3.10 Aromatic chemistry

Question 1

What is the molecular structure for benzene?

Answer 1

C6H6

Question 2

What is the other name for the benzene prefix?

Answer 2

Phenyl

Question 3

What is the name of the old structure for benzene?

Answer 3

Kekule

Question 4

What are the 4 reasons to disprove Kekules structure of benzene?

Answer 4

Bond length
Reactivity with Br2
Enthalpy of hydrogenation
Mechanism of benzene

Question 5

Explain the bond lengths

Answer 5

The bond length seen in the benzes structure is between a double and a single bond showing that all the bond lengths are the same

Question 6

Explain the enthalpy of hydrogenation point

Answer 6

The enthalpy change of hydrogenation is less exothermic than expected when compared to the hydrogenation of kekules structure because benzes has a delocalised ring in it which makes it more stable

Question 7

Explain the mechanism point

Answer 7

Benzez undergoes substitution not electrophilic addition

Question 8

How is the delocalised ring formed in benzene?

Answer 8

Benzene has 6 carbon atoms, each connected to a hydrogen atom, this only used 3 out of the 4 electrons in each carbon. The fourth electron is in the p orbital, therefore all together, there are 6 unused electrons. When the 2 p orbitals overlap, the pi bond is formed and form a delocalised ring. The electrons are free to move around the ring.

Question 9

State the 3 key facts about bonding

Answer 9

• Each C atom has 3 covalent bonds
• Spare electrons in the p orbital overlap to form a pi cloud
• The delocalised ring makes it more stable

Question 10

State the 3 key facts about the shape

Answer 10

• Benzene is planar and flat whereas Kekules isn’t
• It is a hexagon shape with 120 bond angles
  The C-C bonds have equal length which are between double and single

Question 11

State the 3 ke facts about stability

Answer 11

• The expected enthalpy change is less exothermic by 152 Kj mol-1
• Expected enthalpy change of hydrogen of Kekules is -360Kj mol-1
• Benzene is lower in energy than Kekules because of the delocalised ring

Question 12

State the 3 physical properties of arenes

Answer 12

• Colourless liquid at room temperature
• BP of 353K, similar to hexane
• MP of 279K, a low higher than hexane because it is a planar molecule

Question 13

State the 2 important features of the reactions of aromatic molecules

Answer 13

• The ring is an area of high electron density due to its delocalisation, arenes react with electrophiles
• The aromatic ring is very stable

Question 14

Why are arenes rarely burnt?

Answer 14

They are extremely useful substances in the pharmaceutical world, so burning them is wasteful. They also undergo incomplete combustion so you don’t get as much energy out of them as could be.

Question 15

What are the 2 versions of electrophilic substitution?

Answer 15

• Nitration
• Friedel-Crafts Acylation

Question 16

Explain nitration

Answer 16

You replace one of the H atoms on a benzene ring with a NO2 group

Question 17

What are the 2 reagents in nitration?

Answer 17

Conc H2SO4
Conc HNO3

Question 18

What is the overall equation for nitration of benzene?

Answer 18

H2SO4 + HNO3 –> H2NO3+ + HSO4-
H2NO3+ –> H2O + NO2+
= H2SO4 + HNO3 –> HSO4- + H2O + NO2+

Question 19

What is the name of the product of electrophilic substitution?

Answer 19

Nitro benzene

Question 20

Write the equation to show how H2SO4 reforms

Answer 20

HSO4- + H+ –> H2SO4

Question 21

What are the uses of nitrated arenes?

Answer 21

Can be used to produce explosives (TNT)