Aromatic chemistry

Describe the bonding in benzene

Each C has 3 covalent bonds

Spare electrons in p orbital overlap causing delocalisation

Describe the shape of benzene

Planar molecule, 6-carbon ring, C-C bonds equal in length (intermediate between C-C and C=C)

Describe the stability of benzene

The expected enthalpy of hydrogenation of cyclohexatriene is -360 kJ mol^-1, but the actual value for benzene is less exothermic by 152 kJ mol^-1, so benzene is more stable

Why do substitution reactions occur in benzene in preference to addition reactions?

Addition reactions would involve breaking up the delocalised system

Which form of substitution reactions does benzene undergo and why?

Electrophilic, because benzene has a high electron density and so attracts electrophiles

Give the change in functional group during the nitration of benzene

Benzene to nitrobenzene

Give the reagents needed for the nitration of benzene

Conc. nitric acid in the presence of conc. Sulfuric acid catalyst

State the name of the mechanism of the nitration of benzene

Electrophilic substitution

Give the formula of the electrophile used in the nitration of benzene

+NO2

Give the overall equation for the formation of the electrophile in nitration of benzene

HNO3 + 2H2SO4 —> +NO2 + 2HSO4- + H2O

Draw the 2-step mechanism for the formation of the electrophile in nitration of benzene

What temperature is the nitration of benzene carried out at?

60 °C

Why aren’t temperatures higher than 60°C used during nitration of benzene for production of aromatic amines?

At higher temperatures, multiple substitutions can occur and it is vital only one occurs for production of aromatic amines

Give the change in functional group during friedel crafts acylation of benzene

Benzene to phenyl ketone

Give the equation for the formation of the electrophile used in acylation of benzene

AlCl3 + R-COCl —> R-CO+ + AlCl4 -

Draw the mechanism for the acylation of benzene using

State the type of mechanism for acylation of benzene

Electrophilic substitution

Give the reagents and conditions needed for acylation of benzene

acyl chloride in presence of anhydrous aluminium chloride catalyst, heat under reflux

Give the equation for the reform of AlCl3 catalyst in acylation

H+ + AlCl4 - —> AlCl3 + HCl

Give the reagents and conditions for the reduction of nitroarenes to aromatic amines

Sn/HCl OR H2/Ni, heating

Give the full equation for the reduction of nitrobenzene

C6H5(NO2) + 6 [H] —> C6H5(NH2) + 2H2O

How does the delocalisation affect the C-Cl bond in chlorobenzene?

Made stronger, so typical haloalkanes substitution and elimination reactions don’t occur, the electron rich benzene ring will repel nucleophiles

How does delocalisation affect the C-O and O-H bonds in phenol?

Makes C-O stronger and O-H weaker

How does delocalisation affect the base strength of phenylamine?

Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton

Give a use of phenyl ketones

Pharmaceuticals