Electrophilic addition in alkenes

Question 1

What is an electrophile?

Answer 1

• Electrophile is an electron pair acceptor and is attracted to an electron-rich centre.
• Positively charged ions are electrophiles.
• Polar molecules can also be electrophiles as the partially positive atom is attracted to places with electrons.

Question 2

Electrophilic addition

Answer 2

• The double bond in an alkene represents a region of high electron density because of the presence of pi-electrons.
• This high electron density of pi-electrons attracts electrophiles.
• The double bond in alkene opens up and atoms are added to the carbon atoms.

Question 3

Mechanism of electrophilic addition of polar molecule

Answer 3

• 2 electrons from the C=C attack the partially positive X atom, creating new bond between Carbon 1 and X atom.
• X-Y bond breaks by HETEROLYTIC fission.
• Electrons are taken by Y atom to form a negative ion w/ lone pair.
• Carbon 2 is electron deficient as C1 took the electrons to form bond w/ X atom when double bond broke.
• So C2 is a positively charged CARBOCATION.
• Y atom acts as a NUCLEOPHILE, donates its lone pair and forms bond w/ C2.

Question 4

What is a nucleophile and carbocation?

Answer 4

• Nucleophile is an electron pair donor.

- Carbocation is an organic ion containing a positively charged carbon atom.

Question 5

Mechanism for electrophilic addition of non-polar halogens

Answer 5

• When halogen approaches an alkene, the double bond repels the electrons in halogen molecule, polarising Hal-Hal. This is an INDUCED DIPOLE.
• HETEROLYTIC fission of halogen molecule. The partially pos. halogen bonds to C1.
• Positively charged carbocation attracts the negative halogen.

Question 6

Markownikoff’s rule

Answer 6

• If hydrogen halide adds to symmetrical alkene like ethene, only 1 possible product. If it adds to an unsym. alkene, 2 possible products w/ halogen on either carbons.
• Markownikoff’s rule says that the major product from addition of a hydrogen halide to an unsymmetrical alkene, is the one where hydrogen adds to the carbon w/ the most hydrogens already attached.

Question 7

Carbocation stability

Answer 7

• Amount of each product formed depends on how stable the carbocation formed is.
• 3 possible carbocations: primary (2H 1 Alkyl), secondary (1H 2 Alkyl) and tertiary (0H 3 Alkyl).