Electrophilic addition in alkenes Flashcards Preview

CHEM - 4.1.3 Alkenes > Electrophilic addition in alkenes > Flashcards

Flashcards in Electrophilic addition in alkenes Deck (7)
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What is an electrophile?

- 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.


Electrophilic addition

- 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.


Mechanism of electrophilic addition of polar molecule

- 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.


What is a nucleophile and carbocation?

- Nucleophile is an electron pair donor.
- Carbocation is an organic ion containing a positively charged carbon atom.


Mechanism for electrophilic addition of non-polar halogens

- 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.


Markownikoff's rule

- 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.


Carbocation stability

- 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).