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SN1Substitution Nucleophilic Unimolecular (1)
SN2Substitution Nucleophilic Bimolecular (2)
파일
• Two step
• Formation of carbocation intermediate
• One step
• Unstable transition state
https://lh7-us.googleusercontent.com/gepkW_X3ZAUz_zFwmRBWtnzu6ytXUqk43HM6zQJ41EyDRzKIfpm3MlcbtGAZtR9o4IXdks3L9LExNnzDqGB4bZ97TCGuzcTU5bNbH8UIWwM_81cFCBGaDJfaYNpsQtwCTRwfkmEsjJGiaYQ-Z7YahaA
https://lh7-us.googleusercontent.com/vDg978BBT3yswKQHow43lFQ0dzEktNXQ2RIx_oKteCwRi-oeh5BKhnooXE2svzWiPWk0-eZbbxVZs1xczE0sxv0xzif0PpwtbHgmyg80AcVlinHrHmFilQ9A7ZggDLk54wc4QwwCa1aDq-PItS0W-jc
Unimolecular
Rate = k [halogenoalkane]
Biomolecular
Rate = k[halogenoalkane][nucleophile]
1. SN1 > SN2 (in alkaline/ethanolic solution)
• Formation of transition state requires more Ea
2. Leaving group: F < Cl < Br < I HalogenoalkanePolarity & ease to be nucleophilic attacked (n.a.)Bond strength & ease to break C-Xfluoro-↑ (most polar and
prone to n.a.)*
↓ (least polar and less prone to n.a.)↑ (strongest and hardest to break)
↓ (weakest and easiest to break)chloro-bromo-iodo-
*though C-F is the most polarized bond and susceptible to n.a, ultimately the C-X bond needs to be broken in the reaction, and due to C-F being a very strong bond it reacts very slowly.
3. Solvent
*Measured by colorimetry of silver halide precipitate formation upon rxn with AgNO3
polar, protic solvents (H-bonding)
• stronger ion-dipole to solvate/stabilise carbocation intermediate
e.g. Water, alcohol, carboxylic acid
polar, aprotic solvents (non H-bonding)
• Polar enough to solvate the nucleophile but does not participate in H-bonding with the nucleophile (would make Nu less reactive)
e.g. (CH3)2CO, CH3CN
• 50/50 mix of enantiomers
(racemic mixture - optically inactive)
• Inversion of configuration
(optically active)