3.4.2 mechanisms for addition reactions of alkenes

electrophilic addition of hydrogen halides to alkenes

overall equation:

mechanism:

• halogen atoms are more electronegative than hydrogen atoms, so hydrogen halide molecules have a permanent dipole

• H is δ+ and Br is δ-

• the δ+ H acts as an electrophile: it is attracted to the high electron density of the C=C bond, and accepts the electron pair in the pi bond

• as this C-H bond forms, the bonding pair between H and Br moves completely onto Br, so the H-Br bond breaks by heterolytic fission

• a Br^- ion and a carbocation intermediate are formed

• the C atom in the carbocation is positive as it has lost its share of the electron pair that was in the pi bond (it has lost an electron)

• the lone pair of the Br^- ion is attracted to the C⁺ in the carbocation, forming a C-Br bond.

Markovnikov’s rule:

• when a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen is more likely to bond to the carbon which is already attached to the greater number of hydrogen atoms.

  • this will form a more stable carbocation so will form the major product.

  • tertiary carbocations are the most stable.

electrophilic addition of halogen molecules to alkenes

overall equation:

mechanism:

• Br₂ is non-polar.

• the high electron density of the C=C bond repels the electron pair of the Br-Br bond.

• Br-Br becomes polarised and now has an induced dipole.

• δ+ Br acts as an electrophile and is attracted to C=C, and it accepts the electron pair in the pi bond.

• as this C-Br bond forms, the bonding pair in the bromine molecule moves completely onto the other Br atom, so the Br-Br bond breaks by heterolytic fission.

• a Br^- ion and a carbocation intermediate are formed.

• the C atom in the carbocation is positive as it has lost its share of the electron pair that were in the pi bond (it has lost an electron).

• the lone pair of the Br^- ion is attracted to the C⁺ in the carbocation, forming a C-Br bond.

only one possible product

electrophilic addition of sulfuric acid to alkenes

overall equation:

mechanism:

• in the sulfuric acid, one of the δ+ H acts as an electrophile: it is attracted to the high electron density of the C=C bond, and accepts the electron pair in the pi bond.

• as this C-H bond forms, the bonding pair between H and O moves completely onto O; this is heterolytic fission.

• a HO-O₂S-O^- ion (O^-SO₂OH, hydrogen sulfate ion) and a carbocation intermediate are formed.

• the C atom in the carbocation is positive as it has lost its share of the electron pair that were in the pi bond (it has lost an electron).

• the lone pair of the O^- in the hydrogen sulfate ion is attracted to the C⁺ in the carbocation, forming a C-OSO₂OH bond.

apply Markovnikov’s rule

hydration of alkenes - electrophilic addition of water to alkenes

overall equation:

mechanism:

• H⁺ from phosphoric/sulfric acid catalyst acts as an electrophile, it is attracted to the high electron density of the C=C bond, and accepts the electron pair in the pi bond.

• C-H bond forms, so a carbocation intermediate and a dihydrogen phosphate ion is formed.

• a lone pair on a water molecule joins to C⁺ of the carbocation

• an intermediate molecule with a O+ is formed; the O is + as water is neutral, but now O has lost an electron/is sharing it with C.

• H⁺ is lost from the intermediate and bonds to dihydrogen phosphate ion, so the acid catalyst is regenerated.

• as H⁺ leaves the intermediate, the bonding pair in the O-H bond moves completely to O: heterolytic fission.

apply Markovnikov’s rule