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Advanced Level Organic Chemistry: Structure of benzene and aromaticity of compounds

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 7.2 Proof of benzene structure, aromaticity and introduction to electrophilic substitution reactions

INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

Sub-index for this page

7.2.1 Comparison of cyclic hydrocarbon molecules

7.2.2 Enthalpy of hydrogenation evidence for the 'real' structure of a benzene ring

7.2.3 X-ray crystallography - shape and bond lengths evidence for the 'real' structure of a benzene ring

7.2.4 Resonance structures and lack of 'triene' isomers in disubstituted benzene compounds as more evidence for benzene's structure

7.2.5 Influence of the stability of the benzene ring on the chemistry of aromatic compounds - more evidence - electrophilic substitution rather than addition

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7.2.1 A comparison of cyclic hydrocarbon molecules

REMINDERS

Aliphatic compounds are considered to be relatively simple compounds that have straight or branched chains or rings of carbon atoms e.g. alkanes, alkenes, halogenoalkanes, alcohols, ketones, carboxylic acids and amines.

alkanes structure and naming (c) doc b  (c) doc b  (c) doc b  alcohols and ether structure and naming (c) doc b

 aldehydes and ketones nomenclature (c) doc b  (c) doc b  (c) doc b 

The study of their chemical properties is called aliphatic chemistry, and their chemistry is not based on the presence of a benzene ring.

Aromatic compounds contain at least one benzene ring, a ring of 6 carbon atoms with three σ bonds for each carbon atom between adjacent carbon atoms and a hydrogen atom.

The 4th electron from each carbon atom becomes part of a delocalised π bond ring system - all of this described and explained on this page along with the experiment evidence for the structure of a benzene ring.

This pi bond system gives these molecule a particular chemical character known as aromatic chemistry - aromaticity, which you need to be able to distinguish from the functional group chemistry of aliphatic compounds.

Aromatic compounds can incorporate all of the functional groups you find in aliphatic chemistry.

(c) doc b    (c) doc b    (c) doc b    (c) doc b   (c) doc b  (c) doc b    (c) doc b   (c) doc b 

BUT, when these functional groups are attached directly to a benzene ring, although the chemistry is essentially the same, chemical differences can show up e.g. reactivity or a reaction has become impossible for some reason.

 

The chemistry of natural products often involves more complex molecules e.g. chlorophyll, DNA, flavourings, glyceride ester fats/oils, hormones, pheromones, proteins.

 

Three types of relevant cyclic hydrocarbon molecules are illustrated below to illustrate the difference between cyclic aliphatic compounds (alicyclic) and aromatic compounds with a benzene ring.

 

1. Aliphatic saturated ring hydrocarbon compounds

cyclohexane, C6H12 structural formula for cyclohexane C6H12 or  skeletal formula of cyclohexane C6H12  AND  methylcyclopentane, C6H12  structural formula for methylcyclopentane C6H12  or  skeletal formula of methylcyclopentane C6H12

For more details see Molecular Structure and naming of ALKENES

 

2. Aliphatic cycloalkene hydrocarbons e.g. with one or two C=C double bonds.

cyclohexene, C6H10 structural formula for cyclohexene C6H10  or  skeletal formula of cyclohexene C6H10   AND  cyclohexa-1,3-diene, C6H8  structural formula for cyclohex-1,3-diene C6H8  or  skeletal formula of cyclohexa-1,3-diene C6H8 1,3-cyclohexadiene

Examples 1. and 2. are also referred to as alicyclic compounds because they are cyclic aliphatic compounds.

For more details see Molecular structure and naming of ALKENES

 

3. Arenes - aromatic hydrocarbon compounds with one or more benzene rings.

benzene C6H6  structural formula for benzene C6H6  or  (c) doc b   AND  methylbenzene C7H8  structural formula of methylbenzene C7H8   or (c) doc b

These represent the real structure of benzene and methylbenzene (as I put the cart before the horse!).

All the C-C bond lengths are the same with a C-C-C bond angle of exactly 120o.

The cyclic structure of benzene was first proposed by the German chemist Kekule in 1865, but he assumed that the ring consisted of alternate single (C-C) and double bonds( C=C).

Representations of Kekule structures of benzene are shown on the right AND they are still widely used in aromatic chemical equations and mechanisms, so take care!

The arguments for the real arene structures are presented in the next two sections 7.2.2 and 7.2.3 on this page.

For more on naming see Molecular structure and naming of AROMATIC COMPOUNDS


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7.2.2 The evidence from a comparison of enthalpies of hydrogenation

Below are five hydrogenation equations, together with the enthalpies of hydrogenation.

I've used the latest enthalpy values from https://webbook.nist.gov/chemistry/

The 5th equation assumes that benzene really is a 'triene', so we will show in sections 7.2.2 and 7.2.3 that this is not the real structure of benzene.

 

(1) +  H2  ===>  alkanes structure and naming (c) doc b   or   alkenes structure and naming (c) doc b + H2 ===> alkanes structure and naming (c) doc b

hydrogenation of cyclohexene to cyclohexane

ΔHhydrogenation(cyclohexene) = -118 kJ mol-1

This is a typical value for the hydrogenation of an alkene with one C=C bond.

 

(2a) structural formula for cyclohex-1,3-diene C6H8 1,3-cyclohexadiene + 2H2 ===> alkanes structure and naming (c) doc b   or   + 2H2 ===> alkanes structure and naming (c) doc b

Hydrogenation of a 1,3-cyclohexadiene to cyclohexane

ΔHhydrogenation(cyclohexa-1,3-diene) = -229 kJ mol-1

This is just below the expected value for hydrogenating two C=C bonds (2 x -118 = -236 kJ mol-1).

 

(2a) structural formula for cyclohex-1,4-diene C6H8 1,4-cyclohexadiene + 2H2 ===> alkanes structure and naming (c) doc b   or   skeletal formula of cyclohexa-1,3-diene C6H8 1,3-cyclohexadiene + 2H2 ===> alkanes structure and naming (c) doc b

Hydrogenation of a 1,4-cyclohexadiene to cyclohexane

ΔHhydrogenation(cyclohexa-1,4-diene/) = -233 kJ mol-1

This is just below the expected value for hydrogenating two C=C bonds (2 x -118 = -236 kJ mol-1).

 

(3)   +  3H2 ===>  alkanes structure and naming (c) doc b    or    skeletal formula of benzene C6H6 + 3H2 ===> alkanes structure and naming (c) doc b

hydrogenation of the 'real' benzene to cyclohexane

ΔHhydrogenation(benzene) = -208 kJ mol-1

This is well below the expected value for hydrogenating three C=C bonds (3 x -118 = -354 kJ mol-1).

 

(4) (c) doc b  +  3H2  ===> alkanes structure and naming (c) doc b   or    skeletal formula of methylbenzene C7H8 +  3H2  ===> 

hydrogenation of methylbenzene to methylcyclohexane

ΔHhydrogenation(methylbenzene) = -205 kJ mol-1

This is considerable below the expected value for hydrogenating three C=C bonds (3 x -118 = -354 kJ mol-1).

It should be noticed that this 'addition' reaction requires a heated nickel catalyst because of the stability of the benzene ring.

The hydrogen molecules are adsorbed on the Ni surface and split into atoms.

These atoms are effectively very reactive hydrogen radicals (H) which can break open the pi bond system and form new C-H bonds until the saturated cycloalkane is formed.

 

(5)   +  3H2  ===>  alkanes structure and naming (c) doc b  or  +  3H2  ===> alkanes structure and naming (c) doc b  

 This is a 'fictitious' theoretical equation

The enthalpy of hydrogenation for this is theoretically about -354 kJ mol-1.

If we assume (incorrectly) that benzene has a cyclotriene structure with alternating single and double bonds.

 

Discussion of the above enthalpy of hydrogenation values:

Apart from the first and simplest alkene, ethene, most enthalpies of hydrogenation are about -1206 kJ mol-1 (for 1 mole H2 per mole alkene) and in the case of dienes about double that for two moles of hydrogen per diene, which is what you might reasonably expect.

The special case of benzene - the aromatic ring structure of arenes - aromaticity

However, on the basis of these trends, the expected value for the complete hydrogenation of benzene and other aromatic compounds with a single benzene ring would be around 3 x -118 = ~-354 kJ mol-1, but not so!

In fact the energy released on hydrogenating benzene (208 kJ/mole) is even less than hydrogenating a diene!

So, something must be different about benzene but it can be explained with the enthalpy level diagram shown below and an examination of possible molecular structures.

Also note the comparison of equations 8. and 10. from above.

 (c) doc b + 3H2 ==> alkanes structure and naming (c) doc b (actual) and theoretical for a cyclotriene + 3H2 ==> alkanes structure and naming (c) doc b ('fictitious')

enthalpy level diagram for hydrogenation of benzene evidence of hexagon shape delocalised pi ring of electrons

The 'top' molecule in the diagram shows the theoretical structure of a triene with the same molecular formula of benzene (C6H6) and, if it existed in this form it would be called cyclohexa-1,3,5-triene or 1,3,5-cyclohexatriene.

This molecular structure assumes there are simple alternate single (C-C) and double (C=C) carbon-carbon bonds.

BUT, according to the actual thermochemical data calculated and derived from e.g. enthalpies of hydrogenation, benzene is already more stable by ~149 kJ mol-1, so, whatever its structure, it cannot have this 'triene' structure.

This lowering of the potential energy of the benzene molecules is referred to as the resonance stabilisation energy.

The concept of resonance structures is discussed in section 7.2.4

(c) doc b What happens in reality, is that the equivalent of 3 double bonds (C=C) and 3 single bonds (C-C) 'merge' to form a six equal bonds each involve an average of 3 shared electrons.

Two of the electrons for each bond are concentrated between the two carbon atoms OR between a carbon atom and hydrogen atom, both equivalent to a single bond known as a sigma (σ) bond.

The 4th electron per carbon atom is located in a ring orbital, of two sections, above and below the plane of the hexagonal ring of carbon atoms.

These are known as pi (π) orbitals and each one contains 3 pi (π) electrons which are delocalised around the ring.

This is indicated by the ring in the centre of the ring either in skeletal formula or structural formula and is a symmetrical planar hexagonal molecule.

Whenever charge is delocalised or 'spread out' the potential energy of the system is lowered and in the case of benzene about 149 kJ per mole compared to the triene structure of alternate single C-C and double C=C bonds.

 


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7.2.3 X-ray crystallography - electron density maps, shape and bond lengths

as evidence for the 'real' structure of a benzene ring

Apart from the thermochemical evidence argued above, X-ray crystallography has shown:

The actual bond length measurements show that there are six carbon - carbon bonds all of equal length and intermediate between single and double bonds.

The electron density map shows a symmetrical distribution of the electron clouds that fitted a symmetrical hexagonal planar ring of six carbon atoms.

BUT, the electron density map also showed significant electron density in a ring above and below the plane of the ring of 6 carbon atoms.

X-ray crystallography also showed all the bond angles are 120o, that is C-C-C  or  C-C-H due to 3 groups of bonding electrons around each carbon atom of the benzene ring

It was also shown that benzene is a planar molecule giving the 'benzene or aromatic ring' a symmetrical hexagonal shape.

Summary of types of carbon - carbon bonds.

σ = single sigma bond; π = delocalised pi bond system

There term bond order refers to the 'electronsworth' of the bond i.e. the average number of shared electrons involved in that particular bond.

Bond description Bond order Bond length/nm Bond enthalpy/kJmol-1
σ single bond, C-C 1.0 0.154 348
σ plus π, 1.5 0.139 518
σ plus π double bond, C=C 2.0 0.134 612
σ plus π, triple bond, CC 3.0 0.120 837

Typical bond lengths: single bond C-C is 0.154 nm (bond order 1) e.g. in typical alkanes.

An aromatic ring bond is 0.139 nm in length (bond order 1.5) e.g. in arenes like benzene and methylbenzene. X-ray diffraction has shown this bond is shorter than a single C-C bond, but not as short as an alkene C=C bond.

A double bond C=C is 0.134 nm in length (bond order 2)  e.g. in typical alkenes.

Incidentally the triple bond in alkynes has a typical length of 0.120 nm in length (CC, bond order 3).

There is a clear trend of decreasing bond length with increasing in bond order and increasing bond strength shown by the increasing bond enthalpy.

How do not always think a high bond strength is always associated with low reactivity e.g. compare the reactivity of alkanes and alkenes.

diagram of the rings of pi orbitals of benzene aromatic compounds aromaticity above and below a hexagonal ring of carbon atoms


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7.2.4 Resonance structures and lack of 'triene' isomers in disubstituted benzene compounds - more evidence for benzene's structure

Resonance structures and aromaticity

alternating resonance structures for benzene real aromatic structure is a resonance hybrid

Another approach to understanding the structure and chemical behaviour of benzene is to consider the possible, but alternating resonance structures (shown in the top half of the diagram above).

You can consider the two resonance structure as the only two possible extremes of a 'triene' structure.

So, for benzene, the real aromatic structure is a resonance hybrid of these two resonance structures.

You can think of the real structure of benzene as a sort of 'blurred' fusion of the two resonance structures.

 

Other evidence against the Kekule structure - the lack of isomers

In this case consider the Kekule structures of a 1,2-disubstituded benzene compound

fictitious isomers of 1,2-dichlorobenzen real aromatic structure skeletal structural formula of 1,2-dichlorobenzene 

If benzene was a triene (previous diagram) with alternate C-C and C=C bonds, then structures 1 and 2 would be genuine positional structural isomers of 1,2-dichlorobenzene.

 Structure 1 has the two chlorine atoms across a C=C double bond.

 Structure 2 has the two chlorine atoms across a C-C single bond.

They are clearly different structures, but no evidence of there existence has ever been found.

Structure 3 is however, the proven structure of 1,2-dichlorobenzene and can only exist as a one unique structure.

However, don't confuse this argument with the three genuine isomeric positional isomers of dichlorobenzene!

1,2 or 1,3 or 1,4-dichlorobenzene, C6H4Cl2  (c) doc b  (c) doc b  and (c) doc b


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7.2.5 Influence of the stability of the benzene ring on the chemistry of aromatic compounds


In chemistry, aromaticity is a property of cyclic, planar structures with pi bonds in resonance that gives increased stability compared to other geometric or connective arrangements with the same set of atoms, bonding with the same valencies.

Aromatic rings e.g. the benzene ring are very stable and do not readily change in a completed reaction.

The hexagonal aromatic ring of carbon atoms e.g. in arenes like benzene and methyl benzene tends to undergo electrophilic substitution reactions, rather than electrophilic addition reactions like alkenes.

It is the very great stability of the benzene ring in aromatic compounds means that, unlike alkenes, they are reluctant to undergo addition reactions, and this is best understood by considering the basics of electrophilic substitution reactions undergone by aromatic compounds..

Addition of a pair of atoms to a benzene ring means the stable aromatic ring is destroyed - this can be done, but not easily e.g. uv light and chlorine or hydrogen, catalyst and a high temperature.

Therefore aromatic compounds tend to undergo electrophilic substitution reactions involving the generation of a powerful electrophile (electron pair acceptor) which subsequently attacks the electron rich π (pi) electron system of the double bond.

In other words, arenes like benzene and methylbenzene tend to undergo substitution, rather than addition, because substitution in the ring allows the very stable benzene ring to remain intact.

diagram of the rings of electrophilic attack on the pi orbitals of benzene aromatic compounds electrophile electron pair acceptor

With the model portrayed above we can now consider the attack of an electrophile on the pi bond electrons.

general diagram for mechanism of electrophilic substitution in benzene ring or methylbenzene electrophile attacks pi electron cloud formation of carbocation expulsion of proton

The general mechanism for electrophilic substitution is shown above, though the source and/or generation of the electrophile is not shown, so it is often, but not always, a 3-step mechanism.

The electrophile E+ is an electron pair acceptor.

The benzene ring is capable of donating a pair of electrons from the pi orbitals of the aromatic ring to the incoming electrophile.

The electrophile E+ attacks the pi orbitals (the electron rich clouds - high electron density) above and below the plane of the benzene ring.

A C-E bond is formed and yielding a carbocation - but notice that the delocalised system only extends across five of the six carbon atoms of the benzene ring - you must draw this accurately in exams.

It is effectively an electrophilic addition, just like with alkenes, but here the similarity ends.

Instead of a further addition of a negative ion, as with electrophilic addition to alkenes, the mechanistic pathway follows a course that reforms the very stable delocalised electron system of the benzene ring, which is at a lower energy than a saturated system - think of the hydrogenation data and argument.

Since, in the carbocation, the stable delocalised system of the benzene ring is broken, a proton (H+) is expelled and the associated C-H bond pair of electrons rejoin the delocalised system, so, the complete benzene ring of pi orbitals is re-formed (aromaticity restored), yielding the stable substituted aromatic molecule.

e.g. a substituted benzene molecule C6H5E  or  a substituted methylbenzene molecule EC6H4CH3.

In the above diagram for methylbenzene I've just assumed that the substituent is on carbon atom 2 in aromatic nomenclature, but other positions are available!

Overall the mechanism  =  an electrophilic addition  +  an elimination (expulsion)  ===>  substitution

Technically, the final step of this electrophilic substitution mechanism involves the expelled proton adding to a base (not shown in the above diagram)

The base is an electron donor, so one of two things can happen.

H+  +  :B  ===>  HB- (anion formed)  OR  H+  +  :B-  ==> HB (neutral molecule formed).

diagram general reaction progress profile diagram for electrophilic substitution mechanism for benzene and aromatic compounds advanced organic chemistry

The diagram above shows a general reaction progress profile for the electrophilic substitution mechanism for aromatic compounds e.g. arenes like benzene and methyl benzene.

I've omitted the source and formation of the electrophile and just used benzene for simplicity.

The dip is decreases in potential energy for the formation of the carbocation.

Ea1 = the much larger initial activation energy for the attacking electrophile to form a C-E bond from benzene's pi electrons.

Ea2 = the much smaller activation energy for the expulsion of a proton (that will combine with a base) and the re-formation of the stable aromatic ring to complete the substitution reaction.


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Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of AROMATIC CHEMISTRY NOTES

 All Advanced Organic Chemistry Notes

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