The Mechanisms of Electrophilic Addition Reactions
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
Different reactions of the alkenes including reagents and conditions
Alkene electrophilic addition reaction overview
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 charge and the H atom a partial positive charge
Partial charges in the H–Br bond
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 mechanism
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
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 partial charges in a halogen molecule are caused by the high electron density in the C=C bond
The temporary (or induced) dipole in a halogen molecule
Exam Tip
- The electrophilic addition reactions of alkenes with hydrogen halides and halogens are the same
- The difference is whether the electrophile is due to a permanent or temporary dipole
Addition of water
- Water is a weak electrophile, so does not undergo addition reactions with alkenes unless in the presence of a strong acid which can act as a catalyst
- H3O+ acts as the electrophile
- The reaction occurs in two steps
- Step 1
- The π electrons in the C=C are attracted to H3O+
- Heterolytic fission occurs and a carbocation is formed
- Step 2
- Water acts as a nucleophile and donates a pair of electrons to the positive carbon atom forming the C-O bond
- An equilibrium is established between the positive product and the deprotonated product (the alcohol)
- The H3O+ is regenerated as the catalyst
- Step 1
Electrophilic addition of water
This mechanism is catalysed by concentrated acid
