Arenes - benzene (Topic 18A)

Aromatic compounds are often based on the structure of benzene; an unsaturated cyclic compound with the molecular formula .

Kekulé proposed an early model for the structure of benzene as a cyclic hydrocarbon containing alternating double and single bonds between carbon atoms.

Kekulé’s model has been disproved over time in light of experimental evidence.

The current model of benzene is the delocalised model. It describes of a cyclic molecule, in which each carbon atom makes three -bonds.

The remaining electron, in each carbon atom’s p orbital becomes delocalised, giving rise to a π-system of six electrons that are shared equally between all carbon atoms in the ring.

The six delocalised electrons being shared among all six carbon atoms gives the benzene ring great stability.

Benzene rings can be represented either by drawing alternating double bonds, or by including a ring inside the structure to denote electron delocalisation.

Either method is acceptable, but if drawing double bonds, it is important to remember that in reality all carbon-carbon bonds are equivalent.

Benzene is a planar molecule with six carbon atoms in a hexagonal ring, each with one bonded hydrogen atom.

All the carbon to carbon bonds in benzene are equivalent; they are the same length and the same strength.

Experimental evidence led to the evolution of the delocalised model for the structure of benzene.

single bonds are longer than double bonds. If the Kekulé model was correct, with alternate single and double bonds, then benzene would show two different bond lengths and be an irregular hexagon.

X-ray crystallography shows the measured bond length in benzene is between the bond lengths of carbon-carbon single, and carbon-carbon double bonds. This gives benzene a regular hexagonal shape.

The lower than predicted enthalpy of hydrogenation of benzene gives evidence of the delocalised model of benzene.

The enthalpy of hydrogenation for the conversion of cyclohexene to cyclohexane by saturating its one double bond is .

As the Kekulé model of benzene has three double bonds, then theoretically the hydrogenation of 1,3,5-cyclohexatriene () will produce

This is visualised in the enthalpy profile diagram below.

In experimental results, benzene shows an enthalpy of hydrogenation of

This means less energy is released than expected, making benzene more stable than the theoretical molecule 1,3,5-cyclohexatriene.

The delocalised electron ring in benzene is very stable.

If electrophiles were to add onto benzene the delocalised ring would become permanently disrupted.

Aromaticity is lost in the addition product but not in the substitution product.

In substitution reactions, the delocalized system is restored in the product. This makes electrophilic substitution reactions favourable over addition reactions.

Alkenes readily undergo bromination via electrophilic addition, whereas benzene reacts via electrophilic substitution and requires the presence of a halogen carrier catalyst.

Bromine can act as an electrophile; it is attracted to, and will be polarised by, parts of molecules that have high electron densities forming a dipole. In an alkene, the -electrons in the double bond are localised between the two carbon atoms. This results in high electron density, capable of polarising the bromine molecule and creating an electrophile.

In contrast, benzene’s -electrons are delocalised across the ring. This reduces the electron density of any given region of the molecule relative to an alkene. Benzene is therefore less able to polarise bromine and a halogen carrier is required to create the electrophile.

Arenes, such as benzene and its derivatives, will burn in oxygen.

Arenes burn with a very smoky (sooty) flame as there is a higher proportion of carbon than in corresponding alkanes. Incomplete combustion results in carbon particulates.

In an electrophilic substitution reaction, one of the hydrogen atoms in the benzene ring is replaced by an electrophile, E, which is an electron pair acceptor.

Monosubstitution is when a single hydrogen atom from the benzene ring is replaced.

Nitration introduces a (nitro) group to the benzene ring.

Nitration reactions are carried out using concentrated nitric acid with a concentrated sulfuric acid catalyst. These conditions are required to generate the electrophile, (the nitronium ion).

Halogenation introduces a halogen atom, often represented as .

A ‘halogen carrier’ such as or is used as a catalyst in these reactions. It reacts with a halogen molecule to produce the electrophile, .

Acylation introduces an acyl group, , to the benzene ring.

Acylation is carried out using an acyl chloride in the presence of a halogen carrier or .

This type of reaction is known as a Friedel-Crafts acylation. These reactions are useful in organic synthesis because they enable the formation of new bonds.

Alkylation introduces an alkyl group to a benzene ring using a halogen carrier catalyst. In the reaction, the electrophile, is formed through reaction of the haloalkane with the halogen carrier.

This type of reaction is known as a Friedel–Crafts alkylation. These reactions are useful in organic synthesis because they enable the formation of new bonds.

Nitration of aromatic compounds is described in three main steps.

Step 1 – generating the electrophile, :

Overall reaction:

Step 2- electrophilic substitution:

• Ensure the curly arrow moves from the inner ring to the electrophile.
• The intermediate must have a “horseshoe” of electrons extending to the positions either side of the site of substitution; positions 2–6.
• The intermediate has a positive charge, sitting within the mouth of the horseshoe.
• Show recovery an electron pair to restore aromaticity. This arrow goes from the bond of the ring hydrogen to the gap in the horseshoe, with the loss of .

Step 3 – regeneration of the catalyst:

The proton released to complete this reaction returns to the reaction mixture resulting in no net change to the amount of present; the sulfuric acid is catalytic.

Halogenation of aromatic compounds

Generating the electrophile: where is a halogen, or )

Electrophilic substitution mechanism:

• Ensure the curly arrow moves from the inner ring to the electrophile.
• The intermediate must have a “horseshoe” of electrons extending to the positions either side of the site of substitution; positions 2–6.
• The intermediate has a positive charge.
• Recover an electron pair from the ring hydrogen with the loss of an .

Regeneration of catalyst:

Acylation of aromatic compounds is described in three main steps.

Step 1 – Generating the electrophile: from an acyl chloride

The reaction occurs in anhydrous conditions, under reflux.

Step 2 -electrophilic substitution:

• Ensure the curly arrow moves from the inner ring to the electrophile.
• The intermediate must have a “horseshoe” of electrons extending to the positions either side of the site of substitution; positions 2–6.
• The intermediate has a positive charge, sitting within the mouth of the horseshoe.
• Show recovery of an electron pair to restore aromaticity. This arrow goes from the bond of the ring hydrogen to the gap in the horseshoe, with the loss of .

Step 3 – regeneration of the catalyst:

This is a very useful reaction for creating a carbon-carbon bond between an arene ring and a carbonyl group.

The product is an aromatic ketone.

Alkylation of aromatic compounds.

Generating the electrophile: from a chloroalkane

In anhydrous conditions under reflux.

Mechanism: electrophilic substitution

Ensure the curly arrow moves from the inner ring to the electrophile.

• The intermediate must have a “horseshoe” of electrons extending to the positions either side of the site of substitution; positions 2–6.
• The intermediate has a positive charge.
• Recover an electron pair from the ring hydrogen with the loss of a proton .

Regeneration of catalyst:

Phenol undergoes electrophilic substitution more readily than benzene.

The lone pair of electrons on phenol’s oxygen atom is incorporated into the aromatic ring’s delocalised -system, thus increasing electron density within the ring.

For this reason, despite oxygen’s electronegativity, the group in phenol is described as being an electron-donating group.

The ring’s increased electron density makes it more susceptible to attack by electrophiles and better at stabilising the positive charge on the intermediate.

Phenol’s greater reactivity compared to benzene can be illustrated by comparing the conditions required for bromination and nitration.

Phenol is more reactive than benzene and undergoes electrophilic substitution reactions under conditions where benzene does not react.

Phenol reacts with bromine water readily, undergoing multiple bromination events via electrophilic substitution. This produces 2,4,6-tribromophenol. No reaction occures with bromine water and benzene.

This reaction can be used as a distinguishing test for phenol. Bromine water loses its orange-brown colour. The product formed is white precipitate in a colourless solution.

Note that the group in phenol is situated in the 1 position on the ring. With excess bromine, atoms will substitute at positions 2, 4, and 6, if they are not occupied by other groups.