benzene, what is aromaticity? · benzene, what is aromaticity? benzene is a special type of...
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Aromatic Compounds
Benzene, what is aromaticity?
Benzene is a special type of compound which is found always as a delocalised structure. Benzene
ismade up od 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 is lower than that of a single bond but higher than that of a
double bond
The energy released when benzene is hydrogenated is much less than 3 times the energy
released
Electrophilic Substituion
Benzene and similar compounds react via electrophilic substitution. The ring has a high elelctron
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.
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 group like the Chlorine can withdraw the electrons f rom 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 e lectrons
from the ring.
Due to these two effects each group can be listed as wither 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.
Methyl Benzene
Methyl benzene has a similar reactivity to benzene, and all the reactions that benzene undergoes
can be achieved with methyl benzene. It is also noted that the methyl group behaves exactly like a
normal alkane.
Reactions on the ring
Nitration
The nitration of methylbenzene give 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
Methylbenene can be oxidised to both an acid and an aldehyde.
The reaction to produce the acid changes colour from purple to brown on addition of the base and
then white on 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 except for one reaction. Even though
halogenoarenes cannot be substituted like normal halogenoalkanes, these can be transformed into a
phenol using pressure and temperature.
Phenols
Preparation
Sulfuric 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
then 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, especially noting the fact that these are more
reactive then normal alcohols.
Chlorination/Bromination
The hydroxyl group activates the ring, and 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 hyodrogenated using similar conditions to the benzene ring.
Benzaldehyde
Benzaldehyde is a normal aldehyde, and therefore it can undergo all the nulceophilic additions that
have been discussed with aldehydes.
Aromatic reactions are similar o 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 conditionswould 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.
Diazonim 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: