Part 7. The chemistry of AROMATIC COMPOUNDS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 7.5 Electrophilic substitution - nitration of benzene & methylbenzene, properties and uses of nitro-aromatics

Sub-index for this page

7.5.1 The reagents, reaction conditions, products and equations for the nitration of benzene and methylbenzene

7.5.2 The electrophilic substitution mechanism of nitration of arenes like benzene and methylbenzene

7.5.3 The physical properties of some nitro-aromatic compounds obtained from arenes

7.5.4 Selected chemical reactions of some nitro-aromatic compounds obtained from arenes

7.5.5 The uses of some nitro-aromatic compounds obtained from nitrating arenes

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7.5.1 The reagents, reaction conditions and equations for the nitration of aromatic compounds

particularly benzene and methylbenzene

Methylbenzene is more reactive towards electrophiles than benzene itself.

The methyl group has a small, but not insignificant effect of raising the electron density around the ring, particularly at the ring carbons 2 (= 6) and 4 (the 'fuzzy' sketch above tries to illustrate the idea).

Nitration of an arene involves heating the compound with a mixture of a concentrated nitric acid and sulfuric acid mixture.

However, the temperature and heating time are important to control whether you want a mono-nitro-substituted compound or an aromatic molecule containing two or more nitro groups in the benzene ring.

Examples of aromatic nitration substitution reactions

(a)   +  HNO₃  ===>   +  H₂O

benzene  +  nitric acid  ===>  nitrobenzene  +  water

The conc. nitric acid, conc. sulfuric acid and benzene are heated together in a flask fitted with a reflux condenser, at ~60^oC, taking less than an hour.

If the temperature rises above 65^oC, a 2nd nitro group is substituted in the ring yielding the main product to be 1,3-dinitrobenzene - see equation (e).

You need a temperature of 90^oC to complete the reaction to mainly 1,3-dinitrobenzene see (b).

(b) +  HNO₃  ===>   +  H₂O

nitrobenzene  +  nitric acid  ===> 1,3-dinitrobenzene  +  water

1,3-dinitrobenzene is the majority product, BUT, you will still get some small quantities 1,2-dinitrobenzene and 1,4-dinitrobenzene.

 

(c)   +  HNO₃  ===>       +  H₂O

methylbenzene  +  nitric acid  ===> methyl-2/3/4-nitrobenzene  +  water

Three structural-positional isomers C₇H₇NO₂ formed in different proportions.

Methylbenzene reacts faster than benzene, because the methyl group activates the ring by increasing its electron density.

The nitration reactivity order is: methylbenzene  >  benzene  >  nitrobenzene

see section 7.14 for reactivity explanations and orientation of products.

 

(d)   +  3HNO₃  ===>   +   3H₂O

The explosive TNT has the structure  , the acronym comes from its historic-trivial name of 2,4,6-trinitotolune, toluene was the old name for methylbenzene.

A more systematic name for TNT is 2,4,6-trinitromethylbenzene.

I've now included the nitration of other types of aromatic compound for completeness and variety, but I'll only cover the electrophilic substitution mechanisms for the nitration of benzene and methylbenzene.

 

(e) +  HNO₃  ===>   +  H₂O

benzoic acid  +  nitric acid  ===> 3-nitrobenzoic acid  +  water

the 3-nitrobenzoic acid is the majority product, BUT, you will also get some 2-nitrobenzoic acid and 4-nitrobenzoic acid.

(f)   +  HNO₃  ===>         +  H₂O

chlorobenzene  +  nitric acid  ===> chloronitrobenzenes  + water

3 structural-positional isomers of C₆H₄NO₂Cl, 1-chloro-2-nitrobenzene (chloro-2-nitrobenzen), 1-chloro-3-nitrobenzene (chloro-3-nitrobenzene), 1-chloro-4-nitrobenzene (chloro-4-nitrobenzene),  formed in different proportions.

 

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7.5.2 The electrophilic mechanism of nitration of arenes like benzene and methylbenzene

The nitrating mixture consists of concentrated nitric acid (source of the nitro group -NO₂) and concentrated sulphuric acid which acts as a catalyst and as a strong acid.

The overall nitration reaction is the substitution of -H in the benzene ring by the nitro group -NO₂ 

Mechanism diagram 19 (above) - illustrates the electrophilic substitution in the nitration of the benzene ring

-R can be H, alkyl or other group including -COOH, -Cl, -Br and even -NO₂ itself.

For mono-nitration of the benzene ring in a C₆H₅-R compound, apart from when R = H, there are three possible substitution products i.e. substitution at the 2, 3 or 4 position in the benzene ring, R is considered position 1 here.

Step (1) The sulphuric acid protonates the nitric acid (strong acid, but weaker than H₂SO₄)

Step (2) The protonated nitric acid loses a water molecule via a sulphuric acid molecule, to generate the electrophile, the nitronium ion, NO₂⁺.

The nitronium cation is a much more powerful electrophile, i.e. its a positive ion and a stronger electron pair acceptor, more so than the original nitric acid, and the NO₂⁺ is needed to attack the very stable aromatic ring of benzene.

Steps (1) and (2) can be written as: 2H₂SO₄ + HNO₃ ==> NO₂⁺ + H₃O⁺ + 2HSO₄^- 

Step (3) The positive nitronium ion attacks the electron rich pi orbital benzene rings of the aromatic compound.

An electron pair from the delocalised pi electrons of the benzene ring forms a C-N bond with the electron pair accepting nitronium ion forming a highly unstable carbocation.

It is very unstable because the stable electron arrangement of the benzene ring is partially broken to give a 'saturated' C (top right of ring).

Step (4) The hydrogensulfate ion (HSO₄^-, formed in step (1), abstracts a proton from the highly unstable intermediate carbocation and simultaneously the nitro-aromatic product (and stable aromatic ring) is formed.

Thus, simultaneously, deprotonation, the -H proton is abstracted by a base (hydrogensulfate ion) reforming the sulfuric acid catalyst and reforming the stable aromatic ring of pi orbitals in the substituted product nitrobenzene.

The hydrogensulfate ion, HSO₄^- has been shown in the style of ^-:O-SO₂-OH, to emphasize the importance a lone pair on the oxygen abstracting the proton from the aromatic carbocation.

Note:

Like alkenes, arenes are susceptible to electrophilic attack because of the high electron density of the delocalised electrons of the pi orbitals involved in the carbon-carbon bonding.

So both show little reactivity towards nucleophilic reagents - electron pair donors that would tend to be repelled.

However two points should be considered because of the particular stability of the aromatic (benzene) ring.

(i) This makes aromatic compounds less reactive than alkenes, which readily undergo addition rather than substitution.

(ii) Unlike alkenes, aromatic compounds do not usually undergo addition, because this will remove the stability conferred on the molecule by the benzene ring.

By under going substitution rather than addition, the stable aromatic ring is preserved.

 

Having introduced the general electrophilic substitution mechanism for introducing a nitro group (NO₂) into a benzene ring, I've now drawn diagrams to illustrate four specific nitrations of aromatic compounds and kept the notes to a minimum, since there is a detailed description above.

To add more notes to the diagrams below

Mechanism diagram 79A shows the electrophilic substitution mechanism for nitrating benzene to yield nitrobenzene.

 

 

Mechanism diagram 79E shows the reaction progress profile for the final two stages of the nitration of benzene.

First the NO₂⁺ electrophile attacks the pi electron cloud of benzene to give the unstable carbocation in which the aromatic pi orbital rings are broken.

Then, simultaneously, the -H proton is abstracted by a base (hydrogensulfate ion) and the stable aromatic ring of pi orbitals is re-formed to yield the substituted product nitrobenzene.

E_a1 = the higher activation energy for the initial electrophile attack on the pi orbitals of the benzene ring - which is the rate determining step of the mechanism - the change is from stable benzene ring to highly unstable carbocation.

E_a2 = the much lower activation energy, for the unstable carbocation, as the proton is expelled, re-forming the stable pi orbital rings of the benzene product (or the benzene ring of any aromatic compound undergoing electrophilic substitution).

This diagram applies to ALL electrophilic substitution nitration reactions of aromatic compounds.

Mechanism diagram 79B shows the electrophilic substitution mechanism for nitrating methylbenzene to yield methyl-2-nitrobenzene.

Mechanism diagram 79C shows the electrophilic substitution mechanism for nitrating benzoic acid to yield 3-nitrobenzoic acid.

Mechanism diagram 79D shows the electrophilic substitution mechanism for nitrating nitrobenzene to yield 1,2-dinitrobenzene.

Mechanism diagram 79F shows the electrophilic substitution mechanism for nitrating chlorobenzene to yield chloro-4-nitrobenzene.

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7.5.3 Physical properties of some nitro-aromatic compounds obtained from arenes

Nitroarenes and other nitroaromatic compounds

Name	Structure	Mpt/oC	Bpt/oC	Comments
nitrobenzene		6	211
methyl-2-nitrobenzene		-3	220
methyl-3-nitrobenzene		15	233
methyl-4-nitrobenzene		52	238
1-methyl-2,4-dinitrobenzene		70	dec. 300
1,2-dinitrobenzene		118	319	Slightly soluble in water.
1,3-dinitobenzene		90	291
1,4-dinitrobenzene		72	299
1-methyl-2,4,6-trinitrobenzene (2,4,6-trinitrotoluene, TNT)		82	explodes at 240
chloro-2-nitrobenzene		-36	158
chloro-4-nitrobenzene		-48	162
chloro-4-nitrobenzene		7	162

Notes on physical properties

(a) They are all more dense than water.

(b) They are generally insoluble in water.

(c) The lower members are colourless or pale yellow liquids at room temperature or low melting solids.

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7.5.4 Selected chemical reactions of some nitro-aromatic compounds obtained from arenes

(a) Reduction to amines

(i) In industry nitro-compounds are reduced by mixing with hydrogen and passing the mixture over heated nickel catalyst.

e.g. methyl-2-nitrobenzene to methyl-2-phenylamine

  +  6[H]  == Ni/H₂ ==>   +  2H₂O

(ii) Nitro-compounds can be reduced in laboratory using lithium tetrahydridoaluminate(III)

NaBH₄, is not a powerful enough reducing agent to reduce nitro–aromatic compounds.

LiAlH₄ is a more powerful reducing agent than NaBH₄ and in ether solvent readily reduces nitro–aromatics to primary aromatic amines, the simplified equation for nitrobenzene to phenylamine is ...

C₆H₅NO₂  +  6[H]  ===>  C₆H₅NH₂  +  2H₂O

and methylnitrobenzenes would be reduced to methylphenylamine primary amines, i.e.

CH₃C₆H₄NO₂  +  6[H]  ===>  CH₃C₆H₄NH₂  +  2H₂O

as will any aromatic compound with a nitro group (–NO₂) attached directly to the benzene ring.

(iii)  Nitro-aromatic compounds are reduced by refluxing with tin and hydrochloric acid.

C₆H₅NO₂ + 6[H]  ===>  C₆H₅NH₂  +  2H₂O

but the 'real' equations are rather more complicated, the simplest redox equation I can come up with is

2C₆H₅NO_2(aq) + 14H⁺_(aq) + 3Sn_(s) ===> 2C₆H₅NH₃⁺_(aq) + 3Sn⁴⁺_(aq) + 4H₂O_(l)

The nitro group is reduced and the tin oxidised.

The phenylamine can be separated from the 'messy' reaction mixture by steam distillation.

For full details see preparation of phenylamine from nitrobenzene

Amines are very important compounds for the manufacture products as diverse as drugs and dyes.

 

(b) Electrophilic substitutions in the benzene ring

(i) Nitration

This involves further nitration of an already nitrated aromatic compound using the concentrated nitric acid and sulfuric acid mixture.

Previously nitrated aromatic compounds can be further nitrated to introduce another nitro group into the benzene ring e.g.

+  HNO₃  ===>   +  H₂O

nitrobenzene  +  nitric acid  ===> 1,3-dinitrobenzene  +  water

1,3-dinitrobenzene is the majority product, BUT, you will still get some 1,2-dinitrobenzene and 1,4-dinitrobenzene.

 

(ii) Halogenation

Nitro-aromatic compounds will undergo halogenation when refluxed with aluminium chloride and chlorine passed into the mixture e.g. starting with nitrobenzene.

  +  Cl₂   ===>    +  HCl

The principal product is chloro-3-nitrobenzene.

You can synthesise bromo-3-nitrobenzene by refluxing nitrobenzene with bromine and iron(III) bromide catalyst.

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7.5.5 The uses of some nitro-aromatic compounds obtained from arenes

Nitroaromatics are one of the most important groups of intermediate aromatic compounds used in industrial organic synthesis.

Reduction to amines, see 7.5.4 reaction (a), which are used in many pharmaceutical products, dyes and polyamide polymers..

Nitro-aromatics are used in explosives, the best known being TNT.

Need x-reference with phenols and aromatic amines.

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