This week post is about Organic Chemistry: Reactions of alkenes.
Last week, we talked about the reactions of alkanes. This week, we’re diving into alkenes!
First reaction
Combustion:
Complete Combustion: When alkenes burn in the presence of excess oxygen, they produce carbon dioxide and water. This reaction is similar to the combustion of alkanes but tends to be more sooty due to the higher carbon-to-hydrogen ratio in alkenes.
Incomplete Combustion: If the oxygen supply is limited, combustion of alkenes results in the production of carbon monoxide and potentially more soot, alongside water. This happens because the double bonds in alkenes make them more reactive compared to alkanes.
Addition Reactions
Hydrogenation:
Alkenes can react with hydrogen in the presence of a catalyst (typically platinum, palladium, or nickel) to form alkanes. This is used in the industrial preparation of saturated hydrocarbons.
Conditions: Temperature of 200 °C and presence of nickel catalyst.
Halogenation:
Alkenes react with halogens (such as bromine or chlorine) to form dihalogenated compounds. For instance, ethene reacts with bromine to form 1,2 dibromoethane, a reaction that is also a test for unsaturation.
Reddish brown aqueous bromine decolourises/turns colourless in the presence of alkenes. This is a common test to determine the presence of unsaturated hydrocarbons.
Hydration:
Water can be added to alkenes in the presence of an acid catalyst to form alcohols. Example, ethene can be hydrated to produce ethanol.
Conditions: Temperature of 300 °C, 60 atm and presence of phosphoric (V) acid catalyst.
Third reaction
Polymerization:
This process involves the joining of many small alkene molecules (monomers) to form long polymer chains. For example, polymerization of ethene under high pressure and temperature in the presence of a catalyst produces polyethylene, a widely used plastic.
Conditions: Typically, temperatures between 100 to 300 degrees Celsius and pressures from 1000 to 3000 atmospheres are used, with catalysts such as peroxides helping to speed up the reaction.
That’s it!
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