Class notes
CHM 121 Chapter 13 Notes
- Course
- Chemistry 121
- Institution
- Harper College
This is a comprehensive and detailed note on chapter 13; Arrangement of Arenes; Electrophilic Aromatic Substitution. *Essential!!
[Show more]
Content preview
CH. 13Chapter 13
Reactions of Arenes
Electrophilic Aromatic Substitution
Electrophiles add to aromatic rings in a fashion somewhat similar to the addition of
electrophiles to alkenes.
Recall:
R3 R4 E Y E
Y
C C + E Y R1 C C R4 R1 C C R4
R2 R1 δ+ δ−
R2 R3 R2 R3
In aromatic rings, however, we see substitution of one of the benzene ring hydrogens for
an electrophile.
H E
+ E Y + Y H
δ+ δ−
The mechanism is the same regardless of the electrophile. It involves two steps: (1)
Addition of the electrophile to form a high-energy carbocation. (2) Elimination of the
proton to restore the aromatic ring system.
H H
H H
E Y slow
step 1 + + Y
E
δ+ δ−
high energy arenium ion
H
step 2 H E
fast
+ Y + Y H
E
The first step is the slow step since the aromaticity of the benzene ring system is destroyed
on formation of the arenium ion intermediate. This is a high energy species but it is
stabilized by resonance with the remaining two double bonds. The second step is very fast
since it restores the aromatic stabilization.
1
, CH. 13
H H H
H H H
E E E
There are five electrophilic aromatic substitution reactions that we will study.
(1) Nitration
H NO2
H2SO4
+ HNO3
(2) Sulfonation
H SO3H
+ H2SO4
(3) Halogenation with bromine or chlorine
H X
FeX3 X = Br, Cl
+ X2
(4) Friedel-Crafts Alkylation
H R
+ AlX3
RX
(5) Friedel-Crafts Acylation
O
H O C
AlX3 R
+ Cl C R
2
, CH. 13
Nitration of Benzene
We generate the highly electrophilic nitronium ion in situ using sulfuric acid to dehydrate
nitric acid. The nitronium ion is then attacked by the π−electrons to give the arenium ion,
which then loses a proton to regain its aromaticity.
O O O 2
H
HO N + HO S OH O N O N O O N O
O H O
O nitronium ion
very electrophilic nitrogen deficient nitrogen
H H
H H O
H OH2
+ O N O O N
N O
O
Sulfonation of Benzene
H H O
H O
O H O S OH
H O S OH
S S O
+ O O O
O
O
The sulfonation reaction is reversible. Heating the benzene sulfonyl compound in aqueous
acid removes the sulfonyl group and replaces it with a proton.
O
S OH H3O+ H
O + H2SO4
H2O
Halogenation of Benzene
Generally a Lewis acid catalyst is needed to activate the bromine and make it into a better
electrophile. Bromine itself does not react with benzene.
3
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller anyiamgeorge19. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $11.99. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
53022 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now