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