Chapter 25 - Aromatic Compounds

Question 1

What is the molecular formula of benzene?

Answer 1

C6H6

Question 2

What is Kekule’s model?

Answer 2

Suggested the structure of benzene was based on a six membered ring of carbon atoms joined by alternate single and double bonds.

Question 3

What is the evidence to disprove Kekule’s model?

Answer 3

1. lack of reactivity in benzene
2. lengths of carbon carbon bonds in benzene
3. hydrogenation enthalpies

Question 4

How does the lack of reactivity of benzene disprove Kekule’s model?

Answer 4

If benzene contained C=C bonds in Kekule’s model it should decolourise bromine in an electrophilic addition reaction. But benzene does not decolourise bromine or undergo electrophilic addition reactions.

Question 5

How do the lengths of the carbon carbon bonds in benzene disprove Kekule’s model?

Answer 5

Bond length was all the same, and between the length of a single bond and a double bond.

Question 6

How do hydrogenation enthalpies disprove Kekule’s model?

Answer 6

If benzene did have the Kekule structure then it would be expected to have an enthalpy change of hydrogenation 3 times of cyclohexene, but it’s actual enthalpy change is less than expected. The actual structure of benzene is therefore more stable than the theoretical Kekule model

Question 7

What is the delocalised model of benzene?

Answer 7

P orbitals of C atoms overlap sideways above and below the planar ring to form a pi system in which electrons are delocalised

Question 8

How do you name aromatic compound?

Answer 8

Benzene forms the root of the name, with everything else being named as a side chain

Question 9

When does benzene stop being the parent chain?

Answer 9

• when the substituent group contains organic functional group
• when substituent group contains 7 or more C atoms

Question 10

What is the prefix for benzene?

Answer 10

Phenyl

Question 11

How are aromatic compounds with more than one substituent group named?

Answer 11

Ring becomes numbered and substituent groups are listed in alphabetical order

Question 12

What happens in the nitration of benzene? (catalyst, reagents, conditions)

Answer 12

Benzene reacts slowly with nitric acid to form nitrobenzene.
Catalyst = conc H2SO4
Reagent = conc HNO3
Conditions = 50°C

Question 13

What happens if the temperature rises above 50°C in the nitration of benzene?

Answer 13

Further substitution reactions may occur leading to the production of dinitrobenzene

Question 14

What are the steps in the nitration of benzene?

Answer 14

1. HNO3 + H2SO4 → NO2+ + HSO4- + H2O
2. Electrophilic substitution of NO2+ onto benzene, displacing a H+ ion
3. H+ + HSO4- → H2SO4 (regeneration of catalyst)

Question 15

What is required for halogens to react with benzene?

Answer 15

Halogens will not react with benzene unless halogen carrier present
e.g. AlCl3 or FeBr3

Question 16

What happens in the halogenation of benzene? (catalyst, reagents, conditions)

Answer 16

Benzene reacts in electrophilic substitution reaction to form halobenzene.
Catalyst = halogen carrier e.g. AlBr3
Reagents = Halogen
Conditions = room temp and pressure

Question 17

What are the steps for the halogenation of benzene? (use FeBr3 as halogen carrier)

Answer 17

1. Br2 + FeBr3 → FeBr4- + Br+
2. Electrophilic substitution of Br+ onto benzene ring, displacing H+
3. H+ + FeBr4- → FeBr3 + HBr

Question 18

What happens in the Friedel Craft alkylation of benzene? (catalyst, reagent, conditions)

Answer 18

Substitution of a hydrogen atom in the benzene ring by an alkyl group.
Catalyst =AlCl3
Reagents = haloalkane e.g. CH3CH2Cl
Conditions = n/a

Question 19

What happens in the Friedel Craft acylation of benzene? (catalyst, reagent, conditions)

Answer 19

Benzene reacts with acyl chloride to form aromatic ketone
Catalyst = AlCl3
Reagents = acyl chloride e.g. CH3COCl
Conditions = n/a

Question 20

Why does benzene not react with bromine unless a halogen carrier is present?

Answer 20

Because benzene has delocalised pi electrons spread above and below the plane of the C atoms in the ring structure. The electron density around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene so cannot polarise the bromine molecule

Question 21

What is a phenol?

Answer 21

A type of organic chemical containing a hydroxyl -OH functional group directly bonded to an aromatic ring

Question 22

How does phenol’s solubility in water compare to alcohols, and why?

Answer 22

It is less soluble in water due to the presence of the non polar benzene ring

Question 23

What does the partial dissociation of phenol form?

Answer 23

Phenoxide ion and a proton
(classified as a weak acid!)

Question 24

What reaction can you use to distinguish between a phenol and carboxylic acid?

Answer 24

Sodium carbonate (weak base)
Carboxylic acid will react with sodium carbonate to produce CO2 gas, phenol will not

Question 25

How does phenol react with sodium hydroxide?

Answer 25

Forms the salt sodium phenoxide and water (neutralisation reaction)

Question 26

What happens in the bromination of phenol?

Answer 26

Reacts with aqueous solution of bromine to form a white precip of 2,4,6-tribromophenol (decolourises bromine water). Halogen carrier not required, reaction occurs at room temp

Question 27

What happens in the nitration of phenol?

Answer 27

Reacts readily with dilute nitric acid at room temp. A mixture of 2-nitrophenol and 4-nitrophenol is formed

Question 28

What is the difference in reactivity between phenol and benzene?

Answer 28

Bromine and nitric acid react more readily with phenol than they do with benzene, and can use dilute solutions instead of concentrated.

Question 29

Why is phenol more reactive than benzene?

Answer 29

Due to the lone pair electrons from the oxygen p orbital in phenol OH group being donated into the pi system of phenol. The electron density of the benzene ring in phenol is therefore increased, meaning it will attract electrophiles more strongly and can polarise molecules such as bromine.

Question 30

How could you use a reaction to distinguish phenol/alkene?

Answer 30

Add bromine water, both decolourised but phenol would form white precip

Question 31

What is the difference between 2 and 4 directing groups to 3 directing groups?

Answer 31

All 2 and 4 directing groups are activating groups so the aromatic ring reacts more readily with electrophiles. All 3 directing groups are deactivating, meaning the aromatic ring reacts less readily with electrophiles

Question 32

What are the 2 and 4 directing groups?

Answer 32

-NH2 / -NHR
-OH
-OR
-R OR -C6H5
-F, -Cl, -Br, -I

Question 33

What are the 3 directing groups?

Answer 33

RCOR
-COOR
-SO3H
-CHO
-COOH
-CN
-NO2
-NR3+