10: Aromatic Compounds

Question 1

Benzene is planar - what does this mean?

Answer 1

Flat molecule

Question 2

Benzene has a cyclic structure - what does this mean?

Answer 2

6 carbon atoms joined in a flat ring.

Question 3

Describe the bonding in benzene.

Answer 3

Each carbon covalently bonded to 1 hydrogen and 2 other carbon atoms.

Question 4

How many electrons remain unpaired for each carbon in benzene?

Answer 4

6

Question 5

What orbital is the unpaired electron for each carbon atom in in benzene?

Answer 5

p

Question 5

Describe the ring seen in benzene.

Answer 5

Shows the 6 delocalised electrons each from the p orbital.

Question 5

Why are the electrons considered delocalised in benzene?

Answer 5

The electrons do not belong to a specific carbon atom.

Question 5

Describe the length of bonds in benzene.
Compare this to the normal length of a C=C bond and C-C bond.

Answer 5

Bond length is halfway between the length of a normal C-C and C=C bond.

Question 6

State the bond angle in benzene.

Answer 6

120

Question 7

Name the molecule that would be similar to benzene, consisting of alternating double and single bonds.

Answer 7

Cyclohexa-1,3,5-triene

Question 8

Name the process that allows us to compare the stability of theoretical cyclohexa-1, 3, 5-triene and benzene.

Answer 8

Hydrogenation.

Question 9

Is benzene or theoretical cyclohexa-1, 3, 5-triene more stable?

Answer 9

Benzene

Question 10

Compare the hydrogenation of cyclohexene (-120 kJmol-1) and theoretical cyclohexa-1, 3, 5-triene (-360kJmol-1) to benzene (-208 kJmol-1). Refer to the enthalpy changes.

Answer 10

Cyclohexene has 1 double bond with an enthalpy change of -120 kJmol-1 when hydrogenated.
If benzene had 3 double bonds, you’d expect an enthalpy of hydrogenation to be -360kJmol-1 but hydrogenation of benzene is -208kJ mol-1.
The difference in energy between theoretical cyclohexa-1, 3, 5-triene and benzene indicates benzene is of higher stability.

Question 11

Explain why benzene is more stable than theoretical cyclohexa-1, 3, 5-triene.

Answer 11

The delocalised ring of electrons means that electron density is shared over more atoms, so the energy of the molecule is lowered and therefore it becomes more stable.

Question 12

When naming compounds where the benzene ring is the secondary functional group, what do we call benzene as a prefix?

Answer 12

Phenyl

Question 13

What type of molecule do benzene rings attract and why?

Answer 13

Electrophiles, they are electron pair acceptors and are usually short of electrons, so attract to regions of high electron density

Question 14

Name the mechanism for the reaction of benzene with electrophiles.

Answer 14

Electrophilic substitution

Question 15

Name 2 common electrophiles that are often involved in reactions with benzene.

Answer 15

H+, NO2

Question 16

1. Electrophilic substitution
   Why do the electrophiles attract to benzene?

Answer 16

They are attracted to the dense region of negative charge

Question 17

2. Electrophilic substitution
   How does the electrophile bond to the benzene?

Answer 17

It takes an electron pair from the centre of the molecule.

Question 18

3. Electrophilic substitution
   Why does the benzene molecule become positively charged once the electrophile has bound?

Answer 18

Once the electrophile has taken the electron pair from the centre, this breaks the ring.

Question 19

4. Electrophilic substitution
   Why is a hydrogen lost once the electrophile has bound?

Answer 19

To regain the stability of the molecule and ensure only 6 atoms are bound.

Question 20

Give the conditions for a nitration reaction.

Answer 20

Concentrated H2SO4 catalyst, 55C, nitric acid

Question 21

Why do we need sulfuric acid during nitration of benzene?

Answer 21

It helps to form the nitronium ion NO2+.

Question 22

Give an equation for the formation of a nitronium (NO2+ ion)

Answer 22

HNO3 + H2SO4 -> HSO4- + NO2+ + H2O

Question 23

During nitration, if we only want 1 NO2+ to bond then what temperature must the reaction stay below?

Answer 23

55C

Question 24

Reduction of nitrobenzene containing compounds can form molecules with what functional group?

Answer 24

Aromatic amines

Question 25

Give some uses for nitrobenzenes.

Answer 25

Pharmaceuticals, manufacturing dyes, explosives

Question 26

Give an equation for the reformation of H2SO4 in nitration.

Answer 26

H+ + HSO4- -> H2SO4

Question 27

Give the general name of the product formed after Friedel Crafts acylation with benzene.

Answer 27

Phenylketone (+ HCl)

Question 28

Name the reaction where benzene reacts with acyl chlorides.

Answer 28

Friedel Crafts

Question 29

How can strength of an electrophile be increased?

Answer 29

By reacting it with a halogen carrier catalyst.

Question 30

Name a halogen carrier.

Answer 30

AlCl3

Question 31

Name the funcional group that binds to benzene after a Friedel Crafts reaction has taken place.

Answer 31

RCO

Question 32

In Fridel-Crafts, name the carbocation that attracts to the electron ring inside the benzene.

Answer 32

RC+O

Question 33

In Fridel-Crafts, why does the RCO+ carbocation attract to the benzene?

Answer 33

The positive carbocation attracts to the region of electron density in the ring.

Question 34

State the products formed after a Friedel Crafts reaction.

Answer 34

BenzeneRCO + HCl + AlCl3