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Dr. Hussein Al-Mughaid Just Chemistry Department Organic Chemistry 217 Aromatic Compounds Chapter 4 اء عضوية كيمي ك217 د. حسين المغيض
Dr. Hussein Al-Mughaid
Just
Chemistry Department
Organic Chemistry 217
Aromatic Compounds
Chapter 4
كيمياء عضوية217ك
حسين المغيض. د
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Benzene is not an Alkene
No Reaction
resonance representation
bond order = 1.5
Three double bonds
(6 p electrons)
HBr
Benzene (C6H6)
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Nomenclature of Benzene Derivatives
1. Benzene ring with one substituent
name the substituent and add the word benzene
Alkylbenzene
halobenzene bromobenzene chlorobenzene
methylbenzene
ethylbenzene
propylbenzene
Iodobenzene
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Many monosubstituted benzenes have common names.
memorize
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The Phenyl group
When a benzene ring is a substituent, the term phenyl is used (for C6H5) You may also see “Ph” or in place of “C6H5”
-Hsubstituent
Name the following molecules?
Phenyl group benzene
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The Benzyl group
“Benzyl” refers to “C6H5CH2”
benzyl alcohol
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Disubstituted Benzenes
Relative positions on a benzene ring [ortho (o), meta(m), para(p)]
• Numbers can also be used to identify the relationship between the groups;
ortho- is 1,2-disubstituted, meta- is 1,3, and para- is 1,4.
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Examples
What about this?
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Closing-Up
Name the following molecules?
Polysubstituted derivatives of benzene
The descriptors: ortho, meta, and para can not be used when naming a benzene ring bearing three or more substituents
1.
Name the following molecule?
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2.
Name the following molecule?
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How Benzene reacts
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•The characteristic reaction of benzene is electrophilic aromatic substitution— (a hydrogen atom is replaced by an electrophile).
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E E E
Step 1: Attack on the electrophile forms the sigma complex
Step 2: Loss of a proton gives the substitution product.
Mechanism of Electrophilic Aromatic Substitution
(carbocation intermediate, stabilized by resonance.
1. Bromination of Benzene
• Before the electrophilic aromatic substitution can take place, the electrophile must be
activated.
• A strong Lewis acid catalyst, such as FeBr3, should be used.
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Step 1: Electrophilic attack and formation of the sigma complex.
Step 2: Loss of a proton to give the products.
Mechanism for the Bromination of Benzene:
Steps 1 and 2
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Energy Diagram for Bromination
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2. Chlorination of Benzene
• Chlorination is similar to bromination. AlCl3 is most often used as catalyst, but FeCl3 will also work.
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3. Nitration of Benzene
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• Sulfuric acid acts as a catalyst, allowing the reaction to be faster and at lower temperatures.
• HNO3 and H2SO4 react together to form the electrophile of the reaction: nitronium ion
(NO2+).
Protonation of nitric acid by the stronger sulfuric acid allows for the elimination of water and generation of the very electrophilic nitronium ion
electrophile
sulfuric acid nitric acid HNO3 + H2SO4 NO2 + H2O
+
Mechanism for the EAS Nitration of
Benzene
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+
+
+
+
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Fuming sulfuric acid ( 7% mixture of SO3 and H2SO4 ) is required as a catalyst to generate a more reactive electrophile [sulfonium ion = (+SO3H)]
4. Sulfonation of Benzene
5. Friedel-Crafts Acylation
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Step-1: Role of AlCl3 AlCl3 (Lewis acid) catalyst is required to generate the resonance
stabilized acylium ion (electrophile)
electrophile
complex
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Problem: Identify the acyl chloride that might be used in a Friedel-Crafts acylation for the following acylbenzenes:
6. Friedel-Crafts Alkylation
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• Reactive species is a carbocation (electrophile).
(Synthesis of alkyl benzenes)
• a Lewis acid, usually AlCl3 = Ionizes the alkyl halide( R-X).
gives
electrophile Lewis A-B complex
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Step 1: Attack on the carbocation forms the sigma complex
Step 2: Loss of a proton gives the substitution product.
Mechanism
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Example:
Comment on the following reaction?
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Substituents Effect
It depends on the nature of S
Second electrophilic aromatic substitution
The carbons are no
longer the same
Where S (substituent) direct the incoming(second) electrophile?
S
Electron withdrawing (EW)
Electron donating (ED)
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Q:How do I Know if the substituent(S) is ED or EW?
Look at the first atom attached to benzene ring.
Deactivate the ring(EWG)
S Have lone pair (at least)
Have positive or partially positive
Activate the ring(EDG)
O, N
O,P - director m- director
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What about this one?
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Ortho-para director
Nitration of Anisole
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Ortho 31%
Para 67%
+
Role of Oxygen atom
Look how O atom stabilize the carbocation
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A C B D
A1 C1 B1 D1
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Exceptions:
An alkyl substituent that is bonded to a benzene ring is more electron donating than a hydrogen
1. Alkylbenzenes 2. halobenzenes
1. Alkyl groups(R)are weak activators
Ortho-para directors!
Electron Donating. How?
No lone pairs
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Ortho attack
para attack
meta attack
para attack
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2. Halogens are: weakly Deactivating but ortho-para directors
donate electrons into the ring via resonance(effect)
withdraw electrons inductively(Inductive effect)
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Which one is more reactive in electrophilic aromatic substitution reaction?
X = halogen
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Relative Reactivity of Substituted Benzenes
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• Substituents on a benzene ring can affect two things: 1. Location of subsequent substitution rxns
2. Reactivity of ring toward further substitutions
More reactive Intermediate in reactivity less reactive
phenyl
The Power of S (Common substituents): Activator (strong, moderate, weak); deactivator (strong moderate, weak)
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Arrange the following compounds in order of decreasing reactivity toward electrophilic aromatic substitution.
For the two molecules shown below:
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(A) (B)
(1) More reactive in EAS
(2) in B, is the substituent
ortho/para or meta director?
Give the product(s) obtained from the reaction of the following compound with one equivalent of Br2 and employing FeBr3 as a catalyst?.
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+
FeBr3
Br2(1 eq.)
FeBr3
Br2(1 eq.) +
+ Ortho isomer
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Reverse the order
Synthesis of Disubstituted Benzenes
Look how it does make a difference
Route 1
Route 2
Which substituent you have to introduce first
How the following synthesis might be accomplished?
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