An electrophile is something which is attracted to electron-rich regions in other molecules or ions. Because it is attracted to a negative region, an electrophile must be something which carries either a full positive charge, or has a slight positive charge on it somewhere.
"] Ethene and the other alkenes are attacked by electrophiles. The electrophile is normally the slightly positive (+) end of a molecule like hydrogen bromide, HBr.
Electrophiles are strongly attracted to the exposed electrons in the pi bond and reactions happen because of that initial attraction - as you will see shortly.
You might wonder why fully positive ions like sodium, Na+, don't react with ethene. Although these ions may well be attracted to the pi bond, there is no possibility of the process going any further to form bonds between sodium and carbon, because sodium forms ionic bonds, whereas carbon normally forms covalent ones.
In a sense, the pi bond is an unnecessary bond. The structure would hold together perfectly well with a single bond rather than a double bond. The pi bond often breaks and the electrons in it are used to join other atoms (or groups of atoms) onto the ethene molecule. In other words, ethene undergoes addition reactions.
For example, using a general molecule X-Y . . .
Summary: electrophilic addition reactions
An addition reaction is a reaction in which two molecules join together to make a bigger one. Nothing is lost in the process. All the atoms in the original molecules are found in the bigger one.
An electrophilic addition reaction is an addition reaction which happens because what we think of as the "important" molecule is attacked by an electrophile. The "important" molecule has a region of high electron density which is attacked by something carrying some degree of positive charge.
Understanding the electrophilic addition mechanism
The mechanism for the reaction between ethene and a molecule X-Y
It is very unlikely that any two different atoms joined together will have the same electronegativity. We are going to assume that Y is more electronegative than X, so that the pair of electrons is pulled slightly towards the Y end of the bond. That means that the X atom carries a slight positive charge.
The slightly positive X atom is an electrophile and is attracted to the exposed pi bond in the ethene. Now imagine what happens as they approach each other.
You are now much more likely to find the electrons in the half of the pi bond nearest the XY. As the process continues, the two electrons in the pi bond move even further towards the X until a covalent bond is made.
The electrons in the X-Y bond are pushed entirely onto the Y to give a negative Y- ion.
In the final stage of the reaction the electrons in the lone pair on the Y- ion are strongly attracted towards the positive carbon atom. They move towards it and form a co-ordinate (dative covalent) bond between the Y and the carbon.
How to write this mechanism in an exam
The movements of the various electron pairs are shown using curly arrows.
Don't leave this page until you are sure that you understand how this relates to the electron pair movements drawn in the previous diagrams.
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