Electrophilic addition in alkenes

• A reaction mechanism is a series of steps that shows how a reaction takes place. Electrophilic addition is the mechanism for the addition reactions that alkenes take part in to form saturated compounds.
• The double bond in an alkene represents a region of high electron density because of the presence of the Pi electrons

  The high electron density of the double bond attracts electrophiles
• An electrophile is an atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair. An electrophile is usually a positive ion or a molecule containing an atom with a partial positive charge.
• Mechanism for the reaction between but-2-ene and hydrogen bromide

  1. Bromine is more electronegative than hydrogen, so hydrogen bromide contains the dipole H^+-Br^-

  2. The electron pair in the Pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break

  3. A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond

  4. The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom

  5. A bromine ion and a carbocation are formed. A carbocation contains a positively charged carbon atom

  6. The Br^- ion reacts with the carbocation to form the addition product
• Mechanism for the reaction between propene and bromine

  1. Bromine is a non-polar molecule

  2. Pi electrons interact with electrons in Br-Br bond

  3. Polarisation of Br-Br bond, one end slightly positive, other slightly negative

  4. Electron pair in Pi bond attracted to slightly positive end, double bond breaks

  5. Br-Br bond breaks by heterolytic fission, electron pair goes to slightly negative Br

  6. Bromide ion and carbocation formed

  7. Br ion reacts with carbocation to form addition product
• Markownikoff's rule:

  When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms
• In the reaction between propene and a hydrogen halide, two carbocations are possible: a primary and a secondary carbocation.
• In a primary carbocation the positive charge is on a carbon atom at the end of the chain
• In a secondary carbocation, the positive charge is on a carbon atom with two carbon chains attached
• In a tertiary carbocation, the positive charge is on a carbon atom with three carbon chains attached
• When propene reacts with hydrogen bromide, the major product is 2-bromopropane, formed from the secondary carbocation. The yield of 1-bromopropane is much smaller
• Carbocations are classified by the number of alkyl groups attached to the positively charged carbon atom. An alkyl group is usually represented by the symbol -R.
• Tertiary carbo cations are the most stable and primary carbocations are the least stable
• Carbocation stability is linked to the electron-donating ability of the alkyl groups. Each alkyl group donates and pushes electrons towards the positive charge of the carbocation. The positive charge is spread over the alkyl groups. The more alkyl groups attached to the positive carbon atom, the more stable the carbocation.
• Addition of a hydrogen halide to an unsymmetrical alkene forms the major product via the most stable carbocation