The Reaction Between Symmetrical Alkenes and the Hydrogen Halides
This page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactions between the hydrogen halides and alkenes like ethene and cyclohexene. Hydrogen halides include hydrogen chloride and hydrogen bromide. If you want the mechanisms explained to you in detail, there is a link at the bottom of the page.
Electrophilic Addition Reactions Between Symmetrical Alkenes and the Hydrogen Halides
Alkenes react with hydrogen bromide in the cold. The double bond breaks and a hydrogen atom ends up attached to one of the carbons and a bromine atom to the other.
In the case of ethene, bromoethane is formed.
Note: Be careful when you write the names of the addition products that you change the ene ending in the original alkene (showing the C=C) into an ane ending (showing that it has been replaced by C-C).
With cyclohexene you get bromocyclohexane.
The structures of the cyclohexene and the bromocyclohexane are often simplified:
Note: Each corner in one of these diagrams represents a carbon atom. Each carbon atom has enough hydrogens attached to make the total number of bonds up to 4.
In the case of the bromocyclohexane, it isn't necessary to write the new hydrogen into the diagram, but it is helpful to put it there to emphasise that addition has happened.
Be sure that you understand the relationship between these simplified diagrams and the full structures.
The reactions are examples of electrophilic addition.
With ethene and HBr:
and with cyclohexene:
Electrophilic Addition Reactions Involving the Other Hydrogen Halides
Hydrogen chloride and the other hydrogen halides add on in exactly the same way. For example, hydrogen chloride adds to ethene to make chloroethane:
The only difference is in how fast the reactions happen with the different hydrogen halides. The rate of reaction increases as you go from HF to HCl to HBr to HI.
The reason for this is that as the halogen atoms get bigger, the strength of the hydrogen-halogen bond falls. Bond strengths (measured in kilojoules per mole) are:
/ kJ mol-1
Note: You may find slightly different values depending on which data source you use. It doesn't matter – the differences are minor and the pattern is always the same.
As you have seen in the HBr case, in the first step of the mechanism the hydrogen-halogen bond gets broken. If the bond is weaker, it will break more readily and so the reaction is more likely to happen.
The reactions are still examples of electrophilic addition.
With ethene and HCl, for example:
This is exactly the same as the mechanism for the reaction between ethene and HBr, except that we've replaced Br by Cl.
All the other mechanisms for symmetrical alkenes and the hydrogen halides would be done in the same way.
Suggestion: Find out if your syllabus mentions a particular hydrogen halide, and learn that mechanism. You can simply swap the halogen atom if a different hydrogen halide comes up in an exam.
Haven't got a syllabus? If you are working towards a UK-based exam, follow this link to find out how to get one.
Click on the structures to see them in 3D.
Click on the reaction arrows to see an animation of the reaction.
Reproduced with permission from ChemTube3d.com