Benzene, what is aromaticity?
Benzene is a special type of compound which is found always as a delocalised structure. Benzene is made up of 6 Carbon atoms, with 3 double bonds in a ring, and a proposed structure for benzene is
This structure has 3 double bonds which are connected to each other, and therefore the electrons can be seen as a cloud of electrons above the Carbon, delocalising the electrons between all the pi orbitals. This can be drawn as follows:
A number of proofs that the actual structure of benzene is this can be found. These are:
- The bond length between C – C in benzene is 1.40 Å, lower than that of a single bond (1.54 Å) but higher than that of a double bond (1.34 Å).
- The energy released when benzene is hydrogenated is much less than 3 times the energy released.
- Benzene does not undergo electrophilic addition.
- There is only 1 chloro compound and not two.
Electrophilic Substituion
Benzene and similar compounds react via electrophilic substitution. The ring has a high electron density and therefore electrophilic attack is expected. The substitution occurs because it is too energy demanding to lose the aromaticity and therefore it will be much more energy efficient to break a C – H bond, even if these are very stable.
Reaction
Substitution reactions on benzenes occur only through the electrophilic pathway. The same reaction can be used to introduce the second substituent on the ring, although positional rules discussed in a latter part of the chapter need to be used.
The most common reaction for the bezene ring can be summarised in the Figure below.
Nitration
The reaction will give the tri-nitro benzene if the temperature is above 60o C.
The mechanism for the reaction is as follows:
Chlorination/Bromination
For either chlorination or bromination, a catalyst need to be used. This catalyst is important in the preparation of the electrophile, and without it, no reaction would take place
The mechanism for this reaction is as follows:
Friedel Crafts Alkylation
Halogenalkanes can be used to alkylate the benzene ring. This can be achieved with the aid of a catalyst such as AlCl3 which has a vacant p orbital and is electron deficient.
With the mechanism being:
Friedel Crafts Acylation
A similar reaction to alkylation is acylation, where instead of using a halogenoalkanes, an acid chloride is used.
With the mechanism being:
Sulfonation
Another reaction of benzene is with sulphuric acid, with the reaction pathway being the following:
Hydrogenation
Even though benzene rings are highly stable, it is possible to hydrogenate them with the aid of a catalyst and a temperature of 150 oC.
Chlorination
Benzene can also be chlorinated in an addition reaction by using Chlorine and UV light while boiling it.
2,4-directing groups vs 3 directing groups
Reaction do not need to stop after the first substituent has been added, and different groups have different effects on the reactivity of the benzene ring. Two of these effects are the mesomeric effect and the inductive effect.
Inductive effect
The inductive effect is the push and pull of electrons by a substituent group. Groups can either push electrons into the ring, like the alkyl groups, making the ring more electron rich, therefore making it more reactive. Other groups like the Chlorine can withdraw the electrons from the ring making it less reactive.
Mesomeric effect
Lone pairs and double bonds can interact with the ring by taking part in the delocalisation of electrons. Lone pairs tend to put electrons in the ring while double bonds tend to take electrons from the ring.
Para/Ortho Directing groups
Meta Directing groups
Due to these two effects, each group can be listed as either a 2,4-directing group or a 3-directing group. Groups with lone pairs are 2,4-directing while groups with double bonds are 3-directing.
Methylbenzene
Methylbenzene has a similar reactivity to benzene, and all the reactions that benzene undergoes can be achieved with methylbenzene. It is also noted that the methyl group behaves exactly like a normal alkane.
Reactions on the ring
Nitration
The nitration of methylbenzene gives rise to two main products, ortho (2-position) and para (4-position) as discussed in the directing effect of substituent on the ring
Chlorination/Bromination
Chlorination and bromination take place in the same manner as the reaction with benzene
Friedel Crafts Alkylation
Friedel Crafts Acylation
Chlorination of the methyl group
The methyl group can be chlorinated in a radical substitution mechanism just like any other alkane.
Oxidation
Methylbenzene can be oxidised to both an acid and an aldehyde.
The reaction to produce the acid changes colour from purple to brown on the addition of the base and then white on the addition of the acid.
Aldehydes can be produced by the use of a milder oxidising agent.
Halogenoarenes
Halogenoarenes react the same as all other phenyl compounds with the addition of one reaction. Whilst halogenoarenes cannot be substituted under conditions similar to aliphatic halogenoalkanes, these can be transformed into a phenol using high pressure and temperature.
Phenols
Preparation
Sulfonic acids
Halogenoarenes
Diazonium salts
Reactions
Acidity
Phenols are more acidic then normal alcohols since the negative charge produced when an H+ is lost can be delocalised on the ring. This delocalisation increases stability making it much more acidic than normal alcohols.
Oxidation
If left in air phenols tend to oxidise to form a dark coloured polymer.
Esterification
Phenols can take part in esterification reactions, but these are much slower than normal alcohols as the lone pairs are being shared with the ring.
Chlorination/Bromination
The hydroxyl group activates the ring, and the addition of chlorine and bromine cannot be controlled and normally the tri-brominated/chlorinated product will be obtained.
Reaction on the ring
Other reactions are the same as other aromatic compounds.
Hydrogenation
Phenol can be hydrogenated using similar conditions to the benzene ring.
FeCl3
When phenol reacts with FeCl3 it produces a purple solution.
Benzaldehyde
Benzaldehyde raects as a normal aldehyde, and therefore it can undergo all the nucleophilic additions that have been discussed with aldehydes.
Aromatic reactions are similar to those of other aromatic rings
Phenylamine
Preparation
Phenylamine can be prepared by the reduction of nitrobenzene using tin and acid.
Basicity
Phenylamine is a very weak base, as it is weakened by the electron delocalisation to the ring, in contrast to the strengthening effect of the acidity on the phenol. The lone pair can be delocalised on the ring making it less available for Hydrogen acceptance.
Reactions of phenylamine are similar to those of other aromatic compounds except for nitration since the conditions would otherwise oxidise the amine itself.
Nitration
Nitration can be achieved by first protecting the amine group and producing an amide. After the amide is produced nitration can take place, with the amide being 2,4-directing. The amide can then be removed by hydrolysis.
Diazonium salts
Diazonium salts are slats that can be prepared from phenylamine via the reaction with HNO2. This product is a salt that can decompose to give Nitrogen and a cation on the aromatic ring, making it possible to be attacked by nucleophiles.
Preparation
Reaction
Once the diazonium salt is made at 0 oC it can be used for reactions. The first step is to decompose it, which would then be followed by attack by a nucleophile. The mechanism for decomposition is:
While the reactions that a diazonium salt can undergo are the following:
