Electrophilic Addition Mechanism
Electrophilic Addition
- Electrophilic addition is the addition of an electrophile (or Lewis acid) to an alkene double bond, C=C
- The alkene double bond, C=C, is an area of high electron density which makes it susceptible to attack by electrophiles
- The C=C bond breaks forming a single C-C bond and 2 new bonds from each of the two carbon atoms
- Electrophilic addition reactions include the addition of:
- Hydrogen, H2 (g)
- Steam, H2O (g)
- Hydrogen halides, HX
- Halogens, X2
Alkene electrophilic addition reaction overview
Electrophilic addition of hydrogen halides
- A hydrogen halide molecule is polar as the hydrogen and halogen atoms have different electronegativities
- For example, in a molecule of hydrogen bromide, HBr, the bromine atom has a stronger pull on the electrons in the H-Br bond
- As a result of this, the Br atom has a partial negative and the H atom a partial positive charge
Due to differences in electronegativities of the hydrogen and bromine atom, HBr is a polar molecule
- In electrophilic addition reactions with hydrogen halides, the H atom acts as an electrophile and Lewis acid by accepting a pair of electrons from the C=C bond in the alkene
- The H-Br bond breaks heterolytically, forming a Br- ion
- This results in the formation of a highly reactive carbocation intermediate which reacts with the bromide ion, Br-
- For example, the mechanism for the electrophilic addition of hydrogen bromide and ethene is:
Electrophilic addition reaction of HBr and ethene to form bromoethane
Exam Tip
For electrophilic addition mechanisms, the curly arrows must:
- Be double-headed to show the movement of a pair of electrons
- Start from a lone pair of electrons or an area of high electron density, e.g. the C=C bond
- Move towards a δ+ electrophile or the positive charge of a carbocation
Examiners often comment about the poor and incorrect use of curly arrows in organic mechanisms
Electrophilic addition of halogens
- The mechanism for the electrophilic addition of halogens (and hydrogen) is the same as the electrophilic addition of hydrogen halides with one key exception:
- Hydrogen halide molecules have a permanent dipole (as shown above)
- Halogen molecules have a temporary (or induced) dipole caused by the repulsion of the halogens electrons by the high electron density C=C bond
The temporary (or induced) dipole in a halogen molecule
Electrophilic addition of interhalogens
- Interhalogens are compounds that contain two or more different type of halogens
- The mechanism for the electrophilic addition of interhalogens is the same as the electrophilic addition of hydrogen halides
- Just like hydrogen halide molecules, interhalogens have a permanent dipole
- Differences between the electronegativity of the halogens determine which halogen will become the δ+ electrophile
- The electronegativity increases as you move up the halogens, F > Cl > Br > I
The polarity of interhalogen molecules
Exam Tip
The electrophilic addition reactions of alkenes with hydrogen halides, halogens and interhalogens are the same. The difference is whether the electrophile is due to a permanent or temporary dipole