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Reaction mechanisms. Ionic Reactions. Ionic Reactions. Ionic Reactions. Ionic Reactions. Bond Polarity. Partial charges. Nucleophiles and Electrophiles. Leaving Groups. Radical Reactions. Type of Reactions. Nucleophilic reactions: nucleophilic substitution (S N ).

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slide6

Bond Polarity

Partial charges

slide11

Nucleophilic reactions: nucleophilic substitution (SN)

  • in the following general reaction, substitution takes place on an sp3 hybridized (tetrahedral) carbon

Nucleophilic substitution: -> reagent is nucleophil

-> nucleophil replaces leaving group

-> competing reaction (elimination + rearrangements)

nucleophilic substitution
Nucleophilic Substitution
  • Some nucleophilic substitution reactions
mechanism
Mechanism
  • Chemists propose two limiting mechanisms for nucleophilic displacement
    • a fundamental difference between them is the timing of bond breaking and bond forming steps
  • At one extreme, the two processes take place simultaneously; designated SN2
    • S = substitution
    • N = nucleophilic
    • 2 = bimolecular (two species are involved in the rate-determining step)
    • rate = k[haloalkane][nucleophile]
  • In the other limiting mechanism, bond breaking between carbon and the leaving group is entirely completed before bond forming with the nucleophile begins. This mechanism is designated SN1 where
    • S = substitution
    • N = nucleophilic
    • 1 = unimolecular (only one species is involved in the rate-determining step)
    • rate = k[haloalkane]
slide14

SN2 reaction: bimolecular nucleophilic substitution

  • both reactants are involved in the transition state of the rate-determining step
  • the nucleophile attacks the reactive center from the side opposite the leaving group
s n 2
SN2
  • An energy diagram for an SN 2 reaction
    • there is one transition state and no reactive intermediate
slide16

SN1 reaction: unimolecular nucleophilic substitution

  • SN1 is illustrated by the solvolysis of tert-butyl bromide
    • Step 1: ionization of the C-X bond gives a carbocation intermediate
s n 1
SN1
  • Step 2: reaction of the carbocation (an electrophile) with methanol (a nucleophile) gives an oxonium ion
  • Step 3: proton transfer completes the reaction
s n 118
SN1
  • An energy diagram for an SN1 reaction
s n 119

C

H

C

H

6

5

6

5

+

CH

O

C

C

OCH

3

3

H

H

Cl

Cl

(S)-Enantiomer

(R)-Enantiomer

A racemic mixture

SN1
  • For an SN1 reaction at a stereocenter, the product is a racemic mixture
  • the nucleophile attacks with equal probability from either face of the planar carbocation intermediate
effect of variables on s n reactions
Effect of variables on SN Reactions
  • the nature of substituents bonded to the atom attacked by nucleophile
  • the nature of the nucleophile
  • the nature of the leaving group
  • the solvent effect
effect of substituents on sn reactions
Effect of substituents on SN reactions
  • SN1 reactions
    • governed by electronic factors, namely the relative stabilities of carbocation intermediates
    • relative rates: 3° > 2° > 1° > methyl
  • SN2 reactions
    • governed by steric factors, namely the relative ease of approach of the nucleophile to the site of reaction
    • relative rates: methyl > 1° > 2° > 3°
effect of substituents on s n reactions
Effect of substituents on SN reactions
  • Effect of electronic and steric factors in competition between SN1 and SN2 reactions
nucleophilicity
Nucleophilicity
  • Nucleophilicity: a kinetic property measured by the rate at which a Nu attacks a reference compound under a standard set of experimental conditions
    • for example, the rate at which a set of nucleophiles displaces bromide ion from bromoethane
  • Two important features:
  • An anion is a better nucleophile than a uncharged conjugated acid
  • strong bases are good nucleophiles
the leaving group
The Leaving Group
  • the best leaving groups in this series are the halogens I-, Br-, and Cl-
  • OH-, RO-, and NH2- are such poor leaving groups that they are rarely if ever displaced in nucleophilic substitution reactions
solvent effect
Solvent Effect
  • Protic solvent: a solvent that contains an -OH group
    • these solvents favor SN1 reactions; the greater the polarity of the solvent, the easier it is to form carbocationsin it
solvent effect32
Solvent Effect
  • Aprotic solvent: does not contain an -OH group
    • it is more difficult to form carbocations in aprotic solvents
    • aprotic solvents favor SN2 reactions
competing reaction elimination
Competing Reaction: Elimination

-Elimination: removal of atoms or groups of atoms from adjacent carbons to form a carbon-carbon double bond

  • we study a type ofb-eliminationcalled dehydrohalogenation (the elimination of HX)
b elimination
b-Elimination
  • There are two limiting mechanisms for β-elimination reactions
  • E1 mechanism:at one extreme, breaking of the C-X bond is complete before reaction with base breaks the C-H bond
    • only R-X is involved in the rate-determining step
  • E2 mechanism:at the other extreme, breaking of the C-X and C-H bonds is concerted
    • both R-X and base are involved in the rate-determining step
e2 mechanism
E2 Mechanism
  • A one-step mechanism; all bond-breaking and bond-forming steps are concerted
e1 mechanism
E1 Mechanism
  • Step 1: ionization of C-X gives a carbocation intermediate
  • Step 2: proton transfer from the carbocation intermediate to a base (in this case, the solvent) gives the alkene

Nucleophile

-> acting as a strong base

elimination
Elimination
  • Saytzeff rule: the major product of a elimination is the more stable (the more highly substituted) alkene
elimination reactions
Elimination Reactions
  • Summary of E1 versus E2 Reactions for Haloalkanes
substitution vs elimination
Substitution vs Elimination
  • Many nucleophiles are also strong bases (OH- and RO-) and SN and E reactions often compete
    • the ratio of SN/E products depends on the relative rates of the two reactions
  • What favors Elimination reactions:
  • attacking nucleophil is a strong and large base
  • steric crowding in the substrate
  • High temperatures and low polarity of solvent
s n 1 versus e1
SN1 versus E1
  • Reactions of 2° and 3° haloalkanes in polar protic solvents give mixtures of substitution and elimination products
s n 2 versus e2
SN2 versus E2
  • It is considerably easier to predict the ratio of SN2 to E2 products
summary of s vs e for haloalkanes
Summary of S vs E for Haloalkanes
  • for methyl and 1°haloalkanes
summary of s vs e for haloalkanes44
Summary of S vs E for Haloalkanes
  • for 2° and 3° haloalkanes
summary of s vs e for haloalkanes45
Summary of S vs E for Haloalkanes
  • Examples: predict the major product and the mechanism for each reaction

Elimination, strong base, high temp.

SN2, weak base, good nucleophil

SN1 (+Elimination), strong base, good nucleophil, protic solvent

No reaction,

I is a weak base (SN2)

I better leaving group than Cl

carbocation rearrangements
Carbocation rearrangements

Also 1,3- and other shifts are possible

The driving force of rearrangements is -> to form a more stable carbocation !!!

Happens often with secondary carbocations -> more stable tertiary carbocation

carbocation rearrangements in s n e reactions wagner meerwein rearrangements
Carbocation rearrangements in SN + E reactions -> Wagner – Meerwein rearrangements

Rearrangement of a secondary carbocations -> more stable tertiary carbocation

Plays an important role in biosynthesis of molecules, i.e. Cholesterol -> (Biochemistry)