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Elimination Reactions of Alkyl Halides. Chapter 9. “A pessimist finds difficulty in every opportunity. An optimist finds opportunity in every difficulty.” A. Lincoln. Elimination Reactions. Elimination Rx can be of two types E1 mechanism (unimolecular) E2 mechanism (bimolecular).

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elimination reactions of alkyl halides

Elimination Reactions ofAlkyl Halides

Chapter 9

“A pessimist finds difficulty in every opportunity. An optimist finds opportunity in every difficulty.”

A. Lincoln

elimination reactions
Elimination Reactions

Elimination Rx can be of two types

  • E1 mechanism (unimolecular)
  • E2 mechanism (bimolecular)
e2 eliminations
E2 Eliminations

Concerted one step reaction

e2 eliminations1
E2 Eliminations
  • Called “dehydrohalogenations”
  • Called a /  Eliminiation
  • Called 1-2 Elimination
e2 eliminations2
E2 Eliminations

Relative reactivities of RX in an E2 reaction:

(most reactive) RI > RBr > RCl > RF (least reactive))

F- is a better base than Br-

e2 eliminations3
E2 Eliminations

Sometimes only one elimination product is possible

Sometimes More than one product may result

e2 eliminations4
E2 Eliminations

Kinetic and Thermodynamic

Product

(A regioselective reaction !)

zaitsev s rule
Zaitsev’s Rule

The more highly substituted product is obtained when a proton is removed from the b-carbon that is bonded to the fewest hydrogens

reactivity considerations
Reactivity Considerations
  • The more substituted product is not always the most rapidly formed…

That Conjugated Diene thing just keeps showing up…….

reactivity considerations1
Reactivity Considerations
  • The more substituted product is not always the most rapidly formed…

That Pesky Bulky Base – Changes things……

reactivity considerations2
Reactivity Considerations
  • Steric hindrance may dictate removal of a more accessible H

(The most substituted is not formed here.)

slide13
*

(What does this mean?)

alkyl fluorides
Alkyl fluorides
  • Dehydrohalogenation of R-I, R-Br, R-Cl yield more substituted alkene
  • Dehydrohalogenation of RF yields the less substituted alkene
elimination of alkyl fluorides1
Elimination of Alkyl fluorides
  • Fluoride is a strong base
  • Fluoride is a poor leaving group
  • A partial negative charge develops on the carbon that lost the proton (a carbanion)
carbo ions

-

:

R’

C

R

R”

Carbo - Ions

Carbocations are stabilized by alkyl groups

Carbanions are destabilized by alkyl groups

3O < 2O < 1O

Increasing stability of Carbanions

elimination considerations
Elimination Considerations
  • The major product is the more substituted alkene unless…
    • the alkyl halide contains conjugation
    • the base is large
    • the alkyl halide is an alkyl fluoride
e1 mechanism1
E1 Mechanism

Major product will generally be the more substituted - Zatsev

e1 mechanism2
E1 Mechanism

Product depends on stability of the carbocation

e1 mechanism3
E1 Mechanism
  • Because a carbocation is formed – rearrangement is possible
e1 mechanism4
E1 Mechanism
  • Because a carbocation is formed – rearrangement is possible
review of s n reactions
Review of SN Reactions

SN1 is Favored By

More Substituted Carbocation

Weak Nucleophile

Polar Protic Solvent

SN2 is Favored By

Less Substituted Carbocation

Strong Nucleophile

Polar AProtic Solvent

stereochemistry of e2
Stereochemistry of E2
  • In an E2 reaction, 2 groups are eliminated in one step
  • The 2 groups MUST be located in the same plane (periplanar)

(anti-periplanar)

faster

(syn-periplanar)

slower

e2 is stereoselective
E2 Is Stereoselective

The conformers that have the

bulkiest groups on opposite

sides will be the major product.

stereochemistry of e21
Stereochemistry of E2

Note: E isomer is major product because bulky groups, methyl and t-butyl, are anti.

stereochemistry of e22
Stereochemistry of E2

Important note: use models!E2 for 2-bromo-3-phenylbutane

only

E isomer

(2S, 3S)

only

Z isomer

(2S, 3R)

e1 eliminations

stereoselective

E1 Eliminations
  • planar, carbocation intermediate
  • synandanti elimination can occur

bulkiest groups on opposite sides

(Major product is the one with bulkiest groups on opposite sides.)

elimination from cyclics e2
Elimination from Cyclics - E2
  • In an E2 reaction, both groups must be periplanar and anti to each other.
  • This is NOT the most stable conformer.
elimination from cyclics e22
Elimination from Cyclics - E2

More stable conformer has groups equatorial.

Occurs 200 x faster!

elimination from cyclics e23
Elimination from Cyclics - E2
  • In an E2 reaction, the H is removed from the most substituted b-carbon unless that hydrogen is not in the axial
elimination from cyclics e1
Elimination from Cyclics - E1
  • In an E1 reaction, the elimination is not concerted.
  • Intermediate is a carbocation
  • Groups do not need to be in the axial position
elimination from cyclics e11
Elimination from Cyclics - E1
  • Carbocation intermediates may rearrange.

(Zaitsev’s rule is followed, but first watch out for a rearrangement.)

using isotopes in chemistry
Using Isotopes In Chemistry
  • Look at mechanism
  • Look at research handout
  • Look at medical questions handout
substitution vs elimination
Substitution vs. Elimination

E1 vs SN1

or

E2 vs SN2

steps to deciding
Steps To Deciding
  • Decide if it favors Sn2/E2 or Sn1/E1 conditions
  • Decide if it favors Substitution or Elemination

…….. That’s all there is to it 

From Yahoo Images

deciding between first or second order
Deciding Between First or Second order
  • Substrate – 1st order – primary and secondary 2nd order – secondary and tertiary
  • Nucleophile 1st order – weaker nuclephile

2nd order – strong nucleophile

  • Solvent 1st order – polar protic solvent

2nd order – polar aprotic or neutral

2 out of three ain’t bad………… or is that just a long lost song

deciding between sn2 and e2
Deciding Between Sn2 and E2
  • Substrate
  • Base
  • Temperature

From Yahoo Images

deciding between sn2 and e21
Deciding Between Sn2 and E2
  • Substrate Sn2 – primary alkyl halide E2 – secondary alkyl halide
deciding between sn2 and e22
Deciding Between Sn2 and E2
  • Nuclophile Sn2 – Weaker and Smaller E2 – stronger and Bulkier
deciding between sn2 and e23
Deciding Between Sn2 and E2
  • Temperature Sn2 – Lower Temperature E2 – Higher Temperatue
s n 1 e1 conditions
SN1 / E1 Conditions
  • All alkyl halides have same order of reactivity for SN1 or E1
  • Substitution is favored at lower temperatures
practice activity
Practice Activity
  • Each person create a reaction and conditions
  • Have your neighbor decide which mechanism it will undergo
  • Switch and do it again

From Yahoo Images

williamson ether synthesis

CH3OH + Na CH3O- + Na+ + 1/2 H2

Williamson Ether Synthesis
  • Williamson ether synthesis
    • Alkyl halide with alkoxide (SN2 reaction)
    • Alexander Williamson (1850!)
    • React alcohol with sodium metal or sodium hydride

NaH

williamson ether synthesis1
Williamson Ether Synthesis
  • If you want to prepare n-propyl n-butyl ether, there are two methods.
williamson ether synthesis2
Williamson Ether Synthesis
  • If you want to prepare ethyl t-butyl ether, there are two methods… HOWEVER, if you use t-butyl halide, you will obtain primarily the elimination product.
alkyne syntheses
Alkyne Syntheses
  • Elimination Reactions involving geminal dihalides

(note: use a very strong base NH2 , not OH-, or

it stops at vicinyl dihalide!)

synthesis problems
Synthesis Problems
  • Synthesize 1,3-cylcohexadiene from cyclohexane.
synthesis problems1
Synthesis Problems
  • Synthesis Pathway
synthesis problems2
Synthesis Problems
  • Synthesize trans-1,2-dibromo-1-methylcylcohexanefrom methylcyclohexane.
synthesis problems3
Synthesis Problems
  • Synthesis Pathway
synthesis problems4
Synthesis Problems
  • Synthesize 2-butanone from 1-bromobutane.
synthesis problems5
Synthesis Problems
  • Retrosynthetic Analysis
synthesis problems6
Synthesis Problems
  • Synthetic pathway
slide61
END

“A pessimist finds difficulty in every opportunity. An optimist finds opportunity in every difficulty.”

A. Lincoln