Chapter 25 - Aromatic Compounds

Question 1

What is benzene?

Answer 1

A cyclic hydrocarbon compound C6H6

Question 2

Define arene

Answer 2

An aromatic hydrocarbon containing one or more benzene rings

Question 3

Define aromatic

Answer 3

Any compound containing one or more benzene rings

Question 4

What is the electrophilic addition argument against benzene having the Kekule structure?

Answer 4

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

Question 5

Outline the stability argument against benzene having the Kekule structure

Answer 5

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

Question 6

What causes benzene to have its unexpected properties?

Answer 6

Localised p-orbitals overlap and form delocalised π orbitals above and below the carbon structure (rings)

Question 7

Define electrophile

Answer 7

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

Question 8

What are the conditions for nitration of benzene?

Answer 8

Benzene + HNO3

with H2SO4 at 50C

Question 9

As halogens cannot react on their own with benzene, what is required?

Answer 9

Halogen carrier

Question 10

Give two examples of chlorine carriers

Answer 10

AlCl3

FeCl3

Question 11

What is formed as a byproduct of halogenation of benzene?

Answer 11

HX

Question 12

What are the two types of Friedel Crafts reactions of benzene?

Answer 12

Alkylation

Acylation

Question 13

Outline alkylation of benzene

Answer 13

Benzene + RCl with FeCl3

Forms alkylbenzene + HCl

Question 14

Outline acylation of benzene

Answer 14

Benzene + RCOCl with FeCl3

Forms Phenylalkanone + HCl

Question 15

Name the mechanism for bromination of benzene

Answer 15

Electrophilic aromatic substitution

Question 16

Outline electrophilic aromatic substitution

Answer 16

Electrophile near benzene
Arrow from benzene to electrophile
Intermediate ion of benzene
Arrow from H-C bond to ring
Product and H+

Question 17

How does a nitrating mixture produce an electrophile?

Answer 17

H2SO4 + HNO3 -> NO2+ + HSO4- + H2O

Nitronium ion is electrophile

Question 18

How is the H2SO4 catalyst regenerated?

Answer 18

HSO4- + H+ -> H2SO4

H+ from benzene

Question 19

How is an electrophile formed in reaction with a halogen carrier?

Answer 19

X2 + AlX3 -> X+ + AlX4-

Question 20

How is the halogen carrier catalyst regenerated?

Answer 20

AlX4- + H+ -> AlX3 + HX

Question 21

What is the structure of phenol?

Answer 21

C6H6O

OH attached to benzene ring

Question 22

What are the products of phenol reacting with NaOH?

Answer 22

Sodium phenoxide: C6H5O-Na+

Water

Question 23

Why does phenol not react with Na2CO3?

Answer 23

Too weak of an acid to react with weak base

Question 24

What is formed on reaction of phenol with excess Br2?

Answer 24

C6H3OBr3 - OH in position 1 and Br in positions 2,4 and 6

3HBr

Question 25

What are the reaction conditions of bromination of phenol?

Answer 25

Room temperature

No catalyst

Question 26

Why does phenol react more readily with bromine/chlorine than benzene reacts with chlorine/bromine?

Answer 26

• 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

Question 27

What is the term for OH increasing benzene’s reactivity?

Answer 27

Activating group

Question 28

Define activation

Answer 28

An activating group increases the rate of electrophilic aromatic substitution compared to that of benzene

Question 29

Define deactivation

Answer 29

A deactivating group decreases the rate of electrophilic aromatic substitution compared to that of benzene

Question 30

Outline differences between phenol and nitrobenzene reacting with bromine

Answer 30

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

Question 31

What do activating groups do with their electrons in benzene?

Answer 31

Donate electrons

Question 32

What do deactivating groups do with their electrons in benzene?

Answer 32

Withdraw electrons

Question 33

Where do the NH2 and OH groups direct on benzene?

Answer 33

2,4 and 6 positions

Question 34

Where does the NO2 group direct on benzene?

Answer 34

3 and 5 positions

Question 35

Describe the delocalised model of benzene

Answer 35

• 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.

Question 36

Explain the evidence that led scientists to doubt the model proposed by kekule.

Answer 36

• 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

Question 37

Where do Halogens direct on Benzene?

Answer 37

2,4,6 positions

Question 38

What do Halogens, NH2 and OH do with electrons?

Answer 38

Donate Electrons

Question 39

What do COOH, NO2, CN do with electrons?

Answer 39

Withdraw Electrons