Aromatic Compounds

Question 1

Explain the mechanism in Friedyl Crafts Acylation

Answer 1

Benzene reacts with an acylchloride with a halogen carrier to form a phenyl ketone and hydrochloric acid

Question 3

Explain why bromination of Benzene might not have 100% yield with a halogen carrier

Answer 3

The stability of the H and E+ can collapse and revert back to the original Benzene.

Question 5

Compare the ease of electrophilic substitution between phenol and benzene (in terms of electron pair donation to the pi system from an oxygen p orbital in phenol)

Answer 5

Both have an aromatic structure with pi electrons delocalised into a ring, above and below the molecular plane. The oxygen in phenol has its lone pair partially delocalised within the ring which increases its electron density and allows it to polarise the electrophile without the need for a halogen carrier. It’s greater electron density also increases its reactivity towards all electrophiles. Therefore it undergo electrophilic substitution more readily

Question 6

Explain why phenol is acidic but ethanol is not

Answer 6

The phenoxide ion is more stable than ethanoxide ion. This is because in phenoxide the negative charge is delocalised and spread along the Benzene ring whereas in ethanoxide, the charge is localised on the oxygen atom

Question 7

Explain the structure of a benzene compound including the electron organisation

Answer 7

Benzene is a flat, regular hexagon, with six electrons in the delocalised pi ring above and below the molecular plane. It has 6 p orbitals which overlap to form a delocalised pi electron ring. Since it has only one electron in each orbital instead of 2, it has a lower electron density and so great stability.

Question 8

Explain why electrophilic substitution in arenes for nitration and halogenation

Answer 8

Due to Benzene’s lower pi electron density it requires a halogen carrier to previously induce the electrophile for the reaction to kick off.

Question 9

Explain why some groups are activating and others are deactivating

Answer 9

Substituents that attach to the Benzene ring can either be activating or deactivating.

Activating groups (e.g alkyl, amines, esters, hydroxy) have a positive inductive effect as they are electron donating groups and so increase electron density which increases the reactivity. - 2,4 directing

Deactivating groups (e.g NO2 SO2 CO) are all electron withdrawing and due to the mesomeric effect, they reduce reactivity. 
- 3 directing

Question 10

Explain experimentally why phenol is a weak acid

Answer 10

When dissolved in water Benzenes acts as a weak acid as it partially dissociates in water and so relative to water, it is acidic as it forms H3O+
It also neutralises NaOH to form sodium phenoxide ion

Question 14

Explain the solubility of Phenol

Answer 14

Phenol is moderately soluble (relatively soluble compared to Benzene) due to the hydrogen bonding and polarity which allows it to bond with the water molecules.

Question 17

Compare the Kekulé model to the modern model for Bromine

Answer 17

1) Doesn’t have a regular bond length
X Ray diffraction shows bonds in benzene are of equal length whereas the Kekulé structure is a mixture of sigma and pi bonds

2) Turns bromine water colourless
Benzene, unlike alkanes, does not decolourise bromine water as it doesn’t undergo electrophilic addition since it is too stable

3) Enthalpy of hydrogenation is less exothermic
(less negative) than the actual value for Benzene due to Benzene’s greater stability

Question 19

Explain why Benzene cannot de colourise bromine water

Answer 19

Benzene doesn’t decolourise bromine water / undergo an electrophilic reaction with pure bromine. Because the delocalised pi electrons ring does not have a high enough electron density to polarise the bromine and induce a dipole. Alkanes can as their pi electrons are localised between two carbon atoms and have a relatively high electron density that can polarise bromine as it approaches.