Attack Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring Mechanism gt Bromination of Benzene Requires a stronger electrophile than Br 2 Use a strong Lewis acid catalyst FeBr ID: 592205
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Slide1
Electrophilic
AttackSlide2
Electrophilic Aromatic Substitution
Electrophile substitutes for a hydrogen on the benzene ring.Slide3
Mechanism
=>Slide4
Bromination of Benzene
Requires a stronger electrophile than Br
2.Use a strong Lewis acid catalyst, FeBr3.Slide5
Energy Diagram for Bromination
=>Slide6
Chlorination and Iodination
Chlorination is similar to bromination. Use AlCl
3 as the Lewis acid catalyst.Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion.Slide7
Nitration of Benzene
Use sulfuric acid with nitric acid to form the nitronium ion electrophile.
NO
2
+
then forms a
sigma complex with
benzene, loses H
+
to
form nitrobenzene. =>Slide8
Sulfonation
Sulfur trioxide, SO
3, in fuming sulfuric acid is the electrophile. Slide9
Nitration of Toluene
Toluene reacts 25 times faster than benzene. The methyl group is an activator.
The product mix contains mostly ortho and para substituted molecules.Slide10
Sigma Complex
Intermediate is more stable if nitration occurs at the
ortho or para position.Slide11
Energy Diagram
=>Slide12
Friedel-Crafts Alkylation
Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl
3.Reactions of alkyl halide with Lewis acid produces a carbocation which is the electrophile.Other sources of carbocations: alkenes + HF or alcohols + BF3. Slide13
Examples of Carbocation Formation
=>Slide14
Formation of Alkyl Benzene
+
-Slide15
Limitations of Friedel-Crafts
Reaction fails if benzene has a substituent that is more deactivating than halogen.
Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl3 produces isopropylbenzene.The alkylbenzene product is more reactive than benzene, so polyalkylation occurs. Slide16
Friedel-Crafts Acylation
Acyl chloride is used in place of alkyl chloride.
The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation.The product is a phenyl ketone that is less reactive than benzene. Slide17
Mechanism of AcylationSlide18
Clemmensen Reduction
Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc.Slide19
Gatterman-Koch Formylation
Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst.
Product is benzaldehyde.Slide20
Activating,
O-, P-Directing Substituents
Alkyl groups stabilize the sigma complex by induction, donating electron density through the sigma bond.Substituents with a lone pair of electrons stabilize the sigma complex by resonance.Slide21
The Amino Group
Aniline reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the HBr that’s also formed.
=>Slide22
Summary of ActivatorsSlide23
Deactivating Meta-Directing Substituents
Electrophilic substitution reactions for nitrobenzene are 100,000 times
slower than for benzene.The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers.Meta-directors deactivate all positions on the ring, but the meta position is less deactivated. Slide24
Ortho Substitution on NitrobenzeneSlide25
Para Substitution on Nitrobenzene
=>Slide26
Meta Substitution on NitrobenzeneSlide27
Energy DiagramSlide28
Structure of Meta-Directing Deactivators
The atom attached to the aromatic ring will have a partial positive charge.
Electron density is withdrawn inductively along the sigma bond, so the ring is less electron-rich than benzene. Slide29
Summary of DeactivatorsSlide30
More DeactivatorsSlide31
Halobenzenes
Halogens are deactivating toward electrophilic substitution, but are ortho, para-directing!
Since halogens are very electronegative, they withdraw electron density from the ring inductively along the sigma bond.But halogens have lone pairs of electrons that can stabilize the sigma complex by resonance. Slide32
Sigma Complex for Bromobenzene
Ortho and para attacks produce a bromonium ion
and other resonance structures.
No bromonium ion
possible with meta attack.Slide33
Energy DiagramSlide34
Summary of Directing EffectsSlide35
Multiple Substituents
The most strongly activating substituent will determine the position of the next substitution. May have mixtures.Slide36Slide37
37
II. Electrophilic Addition
“Loose” p electrons are nucleophilic (Lewis bases), react with electrophiles (Lewis acids).Slide38
38
II. Electrophilic Addition
A. Addition of hydrogen halides
(X = Cl, Br, I)
Reactivity: HI > HBr > HCl >> HF (stronger acid = better electrophile)Slide39
39
II. Electrophilic Addition
A. Addition of hydrogen halides1. Markovnikov’s rule
In the addition of HX to an alkene, the H goes to the carbon with more H’s.
Question 6-2.
Draw the products. Click on the arrow to check answers.
Check
AnswerSlide40
40
II. Electrophilic Addition
A. Addition of hydrogen halides1. Markovnikov’s rule
In the addition of HX to an alkene, the H goes to the carbon with more H’s.
Answer 6-2.
Slide41
41
II. Electrophilic Addition
A. Addition of hydrogen halides2. mechanism
Mechanistic interpretation of Markovnikov’s rule: The reaction proceeds through the
more stable carbocation intermediate.Slide42
42
II. Electrophilic Addition
A. Addition of hydrogen halides2. mechanism
lower
E
a
faster rate of
formation