Reactions

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Chapter 16 Reactions of Substituted Benzenes
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Examples of Substituted Benzenes
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The relative positions of the two substituents
are indicated by numbers or by prefixes
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The two substituents are listed in alphabetical
order
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How to name substituted benzene?
Unfortunately we must memorize these names
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Unfortunately we must memorize these names
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If one of the substituents can be incorporated
into a name, that name is used and the
incorporated substituent is given the 1-position
2-chlorotoluene ortho-chlorotoluene
4-nitroaniline para-nitroaniline
2-ethylphenol ortho-ethylphenol
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Some disubstituted benzenes have names that
incorporate both substituents
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Naming Polysubstituted Benzenes
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The incorporated substituent is given the
1-position the ring is numbered in the
direction that yields the lowest possible number
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Reaction of Alkyl Substituents
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The halogen in the benzylic position can be
replaced by a nucleophile
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A halo-substituted alkyl group can undergo
elimination
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Reduction of Unsaturated Substituents
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Oxidation of the Alkyl Substituent
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The same reagent that oxidizes the alkyl
substituent will oxidize the alcohol
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Reduction of a Nitro Substituent
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A substituent can also donate electrons into the
ring by delocalizing its lone-pair electrons

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Electron withdrawing by resonance occurs when the
p electrons from the ring are delocalized onto
the substituent
Substituents such as CO, CN, and NO2
withdraw electrons by resonance
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The strongly activating substituents make the
benzene ring more reactive toward electrophilic
substitution
All the strongly activating substituents donate
electrons by resonance and withdraw electrons
inductively
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The moderately activating substituents can
donate electrons into the ring and away from the
ring
Overall, they donate electrons by resonance
more strongly than they withdraw electrons
inductively
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These substituents are slightly more electron
donating than they are electron withdrawing
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These substituents donate into the ring by
resonance and withdraw electrons from the ring
inductively
They withdraw electrons inductively more strongly
than they donate electrons by resonance
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These substituents withdraw electrons both
inductively and by resonance
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These substituents are powerful
electron-withdrawing groups
These substituents withdraw electrons both
inductively and by resonance
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The substituent already attached to the benzene
ring determines the location of the new
substituent
All activating substituents and the weakly
deactivating halogens are orthopara directors
All substituents that are more deactivating than
halogens are meta directors
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• The relative stabilities of the carbocations
  formed from
• the electrophilic substitution of the substituted
  benzene
• determine the preferred reaction pathway

• Any substituent that donates electrons
  inductively is an
• orthopara director

• All substituents that donate electrons by
  resonance are
• orthopara directors

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Electron withdrawal decreases reactivity toward
electrophilic substitution and increases acidity
Electron donation increases reactivity toward
electrophilic substitution and decreases acidity
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The orthopara product ratio decreases with an
increase in the size of the substituents
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Methoxy and hydroxy substituents are so
strongly activating that halogenation is carried
out without Lewis
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Aniline and N-substituted anilines do not
undergo FriedelCrafts reaction
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Aniline cannot be nitrated but tertiary aromatic
amines can be nitrated
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In designing a disubstituted benzene, the order
in which the substituents are to be placed on the
ring must be considered
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Synthesis of Trisubstituted Benzenes
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Steric hindrance makes the position between
the substituents less accessible
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A strongly activating substituent will win out
over a weakly activating substituent or a
deactivating substituent
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If the two substituents have similar activating
properties, neither will dominate
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Synthesis of Substituted Benzenes Using
Arenediazonium Salts
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Preparation of the Diazonium Salt
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Consider the synthesis of para-chloroethylbenzene
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Fluorination of Benzene
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The Arenediazonium Ion as an Electrophile
Only highly activated benzene rings can undergo
this reaction
Substitution takes place preferentially at the
para position
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However, if the para position is blocked
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Reaction of Amines with Nitrous Acid
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Nucleophilic Aromatic Substitution Reactions
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SNAr
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The electron-withdrawing substituents must be
ortho or para to the site of nucleophile attack
The electrons of the attacking nucleophile can be
delocalized
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Formation of Benzyne
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Benzyne Is an Extremely Reactive Species
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Polycyclic Benzoid Hydrocarbons
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Electrophilic substitution reactions of
naphthalene and substituted naphthalenes
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1-substituted naphthalenes are easier to
form 2-substituted naphthalenes are more stable
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In substituted naphthalenes, the nature of the
substituent determines which ring will undergo
electrophilic substitution