Alkenes

Alkenes are a homologous series of hydrocarbons, with the functional group of C=C in all their molecules. They are the building blocks for the manufacturing of polymers and hence plastics. 

The simplest alkene known is ethene H2c=cH2 the next alkene in the homologous series is propene with three carbons. The general formula for alkenes are CnH2n  because of there double bond they therefore carry less than the maximum amount of hydrogen's thus being called unsaturated. When it comes down to naming Alkenes its the same general rule as the naming goes for alkanes but with "ene" added instead of "ane" so the first four alkenes in the homologous series are ethene, propene, butene, pentene. The position of the double bond in the carbon chain indicates the name of the alkene with numbering. for example  CH3-CH=CH2 is known as prop-1-ene, as the double bond is located on the first carbon, it is prop due to it being short for propene. 

The C=C consists of a  σ (sigma) bond and a  π (pi bond), the sigma bond is present along the bond between two carbons, where as the pi conds are formed by the overlap of two p-orbitals of the the carbon atoms.

Due to alkenes double bond, alkene exhibit cis-trans isomerism (gemoteric isomerism)

Alkenes and their reactions

Alkenes undergo addition reactions, where the alkene reacts with another compound to form one single product molecule. 

Reaction with bromine water

Alkenes can undergo addition reactions where the alkene reacts with another compound to form one single product molecule. The reaction involves an electrophile (a positive or electron loving molecule) reacting with a alkene, examples of electrophiles are polarized Br2, H+ in either HBr or HCl. Although the reaction to test for an alkene and the compound that is usually used in electrophilic addition is Br2 (bromine).

The test for a alkene take place with bromine in the dark. The presence of a alkene will then produce a colour change, bromine water being orange/brown will change to a colourless solution,the bromine adds to the double bond of the alkene. 

CH2=CH2 + Br2 → BrCH2-CH2Br 

The Pi bond produces two regions of high electron density in the molecules, this is the bond that is responsible for the attraction of electron loving electrophiles or positive ions. The pi bond is actually forming a covalent bond with the attacking molecule (positive). Because the alkene is reacting with an electrophile the mechanism is known as electrophilic addition.

Reactions with other Halogens

Alkene does react with bromine in an addition reaction, the bromine is added to the alkene, chlorine can also react with alkenes, using the alkene example ethene with chlorine we get the reaction as follows. 

CH₂=CH₂   +   Cl2   ——>   CH2Cl–CH2Cl  

This reaction occurs under normal conditions, the two chlorine atoms get added to each side of the double bond. The product formed in this reaction is known as 1,2-dichloroethane.

Reactions with hydrogen (Hydrogenation)

When a alkene such as ethene reacts with hydrogen it must be reacted under certain conditions. In order for the reaction to take place there must be a metal nickel catalyst present, as well as the temperature of around 140°C, the result of this reaction is ethane. 

CH₂=CH₂   +   H2   ——>   CH3–CH3

This reaction is used in the food industry to convert polyunsaturated vegetable oils into margarine. This is due to the carbon chains becoming saturated, the melting point is increased and therefore will become a solid such as margarine. 

Saturated fats are known to be linked with heart and circulatory diseases, vegetable oils have more unsaturated fats meaning more double bonds, hence the reason why alkenes are known as unsaturated hydrocarbons. 

Reactions with water (hydration)

Alkenes can react with water (in industries steam) to form alcohols. With the presence of  phosphoric acid  acting as a catalyst, under high pressure and temperature. 

R-CH₂=CH₂ + H₂O  ——>    R-C(-OH)-CH₃ 

    Acid

In a water molecule the oxygen is more electronegative than the hydrogen attracting the electrons to its self. The double bond of the alkene (negative electrons are present) attacks the positive proton (hydrogen of the acid catalyst), this creates a positive area on the alkene thus being attacked by the water molecule. Another water molecule will come along and take the extra proton creating three bonds, thus leaving the OH on the alkane, creating a alcohol. 

Reactions with Hydrogen halides (hydrohalogenation)

Alkenes such as ethene can react with concentrated aqueous solutions of hydrogen halides such as HBr or HCl. 

The hydrogen Ions are what acts as elcctrophiles by attacking the double bond and forming a cation, the ion then reacts with a halide ion such as chloride or bromide which forms the product.

The reaction hydrogen bromide and ethene would produce bromoethene.  

CH₂=CH₂   +   HBr   ——>   CH3CH2Br

Addition polymerisation

Alkenes have a double bond present in their compound, under high pressure and temperature alkenes can open up their double bonds and form a long single chain. The alkene is known as a monomer where as the longer chain is known as the polymer.