Alkenes (OCR GCSE Chemistry A (Gateway))

Alkenes Structure & Formula

• Alkenes are unsaturated hydrocarbons

• All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C

• All alkenes contain a double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot

• The names and structure of the first four alkenes are shown below:

Table of alkenes

Displayed formula	Name	Molecular formula
	ethene	C2H4
	propene	C3H6
	but-1-ene	C4H8
	pent-1-ene	C5H10

The first four members of the alkene homologous series

• Compounds that have a C=C double bond are also called unsaturated compounds

• That means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group

• Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds

• This makes them much more reactive than alkanes

A carbon-carbon double can break and form a single bond, allowing more atoms to attach to the carbon atoms

Reactions of Alkenes

Combustion of Alkenes

• These compounds undergo complete and incomplete combustion but because of the higher carbon to hydrogen ratio they tend to undergo incomplete combustion, producing a smoky flame in air.

• Complete combustion occurs when there is excess oxygen so water and carbon dioxide form e.g:

C₄H₈           +          6O₂       →      4CO₂       +         4H₂O

butene      +      oxygen    →  carbon dioxide   +  water

• Incomplete combustion occurs when there is insufficient oxygen to burn so a mixture of products can form, e.g:

C₄H₈     +          4O₂          →        4CO           +         4H₂O

butene      +      oxygen    →  carbon monoxide   +  water

• In addition to carbon monoxide, carbon in the form of soot can be produced:

C₄H₈     +          2O₂          →        4C           +         4H₂O

butene      +      oxygen    →  carbon   +     water

• This is more likely to occur in higher alkenes with larger number of carbons

• This is seen as smoky yellow flames when the alkenes burn

Addition Reactions

• The chemistry of the alkenes is determined by the C=C functional group

• Since all members of the alkene homologous series contain the same functional group then they all react similarly

• Alkenes mainly undergo addition reactions in which atoms of a simple molecule add across the C=C double bond

• The carbon-carbon double bond opens up, forming a single bond between the carbons allowing for two more atoms to bond, one on each carbon

  

  Diagram showing the general equation for the addition reaction of alkenes

Hydrogenation

• Alkenes undergo addition reactions with hydrogen in which an alkane is formed

• These are hydrogenation reactions and occur at 150ºC using a nickel catalyst

• Hydrogenation reactions are used to manufacture margarine from vegetable oils

  • Vegetable oils are polyunsaturated molecules which are partially hydrogenated to increase the Mr and turn the oils into solid fats

Hydrogen atoms add across the C=C in the hydrogenation of ethene to produce ethane

Halogenation

• The halogens also participate in addition reactions with alkenes

• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond

• The reaction occurs readily at room temperature

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

Bromine Water Test

• Alkanes and alkenes have different molecular structures

• All alkanes are saturated and alkenes are unsaturated

• The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot

• This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test

Diagram showing the result of the test using bromine water with alkanes and alkenes

• Bromine water is an orange coloured solution

• When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution

• But when bromine water is added to an alkene, the bromine atoms add across the C=C bond, hence the solution no longer contains free bromine so it loses its colour