© prentice hall 2001chapter 101 on line course evaluation for chemistry 350/section 26260 we are...
Post on 14-Dec-2015
214 Views
Preview:
TRANSCRIPT
© Prentice Hall 2001 Chapter 10 1
On Line Course Evaluation for Chemistry 350/Section 26260 We are participating in the online course
evaluation Please log on http://web.utk.edu/~SAIS/ Click on Spring 2003 Evaluation That will take you to http://ecommerce.
cas.utk.edu/SAIS/NetIDinput.asp Your password is your email address
© Prentice Hall 2001 Chapter 10 2
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
© Prentice Hall 2001 Chapter 10 3
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
© Prentice Hall 2001 Chapter 10 4
The E2 Reaction: Stereochemistry The E2 reaction is regioselective (Zaitsev’s
rule)
The E2 Reaction is also stereoselective
© Prentice Hall 2001 Chapter 10 5
The E2 Reaction: Stereochemistry The E isomer is preferred,as it has the
bulkier groups on opposite sides of the double bond
© Prentice Hall 2001 Chapter 10 6
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
© Prentice Hall 2001 Chapter 10 7
The E2 Reaction: Stereochemistry
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
© Prentice Hall 2001 Chapter 10 8
The E1 Reaction: Stereochemistry
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
© Prentice Hall 2001 Chapter 10 9
Stereochemistry of Elimination Reactions
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
© Prentice Hall 2001 Chapter 10 10
Stereochemistry of Elimination Reactions
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)
© Prentice Hall 2001 Chapter 10 11
E2 Reactions of Cyclic Compounds E2 reaction of cyclic compounds
follows the same stereochemical rules as from open-chain compounds
© Prentice Hall 2001 Chapter 10 12
E2 Reactions of Cyclic Compounds
The E2 reaction of menthyl chloride violates Zaitsev’s rule
© Prentice Hall 2001 Chapter 10 13
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
© Prentice Hall 2001 Chapter 10 14
E1 Reactions of Cyclic Compounds Carbocation rearrangements must be
considered for E1 reactions
© Prentice Hall 2001 Chapter 10 15
Competition Between Substitution and Elimination
The E2 reaction
The SN2 reaction
© Prentice Hall 2001 Chapter 10 16
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
© Prentice Hall 2001 Chapter 10 17
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
© Prentice Hall 2001 Chapter 10 18
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
© Prentice Hall 2001 Chapter 10 19
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
© Prentice Hall 2001 Chapter 10 20
Williamson Ether SynthesisWilliamson Ether Synthesis
The Williamson ether synthesis is a good way to prepare ethers
© Prentice Hall 2001 Chapter 10 21
Williamson Ether SynthesisWilliamson Ether Synthesis
This reaction is an SN2 reaction and therefore benefits from a high concentration of alkoxide ion
© Prentice Hall 2001 Chapter 10 22
Williamson Ether SynthesisWilliamson Ether Synthesis
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
top related