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 is expected, but not there, which is why bromine molecule cannot be polarised to undergo addition reaction.
This only undergoes electrophilic substitution reaction
No reaction with bromine water at room temperature
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
What is formed on reaction of phenol with excess Br2?
C6H3OBr3 - OH in position 1 and Br in positions 2,4 and 6
3HBr
What are the reaction conditions of bromination of phenol?
Room temperature
No catalyst
Why does phenol react more readily with bromine/chlorine than benzene reacts with chlorine/bromine?
- In phenol, lone pair of electrons on Oxygen is partially delocalised into the ring
- This increases the electron density in the ring
- Therefore Electrophile (Cl+/Br+) is more polarised
- The aromatic ring in phenol is therefore more susceptible to attack from electrophiles than benzene
What is the term for OH increasing benzene’s reactivity?
Activating group
Define activation
An activating group increases the rate of electrophilic aromatic substitution compared to that of benzene
Define deactivation
A deactivating group decreases the rate of electrophilic aromatic substitution compared to that of benzene
Outline differences between phenol and nitrobenzene reacting with bromine
Phenol is rapid reaction, nitro is slower
Nitro needs halogen carrier
Phenol at room temp, nitro needs high temp
Phenol can have multiple substitutions, nitro can have only one
What do activating groups do with their electrons in benzene?
Donate electrons
What do deactivating groups do with their electrons in benzene?
Withdraw electrons
Where do the NH2 and OH groups direct on benzene?
2,4 and 6 positions
Where does the NO2 group direct on benzene?
3 and 5 positions
Describe the delocalised model of benzene
- Benzene is a planar, cyclic, hexagonal hydrocarbon, C6H6.
- Each C atom uses 3 of its available 4 electrons in bonding to two other C atoms and to one H atom.
- Each C atom has one electron in a p-orbital at right angles to the plane of the bonded C and H atoms.
- Adjacent p-orbital electron overlap sideways, in both directions, above and below the plane of C atoms to from a ring of electron density.
- This overlapping of p-orbitals creates a system of π-bonds which spread over all 6 C atoms in the ring structure.
- The 6 electrons occupying this system of π-bonds are said to be delocalised.
Explain the evidence that led scientists to doubt the model proposed by kekule.
- Bond length intermediate is different from C=C (short) and C-C (long)
- Enthalpy change of hydrogenation is less exothermic than expected when compared to enthalpy change of hydrogenation of cyclohexene.
- Benzene can only react with Br2 at high temp or in presence of a halogen carrier
Where do Halogens direct on Benzene?
2,4,6 positions
What do Halogens, NH2 and OH do with electrons?
Donate Electrons
What do COOH, NO2, CN do with electrons?
Withdraw Electrons
