Chapter 25 - Aromatic Compounds Flashcards
Introducing benzene, Electrophilic substitution reactions of benzene, The chemistry of phenol, Directing groups.
What is benzene?
A cyclic hydrocarbon compound C6H6
Define arene
An aromatic hydrocarbon containing one or more benzene rings
Define aromatic
Any compound containing one or more benzene rings
What is the electrophilic addition argument against benzene having the Kekule structure?
Three C=C bonds so it is expected
No reaction with bromine water at room temperature
No other electrophilic addition reactions
Only substitution reactions
What is the response to the electrophilic addition argument against benzene having the Kekule structure?
Two forms in rapid equilibrium
Approaching Br-Br not attracted to bonds
Bonds change from C=C to C-C before reaction occurs
Bromine doesn’t react
Outline the stability argument against benzene having the Kekule structure
One C=C bond in 6 carbon ring, enthalpy -120kJ/mol
Two C=C bonds in 6 carbon ring, enthalpy -240kJ/mol
Expect 3 C=C bonds to be -360kJ/mol but is actually -208kJ/mol
Delocalisation energy
More stable than expected
What causes benzene to have its unexpected properties?
Localised p-orbitals overlap and form delocalised π orbitals above and below the carbon structure (rings)
Define electrophile
An atom or group of atoms that is attracted to an electron rich centre, where it accepts a pair of electrons to form a new covalent bond
What are the conditions for nitration of benzene?
Benzene + HNO3
with H2SO4 at 50C
As halogens cannot react on their own with benzene, what is required?
Halogen carrier
Give two examples of chlorine carriers
AlCl3
FeCl3
What is formed as a byproduct of halogenation of benzene?
HX
What are the two types of Friedel Crafts reactions of benzene?
Alkylation
Acylation
Outline alkylation of benzene
Benzene + RCl with FeCl3
Forms alkylbenzene + HCl
Outline acylation of benzene
Benzene + RCOCl with FeCl3
Forms Phenylalkanone + HCl
Name the mechanism for bromination of benzene
Electrophilic aromatic substitution
Outline electrophilic aromatic substitution
Electrophile near benzene Arrow from benzene to electrophile Intermediate ion of benzene Arrow from H-C bond to ring Product and H+
How does a nitrating mixture produce an electrophile?
H2SO4 + HNO3 -> NO2+ + HSO4- + H2O
Nitronium ion is electrophile
How is the H2SO4 catalyst regenerated?
HSO4- + H+ -> H2SO4
H+ from benzene
How is an electrophile formed in reaction with a halogen carrier?
X2 + AlX3 -> X+ + AlX4-
How is the halogen carrier catalyst regenerated?
AlX4- + H+ -> AlX3 + HX
What is the structure of phenol?
C6H6O
OH attached to benzene ring
What are the products of phenol reacting with NaOH?
Sodium phenoxide: C6H5O-Na+
Water
Why does phenol not react with Na2CO3?
Too weak of an acid to react with weak base
