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 3

What is the most common type of reaction of benzene?

Answer 3

Substitution (of a H for a different functional group)

Question 4

What is the shape of benzene?

Answer 4

Flat, regular hexagon. Bond angle = 120o

Question 5

What is the bond length between adjacent C 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

Forms 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

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

Answer 8

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

Question 9

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

Answer 9

• 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 10

What is the appearance of benzene at 298K?

Answer 10

Colourless liquid

Question 11

Why does benzene have a relatively high melting point?

Answer 11

Close packing of flat hexagonal molecules when

solid

Question 12

Is benzene soluble in water? Why?

Answer 12

No- non polar

Question 13

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

Answer 13

It is a carcinogen

Question 14

How do you name compounds containing a benzene ring?

Answer 14

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

Question 15

Why is benzene attacked by electrophiles?

Answer 15

High electron density above/below ring due to

delocalised electrons

Question 16

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

Answer 16

The large amount of energy that is needed to

break the aromatic ring apart. Results in the aromatic ring almost always staying intact

Question 17

What is seen when benzene is combusted? Why?

Answer 17

Smoky flames due to soot from unburnt carbon.

This is because of the high Carbon: Hydrogen ratio.

Question 18

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

Answer 18

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

Question 19

How is this NO2+ ion generated? (conditions and

equations)

Answer 19

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

Question 20

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

Answer 20

HSO4- + H+ → H2SO4

H+ from benzene ring

Question 21

What are the uses of nitrated arenes?

Answer 21

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 22

How do substituents with a positive inductive effect

(e.g. alkyl groups) affect further substitution?

Answer 22

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 23

How do substituents with a negative inductive effect

(e.g. NH2) affect further substitution?

Answer 23

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 24

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

Answer 24

A halogen carrier (e.g. AlCl3)

Question 25

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

Answer 25

AlCl3 + RCOCl → AlCl4- + RCO+ (+ on C)

RCO+ can attack benzene

Question 26

What is happening when AlCl4- is formed in terms of

electrons?

Answer 26

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

Question 27

How is the AlCl3 catalyst reformed?

Answer 27

AlCl4- + H+ → HCl + AlCl3

H+ from benzene

Question 28

How could you use a Friedel-Crafts mechanism to

add a methyl group to a benzene ring?

Answer 28

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

Question 29

If you are considering cyclic compounds, what might

happen if two double bonds are next to each other?

Answer 29

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 30

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 30

Makes the molecule more stable; makes enthalpy of hydrogenation more positive