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Characteristics of Substitution Reaction

Halogenation:

C6H6

+   Cl2 & heat
    FeCl3 catalyst

-->

C6H5Cl
Chlorobenzene

+ HCl

Nitration:

C6H6

+   HNO3 & heat
    H2SO4 catalyst

-->

C6H5NO2
Nitrobenzene

+ H2O

Sulfonation:

C6H6

+   H2SO4 + SO3
    & heat

-->

C6H5SO3H
Benzenesulfonic acid

+ H2O

Alkylation:
Friedel-Crafts

C6H6

+   R-Cl & heat
    AlCl3 catalyst

-->

C6H5-R
An Arene

+ HCl

Acylation:
Friedel-Crafts

C6H6

+   RCOCl & heat
    AlCl3 catalyst

-->

C6H5COR
An Aryl Ketone

+ HCl

 

 

 

 

 

 

 

 

 

 

 

 

The conditions generally used for the aromatic substitution reactions discussed here are repeated in the table shown on the right side. The electrophilic reactivity of these distinct reagents varies. We find, for an instance, that nitration of nitrobenzene takes place smoothly at 95 ºC, giving meta-dinitrobenzene, where bromination of nitrobenzene (ferric catalyst) needs a temperature of 140 ºC. Also, as noted before, that than the benzene, toluene goes through nitration about 25 times faster, but chlorination of toluene is over 500 times faster than that of benzene. According to this we may conclude that the nitration reagent is more reactive and less selective than the halogenation reagents.

Both nitration and sulfonation yield water as a by-product. This does not considerably influence the nitration reaction (note the existence of sulfuric acid like a dehydrating agent), but the sulfonation is reversible and is driven to completion by addition of sulfur trioxide, which converts the water to sulfuric acid. The sulfonation reaction's reversibility is occasionally helpful for removing this functional group.

The Friedel-Crafts acylation reagent is generally composed of an acyl anhydride or halide mixed with a Lewis acid catalyst like AlCl3. This generates an acylium cation, R-C≡O(+), or a associated species. Such types of electrophiles are not exceptionally reactive, so the acylation reaction is usually restricted to aromatic systems that are at least as reactive as chlorobenzene. Carbon disulfide is frequently employed as a solvent, because it is unreactive and is simply removed from the product. If the substrate is an extremely reactive benzene derivative, like anisole, carboxylic acids or esters may be the source of the acylating electrophile. Some instances of Friedel-Crafts acylation reactions are displayed in the diagram. The first illustrates that unusual acylating agents may be employed as reactants. The second makes use of anhydride acylating reagent and the third demonstrates the easiness with which anisole reacts, as noticed previous. The H4P2O7 reagent which is used here is an anhydride of phosphoric acid called pyrophosphoric acid. At last, the fourth instance illustrates various significant points. Because the nitro group is a powerful deactivating substituent, the Friedel-Crafts acylation of nitrobenzene does not occur under any conditions. Though, the existence of a second strongly-activating substituent group allows acylation; the site of reaction is that favored by both substituents.

A common feature of the halogenation, nitration, acylation and sulfonation reactions is that they introduce a deactivating substituent on the benzene ring. As an effect, generally we do not have to worry about disubstitution products being formed. Alternatively Friedel-Crafts alkylation, introduces an activating substituent (an alkyl group), so more than one substitution may occur. If benzene is to be alkylated, like in the following synthesis of tert-butylbenzene, the mono-alkylated product is favored by using a large excess of this reactant. When molar ratio of benzene to alkyl halide falls below 1:1, para-ditert-butylbenzene becomes as the main result.

C6H6 (large excess)  +  (CH3)3C-Cl  +  AlCl3   -->   C6H5-C(CH3)3  +  HCl

The carbocation electrophiles necessary for alkylation may be generated from alkyl halides (as above), alkenes + strong acid or alcohols + strong acid. Because 1º-carbocations are prone to rearrangement, it is generally not possible to introduce 1º-alkyl substituents larger than ethyl by Friedel-Crafts alkylation. For an instance, reaction of excess benzene with 1-chloropropane and aluminum chloride gives a good yield of isopropylbenzene (cumene).

C6H6 (large excess)  +  CH3CH2CH2-Cl  +  AlCl3   -->   C6H5-CH(CH3)2  +  HCl

Additional illustrations of Friedel-Crafts alkylation reactions are displayed in the diagram below.

1474_Reactions Characterstics Homework Help.jpg

The first and third illustrations display how alkenes and alcohols may be the source of the electrophilic carbocation reactant. The triphenylmethyl cation formed in the third example is relatively unreactive, because extensive resonance charge delocalization and single substitutes highly activated aromatic rings. The second instance depicts an interesting case in which a polychlororeactant is employed like the alkylating agent. Four fold excess of carbon tetrachloride is employed to avoid tri-alkylation of this reagent, a process that is retarded through steric hindrance. The fourth instance depicts the poor orientational selectivity frequently found in alkylation reactions of activated benzene rings. The huge tert-butyl group ends up attached to the reactive meta-xylene ring at the least hindered site. This might not be the site of initial bonding, because polyalkylbenzenes rearrange under Friedel-Crafts conditions (para-dipropylbenzene rearranges to meta-dipropylbenzene on heating with AlCl3).

A practical concern in the make use of electrophilic aromatic substitution reactions in synthesis is the separation of isomer mixtures. This is specifically true for cases of ortho-para substitution, which frequently produce important amounts of the minor isomer. As a rule, para-isomers predominate apart from for some reactions of toluene and related alkyl benzenes. These mixtures' Separation is aided through the reality that para-isomers have considerably higher melting points than their ortho counterparts; subsequently, fractional crystallization is frequently an effective isolation method. Because meta-substitution favors a single product, separation of trace isomers is generally not a problem.

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