HOME PAGE * KS3 SCIENCES * GCSE BIOLOGY CHEMISTRY PHYSICS * ADVANCED LEVEL CHEMISTRY

Advanced Level Organic Chemistry: The chemistry of nitro-aromatic compounds

AQA Advanced A level Chemistry Edexcel Advanced A level Chemistry OCR Advanced A level Chemistry A OCR Salters Advanced A level Chemistry B

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 the preparation of 1-methyl-2-nitrobenzene apparatus reagents separating funnel fractional distillation

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

(brief mention of nitration of chlorobenzene, benzoic acid and naphthalene)

Sub-index for this page

7.5.1 Reagents, reaction conditions, products and equations for the nitration of benzene,  methylbenzene and naphthalene

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


INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

[SEARCH BOX]



TOP OF PAGE and sub-index


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

particularly benzene and methylbenzene

diagram of electron clouds of delocalised pi orbitals of benzene increased electron density of ring in 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.

preparation of nitrobenzene apparatus chemicals benzene conc. nitric acid conc. sulfuric acid advanced level organic chemistry doc brownExamples of aromatic nitration substitution reactions

(a) (c) doc b  +  HNO3  ===> (c) doc b  +  H2O

benzene  +  nitric acid  ===>  nitrobenzene  +  water

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

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

Nitrobenzene is a pale yellow liquid mpt. 6oC and bpt. 211 oC with a strong smell of almonds.

It is a very important intermediate in the synthesis of dyes and pharmaceutical products including paracetamol.

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

(b) (c) doc b +  HNO3  ===> (c) doc b  +  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) (c) doc b  +  HNO3  ===> (c) doc b      +  H2O

methylbenzene  +  nitric acid  ===> 1-methyl-2/3/4-nitrobenzene  +  water

Three structural-positional isomers C7H7NO2 are formed in different proportions.

The main products are 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene

Methylbenzene reacts faster than benzene, because the methyl group activates the benzene ring by increasing its electron density (+I effect).

This preparation can be done using the same apparatus as above for preparing nitrobenzene, but you don't heat the water in the beaker water bath, in fact you may need to employ ice as a cooling agent and you don't want to make 2,4,6-trinitomethylbenzene, otherwise known as the explosive TNT !!!

The nitration reactivity order is: methylbenzene  >  benzene  >  nitrobenzene

Note that if the atom of the initial substituent group is directly bonded to the benzene ring does not have any π bonding the ring is usually activated compared to benzene itself. The methyl group tends to increase the electron density of the ring and more so at the 2, 4 and 6 positions, compared to the 3 and 5 positions. Therefore in methylbenzene, the 2 and 4 positions become the preferred substitution points for nitration in the benzene ring of methylbenzene.

see section 7.14 for reactivity theory explanations and orientation of products.

More details on preparing 1-methyl-2-nitrobenzene (2-methylnitrobenzene)

using a separating funnel in the preparation and purification of methyl-2-nitrobenzene 2-methylnitrobenzene1. Methylbenzene is nitrated with an equal volume mixture of concentrated nitric and sulfuric acids.

Methylbenzene is more reactive than benzene, so a lower reaction temperature is used.

2. The acid and methylbenzene mixture is gently heated to around 30oC in the reflux water bath system illustrated above - a high temperature will lead to the formation of dinitromethylbenzenes and at a lower temperature the reaction is too slow.

fractional distillation in preparation of methyl-2-nitrobenzene 2-nitromethylbenzene apparatus3. After completion, water is added to dilute and dissolve the acids. Then the mixture is poured into a separating funnel and the bottom layer containing the 1-methyl-2-nitrobenzene drained off via the tap into a beaker.

4. Using the separating funnel the organic liquid is washed with sodium carbonate solution to remove any remaining acid. The organic liquid is further washed with water to remove traces of the sodium carbonate solution.

5. The oily organic layer is dried with lumps of anhydrous calcium chloride.

6. The organic liquid is decanted into a round-bottom flask and fractionally distilled collecting the fraction ~223-225oC the boiling point of 1-methyl-2-nitrobenzene is 225oC. 1-methyl-4-nitrobenzene boils at ~230oC and should condense back into the flask (theoretically!).

 

The product of further nitration of the benzene ring - beware!

(d) (c) doc b  +  3HNO3  ===> structural formula for TNT 2,4,6-trinitrotoluene 2,4,6-trinitromethylbenzene advanced A level organic chemistry  +   3H2O

If the nitration of methylbenzene is allowed to continue you will obtain a high yield of 2,4-dinitrobenzene and some 2,6-dinitrobenzene and eventually 2,4,6-trinitromethylbenzene.

This preparation should not be attempted because the products are highly explosive!

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

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

It is very unstable substance and readily explodes if not handled carefully.

 

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) (c) doc b +  HNO3  ===> (c) doc b  +  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.

NOTE if the atom of the initial substituent group is directly bonded to the benzene ring does have any π bonding the ring is usually deactivated compared to benzene itself.  In this case the -COOH group tends to decrease the electron density of the ring and more so at the 2, 4 and 6 positions, compared to the 3 and 5 positions. Therefore the 3- position become the preferred 2nd substitution point in the benzene ring of benzoic acid.

(See section 7.14 for more details on this electrophilic substitution reaction).

 

 

(f) (c) doc b  +  HNO3  ===> (c) doc b   (c) doc b   (c) doc b  +  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. Major products.

The more electronegative chlorine atom deactivates the benzene ring, but less so at the 2, 4 and 6 positions, hence the 3- substitution yields the minor product.

(For more details on the theory of electrophilic substitution see section 7.14).


The nitration of naphthalene

reaction of naphthalene with concentrated nitric acid to yield 1-nitronaphthalene molecular structure of products structural formula electrophilic substitution mechanism

At room temperature, naphthalene reacts directly with concentrated nitric acid to yield mainly 1-nitronaphthalene.

This reaction is in principle the same electrophilic substitution reaction undergone by benzene and methylbenzene.


TOP OF PAGE and sub-index


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 

organic reaction mechanisms 

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 than nitric acid, 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 covalent (sigma) 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 pi electron ring 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 electron orbitals in the substituted product nitrobenzene.

The hydrogensulfate ion, HSO4- has been shown in the style of -:O-SO2-OH, to emphasize the importance of 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 of pi electrons.

The greater delocalisation in the benzene ring makes it more stable with a lower electron density system for the electrophile to attack.

By under going substitution rather than addition, the stable aromatic ring of pi electrons 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.

mechanism diagram for nitration of benzene electrophilic substitution in aromatic ring concentrated nitric acid sulfuric acid forming nitrobenzene

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

Initially the generation of the nitronium ion, NO2+, a powerful electrophile - electron pair acceptor, more so than the original nitric acid.

The pi electron cloud donates a pair of electrons to the nitronium ion to form a covalent C-N bond, so one of the ring carbons is saturated with loss of the extra stability of the original benzene ring of pi electrons.

A proton is expelled and abstracted by the hydrogensulfate ion to give final product of nitrobenzene as the stable aromatic benzene ring of pi electrons is reformed and the sulfuric acid catalyst is also regenerated.

 

reaction progress profile diagram for nitration of benzene electrophilic substitution in aromatic ring 

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.

Ea1 = 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.

Ea2 = 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 for nitration of methylbenzene electrophilic substitution in aromatic ring concentrated nitric acid sulfuric acid forming methyl-2-nitrobenzene

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

Initially the generation of the nitronium ion, NO2+, a powerful electrophile, a more powerful electron pair acceptor than the original nitric acid.

The pi electron cloud donates a pair of electrons to the nitronium ion to form a covalent C-N bond, so one of the ring carbons is saturated with loss of the extra stability of the original complete benzene ring of pi electrons.

A proton is expelled and abstracted by the hydrogensulfate ion to give final product of methyl-2-nitrobenzene as the stable aromatic benzene ring of pi electrons is reformed and the sulfuric acid catalyst is also regenerated.

The final substitution product has now regained the stable ring of pi electrons of the benzene ring.

You can also get substitution in the 3 and 4 positions, but the mechanism details are the same.

The 2 and 4 positions are favoured in the electrophilic substitution of methylbenzene.

 

mechanism diagram for nitration of benzoic acid electrophilic substitution in aromatic ring concentrated nitric acid sulfuric acid forming 3-nitrobenzoic acid

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

The 3 position is favoured in the electrophilic substitution of benzoic acid.

The mechanism description is identical to those already described for the nitration of benzene and methylbenzene.

 

mechanism diagram for nitration of nitrobenzene electrophilic substitution in aromatic ring concentrated nitric acid sulfuric acid forming 1,2-dinitrobenzene

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

The 3 position is favoured in the electrophilic substitution of nitrobenzene.

The mechanism description is identical to those already described for the nitration of benzene and methylbenzene.

 

mechanism diagram for nitration of chlorobenzene electrophilic substitution in aromatic ring concentrated nitric acid sulfuric acid forming chloro-4-nitrobenzene

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

The 2 and 4 positions are favoured in the electrophilic substitution of chlorobenzene.

The mechanism description is identical to those already described for the nitration of benzene and methylbenzene.


TOP OF PAGE and sub-index


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 (c) doc b 6 211  
methyl-2-nitrobenzene

1-methyl-2-nitrobenzene

(c) doc b -3 220  
methyl-3-nitrobenzene

1-methyl-3-nitrobenzene

15 233  
methyl-4-nitrobenzene

1-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 (c) doc b 90 291  
1,4-dinitrobenzene   72 299  
1-methyl-2,4,6-trinitrobenzene

(2,4,6-trinitrotoluene, TNT)

structural formula for TNT 2,4,6-trinitrotoluene 2,4,6-trinitromethylbenzene advanced A level organic chemistry 82 explodes at 240  
1-chloro-2-nitrobenzene (c) doc b -36 158  
1-chloro-3-nitrobenzene (c) doc b -48 162  
1-chloro-4-nitrobenzene (c) doc b 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.


TOP OF PAGE and sub-index


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

(c) doc b  +  6[H]  == Ni/H2 ==>  (c) doc b  +  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

2C6H5NO2(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.

(c) doc b +  HNO3  ===> (c) doc b  +  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.

(c) doc b  +  Cl2   ===>  (c) doc b  +  HCl

The principal product is chloro-3-nitrobenzene.

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


TOP OF PAGE and sub-index


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.

[SEARCH BOX]

INDEX of AROMATIC CHEMISTRY NOTES

 All Advanced Organic Chemistry Notes

 

KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry 

*

best gift deals latest video game release, best gift deals best bargains in shop sales latest pop music releases, best sales deals download free music, latest film releases, best gifts for teenagers latest high street fashion in clothes, fashionable trending in clothing, best gift deals best bargains in shop salesgirls buy clothes, spend a lot of money on clothes, best sales deals shoes, sweets and chocolates, magazines and make-up best gifts for teenagers Boys buy food and drink, computer games best gift deals best bargains in trainers shop sales DVDs and CDs, girls and boys spend a lot of money on credit for mobile phones best sales deals best bargains in shop sales buses and trains to transport them going out best gifts for teenagers best bargains in shoes shop sales Girls spend a lot of money on clothes best gift deals color colour preferences in clothes, cool sunglasses best sales deals boys buy expensive thins like best gifts for teenagers designer sports clothes and trainers, teenagers save for holidays, best sales deals clothes, mobile phones, birthday presents and electronic goods, teenage accessories, Favourite style of jeans. best gifts for teenagers A typical girl’s shopping list includes mobile phone credit deals best shoes gift deals fashionable quality cool airpods, air pods, fashionable clothes best sales deals the most popular favourite sneakers best gifts for teenagers fancy shoes, sweets, chocolates, magazines, best trainers gift deals best bargains in shop sales lip moisturizer best bargains in  shop sale slots on make-up, well being, teenage decor decorating their room best sales deals teenagers like LED string lantern lights, best gifts for teenagers beauty products for teenagers, denim jackets, scrunchies coolness, fashionable back packs, typical boy’s shopping list includes mobile credit deals, eating out takeaway food and drinks, what teenagers like to buy in clothes best gifts for teenagers computer games, DVDs, CDs, what teenagers talk about best gift deals what teenagers worry about, what teenagers like to do for fun sports best sales deals what cool things do teenagers buy, resale websites like eBay Teenager best high street shop or best online deals currys pc world argos amazon john lewis dell acer samsung raycon bose sony asus huawei HP microsoft in-ear headphones earbuds ipad desktop computer laptop computer for school college university students latest video games consoles apple iphone google high end mobile phones cell phone bargain smartphone xiaomi oppo high tech products latest fashion in trainers latest fashion in shoes latest fashion in mobile phones cell phones

TOP OF PAGE and sub-index