#Alkenes

#ORGANICCHEMISTRY

#Chemistry

Introduction

• Alkenes are unsaturated hydrocarbons with carbon-carbon double bonds (C=C)

• They have covalent bonds

• Their general formula is CnH2n

• The presence of the double bond, C=C, 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

Manufacture of alkenes

• Although there is use for each fraction obtained from the fractional distillation of crude oil, the amount of longer chain hydrocarbons produced is far greater than needed

• These long chain hydrocarbon molecules are further processed to produce other products

• A process called catalytic cracking is used to convert longer-chain molecules into short-chain and more useful hydrocarbons

• Shorter chain alkanes, alkenes and hydrogen are produced from the cracking of longer chain alkanes

• Alkenes can be used to make polymers and the hydrogen used to make ammonia

• Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen

• Cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them

• The vapours then pass over a hot powdered catalyst of alumina or silica

• This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions

• The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes

• Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure

Distinguishing from alkanes

• The presence of the C=C double bond in an alkene but not an alkane allows us to distinguish between them

• To do this, bromine water is used

• Bromine water is an orange coloured solution

• When bromine water is shaken with an alkane the solution remains orange

• When bromine water is shaken with an alkene, the solution will go colourless, as the bromine can add across the double bond meaning it is no longer in solution