Aromatic alcohols Electrophilic substitution

Ethers may be prepared by (1) dehydration of alcohols and (11) Williamson synthesis. The boiling points of ethers resemble those of alkanes while their solubility Is comparable to those of alcohols having same molecular mass. The C-O bond In ethers can be cleaved by hydrogen halides. In electrophilic substitution, the alkoxy group activates the aromatic ring and directs the Incoming group to ortho and para positions. 

Solubilized lignin solutions are easily oxidized and the presence of the aromatic units containing electron-withdrawing ether and alcohol moieties makes it available for electrophilic substitution reactions, such as nitration, halogenation, hydroxylation, etc. 

The oxidation of m-cresol was carried out in Parr autoclave at 353 K using a 3 1 mixture of H2O and acetonitrile as solvent and Sn-silicalites with Si/Sn ratio of 70 as catalysts. A slightly higher efficiency for H2O2 is seen with Sn-ZSM-12 sample (Table 4). The dihydroxylated products, viz., 2-methylhydroquinone and 4-methylcatechol are found to be in excess over the products of side chain oxidation, viz., 3-hydroxybenzyl alcohol and the aldehyde in the product mkture. The aromatic hydroxylation on Sn-silicalites may follow an ionic mechanism as both the -CH3 and -OH groups in m-cresol are favourably placed for electrophilic substitution reaction. Interestingly, the product distribution on all the three Sn-molecular sieves is almost similar. This shows that in all the three types, the Sn ions are dispersed uniformly and possess identical catalytic property due to similar environment around them. 

SO on, loss of a proton leads to aromatization, completing the substitution reaction to form the new carbon bond in 159. Electrophilic aromatic substitution with alkyl carbocations is called Friedel-Crafts alkylation. It was reported by Friedel and Crafts in 1877. The carbocation precursor can be an alkene, an alcohol, or an alkyl halide. 

An aqueous Friedel-Crafts reaction has also been used in polymer synthesis. The acid-catalyzed polymerization of benzylic alcohol and fluoride functionality in monomeric and polymeric fluorenes was investigated in both organic and aqueous reaction media. Polymeric products are consistent with the generation of benzylic cations that participate in electrophilic aromatic substitution reactions. Similar reactions occurred in a water-insoluble Kraft pine lignin by treatment with aqueous acid. A Bisphenol A-type epoxy resin is readily emulsified in aqueous medium with an ethylene oxide adduct to a Friedel-Crafts reaction product of styrene and 4-(4-cumyl)phenol as emulsifier.Electrophilic substitution reaction of indoles with various aldehydes and ketones proceeded smoothly in water using the hexamethylenetetramine-bromine complex to afford the corresponding Z A(indolyl)methanes in excellent yields.InFs-catalyzed electrophilic substitution reactions of indoles with aldehydes and ketones are carried out in water.Enzymatic Friedel-Crafts-type electrophilic substitution reactions have been reported. ... 

The only known example of the catalytic nucleophilic aromatic substitution involves the use of Rh(III) complexes (e.g., 40). The first demonstration was made in 1980 [99]. The metal center is so electrophilic that an internal alcohol nucleophile substitutes directly without an added base. 

Many of the properties of phenols reflect the polarization implied by the contributing structures. The hydroxyl oxygen is less basic, and the hydroxyl proton more acidic, in phenols than in alcohols. Electrophilic aromatic substitution in phenols is much faster than in benzene, indicating that the ring, especially at the positions ortho and para to the hydroxyl group, is relatively electron-rich. ... 

In somewhat related work. Siskin et ah reported that 4-hydroxyphenyl benzyl ether undergoes complete conversion in decalin and water at 343°C (Fig. 9.47) during 2 days to give toluene (63 and 44%), hydroquinone (32 and 35%), and 2-benzylhydroquinone (4 and 12%). It was postulated that the intermediate benzyl alcohol was formed, which subsequently reacted with hydroquinone to produce the 2-benzylhydroquinone. The benzyl carbonium ion was proposed as the reactive species in the aromatic electrophilic substitution reaction. 

The surface (catalysis) Addo-basic Acid catalysis Basic catalysis Activation of alkanes and alcenes Breaking of C-C bonds in aUphatics Formation of C-C bonds in ahphatics Rearrangements of C-C bonds Reactions of isomerisation Reactions of nucleophUic substitution and addition CycUzation reactions Reactions of electrophilic substitution on aromatic rings Dehydrogenation of alcohols Isomerization of the double bonds of olefins... 

This is an electrophilic substitution we have not seen before—but we do know the mechanisms for the various components of it (Figure 13.47). The first step is protonation of the formaldehyde to make it into a better electrophile (when we look at carbonyl chemistry in the next chapter, we will find this is a very common process). This is then attacked by the aromatic ring in a conventional electrophilic substitution to give benzyl alcohol. Under the reaction conditions, the benzyl alcohol undergoes an S,.j2 substitution to give chloromethylbenzene (Figure 13.48). This process can be... 

This process depends on the reaction of phenol with formaldehyde at the ortho- and j ra-positions. The OH group activates the aromatic ring toward electrophilic substitution. Once benzylic alcohols have been produced, these too can act as electrophiles, and a rigid, cross-linked structure is produced. Bakelite was first produced in 1907 and is regarded as the first artificial polymer. It has high thermal and electrical resistivity and is still widely used in electrical fittings, chipboard, and decorative laminates. 

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