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Advanced Organic Chemistry: Explaining the orientation of benzene ring substitution

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 7.14 Electrophilic substitution and explaining the orientation of products on putting a 2nd substituent into a previously monosubstituted benzene derivative

(perhaps aspects of 7.14 go beyond what is normally required in pre-university courses?)

Sub-index for this page

(please read the notes below this sub-index before studying 7.14.1)

7.14.1 Substitution in a previously monosubstituted benzene derivative

7.14.2 The effect of an activating group on substitution orientation in a monosubstituted benzene derivative - 2, 4 and 6 position directing groups (ortho and para substitution positions)

7.14.3 The effect of a moderately deactivating group on substitution orientation in a monosubstituted benzene derivative - 2, 4 and 6 position directing groups (ortho and para substitution positions)

7.14.4 The effect of a strongly deactivating group on substitution orientation of a monosubstituted benzene derivative - 3 and 5 position directing groups (meta substitution positions)

An important note on old nomenclature and terms used:

With respect to an initial single substituent in a benzene ring,

By convention, this is given position 1 (for the moment ignoring the conventions on naming).

Ortho referred to positions 2 and 6 meta referred to positions 3 and 5,  and para referred to position 4.

This convention is still widely used in textbooks, the internet and research papers.

Also the 'terms':

'Activate/activated' means the substituent group of the monosubstituted aromatic compound, makes it more susceptible to electrophilic attack i.e. an increased reaction rate with an electrophile compared to benzene itself - the electron density of the benzene ring is increased compared to benzene itself.

'Deactivate/deactivated' means the substituent group of the monosubstituted aromatic compound, makes it less susceptible to electrophilic attack i.e. a decreased reaction rate with an electrophile compared to benzene itself - the electron density of the benzene ring is decreased compared to benzene itself.

Here an inductive effect (+/- I) refers to the increase or decrease of the electron density of the benzene ring due to the influence of the substituent group (G) already present in the benzene ring C6H5-G.


Sub-sub-index of reactions covered to give a disubstituted benzene derivative

7.14.2 Activating substituent groups (2 and 4 substitution)

nitration of methylbenzene

sulfonation of methylbenzene

benzene ring chlorination of methylbenzene

benzene ring bromination of methylbenzene

nitration of phenol

benzene ring bromination/chlorination of phenol

Electrophilic substitution of the benzene ring of an aromatic ether

7.14.3 Moderately deactivating groups (2 and 4 substitution)

nitration of chlorobenzene

nitration of bromobenzene

ring chlorination of bromobenzene

7.14.4 More strongly deactivating groups (3 substitution)

nitration of nitrobenzene

nitration of benzoic acid


(Need to further x-ref with other pages which have the mechanism on)


INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

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TOP OF PAGE and sub-indexes


7.14.1 Substitution in a previously monosubstituted benzene derivative

There are five possible substitution positions in a monosubstituted benzene derivative.

So, what happens when this benzene derivative undergoes a second substitution reaction i.e. where does the second substituent go in the benzene ring? and why does the substituent end up in that position on the benzene ring?

The diagram below illustrates the situation, where the initial substituent group is shown as G (and for the rest of this page).

pi orbitals of benzene ring numbered sites for elecrophilic attack of a monosubstituted benzene derivative

Obviously there are five possible positions in the benzene ring for an electrophile to attack - denoted by the carbon ring numbers 2 to 6, for the possible 2nd substituent position (the original substituent is by convention, position 1.)

If we assume for a 2nd substituent into the benzene ring from an electrophilic substitution reaction:

(i) the electron density is the same for positions 2 to 6, i.e. assuming group G has no effect on the π orbitals,

(ii) completely random attack collisions of the electrophile i.e. equal probability of hitting any of the π electron cloud adjacent to one of carbon atoms 2 to 6,

(iii) positions 2 and 6 are equivalent and positions 3 and 5 are equivalent too (molecular symmetry, and the only assumption which is actually true!),

(iv) there is no steric hindrance of the nucleophilic attack by the pre-existing substituent group G e.g. most likely to affect benzene ring positions 2 and 6 closest to the substituent G group.

On this hypothetical basis you would expect an electrophilic substitution reaction of a monosubstituted benzene derivative to yield the following:

40% substitution at ring carbon atom 2 (= 6),

40% substitution at ring carbon atom 3 (= 5), and,

20% substitution at ring carbon atom 4.

% percent theoretical yields of disubstituted products from electrophilic attack on a monosubstituted benzene derivative ring positions 2 3 4 5 6

BUT, this is not what you find in the products from aromatic electrophilic substitution reactions.

You can see this from the data table below of TYPICAL YIELDS from further substitution of monosubstituted benzene derivatives.

KEY to three important abbreviations with reference to electrophilic substitution in monosubstituted benzene derivatives:  

(A24) In these molecules the benzene ring is activated by substituent group G compared to benzene and tends to direct the 2nd substituent to the 2 or 4 position in the benzene ring. See section 7.14.2

(D24) In these molecules the benzene ring is moderately deactivated by substituent group G compared to benzene, but still tends to direct the 2nd substituent to the 2 or 4 position in the ring. See section 7.14.3

(D3) In these molecules the benzene ring is more strongly deactivated by substituent group G compared to benzene, and tends to direct the 2nd substituent to the 3 position in the ring. See section 7.14.4

Substituent group

G in C6H5-G

Electrophilic substitution reaction

(details of some reactions given in other sections)

% yield position 2

(ortho-product)

% yield position 3

(meta-product

% yield position 4

(para-product

If G has no effect All 5 substitution position rates equal 40 40 20
–O–CH3  (A24) Nitration of an ether

35

1 64
–O–CH3  (A24) Friedel-Crafts acylation of an ether 7 3 90
–NO2  (D3) Nitration of nitrobenzene 7 90 3
–CH3  (A24) Nitration of methylbenzene 58 2 40
–CH3  (A24) Sulfonation of methylbenzene 32 6 62
–CH3  (A24) Friedel-Crafts acylation of methylbenzene 11 4 85
–Br  (D24) Nitration of bromobenzene 39 2 59
–Br  (D24) Ring chlorination of bromobenzene 40 6 54
-COOH  (D3) Nitration of benzoic acid 22 78 2
-CN  (D3) Nitration of benzonitrile 17 81 2
-CO-CH3  (D3) Nitration of 1-phenylethanone 26 72 2
-COOCH2CH3 Nitration of ethyl benzoate 28 66 6
-CHO  (D3) Nitration of benzaldehyde 19 72 9
-F  (D24) Nitration of fluorobenzene 13 1 86
-Cl  (D24) Nitration of chlorobenzene 35 1 64
-I  (D24) Nitration of iodobenzene 45 1 54
-OH  (A24) Nitration of phenol 63 3 34
-NHCOCH3  (A24) Nitration of N-phenylethanamide 19 2 79
If G has no effect All 5 substitution position rates equal 40 40 20

The reason being that, the 1st substituent group G has a profound effect on the π orbitals (in particular) i.e. the electron density and susceptibility to electrophilic attack of the other five carbon atoms of the benzene ring e.g.

If the carbon atom that is bonded by a sigma bond to the benzene ring also has a pi bond (e.g. >C-C=O), the electron density of the ring is reduced and substitution is more difficult, but less so in the 3 and 5 positions, where electrophilic substitution is more likely to take place.

If the atom that is bonded to the benzene ring does not have any pi bonding, but has at least one non-bonding pair of electrons, the electron density may be either increased (methylbenzene) or decreased (chlorobenzene)  BUT the electron density is relatively greater at the 2, 4 and 6 positions where electrophilic substitution is more likely to take place,

This in turns leads to the orientation of products (positions of 2nd substitution) that you might not expect.

The influence of the substituent group G on product orientation can be due to:

(a) the charge distribution can depends on the polar effect of G, +/- inductive effect.

Strongly electronegative groups reduce the reactivity (deactivate) the benzene ring.

However their effect is varied, the most electronegative groups often strongly deactivate the benzene ring, and orientate substitution to the 3 position.

But, less strongly electronegative groups reduce reactivity less and direct substituents to the 2 and 4 positions.

Also note that non-polar alkyl groups give a small +I effect to increase the electron density and hence the reactivity of the benzene ring.

(b) Sometimes a lone pair of electrons of the atom of G, or a pi bond system in G, which is directly attached to the benzene ring, gives rise to a resonance effect that can decrease or increases the electron densities in the 2, 4 and 6 positions.

These two factors can work in opposition or complement each other, and detailed explanatory arguments are beyond the scope of this page, neither have I gone into too detail on hyperconjugation and mesomeric effects.

However, I have used the concept of 'inductive' effect (+/- I), delocalisation of electrons, and drawn some resonance structures for selected molecules to:

(i)  help understand the relative reactivity of monosubstituted benzene derivatives compared to benzene itself and,

(ii) why certain positions on the benzene ring are most favoured for substitution, which usually means carbon atoms 2, 4, 6  or  3,5 sets of positions.

 

Examples of substituent group G and their orientation effect on the substitution products - three types of effect.

Orientation and reactivity effects of the initial ring substituent group G (R = alkyl)

Activating substituent G

directing to positions 2, 4 and 6

(ortho and para-orientation)

Strongly deactivating substituent G

directing to positions 3 and 5

(meta-orientation)

Moderately deactivating substituents

directing to positions 2, 4 and 6

(ortho and para-orientation

–O (phenoxide)
–OH
(phenol)
–OR
(ether)
–OC6H5
(ether)
–OCOCH
3
(ester)
–NH2 (amine)
–NR
2
–NHCOCH
3
–R
(alkylbenzene)
–C
6H5
–NO2 (nitrobenzene)
–SO
3H
(benzenesulfonic acid)
–SO
2R
–CO2H (benzoic acid)
–CO2R
(benzoate ester)
–CONH2
(benzamide)
–CHO
(benzaldehyde)
–COR
(ketone)
–CN
(benzonitrile)
–F (fluorobenzene)
–Cl
(chlorobenzene)
–Br
(bromobenzene)
–I
(iodobenzene)
–CH2Cl
((chloromethyl)benzene)
For details of some examples see section 7.14.2 For details of some examples see section 7.14.4 For details of some examples see section 7.14.3

Data adapted from https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/chapt15.htm and other 'thick' textbook sources!

 

The following three sections now look at the three types of substituent groups and their effect, with examples explained in more detail, of the effect of the 'G' group.


TOP OF PAGE and sub-indexes


7.14.2 The effect of an activating group on substitution orientation in a monosubstituted benzene derivative - 2, 4 and 6 position directing groups (ortho and para substitution positions)

Certain substituent groups in a benzene ring can increase the electron density of the benzene ring and make the aromatic compound more reactive towards electrophiles.

The effect is 'partial', and seems to enhance the reactivity at the 2 and 4 carbon atom substitution positions more than the 3 substitution position for a 2nd hydrogen substituted group put into a benzene ring.

Groups that increase reactivity are e.g. methyl -CH3, hydroxy (phenol) -OH, amino/amine -NH2, acylated amine -NHCOCH3, which greatly favour substitution at the 2 and 4 positions (often >90% combined and <10% for the 3 position).

This group of substituents all, by some means, have a small, but significant, electron donating (+I inductive effect) on the ring of pi electrons.

This is shown in the diagram below where I've indicated the 1st substituent group as G and its effect on the pi electron clouds - the subsequent effect of the susceptibility of the other 5 ring CH groups to electrophilic attack i.e. their relative reactivity - the activating or deactivating effect of G.

diagram of effect of activating group on the electron density of benzene ring pi orbitals most electrophilic attack at positions 2 4 6 ortho para substitution

The diagrams above and below show the increase in electron density of C6H5-G compared to C6H6 and below a more specific diagram for methylbenzene showing the increased electron density at positions 2, 4 and 6 - the sites more susceptible to electrophilic attack.

diagram of electron clouds of delocalised pi orbitals of benzene increased electron density of ring in methylbenzene

(a) The nitration of methylbenzene

(c) doc b  +  HNO3  ===> (c) doc b      +  H2O

Typical results for nitrating methylbenzene with conc. nitric and sulfuric acids (electrophile NO2+).

The theoretical percentage yields are quoted as if the methyl group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

The actual typical and theoretical yields of products from nitrating methylbenzene
Name of the isomer methyl-2-nitrobenzene methyl-3-nitrobenzene methyl-4-nitrobenzene
Actual yield of each isomer 58% 4% 38%
Actual yield of 2- and 4- isomers 96%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

The methyl group has a small, but significant +I inductive effect that increases the electron density of the benzene ring, particularly at the 2, 4 and 6 positions.

So, methylbenzene is significantly more reactive than benzene and when nitrated, 96% of the products are either methyl-2-nitrobenzene or methyl-4-nitrobenzene.

If the G group is -NO2, you get deactivation.

The diagram below shows the 2, 4 and 6 activated sites on the methylbenzene molecule.

electrophilic substitution activating effect of methyl group CH3 on the reactivity of methylbenzene increased electron density at 2 4 6 positions ortho para substitution

 

The nitration reactivity order is: methylbenzene  >  benzene  >  nitrobenzene

See also Electrophilic substitution - nitration of benzene and methylbenzene, properties and uses of nitro-aromatics

 

(b) The sulfonation of methylbenzene

Benzene and methylbenzene are insoluble in sulfuric acid, but the resulting sulfonic acids are soluble.

Therefore the sulfonation reaction is complete when the hydrocarbon layer disappears.

Using fuming sulfuric acid (extra SO3 electrophile available), at room temperature, benzene reacts in 20-30 minutes, but methylbenzene only takes 1-2 minutes.

(c) doc b  +  H2SO4  == ==> (c) doc b  (c) doc b (c) doc b + H2O

or

(c) doc b  +  SO3  == ==> (c) doc b  (c) doc b (c) doc b 

The yield with fuming sulfuric acid at 35oC are shown in the table below.

The electrophile is actually sulfur trioxide (sulfur(VI) oxide, SO3).

The theoretical percentage yields are quoted as if the methyl group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors - equal chance of electrophilic attack by SO3.

The actual typical and theoretical yields of products from sulfonating methylbenzene
Name of the isomer methyl-2-benzenesulfonic acid methyl-3-benzenesulfonic acid methyl-4-benzenesulfonic acid
Actual yield of each isomer 32% 6% 62%
Actual yield of 2- and 4- isomers 94%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

What you see is the vast majority of the product involves substitution at the 2 and 4 positions of the benzene ring in methylbenzene. There is very little substitution at the 3 position.

This is due, as argued via the diagram above, because of the increased electron density at the 2, 4 and 6 benzene ring positions from the plus inductive effect of the methyl group of methylbenzene.

 

(c) Benzene ring chlorination of methylbenzene

(c) doc b  +  Cl2  == FeCl3 ==>(c) doc b  (c) doc b  (c) doc b  +  HCl

The reagents chlorine and iron(III)/aluminum chloride substitute a hydrogen atom with a chlorine atom in the benzene ring (NOT in the methyl group).

The actual typical and theoretical yields of products from ring chlorination of methylbenzene
Name of the isomer chloro-2-methylbenzene chloro-3-methylbenzene chloro-4-methylbenzene
Actual yield of each isomer 58% <1% 42%
Actual yield of 2- and 4- isomers >99%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

What you see is the vast majority of the product involves substitution at the 2 and 4 positions of the benzene ring in methylbenzene. There is only a trace of substitution on ring carbon 3.

This is due, as argued via the diagram above, because of the increased electron density at the 2, 4 and 6 ring positions from the plus inductive effect of the methyl group of methylbenzene.

The theoretical percentage yields are quoted as if the methyl group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors - equal chance of attack from the Cl+ electrophile.

See also Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides

(d) Benzene ring bromination of methylbenzene

Using bromine and iron(III) bromide you can substitute a hydrogen atom for a bromine atom in the benzene ring (NOT in the methyl group).

diagram equation products % yield of isomers for reaction between methylbenzene and bromine/iron(III) bromide bromo-2-methylbenzene bromo-3-methylbenzene bromo-4-methylbenzene

The theoretical percentage yields are quoted as if the methyl group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

The actual typical and theoretical yields of products from ring bromination of methylbenzene
Name of the isomer bromo-2-methylbenzene bromo-3-methylbenzene bromo-4-methylbenzene
Actual yield of each isomer 33% <1% 67%
Actual yield of 2- and 4- isomers >99%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

What you see is the vast majority of the product involves substitution at the 2 and 4 positions of the benzene ring in methylbenzene and only trace of substitution at the 3 position.

This is due, as argued via the diagram above, because of the increased electron density at the 2, 4 and 6 ring positions from the plus inductive effect of the methyl group of methylbenzene.

See also Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides

 

(e) The nitration of phenol

The presence of the hydroxy group (OH) directly attached to the benzene ring( making the molecule a 'phenol', increase the electron density of the pi orbitals of the ring and greatly increases the reactivity of phenol towards electrophiles.

electrophilic substitution activating effect of hydroxy group OH on the reactivity of phenol increased electron density at 2 4 6 positions ortho para substitution

Phenol is so reactive, that even at low temperature (< 20oC), with dilute nitric acid, electrophilic substitution readily takes place at the 2, 4 and 6 positions on the benzene rings.

The non-bonding electrons of the oxygen atom interact with the pi orbitals of the benzene ring - which outweighs any minus inductive effect from the more electronegative oxygen atom (electronegativities: C = 2.5, oxygen 3.5).

 

diagram equation products % yield of isomers for reaction between phenol and dilute nitric acid 2-nitrophenol 3-nitrophenol 4-nitrophenol

The theoretical percentage yields are quoted as if the hydroxy group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

The actual typical and theoretical yields of products from low temperature nitration of phenol with dilute nitric acid
Name of the isomer 2-nitrophenol 3-nitrophenol 4-nitrophenol
Actual yield of each isomer 53% <1% 47%
Actual yield of 2- and 4- isomers >99%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

What you see is the vast majority of the product involves substitution at the 2 and 4 positions of the benzene ring in phenol.

This is due, as argued via the diagram above, because of the increased electron density, specifically. at the 2, 4 and 6 ring positions from the inductive effect of the hydroxy group of phenol.

the five resonance hybrid structures of phenol diagram for advanced A level organic chemistry

Another approach to explaining the extra reactivity at the 2, 4 and 6 positions of the benzene ring is to consider the three resonance structures of phenol, shown on the right of the diagram above.

You can draw structures that place a negative charge on carbon atoms 2, 4 and 6 of the ring, but not on ring carbons 3 and 5, where little substitution occurs with phenol.

Although the contribution of these three resonance structure is less than the first two, the effect is sufficient to promote much more substitution on the 2, 4 and 6 positions compared to 3 and 5.

The more negative the carbon atom, the more likely it is to allow the attacking electrophile to substitute the hydrogen atom.

 

structural formula of 2,4,6-trichlorophenol 2,4,6-tribromophenol 2,4,6-trinitrophenol molecular structure advanced organic chemistry

With conc. nitric/sulfuric acids phenol gives 2,4,6-trinitrophenol (diagram above) - a very explosive product, but note the substitutions take place at the carbon atoms of highest electron density in the ring.

See also Electrophilic substitution - nitration of benzene and methylbenzene, properties and uses of nitro-aromatics

 

(f) Benzene ring bromination/chlorination of phenol

With excess chlorine water or bromine water room temperature, phenol gives an immediate white precipitate of 2,4,6-trichlorophenol or 2,4,6-tribromophenol, with almost zero substitution at the 3 and 5 positions of the benzene ring (structures in diagram below).

structural formula of 2,4,6-trichlorophenol 2,4,6-tribromophenol 2,4,6-trinitrophenol molecular structure advanced organic chemistry

This further illustrates the much greater reactivity of phenol towards electrophiles compared to benzene itself/

See also Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides

 

(g) Electrophilic substitution of the benzene ring of an aromatic ether

The ether group, like the hydroxy group, increases the electron density of the benzene ring, particularly at the 2, 4 and 6 positions.

electrophilic substitution activating effect of ether group on the reactivity of aromatic ethers increased electron density at 2 4 6 positions in methoxybenzene ethoxybenzene phenoxybenzene ortho para substitution

You can also, like with phenol, construct three resonance structures with a negative charge on benzene ring carbon atoms 2, 4 and 6.

the five resonance hybrid structures of aromatic ether methoxybenzene ethoxybenzene diagram for advanced A level organic chemistry

Resonance structures for aromatic ethers e.g.

for methoxybenzene (R = CH3), ethoxybenzene (R = CH2CH3)  and phenoxybenzene (R = C6H5)

So you would expect the 2nd substituent to go onto benzene ring carbons 2 or 4, and this is born out by the data below and comparable with phenol in terms of very little 3- substitution.

Substituent group

G in C6H5-G

Electrophilic substitution reaction % yield position 2

(ortho-product)

% yield position 3

(meta-product

% yield position 4

(para-product

–O–CH3 Nitration of an ether

35

1 64
–O–CH3 Friedel-Crafts acylation of an ether 7 3 90
-OH Nitration of phenol 61-63 1-3 32-34
If G has no effect Assuming 5 rate factors are all the same 40 40 20

The theoretical percentage yields are quoted as if the RO- group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors in terms of electrophilic attack from the electrophiles NO2+ or RC=O+.

The major products from methoxybenzene are:

e.g. from nitration: 2-methoxy-1-nitrobenzene  and  2-methoxy-1-nitrobenzene


7.14.3 The effect of a moderately deactivating group on substitution orientation in a monosubstituted benzene derivative - 2, 4 and 6 position directing groups (ortho and para substitution positions)

Certain groups, already present in a monosubstituted benzene derivative, can moderately decrease the electron density of the benzene ring and make the aromatic compound less reactive towards electrophiles.

These groups usually have just one electronegative atom e.g. a halogen atom that interacts with the pi orbitals of the benzene ring.

The electronegative substituent group results in a lowering of electron density, deactivating the benzene ring towards electrophiles, but deactivates more at the 3 and 5 positions so the 2, 4 and 6 positions are favoured by electrophilic substitution

diagram of effect of moderately deactivating group on the electron density of benzene ring pi orbitals most electrophilic attack at positions 2 4 6 ortho para substitution

(a) The nitration of chlorobenzene

(c) doc b  +  HNO3  ===> (c) doc b   (c) doc b   (c) doc b  +  H2O

The theoretical percentage yields are quoted as if the chloro group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

The actual typical and theoretical yields of products from nitrating chlorobenzene
Name of the isomer chloro-2-nitrobenzene chloro-3-nitrobenzene chloro-4-nitrobenzene
Actual yield of each isomer 30% <1% 70%
Actual yield of 2- and 4- isomers >99%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

Chlorobenzene is less reactive than benzene, but, as you can see from the yield data, the deactivation is almost complete for the 3 position of the benzene ring (see the diagram below).

electrophilic substitution deactivating effect of chlorine atom chloro group Cl on the reactivity of chlorobenzene decreased electron density 2 4 6 positions ortho para substitution

Although the overall electron density is reduced in the benzene ring of chlorobenzene, it is still the highest at ring carbons 2, 4 and 6, so those positions are more susceptible to electrophilic attack.

the five resonance hybrid structures of chlorobenzene diagram for advanced A level organic chemistry 

The 2nd diagram above, shows on the right, three resonance hybrids of chlorobenzene with a negative charge on carbon atoms 2, 4 and 6 on the benzene ring, favouring electrophilic attack by a positive electrophile, and less so on the 'neutral' carbon atoms 3 and 5.

See also Electrophilic substitution - nitration of benzene and methylbenzene, properties and uses of nitro-aromatics

 

(b) The nitration of bromobenzene

This is done by treating bromobenzene with a mixture of conc. nitric and sulfuric acids.

diagram equation products % yield of isomers for reaction between bromobenzene and concentrated nitric/sulfuric acid bromo-2-nitrobenzene bromo-3-nitrobenzene bromo-4-nitrobenzene

There is very little substitution at the 3 position of the benzene ring.

The theoretical percentage yields are quoted as if the bromo group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

The actual typical and theoretical yields of products from nitrating bromobenzene
Name of the isomer bromo-2-nitrobenzene bromo-3-nitrobenzene bromo-4-nitrobenzene
Actual yield of each isomer 39% 2% 59%
Actual yield of 2- and 4- isomers 98%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

The diagram below indicates the change in electron density around the benzene ring of bromobenzene caused by the bromine atom.

Although the overall electron density is reduced in the benzene ring of bromobenzene, it is still the highest at ring carbons 2, 4 and 6, so those positions are more susceptible to electrophilic attack.

electrophilic substitution deactivating effect of bromine atom bromo group Br on the reactivity of bromobenzene decreased electron density 2 4 6 positions ortho para substitution

The deactivation of the ring by the electronegative bromine atom, greatly affects the 3 position of the benzene ring, so the major products are bromo-2-chlorobenzene and bromo-4-chlorobenzene and the resonance hybrid structures contribute to the effect too.

the five resonance hybrid structures of bromobenzene diagram for advanced A level organic chemistry

The 2nd diagram above, shows on the right, three resonance hybrids of bromobenzene with a negative charge on carbon atoms 2, 4 and 6 on the benzene ring, favouring electrophilic attack by a positive electrophile, and less so on the 'neutral' carbon atoms 3 and 5.

See also Electrophilic substitution - nitration of benzene and methylbenzene, properties and uses of nitro-aromatics

 

(c) The ring chlorination of bromobenzene

This can be accomplished with chlorine and the halogen carrier catalyst aluminium chloride.

diagram equation products % yield of isomers for reaction between bromobenzene and chlorine/aluminium chloride bromo-2-chlorobenzene bromo-3-chlorobenzene bromo-4-chlorobenzene

Although the overall electron density is reduced in the benzene ring of bromobenzene, it is still the highest at ring carbons 2, 4 and 6, so those positions are more susceptible to electrophilic attack from Cl+.

The actual typical and theoretical yields of products from chlorinating bromobenzene
Name of the isomer bromo-2-chlorobenzene bromo-3-chlorobenzene bromo-4-chlorobenzene
Actual yield of each isomer 43% 2% 55%
Actual yield of 2- and 4- isomers 98%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

The theoretical percentage yields are quoted as if the bromo group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

Again, the deactivation of the ring by the electronegative bromine atom, greatly affects the 3 position of the benzene ring, so the major products are bromo-2-chlorobenzene and bromo-4-chlorobenzene and the resonance hybrid structures contribute to the effect too.

See diagrams above for the nitration of bromobenzene to illustrate the orientation effect

 

See also Electrophilic substitution - ring halogenation of benzene/methylbenzene, properties & uses of aryl halides

 


TOP OF PAGE and sub-indexes


7.14.4 The effect of a deactivating group on substitution orientation of a monosubstituted benzene derivative - 3 and 5 position directing groups (meta substitution positions)

Certain groups, already present in a monosubstituted benzene derivative, can strongly decrease the electron density of the benzene ring and make the aromatic compound less reactive towards electrophiles.

These groups have at least one strongly electronegative atom and a double or triple bond that interacts with the pi orbitals of the benzene ring.

This results in a lowering of electron density, deactivating, but deactivates more at the 2, 4 and 6 positions so the 3 and 5 positions are more favoured by electrophilic attack and consequent substitution.

Examples of groups that considerably decrease reactivity, compared to benzene, by some means or other, are nitro -NO2, carboxylic acid COOH, aldehyde -CHO, and sulfonic acid -SO2OH

These favour electrophilic substitution at the 3 position (often > 70%) and their effect does fit in with them all being strongly electronegative groupings giving a -I inductive effect and/or theoretically, resonance hybrids with a positive charge on ring carbon atoms 2, 4 and 6.

These points are illustrated in the diagram below.

diagram of effect of strongly electronegative deactivating group on the electron density of benzene ring pi orbitals most electrophilic attack at positions 3 5 meta substitution

(a) The nitration of nitrobenzene

diagram equation products % yield of isomers for reaction between nitrobenzene and concentrated nitric/sulfuric acid 1,2-dinitrobenzene 1,3-dinitrobenzene 1,4-dinitrobenzene

1,3-dinitrobenzene is the favoured substituent product when nitrobenzene is treated with a concentrated nitric acid and sulfuric acid mixture.

The theoretical percentage yields are quoted as if the nitro group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

The actual typical and theoretical yields of products from nitrating nitrobenzene
Name of the isomer 1,2-dinitrobenzene 1,3-dinitrobenzene 1,4-dinitrobenzene
Actual yield of each isomer 6% 93% 1%
Theoretical yield of each isomer 40% 40% 20%

As you can see, there is only a total of 7% substitution at ring carbons 2 and 4.

electrophilic substitution deactivating effect of nitro group NO2 on the reactivity of nitrobenzene decreased electron density 3 5 positions ortho para substitution

The nitro O-N-O group is shown as delocalised system and merges with the pi bond system of the benzene ring, but it has, overall, an electron withdrawing effect, deactivating the ring.

 

the five resonance hybrid structures of nitrobenzene diagram for advanced A level organic chemistry 

On the right are three resonance structures of nitrobenzene with a positive charge on ring carbon atoms 2, 4 and 6 making them less favourable to attack from an incoming positive electrophile like NO2+.

See also Electrophilic substitution - nitration of benzene and methylbenzene, properties and uses of nitro-aromatics

 

(b) The nitration of benzoic acid

diagram equation products % yield of isomers for reaction between benzoic acid and concentrated nitric/sulfuric acid 2-nitrobenzoic acid 3-nitrobenzoic acid 4-nitrobenzoic acid

The 3- substitution is highly favoured i.e. the major product is 3-nitrobenzoic acid.

The theoretical percentage yields are quoted as if the carboxylic acid group has no effect on the orientation of the product i.e. all 5 positions on the benzene ring have equal partial rate factors.

The actual typical and theoretical yields of products from nitrating benzoic acid
Name of the isomer 2-nitrobenzoic acid 3-nitrobenzoic acid 4-nitrobenzoic acid
Actual yield of each isomer 19% 80% 1%
Theoretical yield of each isomer 40% 40% 20%

The yield of 3-nitrobenzoic acid is double what you expect if the carboxylic acid group had no influence on the rate of nitration at the different ring carbon atoms.

electrophilic substitution deactivating effect of carboxyl group COOH on the reactivity of benzoic acid decreased electron density 3 5 positions ortho para substitution

The >C=O delocalised and merges with the pi orbitals of the benzene ring, but the net effect is a large minus inductive effect, deactivating the ring, particularly at the 2, 4 and 6 positions.

 

See also Electrophilic substitution - nitration of benzene and methylbenzene, properties and uses of nitro-aromatics

 


Future additional work

Images for future work for meta directed substitution - add them plus note

the five resonance hybrid structures of aromatic ketones 1-phenylethanone diphenylmethanone diagram for advanced A level organic chemistry

Resonance structures for aromatic ketones R = CH3, 1-phenylethanone , R = C6H5 diphenylmethanone

 

the five resonance hybrid structures of benzenesulfonic acid diagram for advanced A level organic chemistry

Resonance structures for benzenesulfonic acid

 

and implication of direction orientation for synthesis e.g. how to make 4-nitrobenzoic acid or 4-aminobenzoic acid


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