Aromatic Chemistry: Bonding

Question 1

What is an aromatic compound?

Answer 1

• A compound that contains a benzene ring

Question 2

What is the formula for benzene?

Answer 2

• C6H6

Question 3

Describe the structure of benzene

Answer 3

• Planar cyclic structure (six atoms joined together in a flat ring)
• Each carbon atoms forms single covalent bonds to adjacent carbons and to one hydrogen atom
• Contains ring of delocalised electrons

Question 4

Explain why benzenes have a delocalised ring of electrons

Answer 4

• Each carbon only forms 3 covalent bonds: 2 with other carbons, 1 with hydrogen
• Final unpaired electron on each carbon atom is located in a p-orbital that sticks above and below the plane of the ring
• The p-orbitals on each carbon atom form a ring of delocalised electrons

Question 5

Describe the length of the bonds in benzene rings in relation to single and double carbon bonds

Answer 5

• All bonds in ring are same length (140 pm)

- Lies in between length of single C-C bond (147 pm) and double C=C bond (135 pm)

Question 6

What is the theoretical compound (Kekule structure) for benzene?

Answer 6

• Cyclohexa-1,3,5-triene

Question 7

How is cyclohexa-1,3,5-triene different from the actual benzene structure?

Answer 7

• Cyclohexa-1,3,5-triene has alternate single and double bonds between carbon atoms

Question 8

Which is more stable, benzene or cyclohexa-1,3,5-triene?

Answer 8

• Benzene

Question 9

Why is benzene more stable than cyclo-1,3,5-triene?

Answer 9

• Benzene has ring of delocalised p electrons

Question 10

Use the following enthalpy changes of hydrogenation to justify why benzene is more stable than cyclohexa-1,3,5-triene

• Cyclohexa-1,3,5-triene ΔHº hydrogenation = -360kJmol^-1
• Benzene ΔHº hydrogenation = -208kJmol-1

Answer 10

• Energy is put in to break bonds and released when bonds are made
• More energy put in to break bonds in benzene than in cyclohexa-1,3,5-triene
• Suggests benzene is more stable