4. Arenes

Question 1

Are benzene less dense than water?

Answer 1

Yes

Question 2

Describe the solubility of benzenes.

Answer 2

Benzenes are immiscible in water but soluble in non-polar solvents –> useful non-polar solvent for many organic compounds

Question 3

Describe the chemical properties of benzene.

Answer 3

Each C atom has a p orbital perpendicular to the plane of the six carbon atoms and contains one electron each. There is continuous overlap of p orbitals resulting in the delocalisation of pi electrons above and below the ring which stabilizes benzene –> benezene is highly stable and less susceptible to addition reactions which destroys the aromatic character

Question 4

What reactions can benzenes undergo?

Answer 4

Electrophilic substitution:

• Nitration of benzene
• Halogenation of benzene
• Friedel-Crafts Alkylation
• Frieldel-Crafts Acylation

Question 5

What reactions can methylbenzene undergo?

Answer 5

-free radical substitution of side chain
-oxidation of side chain
-

Question 6

What are the reagents and conditions for nitration of benzene?

Answer 6

Concentrated HNO3, concentrated H2SO4, 55 degrees Celsius

Question 7

What are the reagents and conditions for halogenation of benzene?

Answer 7

X2 / Fe/ FeX3/ Anhydrous AlX3 where X = Cl or Br

Question 8

What are the reagents and conditions for Friedel-Crafts alkylation?

Answer 8

RCl, anhydrous AlCl3

Question 9

What are the reagents and conditions for Friedel-Crafts acylation?

Answer 9

RCOCl (acyl chloride), anhydrous AlCl3

Question 10

What are the reagents and conditions for FRS of side chain of methylbenzene?

Answer 10

Cl2, UV light, excess methylbenzene (for mono-substituted products)

Question 11

What are the reagents and conditions for oxidation of side chain of methylbenzene?

Answer 11

Acidic conditions:
KMnO4 with H2SO4, heat under reflux

Alkaline conditions:
Alkaline KMnO4, heat under reflux

Question 12

What is the product of oxidation of side chain of methylbenzene and ethylbenzene under acidic conditions? State the observations.

Answer 12

Benzoic acid
Purple KMnO4 turns colourless. When the reaction mixture is cooled, white ppt of benzoic acid forms.
For ethylbenzene, if the gaseous product is passed into limewater, a white ppt is also formed with limewater due to CO2 produced.

Question 13

What effects do the groups on the benzene ring have on the electrophilic substitution of benzenes?

Answer 13

1. Reactivity of aromatic ring
2. Site of attack for the substitution reaction
3. Number of substitutions

Question 14

What effect does electron-donating groups have on the benzene ring?

Answer 14

They make the benzene ring more reactive –> increase the electron density on the benzene ring (making the ring more susceptible to electrophilic attack) and stabilize the arenium ion (making the electrophilic substitution reaction energetically more favorable –> reaction conditions are milder

Question 15

What effect does electron-withdrawing groups have on the benzene ring?

Answer 15

They make the benzene ring less reactive –> decrease the electron density on the benzene ring (making the ring less susceptible to electrophilic attack) and destabilizes the arenium ion (making the electrophilic substitution reaction less energetically favorable –> reaction conditions are harsher

Question 16

What temperature is nitration of methylbenzene carried out at?

Answer 16

30 degrees Celsius. As the methyl group is ring activating (due to its electron-donating ability), methylbenzene is more reactive than benzene.

Question 17

Concentrated HNO3, concentrated H2SO4, 55 degrees Celsius

Name the reaction.

Answer 17

Nitration of benzene (electrophilic substitution)

Question 18

X2 / Fe / FeX3 / Anhydrous AlX3 where X = Cl or Br

Name the reaction.

Answer 18

Halogenation of benzene (electrophilic substitution)

Question 19

RCl, Anhydrous AlCl3

Name the reaction.

Answer 19

Friedel-Crafts alkylation

Question 20

ROCl (acyl chloride), anhydrous AlCl3

Name the reaction.

Answer 20

Friedel-Crafts acylation

Question 21

Compare the reactivities of alkenes and benzenes.

Answer 21

The pi electron cloud in benzene is less susceptible to electrophilic attack than that in a C=C –> requires stronger electrophile

Question 22

How to identify methylbenzenes / alkylbenzenes?

Answer 22

Heat under reflux with acidified KMnO4.
Purple KMnO4 turns colourless and white ppt of benzoic acid forms when reaction mixture is cooled.
For ethylbenzene, when the gaseous products are passed into limewater, a white ppt is formed as CO2 is formed.