Physical properties of arenes sources synthesis Friedel Craft alkylation of benzene methylbenzene arene reactions cracking reforming isomerisation combustion use in fuels advanced A level organic chemistry revision notes doc brown

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Advanced Level Organic Chemistry: Arenes - physical properties, sources and synthesis

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 Friedel Crafts alkylation of benzene and methylbenzene physical properties of benzene send methylbenzene

Part 7.3 Sources & synthesis of arenes by alkylation & physical properties of arenes (aromatic hydrocarbons) and use of benzene in fuels

Sub-index for this page

7.3.1 What are arenes?

7.3.2 Structure and physical properties of selected arene hydrocarbons

7.3.3 Arenes from the petrochemical industry - catalytic cracking

7.3.4 Arenes from the petrochemical industry - Reforming

7.3.5 Arenes from the petrochemical industry - Use of disproportionation

7.3.6 Synthesis using an arene and a halogenoalkane (a Friedel Crafts alkylation reaction) and mechanism

7.3.7 Synthesis using an arene and an alkene (a Friedel Crafts alkylation reaction) and mechanism

7.3.8 Combustion of benzene and methylbenzene - use in fuels

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7.3.1 What are arenes?

Arenes are aromatic hydrocarbons, with one or more benzene rings, consisting of molecules made up of only carbon and hydrogen atoms. Examples are listed in the 7.3.2 table below.

The term "aromatic" originally referred to their pleasant smells (e.g., from cinnamon bark, wintergreen leaves, vanilla beans and anise seeds), but now implies a particular sort of delocalized bonding.

Aromatic hydrocarbons (or sometimes called arenes or aryl hydrocarbons) are hydrocarbons with sigma bonds and delocalized π electrons (indicated by the central complete circle) between the carbon atoms forming the benzene ring.

You can also have fused ring polyaromatic arenes e.g. naphthalene which are normally drawn in Kekule style.

The term alkylbenzenes refers to molecules in which a hydrogen atom in the benzene ring is replaced by an aliphatic alkyl group e.g. the arenes methylbenzene and ethylbenzene (above top-right).

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7.3.2 The structure and physical properties of selected arenes

Name of arene	Structure	Mpt/^oC	Bpt/^oC	Comments
benzene		6	80	The simplest arene hydrocarbon and the simplest possible aromatic compound.
methylbenzene		-95	111	The simplest alkyl substituted derivative of benzene
ethylbenzene		-94	136
propylbenzene		-100	159	These first four arenes show an expected steady increase in boiling point with increase in alkyl chain length.
1,2-dimethylbenzene 1,3-dimethylbenzene 1,3-dimethylbenzene		-25 -47 13	144 139 138	You would need a very effective fractional distillation column to separate these three positional isomers of dimethylbenzene. They are isomeric with ethylbenzene.
naphthalene	C₁₀H₈	79	218	Simplest example of a fused system of two or more benzene rings. These are usually displayed in the Kekule style of formula.
anthracene	C₁₄H₁₀	216	340	A fused system of three benzene rings.

Notes of the physical properties of arene hydrocarbons

(a) At room temperature arenes are colourless liquids or white solids.

The main force between arene molecules like benzene and methylbenzene are instantaneous dipole - induced dipole, so their boiling points are relatively low for the size of the molecule (e.g. in number of electrons).

The simplest (smallest) arene is benzene, C₆H₆, has 42 electrons, so the intermolecular forces are strong enough to make it a liquid at room temperature and it boils at 80^oC.

The pentane molecule, C₅H₁₂, also has 42 electrons and boils at 36^oC.

The higher boiling point of benzene suggests that it is more polarizable than pentane because of he delocalised pi electron orbitals, and this also fits in with arenes susceptibility to electrophilic attack.

(b) All arenes are insoluble in water ...

... and the lower members have a density of less than water (<1.0 g/cm³).

Arenes like benzene and methylbenzene are non-polar molecules and cannot hydrogen bond with water, in fact there cannot be any permanent dipole - permanent dipole attractions between arenes and highly polar water, so they are immiscible with water, no solvation process possible.

The hydrophobic hydrocarbon molecules 'attempt' to disrupt the strong hydrogen bonding between water molecules, without any significant compensating solute - solvent intermolecular forces.

(c) They all have a variety of strong hydrocarbon odours - from a petrol like odour to 'mothballs'!

Many more complex aromatic compounds in nature can have pleasant aromas - which is where the term aromatic originates.

The fumes or benzene should not be breathed in, it is a carcinogenic compound, despite being used as a small percentage in petrol.

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7.3.3 Arenes from the petrochemical industry - Catalytic cracking

Catalytic cracking

Catalytic cracking using zeolites produces more branched alkanes, alkenes, cyclic alkanes and cyclic aromatic compounds with a benzene ring.
e.g. aliphatic cycloalkane hydrocarbon, aromatic benzene derivative

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7.3.4 Arenes from the petrochemical industry - Reforming

Converting linear alkane molecules into cyclic hydrocarbons (cycloalkanes and aromatic molecules - have a benzene ring).
The products have the same number of carbon atoms as the reactants, but less hydrogen atoms and a ring structure.
Naphtha is the chemical feedstock (C₆ - C₁₀ alkanes) and passed over a Pt/Al₂O₃ catalyst at 500^oC. The hydrogen formed is recycled to minimise 'coking' (thermal decomposition to give a carbon deposit on the catalyst).
The following equations are depicted using structural formulae and skeletal formulae e.g.

(a) hexane ===> 1st cyclohexane ===> 2nd benzene

1st + H₂

+ H₂

then 2nd + 3H₂

+ 3H₂

This reforming reaction probably goes through the dehydrogenation sequence.
hexane ==> cyclohexane ==> cyclohexene ==> cyclohexa-1,3-diene ==> benzene
or in old 'Kekule' style

(b) heptane or 2-methylhexane or 3-methylhexane ===> methylcyclohexane ===> methylbenzene

or
or

+ H₂ + 3H₂

methylbenzene is usually shown as the benzene ring is 'skeletal' and the methyl group 'structural'
in terms of skeletal formulae this reforming reaction would be shown as ...
or or

+ H₂ + 3H₂

If asked to write the full equation don't forget the hydrogens for these dehydrogenation reactions!

and check whether structural formulae or skeletal formulae are required.

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7.3.5 Arenes from the petrochemical industry - use disproportionation

Disproportionation is said to occur, when a reactant is transformed into two or more dissimilar products.

2 + , ,

(mixture of three isomeric products)

This is not a redox disproportionation reaction, but it does involve two identical molecules reacting together to give two different products with no other reactant or product involved.

The reaction is a way of producing a mixture of 1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene from methylbenzene. If you need less methylbenzene and more benzene or dimethylbenzenes, this is the way to do it.

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7.3.6 Synthesis using an arene plus a halogenoalkane (a Friedel Crafts alkylation reaction)

If an arene like benzene or methyl benzene is refluxed with a liquid halogenoalkane (or gas passed into the mixture) in the presence of an metal halide catalyst, an alkyl group is substituted into the benzene ring.

Catalysts include anhydrous aluminium chloride, aluminium bromide, iron(III) chloride or iron(III) bromide.

You can think of the catalyst AlCl₃, AlBr₃, FeCl₃ or FeBr₃ as the alkyl group carriers.

Despite their catalytic status, these compounds are not easy to recycle, and relatively high temperatures may be required, so the search is on to find much 'greener' catalysts which can be more readily recycled and operate at a lower temperature by offering a lower activation energy mechanistic route.

The halogenoalkanes can be chloro-alkanes (cheaper) or bromoalkanes.

The preparation must be done under anhydrous conditions and dry laboratory apparatus because aluminium chloride hydrolyses in the presence of water.

The process is called alkylation.

Examples of aromatic Friedel Crafts alkylation substitution reactions

(a) + CH₃CH₂CH₂Cl ===> + HCl

benzene + 1-chloropropane == AlCl₃ ==> 2-phenylpropane + hydrogen chloride

(2-phenylpropane is formed in preference to propylbenzene - see mechanism below for reason)

(b) + CH₃Cl ===> + HCl

methylbenzene + chloromethane ===> 1,2- or 1,2- or 1,4-dimethylbenzene + hydrogen chloride

Three positional structural isomers of C₈H₁₀ formed in different proportions.

(c) + CH₃CH₂Br ===> + HBr

methylbenzene + bromoethane == AlBr₃ ==> 1-ethyl-2/3/4-methylbenzene

Three positional structural isomers of molecular formula C₉H₁₂ formed in different proportions.

The alkylation of naphthalene another Friedel-Crafts reaction of an aromatic hydrocarbon

Friedel-Crafts reaction of naphthalene with methyl iodide to yield 1-methylnaphthalene 2-methylnaphthalene aluminium chloride catalyst molecular structure of products structural formula electrophilic substitution mechanism

Naphthalene readily reacts at room temperature with haloalkanes in the presence of aluminium chloride catalyst.

The principal products from iodomethane/AlCl₃ are 1-methylnaphthalene and 2-methylnaphthalene.

The reaction is of little use in synthesis because of further alkylation substitution - the methyl groups release electron charge into the aromatic ring system (+ I effect) making the product even more reactive than the original naphthalene - these reactions are in principle the same electrophilic substitution reactions undergone by benzene and methylbenzene.

The mechanism of alkylation of benzene and methylbenzene

The nature of electrophilic substitution reactions and why aromatic compounds undergo them rather than addition was introduced in section 7.2 Structure of benzene and an introduction to electrophilic substitution in arenes

Note that the catalyst aluminium chloride (AlCl₃) has a vacant orbital and can act as a Lewis acid, accept a pair of electrons, and, in this context, facilitate the formation of a more powerful electrophile [alkyl R]⁺ in step (1) of this acylation of benzene electrophilic substitution mechanism. Its the same situation for the catalysts AlBr₃, FeCl₃ and FeBr₃.

organic reaction mechanisms

The mechanism of a Friedel Crafts alkylation - an electrophilic substitution reaction. R and R' = H or alkyl

[mechanism 23 above] If R' = H, benzene would form methylbenzene if chloromethane was used.

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

The aluminium chloride reacts with the halogenoalkane molecule to form a carbocation which is a much stronger electron pair accepting electrophile than the original haloalkane.

Step (2) An electron pair from the delocalised π electrons of the benzene ring forms a covalent C-C bond with the electron pair accepting carbocation forming a second highly unstable carbocation.

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

Step (3) is a proton transfer, as the tetrachloroaluminate(III) ion [formed in step (1)], abstracts a proton from the highly unstable intermediate carbocation to give the alkyl-aromatic product, hydrogen chloride gas and re-forms the aluminium chloride catalyst.

In forming the substituted product, rather than an addition product, the stable delocalised pi electron system of the benzene ring is reformed.

If R' = CH₃ methylbenzene: C₆H₅CH₃ + R₃C-Cl ===> R₃C-C₆H₄CH₃ + HCl

When R' is alkyl, a mixture of polysubstituted alkyl aromatic compounds are formed.

e.g. using chloromethane, 1,2- or 1,3- or 1,4-dimethylbenzene will be formed,

so if R=H, the mechanism above would show the formation of 1,2-dimethylbenzene.

The overall alkylation reaction is the substitution of -H by -CR₃

It is actually quite difficult to stop the alkylation going beyond one substituent i.e. alkylating benzene will also produce dimethylbenzenes because alkyl substituent activate the benzene ring.

For more on 'activation' of benzene see section 7.14 for reactivity explanations and orientation of products

Below, mechanism diagram 80G shows the mechanistic pathway in more detail of forming ethylbenzene from chloroethene and benzene, full blue arrows clearly showing all the electron shifts and the pairs of electrons involved.

alkylation of benzene using chloroethene to make ethylbenzene electrophilic substitution mechanism hydrogen chloride aluminium chloride ethyl carbocation Friedel-Crafts reaction synthesis

Mechanism diagram 80G shows the alkylation of benzene using chloroethene to make ethylbenzene.

Initially the aluminium chloride catalyst generates the ethyl carbocation - the electrophile - electron pair acceptor, a more powerful electrophile that the original chloroethane.

The ethyl carbocation accepts a pair of electrons from the pi orbital of benzene to form a covalent C-C bond and part of the benzene ring is now saturated - so you no longer have the extra stability of the pi electron ring of the original benzene molecule.

The 2nd carbocation loses a proton to give the three products - the desired benzene compound, hydrogen chloride and the regenerated aluminium chloride catalyst.

In forming the substituted product, rather than an addition product, the stable delocalised pi electron system of the benzene ring is reformed.

It is an electrophilic substitution mechanism using an aluminium chloride catalyst which generates an ethyl carbocation - the electrophile that attacks the pi orbitals of the benzene ring.

It is a typical alkylation Friedel-Crafts reaction synthesis route.

reaction progress profile alkylation of benzene electrophilic substitution mechanism unstable carbocation intermediate Friedel-Crafts reaction synthesis

The reaction progress profile for the alkylation of benzene via an electrophilic substitution mechanism involving an unstable carbocation intermediate. This a typical reaction progress profile for a Friedel-Crafts alkylation synthesis reaction.

Below, mechanism diagram 80J shows the mechanistic pathway in more detail of forming 2-phenylpropane from 1-chloropropane and benzene, full blue arrows clearly showing all the electron shifts and pairs of electrons involved.

alkylation of benzene using 1-chloropropane to make ethylbenzene electrophilic substitution mechanism hydrogen chloride aluminium chloride secondary propyl carbocation Friedel-Crafts reaction synthesis

Mechanism diagram 80J shows the alkylation of benzene using 1-chloropropane to make 2-phenylpropane.

Initially the aluminium chloride catalyst generates the alkyl cation - the electrophile - electron pair acceptor, a more powerful electrophile than the original 1-chloropropane.

The alkyl cation accepts a pair of electrons from the pi orbital of benzene to form a covalent C-C bond and part of the benzene ring is now saturated - so you no longer have the extra stability of the benzene ring of delocalised pi electrons.

The 2nd carbocation loses a proton to give the three products - the desired benzene compound, hydrogen chloride and the regenerated aluminium chloride catalyst.

Note that in forming the alkyl substituted benzene derivative the very stable ring of pi electrons of the benzene ring is reformed - substitution preferred to addition.

The electrophilic substitution mechanism involves an aluminium chloride catalyst which generates a carbocation from the 1-chloropropane molecule.

Note that the more stable secondary propyl carbocation is formed in this Friedel-Crafts alkylation reaction synthesis. This results in very little propylbenzene from a primary propyl cation in the products.

You get the same major product of 2-phenylpropane if you used 2-chloropropane as the alkyl group source.

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7.3.7 Synthesis using an arene plus an alkene (another Friedel Crafts alkylation reaction)

Arenes like benzene and methylbenzene will react with alkenes to produce substituted alkyl-arenes.

The catalyst is a mixture of hydrogen chloride and aluminium chloride and is another example of a Friedel Crafts alkylation reaction e.g.

(i) + CH₃CH=CH₂ ===>

benzene + propene == AlCl₃ ==> propylbenzene and 2-phenylpropane

Two positional/chain structural isomers of molecular formula C₉H₁₂ formed in different proportions.

However, the minority product is propylbenzene and the majority product is 2-phenylpropane (which is also called 1-methylethylbenzene, in industry commonly known as cumene).

For the explanation of the majority product, see the 2nd mechanism further down the page.

(ii) + CH₂=CH₂ ===>

methylbenzene + ethene == AlCl₃ ==> 1-ethyl-2/3/4-methylbenzene

Three positional/chain structural isomers of molecular formula C₉H₁₂ formed in different proportions.

As one of the most important alkylbenzenes, ethylbenzene is predominantly synthesized by alkylation of benzene with ethene using an acid catalyst - this fits in with the mechanism described below where protonation of ethene yields the ethyl carbocation, the electrophile that attacks the pi bond electron cloud of benzene.

Alkylation of aromatic compounds by this method is an important step in the synthesis of certain type of detergent - sulfonic acids and their salts - see 7.7 Ring sulfonation of arenes, properties & uses of sulfonic acids

The first step is to react benzene with a long chain linear alkene of typically 10 to 14 carbon atoms i.e. length of R + R' = 8 to 12 in the diagram below.

synthesis of a detergent sodium alkylbenzenesulfonate LAS surfactant

The alkylbenzene product is then sulfonated and neutralised to make the sodium salt, which acts as a surfactant (lowers surface tension of water) - in this case the sodium alkylbenzenesulfonate salt is an example of a synthetic detergent.

For more on sulfonic acids see 7.7 Ring sulfonation of arenes, properties & uses of sulfonic acids

Mechanism of alkylation of aromatic compounds using alkenes as the alkyl group source

via a AlCl₃/HCl catalytic mixture e.g.

(1) Producing ethylbenzene form benzene and ethene.

Step 1. Generation of a carbocation electrophile

AlCl₃ + HCl + CH₂=CH₂ ===> CH₃-CH₂⁺ + AlCl₄^-

The hydrogen chloride (^δ+H-Cl^δ-) donates a proton to ethene to form an ethyl carbocation.

The ethyl carbocation can be generated with other acid catalysts.

Step 2. Electrophilic attack of the ethyl carbocation on the pi electron cloud of the benzene ring

CH₂-CH₂⁺ + C₆H₆ ===> C₆H₅CH₂CH₃ + H⁺

Step 3. The regeneration of the catalyst from the expelled proton - to complete the 'catalytic cycle'.

H⁺ + AlCl₄^- ===> AlCl₃ + HCl

The tetrachloroaluminate(III) ion picks up the proton before it can combine with the carbocation to form chloroethane.

Below, mechanism diagram 80K shows the mechanistic pathway in more detail of forming ethylbenzene from ethene and benzene, full blue arrows clearly showing all the electron shifts and pairs of electrons involved.

alkylation of benzene using ethene to make ethylbenzene electrophilic substitution mechanism hydrogen chloride aluminium chloride ethyl carbocation Friedel-Crafts reaction synthesis

The alkylation of benzene using ethene to make ethylbenzene. An electrophilic substitution mechanism involving hydrogen chloride, aluminium chloride and the generation of an ethyl carbocation. It is Friedel-Crafts alkylation synthesis reaction.

The 2nd stage of the mechanism is no different from if you had started with chloroethene (see mechanism 80G), so I've haven't repeated the details.

(2) Producing 2-phenylpropane (cumene) from benzene and propene

Step 1. Generation of a carbocation electrophile

AlCl₃ + HCl + CH₃CH=CH₂ ===> CH₃-C⁺HCH₃ + AlCl₄^-

The hydrogen chloride (^δ+H-Cl^δ-) donates a proton to propene to form mainly the secondary carbocation. Note the formation of the more stable secondary carbocation, rather than the less stable primary carbocation CH₃CH₂CH₂⁺, which explains why propylbenzene is the minor product, and why the major product is 2-phenylpropane.

Step 2. Electrophilic attack of the carbocation on the pi electron cloud of the benzene ring

CH₃C⁺HCH₃ + C₆H₆ ===> C₆H₅CH(CH₃)₂ + H⁺

Step 3. The regeneration of the catalyst from the expelled proton - to complete the 'catalytic cycle'.

H⁺ + AlCl₄^- ===> AlCl₃ + HCl

The tetrachloroaluminate(III) ion picks up the proton before it can combine with the carbocation to form 2-chloropropane.

Below, the full mechanism diagram 80L shows the mechanistic pathway in more detail the formation of 2-phenylpropane from propene and benzene, full blue arrows clearly showing all the electron shifts and pairs of electrons involved.

alkylation of benzene using propene to make 2-phenylpropene electrophilic substitution mechanism hydrogen chloride aluminium chloride secondary propyl carbocation Friedel-Crafts reaction synthesis

The mechanistic pathway of alkylation of benzene using propene to make 2-phenylpropene.

The electrophilic substitution mechanism involving hydrogen chloride aluminium chloride and the more stable secondary propyl carbocation. It is an alkylation Friedel-Crafts reaction synthesis.

Note that the more stable secondary propyl carbocation is formed in this Friedel-Crafts alkylation reaction synthesis. This results in very little propylbenzene from a primary propyl cation in the products.

The alkyl carbocation is a more powerful electrophile (electron pair acceptor) than the original alkene molecule.

The carbocation accepts a pair of the pi electrons to form a C-C bond. The 2nd intermediate carbocation now has a saturated carbon atom, destroying the original stable ring of pi electrons.

A proton is lost to reform the stable pi electron ring of the benzene ring of newly formed alkyl substituted benzene derivative. 'Waste' hydrogen chloride is formed and the aluminium chloride catalyst regenerated.

2-phenylpropane (isopropylbenzene, cumene), is manufactured from propene and benzene using a solid phosphoric acid, AlCl₃ or zeolite catalyst.

Unfortunately, because the initial alkyl group activates the ring, you get lots of disubstituted products.

For more on 'activation' of benzene see section 7.14 for reactivity explanations and orientation of products

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7.3.8 Combustion of benzene and methylbenzene

Benzene and alkylbenzenes are used in vehicles fuels to produce various blends of petrol (gasoline)

They both increase the octane rating of petrol.

With excess air (oxygen source) you should get complete combustion to carbon dioxide and water.

Benzene: C₆H₆(l) + 7½O₂(g) ===> 6CO₂(g) + 3H₂O(l)

Methylbenzene: C₆H₅CH₃(l) + 9O₂(g) ===> 7CO₂(g) + 4H₂O(l)

However, if there is a deficiency of oxygen, carbon soot will be formed in a very smoky flame, or particulates from a car exhaust.

e.g. C₆H₆(l) + 1½O₂(g) ===> 6C(s) + 3H₂O(l)

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