lecture 18 10 08 2014i.ppt

CHEM 342 - Fall 2014

PRINCIPLES OF ORGANIC CHEMISTRY I**

**Nucleophilic Substitution

Carbocation StabilityCarbocations may be classified as either primary (1), secondary (2), and tertiary (3)Classification based on number of alkyl groups attached to the charged carbon atom*


Carbocation StabilityAs the number of alkyl groups increases, the stability of the carbocation increasesIncrease in stability due to both inductive effects and hyperconjugation*


Inductive EffectsInductive effects are electronic effects that occur through sigma (s) bondsElectron donating groups stabilize a carbocation while electron withdrawing groups destabilize a carbocation*


Inductive EffectsAlkyl groups are electron donating groups that stabilize a carbocationIncreasing the number of electron-donating alkyl groups increases carbocation stability*


Inductive EffectsElectrostatic plots show a decrease in areas of low electron density (blue) on carbon as alkyl substitution increases*


HyperconjugationHyperconjugation is the delocalization of charge by overlap of an empty p orbital with an adjacent s bond*


HyperconjugationDelocalization of the positive charge stabilizes the carbocationThe greater the number of alkyl groups, the greater the opportunity for hyperconjugation, the larger the stabilization*

**9/26/2014*Nucleophilic Substitution

Hammond PostulateThe transition state resembles that of the structure closet to it in energyFor an exothermic reaction with an early transition state, the transition state resembles starting material *9/10/2014


Hammond PostulateLowering the energy of the products has little effect on the energy of the transition stateThe energy of activation is not affected and the reaction rate is unaffected *9/10/2014


Hammond PostulateThe transition state resembles that of the structure closet to it in energyFor an endothermic reaction with a late transition state, the transition state resembles products

*9/10/2014


Hammond PostulateThe more stable (lower energy) the products, the more stable (lower energy) the transition stateA more stable transition state results in a lower energy of activation and faster reaction rate *9/10/2014


Hammond PostulateIn an SN1 reaction, formation of the carbocation intermediate is an endothermic processConsequently the transition state will resemble the structure of the carbocation *9/10/2014


Hammond PostulateConsequently, the more stable the carbocation the more stable the transition stateA more stable transition state results in a lower energy of activation and faster reaction rate *9/10/2014


Hammond PostulateThe greater the number of alkyl substitutents, the more stable (lower energy) the carbocationThe more stable the carbocation, the more stable (lower energy) the transition state *9/10/2014


Hammond PostulateA lower energy transition state requires less energy of activation and proceeds at a faster rateConsequently the more substituted carbocation will form faster *9/10/2014


Hammond PostulateThe Hammond postulate predicts the more stable tertiary carbocation will form faster that the less stable secondary carbocation *9/10/2014


Structure of the Alkyl HalideFour factors are used to determine the mechanistic pathway followed by a substitution reactionThe structure of the alkyl halidePrimary, secondary, or tertiaryThe nature of the nucleophileStrong or weak The nature of the leaving groupGood or poorThe nature of the solventProtic or aprotic *


The Structure of the Alkyl HalideThe most important factor in determining whether a reaction follows the SN1 or SN2 mechanistic pathway *


The Nature of the NucleophileThe second factor to consider is the relative strength of the nucleophileThe identity of the nucleophile is not important for an SN1 reaction because it does not appear in the rate equation rate = k[RX]The identity of the nucleophile is important for an SN2 reaction because it appear in the rate equation rate = k[RX][Nu-]

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The Nature of the NucleophileStrong nucleophiles present in high concentration favor SN2 mechanistic pathways

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The Nature of the NucleophileWeak nucleophiles favor SN1 mechanistic pathways by decreasing the rate of any competing SN2 reactions

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The Nature of the NucleophileThe identity of the nucleophile is especially important in determining the mechanism of reactions with secondary alkyl halides

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The Nature of the NucleophileA strong nucleophile such as OH favors an SN2 mechanistic pathway with backside attack and inversion of configuration

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The Nature of the NucleophileA weak nucleophiles such as OH favors an SN1 mechanistic pathway that goes through an carbocation intermediate

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The Nature of the NucleophileLoss of the leaving group in A forms the carbocation, which undergoes nucleophilic attack above and below the plane of the ring to give two products C and D

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The Nature of the NucleophileDeprotonation of C and D gives the final product, a pair of diastereomers E (the cis product) and B (the trans product)

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The Nature of the Leaving GroupA better leaving group increases the rate of both SN1 and SN2 reactions

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The Nature of the Leaving GroupFor alkyl halides, the following order of reactivity is observed for the SN1 and SN2 mechanisms

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**Nucleophilic Substitution The SolventTwo classifications of solvents*

**Nucleophilic Substitution The SolventPolar protic solvents are especially good for SN1 reactionsPolar protic solvents are able to solvate cations (carbocations) by ion-dipole interactions*

**Nucleophilic Substitution The SolventPolar protic solvents are especially good for SN1 reactionsPolar protic solvents are able to solvate anions (leaving group) by hydrogen bonding interactions*

**Nucleophilic Substitution The SolventPolar aprotic solvents are especially good for SN2 reactionsPolar aprotic solvents exhibit dipole-dipole interactions but are unable to solvate anions*

**Nucleophilic Substitution The SolventBecause polar aprotic do not solvate anions, they have no impact on the basicity or nucleophilicity of the nucleophile*

**Nucleophilic Substitution Summary of Factors*

**Nucleophilic Substitution Vinyl and Aryl HalidesSN1 and SN2 reactions only occur at sp3 hybridized carbon atomsVinyl and aryl halides have halides attached to sp2 hybridized carbons and do not undergo SN1 and SN2 mechanistic pathways *

**Nucleophilic Substitution Vinyl and Aryl HalidesThe sp2 carbon-halide bond is shorter and stronger than a sp3 carbon-halide bond preventing SN2Shorter and stronger bond due to the higher percent s-character in the hybrid orbital of the carbon atom*

**Nucleophilic Substitution Vinyl and Aryl HalidesHeterolysis of the carbon-halide bond would form a highly unstable vinyl carbocationThe carbocation has only two groups attached to it and is sp hybridized (less stable than 1 carbocation)*

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lecture 18 10 08 2014i.ppt

Documents

stable lower energy

early transition state

late transition state

alkyl substitution increases

transition statea

lower energy of activation

number of alkyl groups

carbocation intermediate