Benzene/ Phenol (Arenes)

In contrast to benzene, the reaction of an alkene with bromine does not need a halogen carrier. Compare the different reactivities of benzene and alkenes towards chlorine.

-benzene electron are delocalised -Alkenes electrons are localised (concentrated btw 2 carbons) Electrophiles are attracted to high electron density in alkenes

Explain the relative resistance to bromination of benzene compared to phenol and compared to cyclohexene.

-benzene electrons are delocalised -phenol has a lone pair electron on the OH group that is partially delocalised into the ring -cyclohexene electrons are localised -benzene has a low electron density -benzene cannot polarise Br2 -phenol/ cyclohexene can polarise Br2

Carbon dioxide is normally a very poor electrophile. However, this reaction does occur because the benzene ring in the phenoxide ion is activated. Explain how the benzene ring in the phenoxide ion is activated.

-electron pair fro oxygen is delocalised into the ring -increases pi electron density - attracts electrophile more

Explain what is meant by the term delocalised π-bond electrons.

-electrons are spread over more than two atoms -pi bonds formed by overlap of p orbitals

The compound iodine monobromide, IBr, also reacts with benzene in an electrophilic reaction. (i) Which compound would be the main product of this reaction, iodobenzene or bromobenzene? Explain your answer.

-iodobenzene because Br is more electronegative than I -so I atom will be the electrophile/positive

Describe with the aid of suitable diagrams the bonding and structure of a benzene molecule.

-ring is planar -p-orbitals overlap -above and below the ring -to form pi bonds - pi bonds are delocalised -sigma bonds are between C-C and C-H

How does the halogen carrier allow the reaction to take place?

AlCl3 + Cl2 —> AlCl4- + Cl+

Compare the reagents and conditions for the nitration of phenol with those used for the nitration of benzene. State and explain the effect of the -OH group on the reactivity of the benzene ring in phenol.

Nitration of Benzene -conc HNO3 -conc. H2SO4 -heating above 50 degrees C Greater reactivity of phenol -lone pair from O atom is delocalised into the ring -greater pi electron density around the ring -benzene ring in phenol is activated -attracts electrophiles

The chemist observed that bromine decolourises when it reacts with phenol. What other observation would she have made?

White precipitate