What happens when alkenes react with bromine

Simple reactions of alkenes

Electrophilic addition

As has already been said in the treatment of ethene, the main reaction of all alkenes is electrophilic addition. The mechanism of this type of reaction is explained in detail elsewhere, I would like to go into more detail on this page about the reaction products that can be synthesized with the help of such an electrophilic addition.

Hydrogenation

The addition of hydrogen to an alkene generally leads to the corresponding alkane; for example, butene becomes butane. Hydrogenation is often used for analytical purposes; it is quantitative (i.e. the hydrogen used is completely attached to the double bonds, no double bond is spared), so it is easy to determine the number of double bonds in an organic molecule in this way. For example, you can find out how many double bonds an unsaturated fatty acid contains. You don't even have to determine the volume of hydrogen consumed; it is often enough to measure the heat of hydrogenation. About 126 kJ / mol are released per hydrogenated double bond, sometimes more, sometimes less. However, the alkene cannot simply be brought into contact with gaseous hydrogen; instead, special catalysts are necessary so that the addition of H2 can take place. This is simply due to the rather high activation energy of the hydrogenation.

For further details see "Hydrogenation of Alkenes" on this homepage.

Halogenation

The addition of chlorine or bromine takes place at room temperature. The mechanism of the bromination of an alkene is described in detail on the corresponding page. Bromination is often used in school lessons to detect C = C double bonds, mostly hexene or cyclohexene is used for this. Afterwards, I like to take two different vegetable oils, one with few unsaturated fatty acids and, for comparison, one with a high proportion of unsaturated fatty acids. Then I show the S. that you can use bromine water to detect the double bonds in a vegetable oil. In good chemistry courses I let the S. develop an experiment with which they can find out quantitatively (or at least to some extent quantitatively) which of the two vegetable oils contains more unsaturated fatty acids.

The result of the halogenation are so-called vicinal dihaloalkanes. The word "vicinal" (lat.vicinus = Neighbor) means that the two halogen atoms are on directly adjacent carbon atoms. The bromination of 2-butene therefore always results in 2,3-dibromobutane and not 2,2-dibromobutane or 1,3-dibromobutane.

For further details see "Bromination of Alkenes" on this homepage.

Hydrohalogenation

If elemental bromine or chlorine is not used for the addition, but instead hydrogen bromide or hydrogen chloride, monobromo- or monochloroalkanes are obtained. The mechanism of this reaction is somewhat simpler than that of bromination or chlorination, because HBr or HCl are already dipole molecules, so the hydrogen can easily be split off as a proton. The proton then attaches itself to the C = C double bond, creating a carbenium ion. With ethene there is only one possibility for the formation of a carbenium ion, because the two carbon atoms of ethene are equivalent (both are primary carbon atoms). In propene, however, one of the carbon atoms is primary, while the other is secondary. Since secondary carbenium ions are more stable than primary ones, the proton usually attaches itself to the primary carbon atom, which then creates a secondary carbenium ion (Markovnikov's rule).

For further details see "Hydrobromination of Alkenes" on this homepage.

When a proton is added to a C = C double bond, the more stable carbenium ion is always formed, if there are several possibilities.

Hydration

Hydration is the addition of water to an alkene. Here, too, a proton initially attaches itself to the C = C double bond, whereby the Markovnikov rule again applies (see Figure 1). In industry, the direct addition of water to alkenes is a major process for the production of alcohols. Alkenes can easily be obtained from natural gas or petroleum, either by direct isolation from the raw materials or by cracking longer-chain alkanes.

Addition of sulfuric acid

One reaction that is not discussed in normal school lessons is the addition of sulfuric acid to the C = C double bond. If an alkene is allowed to react with cold, concentrated sulfuric acid, an alkyl hydrogen sulfate is formed. Liquid alkenes are simply mixed with the sulfuric acid, gaseous alkenes are passed through the sulfuric acid.

Summary: Simple reaction of alkenes

The most important simple reactions of the alkenes

In this picture you can see the five addition reactions of alkenes mentioned in the text. What has not yet been taken into account in the picture are dimerizations and polymerizations of alkenes. I have created two separate pages for this, because these processes are very important for the industrial chemical industry, for example for the manufacture of plastics.