1. Electrophilic addition
Hydrocarbons
27 November, 2018

2. Recap
In the alkenes, within the double bond, one pair of electrons is held on the line between the two carbon nuclei but the other pair is held in a molecular orbital above and below the plane of the molecule ( bond)
A molecular orbital is a region of space within a molecule where there is a high probability of finding a particular pair of electrons.
The electrons are free to move anywhere in this shaded region and can move from one half to the other

3. So what does this mean?
Because the  electrons are not fully under control of the carbon nuclei as the electrons in the sigma bond, and because they lie exposed above and below the rest of the molecule, they are relatively open to attack by other things.

4. Electrophiles
Electrophile definition: An electron deficient species that can accept a lone pair of electrons (it usually carrier a positive charge or has a slight positive charge)
An electrophile will therefore attack the double bond in an alkene.

5. Electronegativity
Electronegativity definition: The pull an atom has on the bonding electrons.
This means that a more electronegative atom will pull the electrons closer to itself and further away from the less electronegative atom.
Atoms have same electronegativity (electrons equally shared)
B is slightly more electronegative so produces partial charges (- and +)
B is far more electronegative so produces complete charges

6. Addition reactions
It is very unlikely that two different atoms joined together will have the same electronegativity.
In this example, we will assume that Y is more electronegative than X, so the pair of electrons is pulled slightly towards the Y end of the bond. This means that the X atom carries a slight positive charge.

7. Addition Reactions
The slightly positive X atom is an electrophile and is attracted to the exposed pi bond in ethene.

8. Addition reactions
The slightly positive X atom eventually forms a bond with ethane.
The electrons in the X-Y bond are then pushed entirely onto the Y to make a Y- ion.
Carbocation

9. Addition reactions
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 dative covalent bond between the Y and the carbon.

10. The carbocation will always form on the most substituted carbon
The mechanism
Worked examples:
Ethene + Chlorine  1,2-dichloroethane
Prop-1-ene + Hydrogen Bromide  2-bromopropane
In an exam, you need to be able to draw the mechanism for the reaction. This is done using curly arrows to show the movement of various electron pairs.
The carbocation will always form on the most substituted carbon

11. Knowledge check
Have a go at drawing the mechanism and naming the product formed in the following reactions:
Bromine (Br2) with propene
Hydrogen bromide (HBr) with (E)-but-2-ene

12. Electrophilic addition mechanism
bromine with propene
CH3 H C
intermediate
carbocation
CH3 H C Br + + - Br Br -
CH3 H C Br Br
1,2-dibromopropane

13. hydrogen bromide with E but-2-ene
Electrophilic addition mechanism
hydrogen bromide with E but-2-ene
CH3 H C
intermediate
carbocation
CH3 H C + - + Br H
CH3 H C Br Br -
2-bromobutane