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Organic Chemistry 4th EditionPaula Yurkanis BruiceIrene LeeCase Western Reserve UniversityCleveland, OH2004, Prentice Hall
Chapter 16
ReactionsofSubstituted Benzenes
Examples of Substituted Benzenes
The relative positions of the two substituents are indicated by numbers or by prefixes
The two substituents are listed in alphabetical order
If one of the substituents can be incorporated into a name, that name is used and the incorporated substituent is given the 1-position2-chlorotolueneortho-chlorotoluene4-nitroanilinepara-nitroaniline2-ethylphenolortho-ethylphenol
Some disubstituted benzenes have names that incorporate both substituents
Naming Polysubstituted Benzenes
The incorporated substituent is given the 1-position; the ring is numbered in the direction that yields the lowestpossible number
Reaction of Alkyl Substituents
The halogen in the benzylic position can be replaced by a nucleophile
A halo-substituted alkyl group can undergo elimination
Reduction of Unsaturated Substituents
Oxidation of the Alkyl Substituent
The same reagent that oxidizes the alkyl substituent will oxidize the alcohol
Reduction of a Nitro Substituent
A substituent can also donate electrons into the ring bydelocalizing its lone-pair electrons
Electron withdrawing by resonance occurs when the pelectrons from the ring are delocalized onto the substituentSubstituents such as CO, CN, and NO2 withdrawelectrons by resonance
The strongly activating substituents make the benzene ring more reactive toward electrophilic substitutionAll the strongly activating substituents donate electronsby resonance and withdraw electrons inductively
The moderately activating substituents can donateelectrons into the ring and away from the ringOverall, they donate electrons by resonance morestrongly than they withdraw electrons inductively
These substituents are slightly more electron donatingthan they are electron withdrawing
These substituents donate into the ring by resonance andwithdraw electrons from the ring inductivelyThey withdraw electrons inductively more strongly thanthey donate electrons by resonance
These substituents withdraw electrons both inductivelyand by resonance
These substituents are powerful electron-withdrawinggroupsThese substituents withdraw electrons both inductivelyand by resonance
The substituent already attached to the benzene ringdetermines the location of the new substituentAll activating substituents and the weakly deactivatinghalogens are orthopara directorsAll substituents that are more deactivating than halogensare meta directors
The relative stabilities of the carbocations formed from the electrophilic substitution of the substituted benzenedetermine the preferred reaction pathway Any substituent that donates electrons inductively is anorthopara director All substituents that donate electrons by resonance areorthopara directors
Electron withdrawal decreases reactivity toward electrophilic substitution and increases acidityElectron donation increases reactivity toward electrophilicsubstitution and decreases acidity
The orthopara product ratio decreases with an increasein the size of the substituents
Methoxy and hydroxy substituents are so stronglyactivating that halogenation is carried out without Lewis
Aniline and N-substituted anilines do not undergoFriedelCrafts reaction
Aniline cannot be nitrated but tertiary aromatic aminescan be nitrated
In designing a disubstituted benzene, the order in whichthe substituents are to be placed on the ring must be considered
Synthesis of Trisubstituted Benzenes
Steric hindrance makes the position between thesubstituents less accessible
A strongly activating substituent will win out over a weakly activating substituent or a deactivating substituent
If the two substituents have similar activating properties,neither will dominate
Synthesis of Substituted Benzenes Using Arenediazonium Salts
Preparation of the Diazonium Salt
Consider the synthesis of para-chloroethylbenzene
Fluorination of Benzene
The Arenediazonium Ion as an ElectrophileOnly highly activated benzene rings can undergo thisreactionSubstitution takes place preferentially at the para position
However, if the para position is blocked
Reaction of Amines with Nitrous Acid
Nucleophilic Aromatic Substitution Reactions
The electron-withdrawing substituents must be ortho orpara to the site of nucleophile attackThe electrons of the attacking nucleophile can be delocalized
Formation of Benzyne
Benzyne Is an Extremely Reactive Species
Polycyclic Benzoid Hydrocarbons
Electrophilic substitution reactions of naphthalene andsubstituted naphthalenes
1-substituted naphthalenes are easier to form2-substituted naphthalenes are more stable
In substituted naphthalenes, the nature of the substituent determines which ring will undergo electrophilic substitution