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 electrophilic substitution of chlorine and bromine into benzene and methylbenzene mechanisms explained

Part 7.6 Electrophilic substitution - ring halogenation of benzene & methylbenzene, properties & uses of chloro-aromatics and other aryl halides (brief mention of naphthalene)

Sub-index for this page

7.6.1 Halogen ring substitution reaction (Cl and Br), reagents, conditions and equations for synthesising aryl halides

7.6.2 The electrophilic substitution mechanism yielding aryl halides

7.6.3 The physical properties of aryl halides - halogenated arenes

7.6.4 Some chemical reactions of aryl halides and theoretical aspects

7.6.5 The uses of aryl halides

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7.6.1 Halogen substitution reaction, reagents, conditions and equations for synthesising aryl halides

Examples of aromatic chlorination/bromination substitution reactions

The arene hydrocarbon is mixed with anhydrous aluminium chloride (catalyst) and chlorine passed through the mixture at room temperature, substitution takes place.

The aluminium chloride catalyst is referred to as the halogen carrier and generates the attacking electrophile.

Iron(III) chloride can also be used as the catalyst, and can be made in situ by adding iron filings to the mixture before bubbling chlorine through: 2Fe  +  3Cl₂  ===>  2FeCl₃

All reagents and glassware must be dry - anhydrous aluminium chloride hydrolyses with water.

The exothermic reaction is quite rapid, even at room temperature and clouds of hydrogen chloride or hydrogen bromide are given off - hopefully in a fume cupboard.

(a)   +  Cl₂  == AlCl₃ /FeCl₃ ===>   +  HCl

benzene  +  chlorine  ===>  chlorobenzene  +  hydrogen chloride

Chlorobenzene is a colourless liquid bpt. 132oC, it is used as a solvent and in the production of pesticides.

 

(b)   +  Cl₂  ===>      +  HCl

methylbenzene  +  chlorine  ===> chloro-2-, chloro-3 or chloro-4-methylbenzene  +  hydrogen chloride

Three positional structural isomers of C₇H₇Cl formed in different proportions. Major products.

You do NOT get (chloromethyl)benzene, (shown on the right), which is a side-chain substitution product.

(chloromethyl)benzene is formed if uv light is used instead of aluminium chloride.

Note that if the atom of 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 halogenation in the benzene ring of methylbenzene (For more details on the theory of electrophilic substitution see section 7.14).

 

(c)   +  Cl₂   ===>       +  HCl

chlorobenzene + chlorine ==> 1,2- or 1,3- or 1,4-dichlorobenzene + hydrogen chloride

Three positional structural isomers of  C₆H₄Cl₂ formed in different proportions.

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 reagents and conditions for bromination

The arene is mixed with anhydrous iron(III) bromide (catalyst) and liquid bromine in a flask fitted with a reflux condenser - with gentle heating.

Iron(III) bromide can be made in situ by adding excess bromine to iron filings.

2Fe  +  3Br₂  ===>  2FeBr₃

The iron(III) bromide catalyst is referred to as the halogen carrier and generates the attacking electrophile (see mechanisms in next section).

All reagents and glassware must be dry - iron(III) bromide hydrolyses with water.

The equations for bromination are identical, except just swap Br for Cl e.g.

(d) benzene  +  bromine  ===>  bromobenzene  +  hydrogen bromide

C₆H₆  +  Br₂  == AlBr₃/FeBr₃ ==>  C₆H₅Br  +  HBr

bromobenzene is used in the production of pharmaceutical products.

 

(e) methylbenzene  +  bromine  ===>  bromo-2/3/4-methylbenzene  +  hydrogen bromide

C₆H₅CH₃  +  Br₂  ===>  CH₃C₆H₄Br  +  HBr

Three positional structural isomers of  CH₃C₆H₄Br formed in different proportions.

The more electronegative bromine atom deactivates the benzene ring, but less so at the 2, 4 and 6 positions, hence the 3- substitution yields the minor product <1% here).

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

 

The halogenation of naphthalene

Naphthalene readily reacts with bromine in a solvent when the mixture is heated under reflux to yield mainly 1-bromonaphthalene.

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

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7.6.2 The electrophilic substitution mechanism yielding aryl halides

Mechanism diagram 21 - A general electrophilic substitution by halogen in a benzene ring.

For Al and Cl you can substitute Fe and Br i.e. using an FeBr₃ catalyst (halogen carrier).

[mechanism 21 above] When R = H, benzene forms chlorobenzene.

Step (1) The non-polar and uncharged chlorine molecule is not a strong enough an electrophile to disrupt the pi electron system of the benzene ring.

The aluminium chloride reacts with a chlorine molecule to form a positive chlorine ion Cl⁺ which is a much stronger electron pair accepting electrophile and a tetrachloroaluminate(III) ion (either this or an Cl₂-AlCl₃ complex acting as a halogen carrier- see version 3 on mechanism diagram 80C further down the page).

Step (2) When the electrophile attacks, an electron pair from the delocalised pi electrons of the benzene ring forms a C-Cl bond with the electron pair accepting positive chlorine ion forming a highly unstable carbocation.

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

Step (3) The tetrachloroaluminate(III) ion, formed in step (1), abstracts a proton from the highly unstable intermediate carbocation to give the chloro-aromatic product (with the stabilising ring of pi electrons intact), waste hydrogen chloride gas and reformed aluminium chloride catalyst.

The mechanism is similar for: C₆H₅CH₃ + Cl₂ ===> ClC₆H₄CH₃ + HCl

where R = CH₃, methylbenzene forms a mixture of chloro-2/3/4-methylbenzene (3 positional isomers).

chloro-3-methylbenzene is the minority product and the mechanism above would show the formation of chloro-2-methylbenzene when R = CH₃.

FURTHER COMMENTS

The overall halogenation reaction is the substitution of -H by -Cl 

Bromination can be carried in a similar way, so, you can write out the mechanism in exactly the same way, but putting in Br instead of Cl and FeBr₃ instead of AlCl₃.

REMINDER: Why do aromatic compounds tend to react by electrophilic substitution BUT alkenes tend to react by electrophilic addition?

They both interact with electrophiles because they both have 'electron rich' electron pair donating bonding systems i.e. the >C=C< double bond in alkenes and the delocalised π electrons of the benzene ring.

But the benzene ring of pi electrons has confers a particularly high stability which is preserved on substitution rather than addition.

For the same reason alkenes are generally more reactive than arenes.

Don't confuse this electrophilic substitution in the benzene ring with what happens if methyl benzene is reacted with chlorine in the presence of uv light, substitution takes place in the alkyl side chain.

In other words it behaves like an alkane and undergoes a free radical substitution reaction.

The initial product is chloromethylbenzene, C₆H₅CH₂Cl, and further substitution products can be formed C₆H₅CHCl₂ and C₆H₅CCl₃.

For more see side-chain chlorination substitution of the methyl group of methylbenzene

 

Some specific mechanism diagrams based on the general mechanism diagram 21

Mechanism diagram 80A The electrophilic substitution of a chlorine atom into the ring of benzene

The chlorine and aluminium chloride generate the attacking electrophile Cl⁺ and the tetrachloroaluminate ion.

The Cl⁺ ion is a much more powerful electrophile than the original Cl₂ chlorine molecule.

The Cl⁺ attacks the pi electron cloud of the benzene ring and accepts a pair of electrons to form a C-Cl covalent bond - so one of the carbon atoms of the ring is saturated, thereby loss of the extra original stability of the aromatic ring of pi electrons.

This combination with benzene forms the 2nd unstable intermediate cation (partially, and temporarily, saturated).

The AlCl₄^- ion then abstracts a proton from the carbocation to yield the desired product chlorobenzene, hydrogen chloride ('waste') and the re-formed AlCl₃ catalyst.

In forming chlorobenzene the very stable aromatic benzene ring of pi electrons is reformed.

Comparing the reactivity of aromatics like benzene/methylbenzene and aliphatic alkenes

A polarised chlorine/bromine molecule can attack an alkene C=C bond successfully. The alkene bond has a high electron density from the pi orbitals above and below of the >C=C< bond system. This high electron density is sufficient to cause on collision, a repulsion in the halogen molecule to generate the polarised attacking electrophile Cl^δ+-Cl^δ- or Br^δ+-Br^δ-.

In alkenes the electron delocalisation is over two carbon atoms but in benzene compounds it is over six carbon atoms giving a lower pi orbital electron density around the ring. As a general rule the larger the delocalised system, the lower its potential energy so benzene is more stable than alkenes. On collision with benzene, the halogen molecule is just repelled by the pi electron clouds - hence the need for a halogen carrier catalyst to get a reaction with benzene.

A note on mechanism styles of presentation

Mechanism diagram 80C Three versions of how an electrophile is generated to attack benzene (and other aromatics) to yield a carbocation

Version 1: This is the simplest approach to expressing this electrophilic substitution reaction and one I am using on this page at the moment.

Version 1 involves a pre-stage step that generates the hypothetical electrophile X⁺ , from an X₂ + MX₃ collision, which attacks the pi electron system of the benzene ring.

The X⁺ ion is a much more powerful electrophile and electron pair acceptor than the original X₂ halogen molecule.

I have adopted the style of version 1 in my mechanistic descriptions.

Version 2: This is found in older textbooks and shows all the electron shifts necessary, but is not considered the best representation of the 'real' mechanism - it implies the simultaneous collision of three particles, which is highly improbable.

Version 3: This is the best correct version that would be presented at university level courses.

Version 3 also involves a pre-stage step that generates the hypothetical electrophile, but not a simple X⁺ ion.

The electrophile is a 'molecular' combination of the halogen X₂ and the catalyst MX₃. and it is this 'halogen carrier' that attacks the pi electron system of the benzene ring and delivers the halogen atom into combination with the benzene ring.

 

Mechanism diagram 80F The reaction progress profile for the electrophilic substitution of a chlorine atom into the ring of benzene

Reaction profiles usually start with the reactants, but here I've started with the arene (e.g. benzene) and the carbocation.

The first activation energy is high because it involves breaking open the stable pi orbital rings of electrons and is the slower of the two steps shown in the diagram.

The final step is faster with a lower activation energy as the proton is removed and the strong C-X bond is formed.

 

Mechanism diagram 80B The electrophilic substitution of a bromine atom into the 2 position of the benzene ring of methylbenzene giving bromo-2-methylbenzene

The bromine and iron(III) bromide generate the attacking electrophile Br⁺ and the tetrabromoiron(III) ion.

The Br⁺ ion is a much more powerful electrophile than the original Br₂ bromine molecule.

The Br⁺ attacks the pi electron cloud of the benzene ring, forms a C-Br covalent bond, combining with methylbenzene to form the unstable intermediate cation, which is partially, and temporarily saturated - at carbon atom 2 in terms of aromatic ring nomenclature.

Note the loss of the complete stable aromatic ring of pi electrons, hence the carbocation's instability.

The FeBr₄^- ion then abstracts a proton from the carbocation to yield the desired product bromo-2-methylbenzene, hydrogen chloride ('waste') and the re-formed FeBr₃ catalyst.

In forming bromo-2-methylbenzene the complete stable aromatic ring of pi electrons is reformed.

Mechanism diagram 80D The electrophilic substitution of a chlorine atom into the 4 position of the benzene ring of methylbenzene

The chlorine and aluminium chloride generate the attacking electrophile Cl⁺ and the tetrachloroaluminate ion.

The Cl⁺ ion is a much more powerful electrophile than the original Cl₂ chlorine molecule.

The Cl⁺ attacks the pi electron cloud of the benzene ring, accepts a pair of electrons to form a C-Cl bond, so combining with methylbenzene to form the unstable intermediate cation, which is partially, and temporarily saturated, i.e. the ring of pi electrons is broken at carbon atom 4 (in terms of aromatic ring nomenclature).

The AlCl₄^- ion then abstracts a proton from the carbocation to yield the desired product chloro-4-methylbenzene, hydrogen chloride ('waste') and the re-formed AlCl₃ catalyst.

In forming chloro-4-methylbenzene the complete stable aromatic ring of pi electrons is reformed.

The other major product is chloro-2-methylbenzene via the same mechanism.

 

Mechanism diagram 80E The electrophilic substitution of a chlorine atom into the 4 position of the benzene ring of chlorobenzene

The chlorine and aluminium chloride generate the attacking electrophile Cl⁺ and the tetrachloroaluminate ion.

The Cl⁺ ion is a much more powerful electrophile than the original Cl₂ chlorine molecule.

The Cl⁺ attacks the pi electron cloud of the benzene ring and combines with chlorobenzene to form the unstable intermediate cation, partially, and temporarily, saturated - at carbon atom 4 in terms of aromatic ring nomenclature.

The AlCl₄^- ion then abstracts a proton from the carbocation to yield the desired product 1,2-dichlorobenzene, hydrogen chloride ('waste') and the re-formed AlCl₃ catalyst.

In forming 1,4-chlorobenzene, the very stable aromatic delocalised ring of pi electrons is reformed.

The other major product via the same mechanism would be 1,2-dichlorobenzene via the same mechanism.

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7.6.3 The physical properties of aryl halides - halogenated arenes

Abbreviations used: mpt = melting point ^oC;  bpt = boiling point ^oC; sub. = sublimes; dec. = thermally decomposes

Name	Structure	Mpt/oC	Bpt/oC	Comments
chlorobenzene		-45	132
1,2-dichlorobenzene		-17	179	3 positional structural isomers of C6H4Cl2
1,3-dichlorobenzene		-24	172
1,4-dichlorobenzene		53	174
chloro-2-methylbenzene				3 positional structural isomers of CH3C6H4Cl
chloro-2-methylbenzene
chloro-2-methylbenzene
bromobenzene		-31	156
1,2-dibromobenzene		7	221	3 positional structural isomers of C6H4Br2
1,3-dibromobenzene		-7	220
1,4-dibromobenzene		87	218
iodobenzene		-31	189
1,2-diiodobenzene		27	286
1,3-diiodobenzene		40	285
1,4-diiodobenzene		129	sub. 285

Notes

(a) All the above aryl halides are insoluble in water.

Generally, they are not highly polar and cannot hydrogen bond with water.

(b) For the size of molecule (e.. in terms of electrons in molecule, they have relatively low melting/boiling points.

Most of the intermolecular force is due to instantaneous dipole - induced dipole interactions, plus a smaller contribution from the permanent dipole - permanent dipole forces ( C^δ+-Cl^δ-) of the polar bond.

(c)

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7.6.4 Some chemical reactions of aryl halides and theoretical aspects

(a) Hydrolysis of aryl halides to phenols - with difficulty!

Bond enthalpies quoted from https://www2.chemistry.msu.edu/courses/cem850/handouts/Ellison_BDEs.pdf

Bond lengths quoted from https://onlinelibrary.wiley.com/doi/pdf/10.1002/9783527616091.app1

Data table

Molecule	Specific C-Cl Bond	Bond enthalpy/ kJ/mol	C-Cl Bond length/ nm	Comments
chloroethane	CH3CH2-Cl	355	0.179	Pure saturated aliphatic σ C-Cl bond, no delocalisation connection. Bond order 1.0, longer than chloroethene.
chloroethene	CH2=CH-Cl	382	0.173	σ C-Cl bond plus a 'partial C-Cl π bond' from the partial overlap of Cl's non-bonding electron orbitals with the delocalised π C=C alkene bond. Bond order >1.0 <1.5
(chloromethyl)benzene	C6H5CH2-Cl	310	0.179	Pure saturated aliphatic σ C-Cl bond, no delocalisation connection. Bond order 1.0, longer than chlorobenzene.
chlorobenzene	C6H5-Cl	406	0.170	σ C-Cl bond plus a 'partial C-Cl π bond' from the partial overlap of Cl's non-bonding electron orbitals with the delocalised aromatic π bond. Bond order >1.0 <1.5

There is a pattern from the table which makes two points as to why chloro-aromatic compounds hydrolyse more slowly than aliphatic halogenoalkanes.

(i) The C-Cl bond in the benzene ring (and chloroethene) is shorter than the C-Cl in aliphatic halogenoalkanes.

(ii) The C-Cl bond in the benzene ring (and chloroethene) is stronger than the C-Cl in aliphatic halogenoalkanes.

(iii) We can also add the point that the C-Cl bond in the benzene ring (and chloroethene) is less polar than the C-Cl in aliphatic halogenoalkanes because the redistribution of the charge compared to the full effect of the more electronegative chlorine atom on the C-Cl bond..

The cause of the stronger C-Cl bond in chlorobenzene and other chloro-aromatics is the non-bonding pairs of electrons on the chlorine atom form a partial pi C-Cl bond making it stronger (diagram below).

The same thing seems to happen with chloroethene, which is also much harder to hydrolyse compared to chloroethane.

        compare with

So we can contrast the slow hydrolysis of aromatic chlorobenzene or isomeric chloro-2/3/4-methylbenzenes (aryl halides) with (chloromethylbenzene), with the relatively much faster hydrolysis an aliphatic halogenoalkane on the basis of the three points above.

Another approach to understanding the lack of reactivity of aryl halides towards nucleophiles is to consider the possible resonance structures that contribute to a 'conceptual' resonance hybrid structure.

Industrial hydrolysis of aryl halides

If heated under pressure, to increase reaction temperature, chlorobenzene can be made to hydrolyse to phenol.

Typical industrial reaction conditions are a temperature of 350^oC, 300 atm pressure and using aqueous sodium hydroxide as the hydrolysing agent.

These more extreme conditions compared to ordinary 'laboratory' reflux preparations are required because of the lesser reactivity of chlorobenzene towards nucleophiles compare to aliphatic halogenoalkanes.

Initially, (i) sodium phenoxide (sodium phenate) is formed in the hydrolysis reaction, and (ii) the phenol released by adding a stronger acid than phenol itself (a very weak acid) - cheap hydrochloric acid will do.

(i) (aq)  +  2NaOH(aq)  ===>  (aq)  +  NaCl(aq)  +  H₂O(l)

ionically:  C₆H₅Cl(aq)  +  OH^-(aq)  ===>  C₆H₅OH(aq)  +  Cl^-(aq)

(ii)   +  HCl  ===>    +  NaCl

ionically:  C₆H₅O^-(aq)  +  H⁺(aq)  ===>  C₆H₅OH(s)

 

(b) Electrophilic substitution - nitration

Aryl halides can be nitrated like many other aromatic compounds e.g.

   +  HNO₃  ===>         +  H₂O

using a mixture of concentrated nitric and sulfuric acids.

Chlorobenzene is less reactive than methylbenzene, so the reaction is slower and the reaction mixture needs heating under reflux.

Also, what is the major product and why?

Although the electronegative chlorine atom reduces the reactivity of chlorobenzene compared to benzene, the reduction is greater at the 3 and 5 positions on the benzene ring - the yields quoted below bare this out.

The typical yields (from left to right in the equation) are chloro-2-nitrobenze (~30%), chloro-3-nitrobenzene (<1%) and chloro-4-nitrobenzene (~70%), so over 99% of the substitution occurs at the 2 (= 6) and 4 positions.

On a purely random basis for a disubstituted aromatic, you would expect 40%, 40% and 20% (diagram below).

 

(c) Electrophilic substitution - further ring halogenation

This reaction has already been discussed in detail in sections 7.6.1 and 7.6.2.

  +  Cl₂  == AlCl₃ ==>      +  HCl

However, what wasn't discussed was, what is the major product and why?

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7.6.5 The uses of aryl halides - aromatic organo halogen compounds

From section 7.6.4 (a) we can see that chlorobenzene is used to manufacture phenol, from which a huge number of other phenolic compounds are made.

See uses of chloro-phenols

Aromatic chloro compounds (aryl halides) are widely used as insecticides, herbicides, fungicides, and bactericides.

However their over-use has serious environmental problems because they are non-biodegradable, soluble in fatty tissue and accumulate up food chains - toxicity increasing, causing genetic damage and death.

Their lack of biodegradability is often due to the strong C-Cl bonds on the aromatic ring.

As we learn more and more about the effect on organisms, more and more of aryl halides are being banned or their use strictly controlled and limited.

For example, hexachlorophene (structure below) is an external bactericide that was widely used in cosmetic preparations such as soaps, deodorants, but evidence has emerged that it can be absorbed through the skin in potentially dangerous amounts, affecting infants and small children.

Note that apart from the C-Cl group, hexachlorophene has two other functional groups i.e. primary aliphatic halogenoalkane and phenol.

Other aryl halides have been used as pesticides like DDT (dichlorodiphenyltrichloroethane, above) and the herbicides 2,4-D (2,4-dichlorophenoxyethanoic acid, above) and 2,4,5-T (2,4,5-trichlorophenoxyethanoic acid, above) have been partially banned for environmental reasons.

DDT still used to control mosquito populations that spread malaria.

2,4-D and 2,4,5-T were used in 'Agent Orange' defoliation missions by the US in the Vietnam War (to deny the Vietcong soldiers cover), and their adverse poisoning effects are still happening today.

Note these two molecules, apart from C-Cl, have two other functional groups i.e. ether and carboxylic acid.

Aryl bromides are used as flame retardants - compounds that inhibit the burning of combustible materials.

Overall use of aryl halides is in decline (thank goodness!) out of concern for their impact on the environment - their lack of biodegradability - persistence in the environment and building up concentrations in food chains - bad for ecological systems.

BUT, they are important intermediates in the manufacture of other useful compounds e.g. phenol from chlorobenzene.

They are rarely biodegradable and many are carcinogenic, though some of the chloro-phenols can be used safely as bactericides-disinfectants.

See uses of chloro-phenols

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