanes bonded by?

Sigma bonds

Describe the sigma bond in alkanes.

• It is a covalent bond

• Which has a direct overlap of orbitals

What do sigma bonds allow?

Free rotation

What is the shape and angle of an alkane and why?

• Tetrahedral with a 109.5 degree bond angle

• Due to there being 4 bonding pairs of electrons repelling equally

How does the boiling point change in straight chain alkanes?

• Boiling point increases as carbon chain gets longer

• Because London forces get stronger as number of electrons in the molecules increases. More energy is needed to overcome the intermolecular forces.

How does the boiling point change in branched isomers? Why?

• Boiling points decrease as degree of branching increases.

• Because more branching means that London forces get weaker as it reduces the surface contact between molecules.

What is the reactivity of alkanes? Why? (3)

• Unreactive

• Due to high bond enthalpies in molecules

• and Polarity in sigma bonds is low as C and H have similar electronegativities

Are sigma bonds in alkanes considered polar or not and why?

• Non polar

• Because the electronegativities of carbon and hydrogen are very similar

When does complete combustion in alkanes occur?

When there is a plentiful supply of oxygen

Why can alkanes be used as fuels?

they release a lot of energy when burned

When do alkanes combust incompletely?

In a limited supply of oxygen

What are the products of incomplete combustion?

Carbon monoxide/Carbon and Water

What are the products of complete combustion?

Carbon dioxide and water

What is harmful about CO?

It is a toxic gas as it reduces the ability of blood to carry oxygen

What is harmful about C (soot)?

Particulates irritate the respiratory system

Alkanes can react with halogens in the presence of ____.

UV light (eg. sunlight)

Which mechanism occurs with a reaction between alkanes and halogens?

Radical substitution mechanism

What are the 2 downsides of the radical substitution mechanism?

1) The substitution can occur anywhere on the carbon chain; not a good method to make a single organic compound

2) If the halogen is in excess, further substitutions are possible

Define homolytic fission

It occurs when a covalent bond breaks and one electron goes to each atom

Define heterolytic fission

Occurs when a covalent bond breaks and the pair of electron goes to one of the atoms

What kind of fission is involved in the initiation stage of radical substitution?

Homolytic