Because they are unsaturated hydrocarbon compounds, alkenes are much more reactive than alkanes. In addition to combustion, the double bonds in alkenes can be broken to facilitate the addition of hydrogen and many other species.
Combustion of Alkenes
Like all other hydrocarbons, alkenes burn in oxygen to produce carbon dioxide and water vapor. Alkenes show a greater tendency than alkanes to undergo incomplete combustion, forming much carbon monoxide and carbon rather than carbon dioxide.
The combustion reaction of alkenes is not as important as that of alkanes. Unlike alkanes, alkenes are not used as fuels. Instead, because of their reactivity, alkenes are used as the starting point in the manufacture of many important chemicals.
An example of the combustion of an alkene is:
C2H4(g) + 3O2(g) ------------> 2CO2(g) + 2H2O(g)
Addition Reactions of Alkenes
Most of the reactions that alkenes undergo involve the breakage of their double bond and the addition of another species. These are addition reactions.
Addition of Hydrogen
Hydrogen can be added to an alkene to form the corresponding alkane. This reaction occurs in the presence of a palladium, platinum or nickel catalyst. Heat is required for this reaction when a nickel catalyst is used.
Hydrogenation, as this reaction is called, is an important process in the food industry when nickel is used as a catalyst. Via hydrogenation, less valuable(commercially), unsaturated, edible oils, such as those present in sunflower seed oil and peanut oil are converted to more valuable, saturated, edible fats such as margarines.
An example of the hydrogenation of an alkene is:
CH2=CH2(g) + H2(g) ------------> CH3-CH3(g)
Addition of Halogens
Halogens react with alkenes forming halogenoalkanes. This reaction can occur without catalysis.
The addition of liquid bromine to an alkene serves as a characteristic test. If liquid bromine, which is a brown liquid, is added to an alkene without light or heat, a colorless solution is formed. This is because the brown liquid bromine reacts with the alkene to form a bromoalkane which is colorless.
An example of this reaction is:
CH2=CH2(g) + Br2(l) -------------> BrCH2CH2Br(l)
Bromine water also reacts and is decolorised by alkenes. However, different products are formed in this reaction. A bromoalcohol and hydrogen bromide are the products formed when bromine water reacts with alkenes.
An example of this reaction is:
CH2=CH2(g) + Br2(aq) + H2O(l) ------------> BrCH2CH2OH(aq) + HBr(aq)
Addition of Hydrogen Halides
Hydrogen chloride, hydrogen bromide and hydrogen iodide all react with alkenes to form halogenoalkanes.
An example of the addition of a hydrogen halide to an alkene is:
CH2=CH2(g) + HBr(g) ------------> CH3CH2Br(l)
Oxidation of Alkenes
Alkenes are oxidized by alkaline potassium manganate(VII), a weak oxidizing agent. When alkenes are added to the purple alkaline potassium manganate(VII), there is a color change to brown as the alkenes are being oxidized. The product formed as a result of this oxidation is a diol. This reaction can be used to test for carbon-carbon double bonds.
An example of the oxidation of an alkene is:
CH2=CH2(g) (+ KMnO4, OH-) ------------> CH2OH-CH2OH(l)
Polymerization
Polymerization of alkenes occurs when many alkene molecules add together to form one large molecule called a polymer.
The polymerization of alkenes is important in the formation of plastics, rope, food boxes, bowls and buckets among many others
An example of a polymerization reaction is:
nCH2=CH2(g) ------------> -[-CH2-CH2-]-n(s)
Alkenes are much more reactive than alkanes because of their unsaturated nature. The breakage of their double bond allows many reactions, other than combustion, to occur in alkenes. Examples of these are addition reactions, oxidation and polymerization.
SOURCES
Ramsden, E.N. (2000) A Level Chemistry. Nelson Thornes Ltd.
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