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TOPIC 6: ALKENE, simple alkenes do not react with nucleophiles, alkenes…
TOPIC 6: ALKENE
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Halogenation
- Halogens add to π bonds
- Halogens are polarizable
- The electron-rich double bond induces a dipole in an approaching halogen molecule
- Making one halogen atom electron deficient and the other electron rich (Xσ+-Xσ–).
- The electrophilic halogen atom is then attracted to the nucleophilic double bond, making addition possible
- Two facts demonstrate that halogenation follows a different mechanism from that of hydrohalogenation or hydration :
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- Only anti addition of X2 is observed
- These facts suggest that carbocations are not intermediates
- Halogenation with Cl2 and Br2 is generally effective
- Halogenation with I2 is too slow
- Halogenation with F2 is too violent
- Halogenation occurs with ANTI addition
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Hybridization
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Each carbon is sp2 hybridized and trigonal planar, with bond angles of approximately 120°.
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Markonikov's Rule
Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom—that is, the carbon that has the greater number of H atoms, to begin with.
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Carbocation Stability
- The H atom is added to the less substituted carbon to form the more stable, more substituted carbocation.
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- The formation of a carbocation in the rate-determining step of the mechanism.
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