Advanced Higher ChemistryAdvanced Higher Chemistry

Unit 3Unit 3

AromaticsAromatics

AromaticsAromaticsAromatics are hydrocarbons containing the Aromatics are hydrocarbons containing the

benzene ring (Cbenzene ring (C66HH66).).

The systematic name for the family of alkyl The systematic name for the family of alkyl substituted aromatic hydrocarbons is the substituted aromatic hydrocarbons is the arenes.arenes.

Bonding in BenzeneBonding in Benzene

Structure (a) was the first sturcture proposed by Kekule in 1865 Structure (a) was the first sturcture proposed by Kekule in 1865 however this structure was proposed incorrect with x-ray diffraction however this structure was proposed incorrect with x-ray diffraction results and the fact that benzene is resistant to addition reactions. (b) results and the fact that benzene is resistant to addition reactions. (b) and (c) are resonance structures and differ only in the and (c) are resonance structures and differ only in the arrangement of the electrons but do not actually exist . The real arrangement of the electrons but do not actually exist . The real structure of benzene is best represented by (d)structure of benzene is best represented by (d)

The six carbons in benzene are spThe six carbons in benzene are sp22 hybridised. hybridised.

The carbons are bonded to each other and the The carbons are bonded to each other and the hydrogen atoms by sigma bonds. hydrogen atoms by sigma bonds.

Each carbon has a p orbital containing a single Each carbon has a p orbital containing a single electron that can overlap with neighbouring electron that can overlap with neighbouring orbitals to form pi bonds.orbitals to form pi bonds.

This area of negative charge is above and This area of negative charge is above and below the ring (delocalised p electron system).below the ring (delocalised p electron system).

The six electrons become delocalised and occupy two continuous doughnut-shaped electron clouds above and below the planar sigma framework.

The delocalisation of the electrons help to bond The delocalisation of the electrons help to bond the atoms more tightly together. The result is a the atoms more tightly together. The result is a completely symmetrical molecule, with completely symmetrical molecule, with considerable stability.considerable stability.

The term ‘aromatic’ can be defined to describe The term ‘aromatic’ can be defined to describe any system that contains a ring of atoms any system that contains a ring of atoms stabilised by delocalised pi electrons.stabilised by delocalised pi electrons.

Molecules with straight or branched chains are Molecules with straight or branched chains are described as aliphatic.described as aliphatic.

Reactions of benzeneReactions of benzeneBenzene tends to undergo electrophilic substitution Benzene tends to undergo electrophilic substitution reactions.reactions.

The areas of high electron density attract positive The areas of high electron density attract positive reagents.reagents.

The electrophile attacks one of the carbon atoms The electrophile attacks one of the carbon atoms forming an intermediate ion with a positive charge.forming an intermediate ion with a positive charge.

The intermediate ion then loses a hydrogen ion.The intermediate ion then loses a hydrogen ion.

The aromatic nature of the hydrocarbon is then restored.The aromatic nature of the hydrocarbon is then restored.

Reactions with bromine or chlorineReactions with bromine or chlorineBenzene can undergo electrophilic substitution by bromine in the presence of a Benzene can undergo electrophilic substitution by bromine in the presence of a catalyst (e.g. iron (III) bromide, iron (III) chloride, aluminium (III) chloride).catalyst (e.g. iron (III) bromide, iron (III) chloride, aluminium (III) chloride).There are parallels in the catalysed reaction of bromine with benzene with the There are parallels in the catalysed reaction of bromine with benzene with the reaction of bromine to an alkene. Since benzene is less reactive than alkenes, reaction of bromine to an alkene. Since benzene is less reactive than alkenes, the catalyst is needed to polarise the bromine molecule creating an electrophilic the catalyst is needed to polarise the bromine molecule creating an electrophilic centre.centre.

The partially positive bromine atom can then attack the benzene ring. Heterolytic The partially positive bromine atom can then attack the benzene ring. Heterolytic fission of the Br-Br bond occurs to form a carbocation, just like the bromination of fission of the Br-Br bond occurs to form a carbocation, just like the bromination of an alkene.an alkene.

However, the similarity ends there and, by losing a hydrogen ion in the final step, However, the similarity ends there and, by losing a hydrogen ion in the final step, the aromatic system is restored and the catalyst regenerated.the aromatic system is restored and the catalyst regenerated.

The overall effect is that one of the hydrogen The overall effect is that one of the hydrogen atoms of the benzene molecule has been atoms of the benzene molecule has been replaced by a bromine atom i.e. an electrophilic replaced by a bromine atom i.e. an electrophilic substitution reaction has taken place.substitution reaction has taken place.

In the dark, chlorine undergoes a similar reaction with benzene to form chlorobenzene.

In the light, chlorine adds to benzene by a different mechanism to form 1,2,3,4,5,6-hexachlorocyclohexane (one of the very few addition reactions of the benzene ring).

Nitration of benzeneNitration of benzene

Benzene reacts with a mixture of conc.nitric acid and conc. sulphuric acid to from nitrobenzene. Benzene reacts with a mixture of conc.nitric acid and conc. sulphuric acid to from nitrobenzene.

This reaction is of great importance to industry since the nitrobenzene can be reduced to produce This reaction is of great importance to industry since the nitrobenzene can be reduced to produce phenylamine, Cphenylamine, C66HH55NHNH22, also known as aniline, which is an important intermediate in the manufacture of , also known as aniline, which is an important intermediate in the manufacture of

dyes.dyes.

The mechanism for nitration involves The mechanism for nitration involves electrophilic attack by the very reactive nitronium electrophilic attack by the very reactive nitronium cation NOcation NO22

++, which is produced when conc. nitric , which is produced when conc. nitric

acid reacts with conc. sulphuric acidacid reacts with conc. sulphuric acid

The nitronium cation is a powerful electrophile The nitronium cation is a powerful electrophile and attacks the benzene molecule as shown.and attacks the benzene molecule as shown.

Further substitution can occur to produce Further substitution can occur to produce di- and tri- compounds.di- and tri- compounds.

Further substitution can occur to produce Further substitution can occur to produce di- and tri- compounds.di- and tri- compounds.

Sulphonation of benzeneSulphonation of benzeneBenzenesulphonic acid can be produced by electrophilic substitution if benzene is heated with conc. Benzenesulphonic acid can be produced by electrophilic substitution if benzene is heated with conc. sulphuric acid under reflux. sulphuric acid under reflux.

The same product is formed in the cold when using fuming sulphuric acid, which is a solution of The same product is formed in the cold when using fuming sulphuric acid, which is a solution of sulphur trioxide in conc. sulphuric acid.sulphur trioxide in conc. sulphuric acid.

The electrophile in sulphonation is believed to be the SO3 molecule, either free or in combination with acid, which helps to turn it into a strong enough electrophile to attack the benzene molecule.

The SO3 molecule is electron deficient and carries a partial positive charge on the sulphur atom.

The mechanism is similar to that for nitration.

This reaction is important in the production of synthetic detergents.

Alkylation of benzene : Friedel-Crafts ReactionAlkylation of benzene : Friedel-Crafts ReactionAs for the reaction of benzene with bromine, a halogenalkane is made polar by the As for the reaction of benzene with bromine, a halogenalkane is made polar by the action of a action of a catalyst (e.g. aluminium (III) chloride).catalyst (e.g. aluminium (III) chloride).

The C-Cl bond is already polar. The catalyst increases the polarity and may even cause the bond to The C-Cl bond is already polar. The catalyst increases the polarity and may even cause the bond to break heterolytically to form a carbocation. break heterolytically to form a carbocation. In either case, the power of the electrophile is increased to allow it to attack the benzene ring to In either case, the power of the electrophile is increased to allow it to attack the benzene ring to form a monoalkylbenzeneform a monoalkylbenzene

This reaction is used industrially in the This reaction is used industrially in the manufacture of synthetic dyes, the production of manufacture of synthetic dyes, the production of phenol and propanone, and in the manufacture of phenol and propanone, and in the manufacture of nylon.nylon.

Although electrophilic substitution is the typical Although electrophilic substitution is the typical reaction of the benzene ring, the presence of reaction of the benzene ring, the presence of substituents on the ring has an influence on the substituents on the ring has an influence on the outcome of this reaction.outcome of this reaction.

Some substituents increase the Some substituents increase the susceptibility to electrophilic attack while susceptibility to electrophilic attack while others decrease it.others decrease it.

On the other hand, the benzene ring also On the other hand, the benzene ring also has an influence on the behaviour of such has an influence on the behaviour of such substituents.substituents.

In the next slides phenol and phenylamine In the next slides phenol and phenylamine (aniline) will be considered.(aniline) will be considered.

Phenol Phenol Phenols are aromatic compounds with a hydroxyl group attached to the benzene ring.Phenols are aromatic compounds with a hydroxyl group attached to the benzene ring.The OH bond is polar and can be broken heterolytically to produce HThe OH bond is polar and can be broken heterolytically to produce H++ ions, under certain ions, under certain circumstances. The OH group is therefore potentially acidic.circumstances. The OH group is therefore potentially acidic.Phenol has a higher Ka value than ethanol therefore is more acidic. Phenol has a higher Ka value than ethanol therefore is more acidic. When phenol acts as an acid, it produces the phenoxide ion in which the negative charge on When phenol acts as an acid, it produces the phenoxide ion in which the negative charge on the oxygen atom can be partly delocalised into the aromatic pi system.the oxygen atom can be partly delocalised into the aromatic pi system.

This delocalisation can be illustrated using possible resonance structures for the phenoxide ion.

The resonance structures show how the negative charge on the oxygen atom is decreased, thus stabilising the phenoxide ion and making it less likely to accept a proton.

This effect is confirmed by phenol’s KThis effect is confirmed by phenol’s Kaa

value lying between an alcohol and an value lying between an alcohol and an alkanoic acid i.e. phenol is more acidic alkanoic acid i.e. phenol is more acidic than an alcohol but less acidic than an than an alcohol but less acidic than an alkanoic acid.alkanoic acid.

PhenylaminePhenylamine

Phenylamine (aniline) is an aromatic, basic amine.Phenylamine (aniline) is an aromatic, basic amine.

The lone pair of electrons on the nitrogen atom can The lone pair of electrons on the nitrogen atom can accept a hydrogen ion and form the accept a hydrogen ion and form the phenylammonium ion.phenylammonium ion.

Phenylamine is a weak base as the KPhenylamine is a weak base as the Kaa of the resulting of the resulting

conjugate acid is high.conjugate acid is high.

BaseBase Conjugate acidConjugate acid KKaa at 25 at 25ooCC

ammoniaammonia Ammonium ionAmmonium ion 5.5. x 105.5. x 10-10-10

ethylamineethylamine Ethylammonium Ethylammonium ionion

1.9 x 101.9 x 10-11-11

phenylaminephenylamine Phenylammonium Phenylammonium ionion

2 x 102 x 10-3-3

Phenylamine is less basic than both ammonia and ethylamine.Phenylamine is less basic than both ammonia and ethylamine.

The decrease in basicity can be explained by looking at its resonance structures.The decrease in basicity can be explained by looking at its resonance structures.

The lone pair of electrons on the nitrogen atom are delocalised into the aromatic pi system therefore the nitrogen atom is The lone pair of electrons on the nitrogen atom are delocalised into the aromatic pi system therefore the nitrogen atom is less able to act as a base.less able to act as a base.

Such delocalisation stabilises the phenylamine ion in a way that is not possible for the phenylammoniumion.Such delocalisation stabilises the phenylamine ion in a way that is not possible for the phenylammoniumion.

ExerciseExercise

Now try the exercise on page 57 of your Now try the exercise on page 57 of your Unit 3(b) notes.Unit 3(b) notes.