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Stereochemistry of Elimination Reactions

If the elimination reaction removes two substituents from the same side of the molecule it is syn elimination

If the elimination reaction removes two substituents from opposite sides of the molecule it is anti elimination


The E2 Reaction: Stereochemistry

In an E2 reaction, the bonds to the eliminated substituents must be in the same plane

H

X

syn-periplanar

H

X

anti-periplanar

eclipsed conformation

staggered conformation


The E2 Reaction: Stereochemistry The E2 reaction is regioselective (Zaitsev’s

rule)

The E2 Reaction is also stereoselective


The E2 Reaction: Stereochemistry The E isomer is preferred,as it has the

bulkier groups on opposite sides of the double bond


The E2 Reaction: Stereochemistry Note that in the formation of 2-pentene

there were two and only 2 hydrogens on the carbon

This is the requirement if a pair of E,Z isomers is to result from predominantly anti elimination



When only one hydrogen is on the carbon predominantly anti elimination leads to high stereoselectivity

(2S,3S)-2-bromo-3-phenylbutane (E)-2-phenyl-2-butene

(2S,3R)-2-bromo-3-phenylbutane (Z)-2-phenyl-2-butene



Because both syn and anti elimination can occur, an E1 reaction forms both E and Z products regardless of whether the -carbon is bonded to one or two hydrogens



When the carbon has two hydrogens, products of the E2 and E1 reactions are stereoselective Both E and Z isomers are formed The major product will have the bulkier groups

on opposite sides of the double bond



When there is just one hydrogen on the carbon, E2 reaction is gives predominantly the anti

elimination product with a high degree of stereoselectivity

E1 reaction is gives mostly the product with the bulkier groups on opposite sides of the double bond (usually E)


E2 Reactions of Cyclic Compounds E2 reaction of cyclic compounds

follows the same stereochemical rules as from open-chain compounds


E2 Reactions of Cyclic Compounds

The E2 reaction of menthyl chloride violates Zaitsev’s rule


E1 Reactions of Cyclic Compounds When a cyclohexyl chloride undergoes

an E1 reaction, there is no requirement that the two groups to be eliminated be diaxial


E1 Reactions of Cyclic Compounds Carbocation rearrangements must be

considered for E1 reactions


Competition Between Substitution and Elimination

The E2 reaction

The SN2 reaction


Competition Between Substitution and Elimination Conditions that favor E2 also favor SN2 Conditions that favor E1 also favor SN1 No need to worry about SN2/E1 or SN1/E2

combinations First decide whether the reaction would favor

SN2/E2 or SN1/E1 reactions If the halide is primary, only SN2/E2 need be

considered If the halide is secondary or tertiary, SN2/E2 or SN1/E1

depends on reaction condition


Competition Between Substitution and Elimination

SN2/E2 reactions are favored by a high concentration of a good nucleophile/strong base

SN1/E1 reactions are favored by a poor nucleophile/weak base


Competition Between SN2 and E2

Primary halides generally undergo substitution, although if the halide or the base is bulky, elimination is possible

Secondary halides are more difficult to predict The stronger and bulkier the base, the more

elimination product is produced The higher the temperature, the more elimination

product is produced Tertiary halides never undergo SN2 reaction

Elimination is the only possibility


Competition Between SN1 and E1

Because SN1 and E1 reactions both proceed through a carbocation, they have the same rate-determining step

Primary halides do not undergo either SN1 and E1 reactions

For secondary and tertiary halides, raising the temperature increases the elimination product


Williamson Ether SynthesisWilliamson Ether Synthesis

The Williamson ether synthesis is a good way to prepare ethers



This reaction is an SN2 reaction and therefore benefits from a high concentration of alkoxide ion



The alkoxide ion is prepared by using sodium metal or sodium hydride to remove a proton from an alcohol

ROH + Na RO + Na + 1/2 H2

ROH + NaH RO + Na + H2

© Prentice Hall 2001Chapter 101 On Line Course Evaluation for Chemistry 350/Section 26260 We are participating in the online course evaluation Please log.

Documents

prentice hall

anti elimination slide

e1 reaction forms

stereoselective slide

butene slide

diaxial slide

syn elimination

double bond slide