25. Benzene and its Compounds

Question 1

Explain the bonding in benzene.

Answer 1

Planar and symmetrical structure.
Each C is sp2 hybridised - one 2s electron is promoted to the empty 2p shell to give 4 unpaired electrons. When three bonds are formed, one s and 2p electrons are used (sp2, bond angle 120).
Sigma bonds with neighbouring atoms, but one spare electron is contributed to a pi system. The p electrons are delocalised because they overlap with each other.

Question 2

Why isn’t benzene ‘double bonded’?

Answer 2

If it were, it would undergo addition reactions like alkenes.
C-C bond lengths lie between the typical C-C and C=C values.
High stability due to delocalisation.

Question 3

Why does halogenation of alkylarenes occur at positions 2 and 4? What about 6?

Answer 3

These are activated by electron-donating groups like alkyls.

Can also occur at position 6 in excess halogen because the 2 and 6 positions are equivalent in substituted benzene.

Question 4

Why does nitration of arenes occur at positions 1, 3 and 5?

Answer 4

NO2 group electron withdrawing, deactivates 2, 4 and 6.

Question 5

What are the relative acidities of phenol, water and ethanol?

Answer 5

Phenol strongest (still weak) - LP on EN oxygen overlaps with delocalised electron system, but nucleus still attracts delocalised electrons, easier for H+ to re-bond. 
H2O middle 
Ethanol weakest - alkyl group attached to O in alkoxide ion, donates electrons.