3.3.10 Aromatic Chemistry

Question 1

What is benzene’s formula and structure?

Answer 1

C₆H₆

Question 2

What is another name for arenes? Why did this come about?

Answer 2

Aromatic compounds, as first found in sweet-smelling dyes

Question 3

What is the most common type of reaction of benzene?

Answer 3

Substitution (of a hydrogen for a different functional group)

Question 4

What is the shape of benzene?

Answer 4

Flat, regular hexagon.
Bond angle = 120°

Question 5

What is the bond length between adjacent carbon atoms?

Answer 5

Intermediate between C-C and C=C

Question 6

What happens to the 4th electron in the p orbital of each C atom in benzene?

Answer 6

It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.

Question 7

What is the effect on benzene’s stability of the rings of electron density?

Answer 7

Makes benzene very stable, even though it is unsaturated (aromatic stability)

Question 8

Draw the skeletal structure of cyclohexa-1, 3, 5-triene.

Answer 8

Question 9

What is the thermochemical evidence that benzene is more stable than cyclohexa-1, 3, 5-triene?

Answer 9

Hydrogenation of cyclohexene = -120 kJ mol⁻¹
Hydrogenation of cyclohexa-1,3,5-triene = -360 kJ mol⁻¹
Benzene hydrogenation = -208 kJ mol⁻¹ so benzene is 152 kJ mol⁻¹ more stable

Question 10

Why else is cyclehexa-1,3,5-triene not a suitable model for benzene?

Answer 10

• Would not be symmetrical (C=C shorter than C-C), but benzene is.
• Would easily undergo addition reactions across the double bonds - benzene does not.
• Would form two isomers on the addition of Br₂ or similar - benzene does not

Question 11

What is the appearance of benzene at 298K?

Answer 11

Colourless liquid

Question 12

Why does benzene have a relatively high melting point?

Answer 12

Close packing of flat hexagonal molecules when solid.

Question 13

Is benzene soluble in water?
Why?

Answer 13

No, because it’s non-polar

Question 14

Dangers of benzene? (why it is not used in schools)

Answer 14

It is a carcinogen

Question 15

How do you name compounds containing a benzene ring?

Answer 15

-benzene, or phenyl-
Can designate position on ring using numbers if there is more than one substituent

Question 16

Why is benzene attacked by electrophiles?

Answer 16

High electron density above/below ring due to delocalised electrons.

Question 17

What is delocalisation energy and what is the effect of this on benzene’s reactions?

Answer 17

The large amount of energy that is needed to break the aromatic ring apart. Results in the aromatic ring almost always staying intact.

Question 18

What is seen when benzene is combusted? Why?

Answer 18

Smoky flames due to soot from unburnt carbon.
This is because of the high Carbon:Hydrogen ratio.

Question 19

Draw a general electrophilic substitution mechanism of benzene, using El⁺ to represent an electrophile.

Answer 19

Question 20

Which ion (name and formula) is used to nitrate benzene?

Answer 20

NO₂⁺ (+ charge is on the nitrogen). Nitronium ion or nitryl cation

Question 21

How is this NO₂⁺ ion generated? (conditions and equations)

Answer 21

Concentrated H₂SO₄ and concentrated HNO₃.
H₂SO₄ + HNO₃ -> H₂NO₃⁺ + HSO₄⁻
H₂NO₃⁺ -> H₂O + NO₂⁺
Overall:
H₂SO₄ + HNO₃ -> H₂NO₃⁺ + HSO₄⁻ + H₂O

Question 22

How is the H₂SO₄ catalyst regenerated in the nitration of benzene?

Answer 22

HSO₄⁻ + H⁺ -> H₂SO₄ (H⁺ from benzene ring)

Question 23

Draw a mechanism and write an overall equation for the nitration of benzene.

Answer 23

Question 24

What are the uses of nitrated arenes?

Answer 24

Production of explosives (e.g. TNT (1-methyl-2,4,6-trinitrobenzene) - releases lots of heat and gas on explosion.
To make aromatic amines that are used for industrial dyes

Question 25

How do substituents with a positive inductive effect (e.g. alkyl groups) affect further substitution?

Answer 25

They release electrons into the delocalised electron ring, increasing the electron density and making further substitution reactions more likely/quick. Direct substituents to the 2,4,6 positions.

Question 26

How do substituents with a negative indictive effect (e.g. NH₂) affect further substitution?

Answer 26

Remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely/quick. Direct substituents to 3,5 positions.

Question 27

What type of catalyst is used for a Friedel-Crafts reaction?

Answer 27

A halogen carrier (e.g. AlCl₃)

Question 28

Write an equation to form an electrophile that could be used to acylate benzene, starting with AlCl₃ and RCOCl.

Answer 28

AlCl₃ + RCOCl -> AlCl₄⁻ + RCO⁺ (+ on carbon) RCO⁺ can attack benzene

Question 29

What is happening when AlCl₄⁻ is formed in terms of electrons?

Answer 29

Chlorine atom's lone pair of electrons is forming a coordinate bond to Al

Question 30

How is the AlCl₃ catalyst reformed?

Answer 30

AlCl₄⁻ + H⁺ -> HCl + AlCl₃ (H⁺ from benzene)

Question 31

How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

Answer 31

Use a halogenoalkane and AlCl₃ to create an electrophile that can attack benzene

Question 32

Draw the mechanism for the acylation of benzene from RCO⁺.

Answer 32

Question 33

If you are considering cyclic compounds, what might happen if two double bonds are next to each other?

Answer 33

C=C bonds are in close proximity, so electrons in pi cloud/p orbitals can partially delocalise and move between the two C=C double bonds.

Question 34

What effect would electrons in p orbitals moving between the two C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation?

Answer 34

Makes the molecule more stable. Makes enthalpy of hydrogenation more positive.