25.1 - Introducing benzene

Question 1

What is the empirical and molecular formula of benzene?

Answer 1

CH and C6H6

Question 2

Give three characteristics of benzene

Answer 2

1. Colourless, sweet smelling, and highly flammable liquid
2. Found naturally in crude oil, is a component of petrol, and found in cigarette smoke
3. Classified as a carcinogen

Question 3

What is Kekulé’s delocalised model of benzene?

Answer 3

Six carbon atoms in a ring joined by alternate single and double bonds

Question 4

What are the three reasons to why Kekulé’s model is incorrect?

Answer 4

1. The lack of reactivity of benzene
2. The lengths of the carbon-carbon bonds in benzene
3. Hydrogenation enthalpies

Question 5

Explain why the lack of reactivity of benzene disprove’s Kekulé’s model

Answer 5

• If benzene contained double bonds as suggested by his model, it should decolourise bromine in an electrophilic addition reaction
• But it does not undergo electrophilic addition reactions and does not decolourise bromine under normal conditions

Question 6

Explain why the lengths of the carbon-carbon bonds in benzene disprove’s Kekulé’s model

Answer 6

X-ray diffraction showed that all the bond lengths in benzene were equal: 0.139nm - which is between the length of a single and double bond

Question 7

Explain why the hydrogenation enthalpies of benzene disprove’s Kekulé’s model

Answer 7

• If benzene had Kekulé’s model, then the hydrogenation enthalpy would be three times of that of cyclohexene, -360kjmol-1
• However the actual enthalpy is -208kjmol-1, suggesting that the actual structure is more stable than the theoretical model, delocalised electrons are more stable

Question 8

What is the delocalised model of benzene?

Answer 8

• Planar, cyclic, hexagonal hydrocarbon containing six carbon and hydrogen atoms
• Each carbon uses three of its four electrons to bond to two other carbon atoms and to one hydrogen atom- The other electron is in the p orbital at right angles to the plane
• The overlap of p-orbitals from each of the six delocalised electrons forms a ring of electron density above an below the plane (pi-bond over all carbons)

Question 9

Draw benzoic acid

Answer 9

:)

Question 10

Draw phenylamine

Answer 10

:)

Question 11

Draw benzaldehyde

Answer 11

:)