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7.3.2 The structure and physical properties of selected arenes

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 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 fits in with arene's 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.

 

(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. cycloalkane, aromatic

 

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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.

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 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 acid chloride.

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

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 pairs of electrons involved.

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.

 

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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.

 

Mechanism

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.

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

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.

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)

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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