The benzene ring, with its electron-rich delocalised system, is subject to attack by electrophilic reagents. Commonly benzene and its derivatives undergo electrophilic substitution reactions in which the delocalised system in the product molecules is restored.
The -OH group and -NO2 group represent two different types of groups that can be attached to the benzene ring. The -OH group is an activating group; it contributes negative charge into the benzene ring making it even more susceptible to electrophilic attack and therefore more reactive. The -NO2 group does the opposite. It is a deactivating group; it withdraws negative charge from the benzene making it less susceptible to electrophilic attack and therefore less reactive.
Activating groups direct substitution to positions to 2- and 4- (and 6-) of the benzene ring. In older terms, they are ortho- and para- directing. Deactivating groups direct substitution to positions 3- and 5- of the benzene ring. In older terms they are meta- directing.
The nitration of the benzene ring results from the attack of the nitronium ion, NO2+. Concentrated nitric(V) acid contains very little of the nitronium ion.
The nitration of Benzene¼
itself requires a considerably higher concentration of NO2+; a 'nitrating mixture' containing equal quantities of concentrated nitric(V) acid and sulphuric(VI) acid carefully mixed together and added to the benzene is refluxed on a water bath maintained at 55-60 °C. The highly acidic sulphuric(VI) acid causes the nitric(V) acid to function as a base. The NO2+ released undergoes substitution into the ring.
HNO3 + 2H2SO4 = NO2+ + H3O+ + 2HSO4-
In the above reaction a little 1,3-dinitrobenzene (m-dinitrobenzene) will also inevitably form along with the monosubstituted derivative. However, an -NO2 group substituted into the benzene ring, being a deactivating group, makes further electrophilic substitution into the ring somewhat more difficult. 1,3-dinitrobenzene can be prepared by refluxing benzene with the nitrating mixture on a water bath maintained at 95 °C or by continued nitration of nitrobenzene at 55-60 °C.
[1,3,5-trinitrobenzene requires treatment of benzene with fuming nitric(V) and sulphuric(VI) acids for five days at 110 °C. Even under these conditions the yield is only about 40%.]
The nitration of Phenol¼
is readily carried out at room temperature using dilute nitric(V) acid. A mixture of 2-nitrophenol and 4-nitrophenol is formed. However, since the concentration of nitronium ions present in dilute nitric(V) acid is very small, the NO2+ ion is unlikely to play a significant part in the nitration process. There is evidence that the electrophile involved is the nitrosonium ion, NO+, substituting an -NO group into the benzene ring. The nitric(V) acid present oxidises this to the -NO2 group, forming the 2- and 4-nitrophenols. More can found out about this reaction in a advanced text book of organic chemistry.
[The use of concentrated nitric(V) acid at room temperature yields 2,4,6-trinitrophenol (picric acid) as the major product. However, nitric(V) acid is a strong oxidising agent and reacts with phenol to form an excessive amount of oxidation products.]
Add some bromine to benzene and no obvious reaction takes place. The bromination of benzene requires the presence of a 'halogen carrier' catalyst, e.g. Fe, FeBr3. (Iron filings react with bromine to form FeBr3.) The catalyst polarises the bromine molecule, forming the complex Brd+¾Brd-----FeBr3 which then ionises to Br+[FeBr4]-. It is not possible to say to what extent, if at all, free Br+ is present.
C6H6 + Br+[FeBr3] ® C6H5Br + HBr + FeBr3
On the other hand, with Phenol, bromination of the aromatic ring occurs at room temperature without the use of a halogen carrier catalyst. If bromine water is added to an aqueous solution of Phenol an immediate white precipitate of 2,4,6-tribromophenol forms. 'Carefully smell the solution and it will remind you of the antiseptic TCP'.
For example, an alkaline solution of phenol reacts with benzene diazonium chloride to form a yellow Azo-dye.
In the coupling reaction, with an alkaline solution of phenol, the benzene diazonium ion becomes a sufficiently powerful electrophile to react with the benzene ring forming an azo-dye via an electrophilic substitution reaction.