3.10 - Aromatic Chemistry

Question 1

What is benzene’s formula?

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 4

What is the most common type of reaction of benzene?

Answer 4

Substitution of a H for a different functional group

Question 5

What is the shape of benzene?

Answer 5

Flat, regular hexagon
Bond angle = 120°

Question 6

What is the bond length between adjacent C atoms?

Answer 6

Intermediate between C-C and C=C

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

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

Answer 9

Hydrogenation of cyclohexane = -120 kJmol-1 –> cyclohexa-1,3,5-triene = -360 kJmol-1
Benzene hydrogenation = -208 kJmol-1 so benzene is 152 kJmol-1 so its more stable

Question 10

Why else is cyclohexa-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 doule 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 MP?

Answer 12

Close packing of flat hexagonal molecules when solid

Question 13

Is benzene soluble in water? Why?

Answer 13

No, its non-polar

Question 14

Dangers of benzene?

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 the 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

Which ion is used to nitrate benzene?

Answer 19

+NO2
Nitronium ion or nitryl cation

Question 20

How is this NO2+ ion generated?

Answer 20

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

Question 21

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

Answer 21

H2SO4- + H+ –> H2SO4
H+ from benzene ring

Question 22

Overall equation of the nitration of benzene

Answer 22

C6H6 + HNO3 –> C6H5NO2 + H2O

Question 23

What are the uses of nitrated arenes?

Answer 23

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 24

How do substituents with a positive inductive effect affect further substitution?

Answer 24

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

Question 25

How do substituents with a negative effect affect further substitution?

Answer 25

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

Question 26

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

Answer 26

A halogen carrier (e.g. AlCl3)

Question 27

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

Answer 27

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

Question 28

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

Answer 28

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

Question 29

How is the AlCl3 catalyst reformed?

Answer 29

AlCl4- + H+ --> HCl + AlCl3 H+ from benzene

Question 30

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

Answer 30

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

Question 31

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

Answer 31

C=C bonds are in close proximity, so electrons in pi cloud/ p orbitals can partially delocalise and move between the 2 C=C double bonds

Question 32

What effect would electrons in p orbitals moving between the 2 C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation?

Answer 32

Makes the molecule more stable makes enthalpy of hydrogenation more positive