Organic Reactions Susbstitution S - Manu's...
Transcript of Organic Reactions Susbstitution S - Manu's...
Organic ReactionsSusbstitution – SN
1
Dr. Sapna Gupta
Kinetics of Nucleophilic Reaction
• Rate law is order of reaction
• 0 order is when rate of reaction is unaffected by change in concentration of the reactants
• 1st order is when rate of reaction doubles when one of the reactants is doubled
• 2nd order is when rate of reaction quadruples when two of the reactants are doubled
• Order of reaction explains the reaction mechanism
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• In this reaction a nucleophile is a species with an unshared electron pair which reacts with an electron deficient carbon
• A leaving group is substituted by a nucleophile
• Examples of nucleophilic substitution
Nucleophilic Substitution Reactions
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SN1 Reaction
• Unimolecular nucleophilic substitution.
• Two step reaction with carbocation intermediate.
• Rate is first order in the alkyl halide, zero order in the nucleophile.
• Racemization occurs.
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The SN1 Reaction Mechanism
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SN1 Energy Diagram
• There are three transition states. Step1 crosses the highest energy barrier and is rate determining
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Stereochemistry of SN1
Racemization: inversion and retention
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Factors Affecting Rates of SN1 Reactions
1. Substrate: 3° > 2° > 1° >> CH3X
2. Leaving group: should be weak base(like SN2)
3. Nucleophile: no effect since its not part of the transition state –but in general weak Nu can also react.
4. Solvent: Polar protic solvent best, it solvates ions strongly with hydrogen bonding.
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1) Nature of Substrate
Substrate: 3° > 2° > 1° >> CH3X
a) Order follows stability of carbocations (opposite to SN2)
b) More stable ion requires less energy to form
c) Resonance can also stabilize cation (e.g. allyl cation – C=C-C+)
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2) Leaving Group
• Leaving group is key in forming the carbocation intermediate for transition state.
• Larger halides ions are better leaving groups
• In acid, OH of an alcohol is protonated and leaving group is H2O, which is still less reactive than halide
• p-Toluenesulfonate (TosO-) is excellent leaving group
3) Nucleophile
• Does not matter since nucleophile is not part of the rate determining step.
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4) Solvent
Polar solvents help with ionization and stability of the carbocation.
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SN2 or SN1?
SN2 SN1
Substrate Primary or methyl Tertiary
Nucleophile Strong nucleophile Weak nucleophile (may also be solvent)
Solvent Polar aprotic solvent Polar protic solvent, silver salts
Kinetics [substrate][Nu] [substrate]
Stereochemistry Inversion Racemic mixture
Rearrangement No Yes
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Solved Problems
• Predict mechanism and stereochemistry of each product
• Predict mechanism of the following reaction
• Predict mechanism and stereochemistry of each product
Cl
CH3OH/ H2O
OH OCH3
HCl+
(R)-2-Chlorobutane
+ +
+ +Na+ CN- Na+ Br -
DMSOBr CN
Br
CH3S-Na+
SCH3
Na+Br-+ +acetone
(R)-2- Bromobutane
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Key Words/Concepts
•Substitution Reaction•Nucleophile•Electrophile•Leaving group•1st order reaction (unimolecular)•2nd order reaction (bimolecular)•Transition state•Rate determining step•Carbocation
•Polar protic solvent•Polar aprotic solvent•Non polar solvent•Solvolysis