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Addition of Hydrogen Halides to Alkenes

HX Addition to Alkenes

Alkenes differ from alkanes in that they contain an additional bond, a π bond, which makes them much more reactive than alkanes. Similar to the free electron pair of Lewis bases, the electron cloud of the π bond is easily accessible to electrophiles. In a reaction with a polar molecule such as hydrogen chloride (HCl), for example, the π bond of an alkene reacts as a nucleophile. Thus, the partially positively charged hydrogen atom of HCl acts as an electrophile protonating the double bond of the alkene.

By adding a proton of a strong acid to a double bond a carbenium ion is formed.
Electrophilic addition of HCl to alkenes

The carbenium ion, which is formed by protonation of an alkene by HCl, is subsequently attacked by the chloride anion resulting in a monohalogenated alkane. The reaction type is A + B → C, an addition reaction. Since the first step in this addition reaction is the interaction of a double bond with an electrophile, the reaction, therefore, is called an electrophilic addition.

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Mechanism of the electrophilic addition of HCl
  • The electron cloud of the π bond (π orbital) is located above and below the molecular plane. Alkenes are nucleophiles because their π electrons are easily accessible to electrophilic reaction partners from a steric as well as energetic point of view.
  • The dissociation energy of the π bond is about 272 kJ/mol and therefore noticeably smaller than the dissociation energy of 350 kJ/mol attributed to a σ bond. Therefore, the π bond can more easily be broken than a σ bond.
The electrophilic addition
A π bond can be broken without attacking the σ bond. The two initially double-bonded carbon atoms each form a new σ bond to a reaction partner. This addition reaction is typical of alkenes.


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