organic chemistry - alkanes and alkenes

Question 1

general formula for alkanes

Answer 1

CnH2n+2

This means that to work out the number of hydrogens, you double the number of carbons and then add two.

Question 2

what are saturated hydrocarbons

Answer 2

A molecule containing only single bonds between carbon atoms. For example, alkanes as described as saturated molecules.

Question 3

what are unsaturated hydrocarbons

Answer 3

A molecule containing a carbon-carbon double or triple bond. For example, alkenes as described as unsaturated molecules.

Question 4

what is the reaction of alkanes with halogens in the presence of UV

Answer 4

• Alkanes react with bromine in the presence of UV light, e.g. sunlight.
• A hydrogen atom in the alkane is replaced by a bromine atom.

This is known as substitution.

Question 5

what functional group do alkenes contain

Answer 5

> C=C<

Question 6

general formula for alkenes

Answer 6

Alkenes have the general formula CnH2n

So an alkene always has twice as many hydrogen atoms as carbon atoms.

Question 7

reaction of alkenes with bromine

Answer 7

• alkenes react with bromine water. UV light is not required for this reaction.
• the double bond is broken and the bromine atoms are added. This is an addition reaction.
• during this reaction there is a colour change from orange to colourless.

This is how we can test for the presence of an alkene or another type of unsaturated molecule.

Question 8

how can bromine water be used to distinguish between alkanes and alkenes

Answer 8

• in the absence of UV light an alkane added to bromine water will not react: the bromine water will stay orange.
• however, alkenes react with bromine water even without UV light. There will be a colour change of orange to colourless.