Chapter 16 Chemistry of Benzene
A.
Electrophillic aromatic substitution of benzene rings
1.
Monosubstitution - If compound does electrophilic substitution rather than addition –
it is most likely aromatic.
- some possibilities
Addition reaction
Substitution reaction
-
less reactive benzene ring requires a Lewis acid catalyst to promote reaction by making
the electrophile more electrophilic.
doubly allylic carbocation – three resonance structures
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Chapter 16 Chemistry of Benzene
Reaction of benzene is slower than alkenes because benzene is more stable.
Both reactions are endergonic to
make carbocation intermediate.
•
•
Benzene has a larger Ea.
Subtitituion of H not
addition at carbocation
get back aromatic stable structure.
If forms the addition
product lose aromatic
stability. Endergonic
reaction is then observed.
This overall is an
exergonic reaction.
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Chapter 16 Chemistry of Benzene
i)
Aromatic bromination: Mechanism.
16.2
Other aromatic substitutions:
These occur by a similar mechanism.
ii)
Halogens (Cl2, I2) - F2 is too reactive and give poor yields –
Anti-anxiety drug – Valium.
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Chapter 16 Chemistry of Benzene
First oxidize Iodine with Cu( II) salt or hydrogen peroxide to give more electrophilic I⊕
iii)
Aromatic Nitration:
The catalyst is a mixture of Conc. H2SO4 & conc. HNO3
to give NO2⊕
Sulfuric acid protonates the HNO3, subsequent loss of water gives the Nitronium ion
Nitronium ion is very electrophilic – deprotonation of the Hydrogen regenerates the catalyst
and forms the substituted product.
•
•
reduction of Nitrobenzene
use Fe, Sn or SnCl2
Aniline for dyes and
pharmaceuticals
More in chapter 24.
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Chapter 16 Chemistry of Benzene
iv)
Aromatic sulfonation – making benzenesulfonic acid.
Catalyst – Fuming sulphuric acid (mixture of H2SO4 and SO3)
Electrophile is HSO3⊕ or neutral SO3
Sulfonation product for
dyes & pharmaceuticals
This reaction is reversible.
Sulfonation product favoured by strong acid… Reverse favored by hot, dilute aqueous acid.
Aromatic sulfonation is a key step towards making Sulfanilamide – an antibiotic - now it is
used to treat meningitis and urinary tract infections.
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Chapter 16 Chemistry of Benzene
16.3
Alkylation and Acylation of Aromatic rings
v)
Friedel-Craft alkylation - Need alkyl chloride and Lewis acid catalyst AlCl3
Electrophile is an alkyl carbocation.
Cl
AlCl3
AlCl4
H
AlCl4
AlCl4
+
HCl
+
AlCl3
Limitations of Friedal-Craft reactions
1.
No reaction with aryl halide or vinylic halide – carbocations are too high energy.
Cl
Cl
2.
No reaction - making disubstituted rings by Friedal-Craft when strongly electron
withdrawing group is present.
Presence of Amino group, or C=O (carbonyl group)
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Chapter 16 Chemistry of Benzene
3.
Difficult to stop with a single substitution. – Poly alkylation is often observed.
- high yield of monosubtitution product is obtained by using large excess of benzene.
4.
Rearrangement of alkyl carbocation when primary alkyl halide is used.
1,2-H-shift to form a more stable carbocation
1-chlorobutane
1,2-methyl shift.
Neopentyl chloride
(1-chloro-2,2-dimethylpropane)
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Chapter 16 Chemistry of Benzene
vi)
Friedal-Craft acylation
No carbocation rearrangement - acyl carbocation is resonance stabilized.
Acyl cation is stabilized by donation of electrons from lone pairs on O to Carbocation's vacant
– p-orbital.
Only monosubstitution – no polysubstitution. - C=O – acylated benzene is less reactive.
Do questions 16.4, 16.5, 16.6
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Chapter 16 Chemistry of Benzene
2.
Disubstitution
16.4
Substituent effects in substituted aromatics rings.
Presence of a substituent has two kinds of effects on the ring
A)
Reactivity of the ring
i)
Activate – make benzene ring more reactive
ii)
Deactivate – make benzene ring less reactive.
B)
Orientation of new substituent: ortho, meta, para.
•
•
The formation of these disubstituted products are not equal.
Nature of the substituent already present will determine the position of second.
Classification of the substituent already present into three categories:
1.
2.
3.
Ortho and para directing activators
Ortho and para directing deactivators
Meta directing deactivators
Note: There are no meta directing deactivators
Directing effect correlate to reactivities.
1.
2.
3.
All meta directing groups are deactivators
Most ortho- and para- directing groups are activators
Halogens are weak ortho- and para- directing deactivators.
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Chapter 16 Chemistry of Benzene
Reactivity and orientation: Two key factors - Inductive effect and resonance effect.
Inductive effect:
Donation or withrdawl of electrons through σ-bond due to electronegativity.
Donating groups: Alkyl groups
Withdrawing groups: Halogens, Cyano (-C≡N), hydroxyl, Carbonyl groups, Nitro
Halogen and Hydroxyl – electronegative atom directly attached.
Rest are -Y-Z type (where Y is attached to ring and Z is electronegative atom).
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Chapter 16 Chemistry of Benzene
Resonance effect:
Withdrawal or donation of electrons through π-bond due to p-orbital
overlap on the subsitutent with p-orbital on aromatic ring.
Withdrawing groups: Carbonyl, Cyano, Nitro – have Y-Z , where Z>Y in electronegativity
These leave a positive charge on the ring.
Donating groups:
Halogen, Hydroxyl, alkoxy, amino
Note: these have at least a lone pair of electrons – these electrons can be pushed into the ring to
give the ring a negative charge.
Note: inductive effect and resonance effect counteract each other for this type of substituent.;
the stronger effect will dominate.``
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Chapter 16 Chemistry of Benzene
16.5
Explanation of substituent effects:
Common characteristics
Activating: electrons donate to ring. This stabilizes carbocation intermediate and lower
activatioin energy for its formation.
-OH, -alkoxy, amino groups – donate π electrons (resonance effect) Stronger than EW.
- alkyl groups donate through inductive effect.
Deactivating: electrons are withdrawn from the ring. This destabilizes the carbocation
intermediate and raises the energy for its formation.
Carbonyl, cyano, nitro groups are examples. Have both inductive effect and resonance effect
to withdraw electrons from the ring
Halogens are deactivating – strong EW (σ- inductive) and weak ED (π-resonance)
There are three possible types of activators and deactivators.
1)
Ortho & Para – directing activators - alkyl groups, OH, and NH2
2)
Ortho & Para – directing deactivators – Halogens
3)
Meta – directing deactivators - Carbonyl, CN, NO2
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Chapter 16 Chemistry of Benzene
Ortho and Para- directing activators – Alkyl groups.
Electron donating inductive effect – no resonance effect.
Nitration of toluene- can occur in the ortho, meta, or para positions.
In each case, the most stable carbocation intermediate is obtained when substitution occurs at
the ortho or para positions. - Tertiary carbocation is more stable than secondary.
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Chapter 16 Chemistry of Benzene
Ortho and para –directing activators - OH and NH2 groups.
Stronger ED π, Weaker EW σ
Stabilization of the carbocation by resonance effect – electron donation of Oxygen's lone pair
to positively charged carbon. – only ortho and para position have this effect.
Ortho and para – directing deactivators: Halogens - Stronger EW σ, and weaker ED π.
Electron donating π is weak but felt more at ortho and para positions.
Stabilize carbocation intermediate when 2nd substituent is placed in ortho or para positions.
Same diagram as above – relative amounts: ortho 35%, para 64%
This is considered deactivating because the EW is stronger than ED.
Activating requires Stronger ED.
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Chapter 16 Chemistry of Benzene
Meta directing deactivators: Have stronger EW (both σ& π)
effect is felt stronger at ortho and para – destabilizes it. so that meta substitution occurs.
Charge on the ipso carbon is not good especially with electron withdrawing group next to it.
This table is a nice summary of different effects on aromatic substitution.
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Chapter 16 Chemistry of Benzene
16.6
Trisubstituted benzene
Resonance and inductive effects still applies. Now, determine additive effect of two groups.
1.
Reinforcement of two groups – direct to same position
2.
Contradictory – more powerful activating group wins.
3.
Steric hindrance between meta substituted group – no substitution
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Chapter 16 Chemistry of Benzene
Do suggested questions in textbook
6th edition: 16.1, 16.4, 16.5, 16.6, 16.7, 16.8, 16.9, 16.10, 16.12, 16.13, 16.14, 16.15, 16.28,
16.29, 16.30, 16.31, 16.32, 16.33, 16.34, 16.35, 16.36, 16.42, 16.51, 16.52, 16.53, 16.64
7th edition: 16.1, 16.4, 16.5, 16.6, 16.7, 16.8, 16.9, 16.10, 16.12, 16.13, 16.14, 16.15, 16.29,
16.30, 16.31, 16.32, 16.33, 16.34, 16.35, 16.36, 16.37, 16.43, 16.52, 16.53, 16.54, 16.65
63