14.2 Reactions of alkenes

• The double bond of an alkene is an area of high electron density, and therefore an area of high negative charge. The negative charge of the double bond may be attacked by electron-deficient species, which will accept a lone pair of electrons. These species have either a full positive charge or slight positive charge on one or more of their atoms. They are called electrophiles, meaning 'electron loving'. Alkenes undergo addition reactions when attacked by electrophiles.
• Electrophilic addition mechanism: 1

  In the first stage of electrophilic addition, the positive charge on the electrophile it attracted to the electron density in the double bond. As the electrophile approaches the double bond, electrons in the A-B bond are repelled towards B. The pi bond breaks, and A bonds to the carbon, forming a carbocation - an ion wih a positively-charged carbon atom. The two electrons in the A-B bond move to the B forming a B- ion.
• Electrophilic addition mechanism: 2

  In the second stage of electrophilic addition, the B- ion acts as a nucleophile and attacks the carbocation.

  The lone pair of electrons on the B- ion are attracted towards the positively-charge carbon in the carbocation, causing B to bond to it.
• Electrophilic addition of hydrogen bromide

  Reagent: hydrogen bromide.. it is electrophilic as the H is slightly positive

  Condition: room temperature

  Equation: C2H4 (g) + HBr (g) -> C2H5Br (l) bromoethane

  Step 1: As the HBr nears the alkene, one of the carbon-carbon bonds breaks. The pair of electrons attaches to the slightly positive H end of the H-Br. The HBr bond breaks to form a bromide ion. A carbocation is formed.

  Step 2: The bromide ion behaves as a nucleophile and attacks the carbocation. Overall there has been an addition of HBr across the double bond.
• Electrophilic addition of bromine

  Reagent: bromine (neat liquid or dissolved in tetrachloromethane, CCl4)

  Condition: room temperature, no catalyst or UV light required!

  Equation: C2H4 (g) + Br2 (l) -> CH2BrCH2Br (l) 1,2-dibromoethane

  It is surprising that bromine should act as an electrophile as it is non-polar.
• Electrophilic addition of bromine

  As a bromine molecule approaches an alkene, electrons in the pi bond of the alkene repel the electron pair in the bromine-bromine double bond thus inducing a dipole. (The electron pair is now nearer one end so the bromine molecule is polar and becomes electrophilic).