Alkenes are susceptible to electrophilic addition because their carbon-carbon double bond contains an exposed, electron-rich pi bond that attracts electron-deficient species called electrophiles. The pi bond can break as new, stronger sigma bonds form across the two carbon atoms.
The Reasoning
A carbon-carbon double bond consists of one sigma bond and one pi bond. The sigma bond forms through head-on orbital overlap, while the pi bond forms through sideways overlap above and below the plane of the carbon atoms.
| Feature | Sigma bond | Pi bond |
|---|---|---|
| Orbital overlap | Head-on overlap | Sideways overlap |
| Electron density | Concentrated along the internuclear axis | Exposed above and below the molecular plane |
| Relative strength | Stronger | Weaker and more easily broken |
The exposed pi electrons create a region of high electron density. An electrophile is an electron-pair acceptor, so it is attracted to this region and accepts an electron pair from the alkene.
For example, when ethene reacts with bromine, the pi-electron density induces a dipole in . The nearer bromine atom becomes , allowing it to act as the electrophile. An electron pair from the pi bond forms a new carbon-bromine bond, and the bromine-bromine bond undergoes heterolytic fission. Further bond formation produces 1,2-dibromoethane.
This is an addition reaction because two reactant species combine to form one main product, with atoms added across the double bond. The sigma framework remains, but the pi bond is replaced by two new sigma bonds.
A common misconception is that the two bonds in a carbon-carbon double bond are equivalent. They are not: the weaker, exposed pi bond accounts for the characteristic reactivity of alkenes.
Exam Technique
This explanation is required at both SL and HL under Reactivity 3.4. At HL, a mechanism question may also require correctly placed curly arrows: each arrow must begin at an electron pair or bond and point to where a new bond forms.
