CHEM C15: Electrophilic Substitution of Arenes

  • Electrophilic Substitution in Arenes
    • General mechanism
    • Halogenation
    • Nitration
    • Friedel-Crafts Alkylation
    • Friedel-Crafts Acylation
    • Orientating effect of side-chains on incoming electrophiles

General mechanism:

Electrophile is formed or suppliedAny species with a positive charge
Electrophile attacks the electron-rich delocalized π-electron ring 
1 pair of π electrons from a C is donated to the electrophile, forming a dative bond. The benzene ring lacks 1 electron – it becomes + charged.

Representing as a Kekule structure, it can be imagined that the neighbouring C is lacking 1 electron.

This structure is an INTERMEDIATE:
Electrons are partially delocalised across only 5 out of the 6 C.
Only 4 π electrons are delocalised.
Only 5 C are left with sp2 orbitals.
The C forming the dative bond now has sp3 orbitals – its bond angles are 109.5°
 
The H on the sp3 C donates an electron to the benzene ring, making it stable again.
The H becomes H+ & is released from the intermediate.
 
An aromatic substituted product is formed. 

Halogenation

FeX3 catalyst polarizes X2 molecule   
Fe has an empty 3d orbital
Dative bond is formed between Fe and X
FeX3 acts as an electron acceptor (Lewis acid / oxidising agent) due to the empty orbital in Fe  

FeX3 + X-X → FeX4 + X+

The X+ released acts as an electrophile, it is electron-deficient.
 
Benzene ring is electron-rich, due to the delocalised ring of π electrons.
There is an attraction between benzene & X+
 
1 pair of π electrons from a C is donated to the X+, forming a dative bond. The benzene ring lacks 1 electron – it becomes + charged  

This structure is an INTERMEDIATE:
Electrons are partially delocalised across only 5 out of the 6 C.
Only 4 π electrons are delocalised.
Only 5 C are left with sp2 orbitals.
The C forming the dative bond now has sp3 orbitals – its bond angles are 109.5°
 
The H on the sp3 C donates an electron to the benzene ring, making it stable again.

The H becomes H+ & is released from the intermediate.
 
H+ reacts with FeX4 to form HX & releasing FeX3 back

H+ + FeX4 → HX + FeX3
Since FeX3 is not used up, we call it a catalyst.
 

Overall mechanism:

The product of halogenation may REPEAT the reaction if X2 is in excess. This is Continued Halogenation

What if there already is a group attached to the benzene ring?
If a C already has an alkyl group attached, the electrophile X+ cannot substitute it. Instead, the presence of a group DIRECTS the X+ to a specific set of of Carbons, depending on the nature of the species. I’ll cover this effect at the end of this post.


Nitration of Benzene
Product is NITROBENZENE

Nitrating mixture contains HNO3 & H2SO4

This reaction releases a nitronium ion:
NO2+ HNO3 + 2H2SO4 → NO2+ + 2HSO4 + H3O+

The NO2+ released acts as an electrophile, it is electron-deficient.
 
Benzene ring is electron-rich, due to the delocalized ring of π electrons.
There is an attraction between benzene & NO2+
 
1 pair of π electrons from a C is donated to the N in NO2+, forming a dative bond
The benzene ring lacks 1 electron – it becomes + charged  

This structure is an INTERMEDIATE:
Electrons are partially delocalised across only 5 out of the 6 C.
Only 4 π electrons are delocalised.
Only 5 C are left with sp2 orbitals.
The C forming the dative bond now has sp3 orbitals – its bond angles are 109.5°
 
The H on the sp3 C donates an electron to the benzene ring, making it stable again.
The H becomes H+ & is released from the intermediate.
 

Overall mechanism:

Conditions:
Temperature: 55 C
Nitrating mixture: conc H2SO4 + HNO3


Friedel-Crafts Reactions

Friedel-Crafts Alkylation of Benzene

Electrophile can be formed via the reaction between AlCl3 & a halogenoalkane:
AlCl3 + R-Cl → R+ + AlCl4

AlCl3 acts as an electron acceptor (Lewis acid / oxidizing agent) due to the empty orbital in Al.

The carbocation on R+ acts as an electrophile, it is electron-deficient.
 
Benzene ring is electron-rich, due to the delocalized ring of π electrons.
There is an attraction between benzene & the carbocation.
 
1 pair of π electrons from a C is donated to the carbocation, forming a dative bond. The benzene ring lacks 1 electron – it becomes + charged.

This structure is an INTERMEDIATE:
Electrons are partially delocalised across only 5 out of the 6 C.
Only 4 π electrons are delocalised.
Only 5 C are left with sp2 orbitals.
The C forming the dative bond now has sp3 orbitals – its bond angles are 109.5°
 
The H on the sp3 C donates an electron to the benzene ring, making it stable again.
The H becomes H+ & is released from the intermediate.
 
H+ reacts with AlCl4 to form HX & releasing AlCl3 back:
H+ + AlCl4 → HCl + AlCl3

Since AlCl3 is not used up, we call it a catalyst.
 

Disclaimers:

  • the catalyst can be any HALOGEN CARRIER: AlX3 or FeX3
  • the electrophile R+ can be generated from any halogenoalkane: CnH2n+1X
  • the halogen on the electrophile & catalyst can be different, such as CH3Cl & FeBr3

Acylation of Benzene

Electrophile can be formed via the reaction between AlCl3 & an acyl chloride: AlCl3 + R-COCl → R-C+O + AlCl4

AlCl3 acts as an electron acceptor (Lewis acid / oxidizing agent) due to the empty orbital in Al.
The carbocation acts as an electrophile, it is electron-deficient.
 
Benzene ring is electron-rich, due to the delocalized ring of π electrons.
There is an attraction between benzene & the carbocation.
 
1 pair of π electrons from a C is donated to the carbocation, forming a dative bond. The benzene ring lacks 1 electron – it becomes + charged.

This structure is an INTERMEDIATE:
Electrons are partially delocalised across only 5 out of the 6 C.
Only 4 π electrons are delocalised.
Only 5 C are left with sp2 orbitals.
The C forming the dative bond now has sp3 orbitals – its bond angles are 109.5°
 
The H on the sp3 C donates an electron to the benzene ring, making it stable again.

The H becomes H+ & is released from the intermediate.
 
H+ reacts with AlCl4 to form HX & releasing AlCl3 back:
H+ + AlCl4 → HCl + AlCl3

Since AlCl3 is not used up, we call it a catalyst.
 

Orientating Effects of Groups in Aromatic Substitution Reactions

Electron-donating groups:
Alkyl groups: -R
Alcohol group: -OH
Ether group: -O-R
Amine group: -NH2
Amide group: -NHCOR
Make the ring more reactive to electrophilic substitution
Activate the 2nd & 4th C
Electron-attracting groups:
Nitro group: -NO2
Ammonium group: -NH3+
Cyanide group: -CN
Aldehyde group: -CHO
Ketone group: -COR
Acid group: -COOH
Ester group: -COOR
Make the ring less reactive to electrophilic substitution
Activate the 3rd (aka the 5th due to symmetry) C
Exception: Halogens (Cl)Make the ring less reactive to electrophilic substitution
Activate the 2nd & 4th C

Why?
Let’s look at the effect of these groups on reactivity first.

  • In general, electron-donating groups supply extra electrons to the delocalised π-electron cloud of the ring. This ACTIVATES the ring, making it MORE electron-dense & MORE reactive towards electrophilic substitution. We call these ACTIVATING GROUPS.
  • Electron-withdrawing groups pull electrons away from the delocalised π-electron cloud of the ring. This DEACTIVATES the ring, making it LESS electron-dense & LESS reactive towards electrophilic substitution. We call these DEACTIVATING GROUPS.

How about the orientating effect?
This one is a bit trickier to explain. Here are some videos explaining it really well:

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