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

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 advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry evidence for the aromatic structure of benzene compounds enthalpies of hydrogenation X-ray crystallography lack of 1,2-disubstituted isomers

Part 7.2 Proof of benzene structure - the evidence discussed, what is aromaticity? and an 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 to 7.2.6 describe evidence of the ring structure of benzene and its presence in other aromatic compounds

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

7.2.6 Can spectroscopy tell us anything about benzene and the structure of aromatic compounds?

[SEARCH BOX]



TOP OF PAGE and sub-index


7.2.1 A comparison of cyclic hydrocarbon molecule structure

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 is altered 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.

naphthalene skeletal formula of naphthalene C10H8 molecular structure advanced level organic chemistry (c) doc brown , C10H8  consists of two fused aromatic rings

anthracene skeletal formula of anthracene C14H10 molecular structure advanced level organic chemistry (c) doc brown or  C14H10  consists of three fused rings.

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


TOP OF PAGE and sub-index


7.2.2 The evidence from a comparison of enthalpies of hydrogenation

7.2.2 to 7.2.6 describe evidence of the ring structure of benzene and its presence in other aromatic compounds

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 -120±6 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.

 


TOP OF PAGE and sub-index


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

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

7.2.2 to 7.2.6 describe evidence of the ring structure of benzene and its presence in other aromatic compounds

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.

(c) doc bThe 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.

skeletal formula hexachlorobenzene molecular structure structural formula C6Cl6 advanced level organic chemistry doc brownX-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 in a trigonal planar arrangement around each carbon atom of the benzene ring.

So, it has been shown conclusively, that benzene is a planar molecule giving the 'benzene or aromatic ring' a symmetrical hexagonal shape.

skeletal formula hexamethylbenzene molecular structure structural formula C6(CH3)6 advanced level organic chemistry doc brownIn 1920s the chemist Kathleen Lonsdale proved from X-ray diffraction that all the six internal bond angles of hexachlorobenzene C6Cl6 and hexamethylbenzene C6(CH3)6 were precisely 120o, since the molecule was derived from benzene, it seemed illogical not to assume that benzene had the same perfect hexagonal ring of carbon atoms.

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, in benzene 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 aliphatic alkenes.

Incidentally the triple bond in aliphatic 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.

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

The final electronic picture of benzene

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

The two pi orbital rings above and below the hexagonal plane of the carbon atoms is considered to be formed by the overlap of the six 2nd 2p orbitals of the carbon atoms (outer electrons 2s22p2) i.e. from the 4th outer valency electron of the carbon atom.

For each carbon atom, three outer electrons (s + s + p) contribute to three sigma bonds, two C--C and one C-H.

The fourth electron (origin 2p orbital) is delocalised in the circular pi orbital, so in total there are three electrons in each pi orbital, six all together and non associated with a particular carbon atom.


TOP OF PAGE and sub-index


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

7.2.2 to 7.2.6 describe evidence of the ring structure of benzene and its presence in other aromatic compounds

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 different 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.

These 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


TOP OF PAGE and sub-index


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

An introduction to the electrophilic substitution reactions of aromatic compounds

7.2.2 to 7.2.6 describe evidence of the ring structure of benzene and its presence in other aromatic compounds

The lack of aromatic ring reactivity compared to the C=C double bond in aliphatic alkenes

Consider a series of addition reaction with solutions of the electrophiles bromine (Br2, Brδ+Brδ-, weak electrophile on collision) and hydrogen bromide (strong acid, Hδ+Brδ-, and stronger electrophile).

All of the alkene electrophilic addition reactions readily occur at room temperature and no catalyst involved either.

alkenes structure and naming (c) doc b +  Br2 ===> (c) doc b  cyclohexene gives 1,2-dibromocyclohexane

alkenes structure and naming (c) doc b +  HBr ===> molecular structure skeletal formula bromocyclohexane molecular formula cyclohexene gives bromocyclohexane

alkene +  2Br2 ===> cyclohex-1,3-dene gives 1,2,3,4-tetrabromocyclohexane

(c) doc b or  (c) doc b  +  Br2  or  HBr ===> no reaction at all

+  Br2  or  HBr ===> expect an addition reaction

Alkenes readily undergo addition reactions with electrophilic reagents like bromine, chlorine and hydrogen halides. The more localised pi electron orbitals of alkenes are much more susceptible to electrophilic attack than benzene ring of aromatic compounds (diagram on the right).

Though only part of the electrophile adds in the first step.

See 2.3 Bonding in alkenes, comparing alkane/alkene reactivity, electrophilic addition with hydrogen halides (HBr, HCl)

However, with aromatic compounds like benzene or methylbenzene, no such addition reactions occur under the same conditions i.e. room temperature, no catalyst and no raised temperature.

If benzene has a cyclo triene structure (last equation above) you would expect the rapid addition of three moles of bromine per benzene molecule as cyclohexene adds one molecule of bromine and cyclohexa-1,3-diene adds two molecules of bromine - so there must be something very different about the reactivity of 'aromatic' benzene compounds compared to 'aliphatic' alkenes.

These observations suggests that benzene compounds have some particular stability built into their structure.

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.

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.

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

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

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

The initial electrophilic reagent attack looks the same as the mechanism for addition of an electrophile to an alkene, but ...

(i) ionic mechanism of electrophilic addition of hydrogen bromide to ethene advanced A level organic chemistry

(i) in the electrophilic addition to alkenes part of the electrophile adds on in the first step (see ethene plus hydrogen bromide mechanism 53 above) and then the 2nd part adds on to give the saturated product.

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

(ii) For benzene compounds, all or part of the electrophile adds on to the benzene ring to give a saturated carbon atom (diagram above), a highly unstable intermediate due to loss of pi electron ring.

This intermediate carbocation expels a proton to give the very stable benzene ring and the substitution product - the retention of aromatic character of the delocalised pi electron ring of benzene/benzene compound.

The above diagram shows substitution in the 2 position of the benzene ring of a monosubstituted benzene compound, but you can also get substitution in the 3 and 4 positions.

In the sections describing the electrophilic substitutions of arenes, I've fully described the formation of the electrophile via the catalyst and its regeneration.

Most electrophilic substitution of benzene and methylbenzene require a catalyst, which is also a reflection of the stability of the benzene ring.

So alkenes react with electrophiles by addition, usually without a catalyst.

Aromatic benzene compounds react with electrophiles by substitution and usually need a catalyst.

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 sigma covalent 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 - also note the loss of the extra stability of the original aromatic benzene ring.

At this point 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 aromatic pi electron system of the benzene ring, which is at a lower energy than a saturated system - think of the hydrogenation data 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 stable 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.


TOP OF PAGE and sub-index


7.2.6 Can spectroscopy tell us anything about benzene and the structure of aromatic compounds?

7.2.2 to 7.2.6 describe evidence of the ring structure of benzene and its presence in other aromatic compounds

 Any evidence for the actual aromatic molecule(c) doc b as opposed to the theoretical but fictitious triene

In terms of spectroscopic evidence, I don't think spectroscopy provides any substantial extra evidence compared to the evidence described in sections 7.2.2, to 7.2.5 (at least not at pre-university level).

Mass spectra

The mass spectrum of benzene doesn't give that much information about its structure.

There is a characteristic peak for the m/z ion of 77 (also the most abundant  base ion peak), but this occurs in the mass spectra of many aromatic benzene compounds, particularly mono-substituted benzene compounds with the C6H5- group (mass = 77).

The mass spectrum of benzene

The m/z 77 ion also appears as a small peak in the mass spectrum of cyclohexene and would probably appear in the theoretical mass spectrum of a symmetrical C6H6 cyclotriene structure, but perhaps not in some isomeric aliphatic di-ynes mentioned below!

The mass spectrum of cyclohexene

Infrared spectra

For benzene the principal absorption band for C=C stretching vibrations is ~1480 cm-1 and is significantly different from that observed for non-conjugated C=C bonds in aliphatic alkenes (cyclic r open chain)

e.g. for cyclohexene, the principal absorption band for C=C stretching vibrations peaks at 1640 cm-1 and is typical for aliphatic mono-alkenes.

The peak at ~1440 cm-1 is attributed to CH2 vibrations, not C=C stretching vibrations with cyclohexene.

The infrared spectrum of benzene

The infrared spectrum of cyclohexene

1H NMR spectra

All protons in benzene are equivalent to each other, so you get a single and unsplit 1H NMR resonance line, implying all the hydrogen atoms are in the same chemical environment.

(But, you might 'theoretically' observe a single 1H NMR line for a symmetrical C6H6 cyclotriene structure too!)

Isomeric C6H6 hexacyclodienes will give more than one 1H NMR resonance line e.g. hexa-1,3-diene alkene has three 1H chemical environments (1:1:2 or 2:2:4 ratio)  and hexa-1,4-diene alkenes structure and naming (c) doc b two 1H chemical environments (1:1 or 4: 4 ratio)

The H-1 NMR spectrum of benzene

The H-1 NMR spectrum of cyclohexene

Some aliphatic C6H6 isomeric structures which do exist (note an ...yne instead of an ...ane or ...ene!)

CH3-C≡C-C≡C-CH3 hexa-2,4-diyne has only one proton chemical environment

HC≡C-CH2-CH2-C≡CH hexa-1,5-diyne has two proton chemical environments

13C NMR spectra

All carbon atoms in benzene are equivalent to each other, so you get a single 13C NMR resonance line, implying all the hydrogen atoms are in the same chemical environment.

(But, you might 'theoretically' observe a single 13C NMR line for a symmetrical C6H6 cyclotriene structure too!)

Isomeric hexacyclodienes (C6H6) would give more than one 13C NMR resonance line.

 alkene three 13C chemical environments  and alkenes structure and naming (c) doc b two 13C chemical environments

The C-13 NMR spectrum of benzene

The C-13 NMR spectrum of cyclohexene

Some aliphatic C6H6 isomeric structures which do exist

CH3-C≡C-C≡C-CH3 hexa-2,4-diyne has three 13C NMR chemical environments

HC≡C-CH2-CH2-C≡CH hexa-1,5-diyne also has three 13C NMR chemical environments


TOP OF PAGE and sub-index


Doc Brown's Advanced Level Chemistry Revision Notes

[SEARCH BOX] ignore ads at top

INDEX of AROMATIC CHEMISTRY NOTES

 All Advanced Organic Chemistry Notes

TOP OF PAGE and sub-index

 

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 

*

TOP OF PAGE and sub-index

best Christmas gift deals latest video game release, best Christmas gift deals best bargains in new year January shop sales latest pop music releases, best New Year sales deals download free music, latest film releases, best Christmas gifts for teenagers latest high street fashion in clothes, fashionable trending in clothing, best Christmas gift deals best bargains in new year January shop salesgirls buy clothes, spend a lot of money on clothes, best New Year sales deals shoes, sweets and chocolates, magazines and make-up best Christmas gifts for teenagers  Boys buy food and drink, computer games best Christmas gift deals best bargains in new year January shop sales DVDs and CDs, girls and boys spend a lot of money on credit for mobile phones best New Year sales deals best bargains in new year January shop sales buses and trains to transport them going out best Christmas gifts for teenagers best bargains in new year January shop sales Girls spend a lot of money on clothes best Christmas gift deals  color colour preferences in clothes, cool sunglasses best New Year sales deals boys buy expensive things like best Christmas gifts for teenagers designer sports clothes and trainers, teenagers save for holidays, best New Year sales deals clothes, mobile phones, birthday presents and electronic goods, teenage accessories, Favourite style of jeans. best Christmas gifts for teenagers A typical girl’s shopping list includes mobile phone credit deals best Christmas gift deals fashionable quality cool airpods, air pods, fashionable clothes best New Year sales deals the most popular favourite sneakers best Christmas gifts for teenagers fancy shoes, sweets, chocolates, magazines, best Christmas gift deals best bargains in new year January shop sales lip moisturizer best bargains in new year January shop sale slots on make-up, well being, teenage decor decorating their room best New Year sales deals teenagers like LED string lantern lights, best Christmas 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 Christmas gifts for teenagers computer games, DVDs, CDs, what teenagers talk about best Christmas gift deals what teenagers worry about, what teenagers like to do for fun sports best New Year sales deals  what cool things do teenagers buy, resale websites like eBay Teenager