ASSIGNMENT CHEMISTRY (ENGLISH) COURSE : SANKALP (VCMP)
REACTION MECHANISM Reaction Mechanism : Substrate (Alkyl Halides)
Reagent
Dominating Mechanism
CH3 – X
Any nucleophile
SN2
1º R – X
Any nucleophile (except hindered base)
SN2 Major
*Hindered base give E-2 reaction.
2º R – X
Strong nucleophile ( SH , RS I , Br , Cl , CN ,NH3, OAc )
SN2 Major > E-2 minor
2º R – X
Strong base ( NH2 , Alc.KOH,MeO–, EtO–, t–BuO–, Et3N)
E2 Major > SN2 minor
2º R – X
Weak nucleophile + Polar Protic solvent (H2O, ROH, RCOOH)
SN1 > E-1
–
3º R – X
Strong nucleophile ( SH , RS I , Br , Cl , OAc )
No reaction
3º R – X
Strong base ( NH2 , alc. KOH, MeO–, EtO–, t-BuO–, Et3N,OH–)
E-2 Only
3º R – X
Weak nucleophile or weak base (H2O, ROH, RCOOH)
SN1 > E-1
Substrate (Alcohol)
Reagent
Dominating Mechanism
ROH
HI, HBr, HCl / ZnCl2
SN1
HI, HBr, HCl
SN2
ROH (1º>2º>3º)
PX3, PX5, P4+ X2, SOCl2/Base
SN2
ROH (1º>2º>3º)
SOCl2 without base
SNi [Retention]
ROH (3º>2º>1º)
Conc.H2SO4,H3PO4,HClO4, KHSO4/
E-1 (Saytzeff)
ROH
Al2O3/, P2O5/ (Saytzeff elimination without rearrangement) dehydration reaction
ROH
ThO2/, WO3/ (Hoffmann elimination without rearrangement) dehydration reaction
–
(3º 2º &1º with -branch) ROH (1º without -branch)
RESONANCE
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Important feature of reaction mechanism SN1
Two steps process/Intermediate C+/Rearrangement/Weak Nu./PPS/Racemisation/ Rate : 3º > 2º > 1º or (Stability of C+),Ag+
SN2
One step process/No intermediate/No rearrangement/Strong Nu–/PAS/Inversion at -carbon/ Rate : CH3 – X > 1ºR–X > 2º R – X/Stereospecific reaction.
E1
Two step process/Intermediate C+/Rearrangement/Weak base//Regioselective reaction saytzeff alkene (more branched) major. Rate : 3º > 2º > 1º or (Stability of C+)
E2
One step process/No rearr./Strong base, /Regioselective, Regiospecific, stereoselective, stereospecific, antielimination reaction. Kinetic primary isotopic effect is observed. Rate of E-2 : 3º > 2º > 1º Smaller base Saytzeff alkene Big base Hofmann alkene.
E1 cB Two step/Intermediate C/Hofmann elimination product. Regioselective, stereospecific, anti elimination reaction. Dexchange at -carbon.
Condition : More acidic -H or e.g. (F, NR 3 , SR 2 ) or EWG at –position. Condition for Hofmann elimination : (1) Hindered strong base (2) Poor leaving group.
(3) When Strong – I groups behave as leaving group. (–F, NR 3 – SR 2 ) (4) When-carbon is quaternary.
RESONANCE
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RESONANCE
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