Alkene

General Formula of Alkene : CnH2n , where n=2,3,4,5,6,7,8,9,10.

Alkenes undergo the following reaction.

• Hydrogenation

• Hydration

• They react with hydrogen halide to make halogen alkane.

• They react with cold dilute acidified potassium dichromate

• They react with hot concentrated acidified potassium dichromate

Hydrogenation

• Alkene + Hydrogen = Alkane

• Nickel is needed as a catalyst.

1. Ethene + Hydrogen = Ethane

2. Cyclohexene + Hydrogen = Cyclohexane

Hydration

Alkenes react with steam in the presence of Phosphoric acid at 300°C temperature and 60 atm pressure to make alcohol.

1. Ethene + Steam = Ethanol

2. Butene + water = butan-1-ol / butan-2-ol

• Butane has two positional isomers (a) But-1-ene , (b) But-2-ene

For But-1-ene the reaction is:

2CH3CH2CH=CH2 + H2O -> CH3CH2CH2CH2OH (Butan-1-ol)

For But-1-ene the reaction could also be:

2CH3CH2CH=CH2 + H2O -> CH3CH2CH(OH)CH3 (Butan-2-ol)

For But-2-ene the reaction could also be:

2CH3CH=CHCH3 + H2O -> CH3CH2CH(OH)CH3 (Butan-2-ol)

Electrophilic Addition

• Alkenes react with hydrogen halide to make halogen alkanes.

• The reaction is electrophilic addition.

Electrophile

• Electrophiles are electron pair acceptors.

• Electrophiles are positively charged ions or neutral molecules that are electron-deficient compounds.

Process of electrophilic addition with hydrogen halide

• The pi electrons of alkene repel the electrons of hydrogen halide.

• Dipoles are induced in the hydrogen halide.

• The double bond breaks and hydrogen form a bond.

• A carbocation is formed.

• Halide ion reacts with the carbocation to form halogen alkane.

Stability of a carbocation

• During electrophilic addition, different carbocations have different stability.

• The Alkyl group is responsible for the stability of a carbocation.

• Alkyl groups are electron-releasing groups.

• As the number of alkyl groups increases the stability of carbonation increases.

• During electrophilic addition, electrophile joins first.

• Greater stable carbocations have products with a higher percentage by mixture.

• Product with more stable carbocation forms the major product.

• A product with less stable carbocation forms the minor product.

• If any alkene reacts with halogen, it reacts by electrophilic addition.

• The dipoles on a halogen are induced by the pi electrons in an alkene.

Oxidation of Alkene

Alkenes react with cold dilute acidified KMnO4 to make diol.

CH2CH2 + [O] + H2O → CH2OHCH2OH

CH3CH=CHCH3 + [O] + H2O → CH3CH(OH)CH(OH)CH3

Alkenes react with hot concentrated acidified KMnO4.

There are three possible products.

Carbon dioxide - If the carbon with double bonds has two hydrogens.

Carboxylic Acid - If the carbon with double bonds has one hydrogen atom.

Ketone - If the carbon with double bonds has no hydrogens.