Alkenes

Topic Content:

Alkenes are a homologous series of unsaturated hydrocarbons with a general molecular formula – C_n H_2n . Alkenes contain carbon-to-carbon double bonds and are sp² hybridized. The bond angle between carbon and hydrogen is 121.3° . Alkenes are also more reactive than alkanes because of the availability of electrons in the double bonds.

Laboratory Preparation of Ethene:

Ethene is prepared in the laboratory by dehydration of ethanol. Ethanol is heated with an excess of concentrated tetraoxosulphate(VI) acid at a temperature of 170°C. The gases produced are passed through sodium hydroxide solution to remove the carbon(IV) oxide and sulphur(IV) oxide produced from side reactions.

The reaction occurs in two stages. On mixing ethanol and concentrated tetraoxosulphate(VI) acid in a volume ratio of 1:2 respectively, ethyl hydrogentetraoxosulphate(VI), is formed.

\( \scriptsize C_2H_5OH_{(aq)}\: + \: H_2SO_{4(aq)} \rightarrow C_2H_5HSO_{4(aq)} \:+\: H_2O_{(l)}\)

Hydrogentetraoxosulphate(VI) decomposes on heating to produce ethene in the presence of tetraoxosulphate(VI) acid to produce ethene.

\( \scriptsize C_2H_5HSO_{4(aq)} \: + \: C_2H_{4(g)} \: +\: H_2SO_{4(aq)} \)

The overall reaction is known as dehydration of ethanol by hot concentrated hydrogentetraoxosulphate(VI) acid due to the fact that the overall change is the removal of water from ethanol