15.4Reactions of Alkenes

15.4 Reactions of Alkenes

Electrophilic addition

Unlike alkanes, alkenes are more reactive because they are unsaturated and contain a C=C bond. A C=C bond contains a σ bond and a π A π bond is a region of high density of electron which is open to attack by something that carries an amount of positive charge, electrophiles.

Alkenes mainly undergo electrophilic addition. An electrophilic addition reaction is the addition of an electrophile across the C=C bond.

The general mechanism of electrophilic addition:
1. The electronegativity difference in an electrophile, say, XY causes the molecule to be Therefore it carries a δ+ and δ- end. The δ+ end of the molecule behaves as an electrophile.
2. The δ+ end is attracted to the π bond, a region with high electron As it approaches the π bond, the electrons in the X-Y bond are repelled further and further to the Y atom.
3. Eventually the electrons are donated to Y, forming Y⁻ ion. The X atom forms a single bond with The electrons needed for this bonding come from the π bond. As a result, the adjacent carbon atom will carry a positive charge because it has lost its π electron to the C-X bond.
4. The carbon-containing species which carries a positive charge is called carbocation. The carbocation is attracted to the negative Y⁻ ion. These two particles eventually bond together via a co-ordinate bond

In the examination, the mechanism can be written like this. The curly arrows show the movement of electrons

The addition of unsymmetrical alkenes follows Markovnikov’s rule:
” In the addition of H-X to a C=C bond of an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom that holds the greatest number of hydrogen atoms. ”

Hydrocarbons can be classified as primary, secondary or tertiary depending on the number of R groups attached to the positive

The Rs are alkyl groups, they can be the same or different. Carbocation is formed as an intermediate when the electrophile bonds with the carbon atom.

The alkyl groups have an electron-pushing effect, they tend to push the electrons towards the carbon atom which carries a positive charge

It follows that tertiary carbocation is the most stable because the positive charge is neutralised by the negative charges. The charge density on the carbon atom is the least

Therefore, if the electrophile is bonded in such a way that a tertiary carbocation is produced, the stability of the intermediate formed can be increased. This can only happen if the hydrogen attaches itself with the carbon with most hydrogen atom(normally at the ends of the molecule). Therefore Markovnikov’s rule is obeyed.

Addition of hydrogen, H2 (Hydrogenation)

Reagent : Hydrogen gas, H2
Condition : Heat in the presence of nickel, Ni catalyst at 150 °C
Product : Alkanes
In hydrogenation of ethene, two hydrogen atoms are added across the C=C double bond. Ethane is produced

In this case, although hydrogen molecule has no polarity, it is still able to behave as an electrophile. This is because as the hydrogen molecule approaches the double bond, a dipole is induced due to the repulsion between the two bond pair of electrons(one from the C=C bond, another from the H-H bond).

Addition of steam, H2O (Hydration)

1) Reagent     : Steam, H2O
Condition : Heat the gaseous alkene at 330 °C and 60 atm in the presence of phosphoric(V) acid, H3PO4 as   catalyst
Product      : Alcohols (compounds that -OH group)
In the hydration of alkene, steam, H-OH is added across the double Ethanol is produced.

Addition of halogen, X2 (Halogenation)

Reagent : Halogen, X2 (in tetrachloromethane or aqueous) Condition : Room temperature in dark
Product : Halogenoalkanes (alkanes with halogens)
When ethene is bubbled into Br2 in CC4 at room temperature in dark, Br-Br is added across the C=C bond. The brown colour of bromine is decolourised. 1,2-dibromoethane is produced

When ethene is bubbled into aqueous Br2 at room temperature in dark, two products are obtained:

Br2 in CCl4 is used as a test to differentiate alkanes and alkenes as they both are colourless. Alkene will decolourise the brown colour of bromine whereas alkane will not

Addition of hydrogen halide, HX

Reagent : Hydrogen halide, HX(g)
Condition : Room temperature
Product : Halogenoalkanes (alkanes with halogens)
When ethene reacts with hydrogen bromide, 1-bromoethane, a halogenoalkane is produced

Oxidation by cold and dilute potassium manganate(VII) solution, KMnO4

Reagent : Cold and dilute potassium manganate(VII) solution, KMnO4
Condition : Room temperature
Product : Alcohols(diols)
When ethene is reacted with cold, dilute acidified KMnO4 solution, the purple colour of KMnO4 is decolourised. Ethane-1,2-diol is produced

This is a redox reaction, the carbon species is oxidised while the oxidising agent is reduced.

This is another test used to differentiate alkanes and alkenes. Only alkene will decolourise the purple colour of KMnO4 whereas alkane will not.

Oxidation by hot and concentrated potassium manganate(VII) solution, KMnO4

Reagent : Hot and concentrated acidifted potassium manganate(VII) solution, KMnO4
Condition : Room temperature
Product : Carbon dioxide/carboxylic acids/ketones
When an alkene is reacted with hot and concentrated acidified KMnO4 solution, the C=C bond in the alkene ruptures. The purple colour of KMnO4 is also The product formed will differ according to the position of the C=C bond.

i) If there are two alkyl groups at one end of the bond, that part of the molecule will give a ketone. For example:ii) If there are one alkyl group and one hydrogen at one end of the bond, that part of the molecule will give a carboxylic acid. For example:
Ethanal(an aldehyde) is further oxidised to ethanoic acid(a carboxylic acid)The net effect is as such:
iii) If there are two hydrogens at one end of the bond, that part of the molecule will give carbon dioxide and water. For example:
This reaction is useful in determining the position of C=C bond in an alkene molecule. This can be done by examining the product(s) formed

Addition polymerisation

Alkenes can open up its C=C bond and join with each other in a long chain to form a A polymer is a long-chain molecule made of repeating units called monomers.