Chapter 12 - Arenes

Question 1

What is the molecular formula of benzene?

Answer 1

C6H6

Question 2

How are principal functional groups (at position 1) of arenes determined?

Answer 2

-CO2H > -OH > -CH3 > -halogen > NO2; rest are named by alphabetical order

Question 3

What is the state and colour of benzene at rtp?

Answer 3

colourless liquid; highly volatile and flammable

Question 4

What is the melting and point of benzene?

Answer 4

Melting point: 5.5ºC

Boiling point: 80ºC

Question 5

Why is the melting and boiling point of benzene low?

Answer 5

Being a non-polar molecule, only a small amount of energy is required to overcome the weak dispersion forces between molecules.

Question 6

What are the health effects of benzene? (4)

Answer 6

From chronic (long term) exposure, benzene is:

1) able to cause harmful effects on the bone marrow
2) able to cause a decrease in red blood cells leading to anaemia
3) carcinogenic
4) able to cause leukaemia

Question 7

Describe the structure of benzene. (3)

Answer 7

1) planar, aromatic molecule
2) all carbon atoms in the ring are sp2, each bonded to 2 carbons and an hydrogen atom
3) geometry around each carbon atom is trigonar planar with C–C–C bond angle of 120º.

Question 8

How does delocalised pi electron clouds form on benzene rings?

Answer 8

the unhybridised p orbitals of each carbon are perpendicular to the plane, overlapping side-on with the unhybridised p orbitals on adjacent carbon atoms. the overlapping p orbitals produce 2 continuous rings of pi electrons above and below the plane of the benzene ring, which is known as the delocalised pi electron cloud. The 6 electrons in the cloud are free to move around.

Question 9

What are 4 implications of the pi electron cloud delocalisation?

Answer 9

1) all carbon-carbon bond lengths in benzene are equal (intermediate between c-c and c=c length), creating a planar, regular hexagonal shape.
2) resonance stabilisation, giving rise to extra stability of benzene (also known as aromaticity of benzene)
3) because of resonance stabilisation, benzene preferably undergoes electrophilic substitution, where one or more hydrogen atoms can be substituted by electrophiles, preserving its aromaticity.
4) due to resonance stabilisation, the pi electron cloud in benzene is less susceptible to attack by electrophiles than C=C, thus requiring a stronger electrophile to react as compared to alkenes.

Question 10

What are the 3 types of electrophilic substitutions benzenes go through?

Answer 10

1) electrophilic substitution with chlorine or bromine (chlorination/bromination)
2) electrophilic substitution with concentrated nitric acid (nitration)
3) electrophilic substitution with halogenoalkanes (friedel-crafts alkylation)

Question 11

State the reagents&conditions needed and observations during electrophilic substitution with chlorine or bromine on benzene.

Answer 11

Reagents&Conditions:
1) Cl2 gas/Br2 liquid, FeCl3/FeBr3/AlX3 (s) as lewid acid catalyst, warm
2) (OR) Cl2/Br2 (g), Fe (s)/Al (s), warm
Observations:
- Decolourisation of greenish-yellow Cl2 (g) / reddish brown Br2 (l)
- white fumes of HBr (g) or HCl (g)

Question 12

Briefly describe the 3-step mechanism of electrophilic substitution with chlorine or bromine on benzene.

Answer 12

Using the example of bromine,

1) FeBr3(s) and Br2(l) reacts to form a strong electrophile Br+.
2) Delocalised pi electron cloud attacks the electrophile Br+. Br+ adds onto the benzene ring, forming a carbocation intermediate. The positive charge on this intermediate is delocalised over the other 5 carbon atoms. Since the delocalised pi system is disrupted, this step has a fairly high activation energy and is the rds.
3) [FeBr4]- from step 1 acts as a Bronsted base and deprotonates the carbonates the carbocation intermediate, regenerating FeBr3, forming bromobenzene and HBr (byproduct). Aromaticity is restored.

Question 13

In electrophilic substitution of chlorine/bromine on benzene, why is FeX3 the lewis acid catalyst?

Answer 13

the Fe in FeX3 can accept a lone pair of electrons of halogen X2

Question 14

Why must In electrophilic substitution of chlorine/bromine on benzene be anhydrous?

Answer 14

In the presence of water, FeX(s) / AlX(s) will accept lone pair of electrons from water instead, and hence can no longer accept lone pairs of electrons from halogen X2 to generate the strong electrophile X+.

Question 15

State the 1 reagent&2 conditions needed and observations during electrophilic substitution with concentrated nitric acid on benzene.

Answer 15

Reagent: concentrated HNO3
Conditions: concentrated H2SO4 as Bronsted-Lowry acid catalyst, maintained at 55ºC
Observation: pale yellow oily liquid formed

Question 16

Why is sulfuric acid used in the electrophilic substitution with concentrated nitric acid on benzene?

Answer 16

It is a stronger Bronsted-Lowry acid than nitric acid, hence sulfuric acid would protonate nitric acid, eventually generating strong electrophile nitronium ion NO2+.

Question 17

State the 1 reagent & 2 conditions needed and observations during electrophilic substitution with halogenoalkanes on benzene. (friedel-crafts alkylation)

Answer 17

Reagent: chloroalkane
Conditions: AlX3 (s) as lewis acid catalyst, warm
Observation: white fumes of HCl (g)

Question 18

What is methylbenzene?

Answer 18

It is a compound obtained when a methyl group (–Ch3) replaces one of the H atoms in benzene. It has the molecular formula C7H8.

Question 19

Briefly describe the physical properties of methylbenzene.

Answer 19

it is a colourless liquid which is insoluble in water. It is relatively less toxic than benzene.

Question 20

What are the 3 types of reactions that the benzene ring on methylbenzene can undergo?

Answer 20

electrophilic substitution of:

1) chlorine/bromine
2) nitration
3) friedel-crafts alkylation

Question 21

What are the 2 types of reactions that the side chain on methylbenzene can undergo?

Answer 21

side chain reactions:

1) free-radical substitution
2) side-chain oxidation

Question 22

State the reagents&conditions needed and observations during electrophilic substitution with chlorine or bromine on methylbenzene.

Answer 22

Reagents & Conditions: Cl2(g)/Br2(l), FeX3 (s) as lewis acid catalyst, room temperature and absence of UV
Observation: decolourisation of greenish-yellow Cl2/reddish-brown Br2, white fumes of HX (g)

Question 23

State the 1 reagent & 2 conditions needed and observations during electrophilic substitution with concentrated nitric acid on methylbenzene.

Answer 23

Reagent: concentrated HNO3
Conditions: Concentrated H2SO4 as Bronsted-Lowry acid catalyst, maintained at 30ºC (milder conditions since –CH3 is activating)
Observation: yellow oily liquid formed

Question 24

State the 1 reagent & 2 conditions needed and observations during friedel-crafts alkylation of methylbenzene.

Answer 24

reagent: chloroalkane
conditions: AlCl3 (l) as lewis acid catalyst, room temperature (activating –CH3 requires milder conditions)
observations: white fumes of HCl (g)

Question 25

What are activating groups?

Answer 25

electron-donating substituents increase the electron density of the benzene ring, thereby increasing the reactivity of the benzene ring.

Question 26

What are deactivating groups?

Answer 26

electron-withdrawing substituents decrease the electron density of the benzene ring, thereby decreasing the reactivity of the benzene ring.

Question 27

How does the inductive effect work on arene substituents?

Answer 27

Inductive is present when there is a difference in electronegativity between 2 covalently bonded atoms. Substituents can inductively donate/withdraw electron density into/from the benzene ring through the sigma bond.

Question 28

How does the resonance effect work on arene substituents? (2)

Answer 28

Resonance effect is present when there is overlapping p orbitals (in this case between carbon of benzene and substituent)

1) substituents with general structure X, where X has at least 1 lone pair residing in the p orbital, can donate a lone pair of electrons into benzene ring via resonance through the pi bond.
2) substituents with general structure –Y=Z, where Z is more electronegative than Y, can withdraw electrons from the benzene ring via resonance through the pi bond.

Question 29

What happens when there are two substituents on the benzene ring? (3 scenarios)

Answer 29

1) if the directing effects of both substituents direct to the same position on the ring, then it is where electrophilic substitution will predominantly occur.
2) If both substituents are activating, and direct to different positions, the directing effect of the more activating substituent will dominate.
3) if the substituents consists of a strongly activating group and a deactivating group, the directing effect of the strongly activating substituent will dominate
4) if the substituents consists of other combinations/similar activating effects/similar deactivating effects, and each group directs towards different positions, it will result in a mixture of products of poor yield. (not ideal)

Question 30

State the reagents & conditions needed and observations during side-chain halogenisation on methylbenzene (FRS).

Answer 30

Reagents and conditions: X2, UV light or heat

Observations: reddish brown Br2/greenish-yellow chlorine decolourises slowly

Question 31

State the reagents & conditions needed and observations during side-chain oxidation of methylbenzene.

Answer 31

Reagents and conditions: aqueous KMnO4, dilute H2SO4/NaOH, heat
Observations: purple KMnO4 decolourises. For the reaction with H2SO4, white precipitate of benzoic acid is formed. For the reaction with NaOH, brown precipitate of manganese dioxide is formed.

Question 32

What is one exception of side-chain oxidation of methylbenzene?

Answer 32

when there are 3 alkyl substituents attached to the C atom beside the benzene ring, the alkylbenzene will NOT undergo side-chain oxidation.

Question 33

How do we differentiate between alkenes and arenes? Describe the procedure and observations. (3)

Answer 33

1) add bromine in tetrachloromethane DROPWISE WITH SHAKING to 1cm3 of each compound in separate test tubes. if it is an arene, the reddish brown Br2 in CCl4 will decolourise. if it is an alkene, it will not decolourise.
2) add aqueous bromine DROPWISE WITH SHAKING to 1cm3 of each compound in separate test tubes. if it is an arene, yellow orange aq bromine will not decolourise, while it will decolourise if it is an alkene.

Question 34

How do we differentiate between certain alkenes and certain alkylbenzenes?

Answer 34

Add 5 drops of KMnO4 to 1cm3 of dilute H2SO4 in a test tube. To 1cm3 of each compound in separate test tubes, add 2-3 drops of the mixture DROPWISE WITH SHAKING. Warm the test tubes in a water bath for a few minutes.
Alkenes with terminal C=C bonds will undergo oxidative cleavage, producing CO2 effervescence and decolourising purple KMnO4 without producing white PPT. Alkylbenzenes will undergo side-chain oxidation, decolourising purple KMnO4 and producing white ppt of benzoic acid, without producing effervescence.

Question 35

Explain why aromatic compounds tend to undergo substitution reactions instead of addition reactions.

Answer 35

Electrophilic addition destroys the delocalisation in the pi electron cloud/disrupts aromaticity which requires a significant amount of energy, whereas electrophilic substitution preserves aromaticity.