13.4 - Electrophilic Addition in Alkenes

Question 1

Define electrophile

Answer 1

Atom or group of atoms that is attracted to an electron-rich centre where it ACCEPTS a pair of electrons.

• Usually a +ve ion or has partially positive charge

Question 2

Why does a C=C attract electrophiles?

Answer 2

High electron density (due to pi bond presence), attracts electrophiles to accept an electron pair.

Question 3

Describe the first step in the mechanism for electrophilic addition.

Answer 3

In the case of butene + hydrogen bromide

• HBr is a dipole due to the difference in electronegativity between the H and Br
  1. Electron pair in pi-bond attracted to partially positive H. This breaks the double bond
  HBr breaks by heterolytic fission
  And the H of the H-Br bonds to the butene chain. This forms a carbocation and a Br-.

Question 4

Describe the second step in the mechanism for electrophilic addition.

Answer 4

Br- reacts with the carbocation forming a haloalkane.

Question 5

How is carbocation stability defined?

Answer 5

Primary carbocation - least stable
Secondary
Tertiary carbocation - most stable

Question 6

What does carbocation stability have to do with electrophilic addition of unsymmetrical alkenes?

Answer 6

• The more stable the carbocation the more likely it is to form.
  Markownikoff’s rule states: The hydrogen will atack the carbon atom with the most amount of hydrogens attached to it and the least amount of carbons.

Question 7

If the addition is between an alkene and a halogen, how is the dipole produced?

Answer 7

It is induced by the electron rich density of the pi bond.