elimination reactions of alkyl halides chapter 9 “a pessimist finds difficulty in every...
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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 Rx can be of two types
• E1 mechanism (unimolecular)
• E2 mechanism (bimolecular)
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 EliminationsSometimes only one elimination product is possible
Sometimes More than one product may result
Zaitsev’s Rule
The more highly substituted product is obtained when a proton is removed from the -carbon that is bonded to
the fewest hydrogens
Reactivity Considerations• The more substituted product is not always the most
rapidly formed…
That Conjugated Diene thing just keeps showing up…….
Reactivity Considerations• The more substituted product is not always the most
rapidly formed…That Pesky Bulky Base – Changes things……
Reactivity Considerations
• Steric hindrance may dictate removal of a more accessible H
(The most substituted is not formed here.)
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 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 - IonsCarbocations are stabilized by alkyl groups
Carbanions are destabilized by alkyl groups
CR’
R”
R
: -3O < 2O < 1O
Increasing stability of Carbanions
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
Competition E1 vs. E2
RX Elimination comments
1o alkyl halides E2 only Difficulty in forming 1o carbocations
2o alkyl halides E1 and E2 Favored by same as SN2 conditions
3o alkyl halides E1 and E2 Favored by same as SN2 conditions
Review of SN Reactions
SN1 is Favored ByMore Substituted Carbocation
Weak NucleophilePolar Protic Solvent
SN2 is Favored ByLess Substituted Carbocation
Strong NucleophilePolar AProtic Solvent
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)
(syn-periplanar)slower
(anti-periplanar)
faster
E2 Is Stereoselective
•
The conformers that have the bulkiest groups on opposite sides will be the major product.
Stereochemistry of E2
Note: E isomer is major product because bulky groups, methyl and t-butyl, are anti.
Stereochemistry of E2
Important note: use models! E2 for 2-bromo-3-phenylbutane
onlyE isomer
onlyZ isomer
(2S, 3S)
(2S, 3R)
E1 Eliminations
– planar, carbocation intermediate– syn and anti elimination can occur
(Major product is the one with bulkiest groups on opposite sides.)
bulkiest groups on opposite sides
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 - E2
• In an E2 reaction, the H is removed from the most substituted -carbon unless that hydrogen is not in the axial
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 - E1
• Carbocation intermediates may rearrange.
(Zaitsev’s rule is followed, but first watch out for a rearrangement.)
Stereochemistry Reviewmechanism products comments
SN1 Both enantiomers formed (more inverted than retained)
Racemization w/ some inversion
•E1Both E and Z formed (more of the stereoisomer with bulkiest groups trans)
Also C+ intermediate
•SN2 Only the inverted product is formed
Inversion
E2 Both enantiomers formed (more of the stereoisomer with bulkiest groups trans)
Using Isotopes In Chemistry
• Look at mechanism
• Look at research handout
• Look at medical questions handout
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
• Substrate –1st order – primary and secondary2nd order – secondary and tertiary
• Nucleophile 1st order – weaker nuclephile2nd order – strong nucleophile
• Solvent 1st order – polar protic solvent2nd order – polar aprotic or neutral
2 out of three ain’t bad………… or is that just a long lost song
SN1 / E1 Conditions
• All alkyl halides have same order of reactivity for SN1 or E1
• Substitution is favored at lower temperatures
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
• Williamson ether synthesis– Alkyl halide with alkoxide (SN2 reaction)
– Alexander Williamson (1850!)– React alcohol with sodium metal or sodium hydride
CH3OH + Na CH3O- + Na+ + 1/2 H2
NaH
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• Elimination Reactions involving geminal
dihalides(note: use a very strong base NH2 , not OH-, or
it stops at vicinyl dihalide!)
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