Reactions of the double bond are the main feature of the chemistry of the alkenes, and these make their chemistry rather different from that of the alkanes. The presence of the double bond has two main effects on the chemistry of the alkenes.
- It means that alkenes exhibit cis–trans isomerism. There is no rotation around the double bond – rotation would place great strain on one of the bonds and break it. As a result, geometric isomers frequently occur amongst the alkenes.
- It causes the alkenes to undergo addition reactions, rather than the substitution reactions of the alkanes. In an addition reaction, two substances react together to form a single product.
Alkenes are more reactive than alkanes, because the energy required to break the double bond is less than twice the energy required to split one single bond
347 kJ mol–1 for C–C, 612 kJ mol–1 for C=C.
The energy input needed to break one component of the double bond is therefore less than that needed to break a single bond, resulting in increased reactivity.
The reactions of the alkenes do not involve radicals. Instead, heterolytic fission of the double bond occurs, and it is this that determines how the molecules behave. The high electron density associated with the double bond means that the alkenes are attacked by both electrophiles (species that ‘love’ negative charge) and oxidising agents.
Reactions with oxygen
The alkenes, just like all the other hydrocarbon families, burn in air to produce carbon dioxide and water. Ethene will react explosively with oxygen in a highly exothermic reaction. However, the alkenes are not used as fuels for two main reasons – they produce a lot of carbon when they burn, and they are too valuable as a feedstock for the chemical industry.
There are a few instances where the reaction of an alkene with oxygen is useful, however. Industrially, oxygen is added to ethene in the presence of a finely divided silver catalyst and at 180oC to produce a ring compound called epoxyethane. This is the starting point for several other industrial syntheses, because the three–membered ring contains bonds that are under a lot of strain, and so epoxyethane is very reactive. These industrial processes include the formation of ethane–l,2–diol, an important component of the antifreeze mixtures that prevent the water in cars freezing in winter, and also a precursor in the manufacture of polyester for the textile market. Reactions between epoxyethane and alcohols are used in the production of solvents, plasticisers and detergents. (The non–systematic name for ethane–1,2–diol is ethylene glycol, a term still used for antifreeze in the motor trade.)
Reactions with halogens
The alkenes react with the halogens very differently from the reaction of the alkanes. For example, when ethene is bubbled through bromine water in the dark, the bromine is decolorised and a colourless liquid is formed that is immiscible with water. This reaction is typical of the alkenes and can be used to demonstrate the presence of a double bond. The alkene undergoes an addition reaction with the bromine; for example, ethene forms 1,2–dibromoethane as shown below.
All alkenes react vigorously with fluorine. The vigour of the reactions with the halogens decreases down the group, and reactions with iodine are relatively slow. Ethene and fluorine react explosively to form two molecules of tetrafluoromethane, while with iodine a very slow reaction takes place to form 1,2–diiodoethane. The reaction of ethene with chlorine shown above is particularly important in the manufacture of chloroethene (non–systematic name vinyl chloride), which is used to make polyvinylchloride or PVC, a widely used plastic.
Reactions with hydrogen halides
The double bond in the alkenes reacts readily with the hydrogen halides, producing the corresponding halogenoalkane. For example, ethene reacts with HBr as shown in the below figure. This reaction proceeds rapidly at room temperature, forming bromoethane.
The addition of hydrogen halides to asymmetrical alkenes like propene can lead to two possible products, as the reaction of hydrogen iodide with propene in the above figure shows. In this case, the major product formed is 2–iodopropane, with a smaller amount of the alternative 1–iodopropane. The likely products of the addition of hydrogen halides to asymmetrical alkenes can be predicted using Markovnikov’s rule:
When HX adds across an asymmetrical double bond, the major product formed is the molecule in which hydrogen adds to the carbon atom in the double bond with the greater number of hydrogen atoms already attached to it.
Like all simple rules in science, this one has its exceptions –however, Markovnikov’s rule does provide a useful way of deciding the most likely product of this type of reaction.
Reactions with hydrogen
The alkenes do not react with hydrogen under normal conditions of temperature and pressure. However, in the presence of a finely divided nickel catalyst and at a moderately high temperature (around 200oC), alkenes undergo an addition reaction with hydrogen to form the equivalent alkane. The reaction of ethene with hydrogen shown in the below figure is a typical example.
Reactions with sulphuric acid
The alkenes will react with concentrated sulphuric acid at room temperature, although the reaction does not proceed very rapidly. When ethene reacts with cold concentrated sulphuric acid, the compound ethyl hydrogensulphate is the result. This can be further reacted with water to produce ethanol. This procedure used to be carried out industrially to manufacture ethanol, but nowadays the direct catalytic hydration of ethene (addition of water across the double bond) is more commonly used. Similarly, propan–2–ol can be produced from the reaction between propene and concentrated sulphuric acid, which is still an industrial process of some importance. The reactions are shown in the below figure. Again, Markovnikov‘s rule says that propan–2–ol rather than propan–2–ol is the preferred product.
Reactions with acidified potassium manganate(VII)
The reaction of the alkenes with acidified potassium manganate(VII) involves both addition across the double bond and oxidation. The products of the reaction are alkanediols, and the purple manganate(VII) solution is decolorised in the process. For example, when ethene is bubbled through an acidified solution of potassium manganate(VII), the solution decolorises and ethane–1,2–diol is formed.
Ethane–l,2–diol, an extremely useful chemical in a variety of industrial processes, is not made in this way industrially because potassium manganate(VII) is a relatively expensive chemical and it is cheaper and more efficient to produce ethane–1,2–diol from epoxyethane, produced in the catalysed reaction of ethene with oxygen described earlier.
The reaction of ethene with potassium manganate(VII) can be used to identify the alkenes from the alkanes and alkynes, as the alkanes do not react and the alkynes react to form organic acids.