3.10 The Structure of Benzene

Question 1

What is the bond angle in a benzene molecule?

Answer 1

120 degrees

Question 2

Describe the stability of benzene.

Answer 2

Benzene is more stable than expected due to its ring of delocalised electrons.

Question 3

Is benzene classified as an alkene?

Answer 3

NO

Question 4

Why does benzene have a less exothermic enthalpy of hydrogenation than expected?

Answer 4

Benzene has a ring of delocalised electrons.

Question 5

List the three pieces of evidence used to prove benzene is not an alkene

Answer 5

Reactivity of bromine, Bond length, Hydration enthalpy.

Question 6

How does bromine water provide evidence that cyclohexa-1,3,5-triene is not the correct structure for benzene?

Answer 6

Benzene does not react with bromine water

Question 7

Describe the shape of benzene

Answer 7

Planar hexagon. All bonds have equal lengths.

Question 8

What is observed when benzene is added to bromine water?

Answer 8

No observation. Benzene does not react with bromine water because it is more stable than an alkene.

Question 9

How many covalent bonds does each C atom in a benzene ring have?

Answer 9

3

Question 10

What is the chemical test for an alkene?

Answer 10

Bromine water, room temperature and shake, Alkenes decolourise bromine water (solution goes from orange to colourless).

Question 11

The enthalpy change for the hydrogenation of cyclohexene is -120 kJmol-1. What is the theoretical enthalpy change for cyclohexa-1,3,5-triene?

Answer 11

-360 kJmol-1 (3 x 120)

Question 12

What can the enthalpy of hydrogenation be used to prove?

Answer 12

That benzene is not an alkene.

Question 13

What feature of the bonding in benzene distinguishes it from an alkene?

Answer 13

It has a ring of delocalised electrons.

Question 14

Which bond is longer, a C-C bond or a C=C bond?

Answer 14

C-C (less attraction)

Question 15

How does bromine water provide evidence that benzene is not an alkene?

Answer 15

Benzene does not react with bromine water

Question 16

What is the molecular formula of benzene?

Answer 16

C6H6

Question 17

Why is cyclohexa-1,3,5-triene considered to be a theoretical molecule?

Answer 17

The p-orbitals of adjacent C atoms overlap to form a delocalised ring. The result is benzene.

Question 18

Why was the theoretical enthalpy of hydrogenation of cyclohexa-1,3,5-triene predicted to be more exothermic than that of benzene?

Answer 18

Cyclohexa-1,3,5-triene does not account for the delocalised ring of electrons that make benzene more stable and less reactive.

Question 19

Benzene has a regular hexagon shape. What length are it bonds?

Answer 19

All its bonds are equal in length. Benzenes bonds are longer than C=C and shorter an C-C bonds

Question 20

What is the enthalpy of hydrogenation?

Answer 20

The enthalpy change when one mole of C=C bonds react with hydrogen.

Question 21

How many C atoms does benzene have?

Answer 21

6

Question 22

Given that benzene has a ring of delocalised electrons how are its properties different from an alkene?

Answer 22

It is more stable and therefore less reactive.

Question 23

What type of molecule is benzene?

Answer 23

Aromatic

Question 24

What is the empirical formula of benzene?

Answer 24

CH

Question 25

Describe the bonding in benzene.

Answer 25

Each C has 3 single covalent bonds. P-orbitals overlap to form a ring of delocalised electrons.

Question 26

Why is benzene less reactive than a molecule that contains C=C bonds?

Answer 26

It has a ring of delocalised electrons

Question 27

Why is benzene not classified as an alkene?

Answer 27

It has a ring of delocalised electrons.

Question 28

All benzenes bonds are equal in length. What does this suggest?

Answer 28

That benzene does not have alternating single and double bonds. It is not an alkene.

Question 29

What is another way of referring to the theoretical molecule cyclohexa-1,3,5-triene?

Answer 29

Kekule benzene.
Kekule incorrectly thought that benzene had alternating single and double bonds.

Question 30

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

Answer 30

It has a ring of delocalised electrons