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.
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.
- 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.