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Benzene and Reactions of Arenes answers.

1.         Identify the following molecules as alkane, alkene, arene or        cycloalkane

a) CH3CH2CH2CH2CH3 

Alkane

 b) C6H5CH3 

Arene 

 c) CH3CH=CHCH2CH3 

Alkene
d) CH3CH(CH3)CH2CH3

Alkane

 e) C6H12

Alkene or Cycloalkene

  

2.         Benzene is classified as aromatic because of the delocalised system of electrons around the benzene ring.

            a)         What is meant by a delocalised system of electrons?

Definition: Delocalised electron systems in organic molecules involve p bonds with all the carbon (or other) atoms in the system in a single plane and in which overlap of p orbitals extends over a significant number of carbon atoms.

            b)         Why is knowledge of bond lengths important in the discussion of the structure of benzene?

Techniques were developed to measure the bond length in benzene, and the results where quite surprising.

Bond Lengths

/nm

C-C cyclohexane

0.154

C=C cyclohexane

0.134

C-C in benzene

0.140

The bond length of C-C bonds in benzene was someway between that of an alkane and an alkene.

c)         What thermochemical data supports the idea of a stable ring structure for benzene with delocalised electrons?

The hydrogenation of cyclohexane is well known.

So if three double bonds are present – as in benzene, then the comparable reaction should liberate 3 times that of cyclohexane.

-(3 × 119) = - 357 kJ mol-1

But the actual value for benzene was found to be different.

So benzene is (357 – 207) = 150 kJ mol-1 more stable than otherwise expected, or if it contained 3 ordinary C=C bonds.

The diagram shows the difference – the stabilisation energy of benzene.

3          Describe the two types of reactions that benzene is normally capable of carrying out. Give an example of each.

4.         Draw a diagram to represent the delocalised structure of benzene. Include the p-orbitals.

5.         What evidence is present to suggest delocalisation is present in benzene? There are at least 3 points.

  • Bond Length somewhere between a sigma bond and a pi bond

  • Stability due to thermochemical delocalisation energy

  • Difficulty of break-up of Aromaticity due to delocalisation

6.         Why should benzene never be drawn as a hexagon?

A plain hexagon = cyclohexane. Benzene should always be drawn as either:

or