Esters, aromatic

Aromatic esters may be prepared by methods similar to those already described for aliphatic esters (see discussion preceding Section 111,95). These include — 

Esterification with an aromatic alcohol may be readily achieved by using an excess of the acid. The latter is readily removed by washing with water and/or treatment with sodium bicarbonate solution, for example  

CH3COOH + CeHjCHjOH Acetic acid Benzyl alcohol 

From the nitrile. By refluxing a mixture of the aromatic nitrile (with —CN group in side chain) with alcohol and concentrated sulphuric acid simul taneous hydrolysis and esterification occurs, for example  

From the acid chioride. The interaction of the acid chloride of an aromatic acid with the calculated quantity of an alcohol or a phenol affords a good yield of the ester, for example  

By the Schotten-Baumann reaction. Under the usual Schotten-Baumann conditions (compare discussion preceding Section IV,52, also Section IV,100,2 and Section IV,114,2), esters are readily formed, for example  

Aromatic Esters. - Aromatic methyl groups can be oxidised to the 

In view of related couplings of enol triflates, it is perhaps not surprising that triflates (304) derived from phenols undergo smooth carbonylation in the presence of palladium acetate and methanol to 

Poly-halobenzenes can be converted into the corresponding polybenzoates [e.g. (306) - (307)] by carbonylation using a cobalt carbonyl 

The useful carbanions (308), obtained from the parent o-toluate using LDA, can be efficiently acylated using N-methoxy-N-methylamides to give the isocoumarin precursors (309) many other more obvious 

On halogenating the homologues of benzene, two series of compounds with very different physical and chemical, but especially physiopathological, properties may be formed, according to whether the halogen atom enters the side chain or the benzene nucleus. While the compounds with a halogen atom in the side chain, like benzyl chloride (I), are efficient lachrymators, those with a halogen atom in the nucleus, like o-chlorotoluene (II), have no lachrymatory action at all. 

Examination of the physiopathological properties of these halogen compounds has shown that the lachrymatory power is increased by an increase in the atomic weight of the halogen present, that is, the iodo-compounds are biologically more efficient than the bromo-compounds and these more efficient than the chloro-compounds. 

In the war of 1914-18 these substances had a limited use. On the one hand, the raw material for their preparation (toluol) was too costly, and on the other their lachrymatory power was soon surpassed by that of other substances. 

Research carried out in the latter part of the war and continued in the post-war period has shown that the introduction of certain radicles into the molecules of these substances considerably increases their aggressive power. The entry of the NO -group into the benzene nucleus in the ortho-position to the side chain containing the halogen, and the introduction of the CN-group into the halogenated side chain itself are particularly efficacious. 

Thus among the halogen-compounds containing the NO -group,. c-nitrobenzyl chloride (I) and bromide (II) are superior to the corresponding simple halogenated derivatives. 

It is possible to alkylate benzoic acids directly, without the need to prepare reactive potassium salts in a separate step, because they can be generated in situ by reacting the acid with a base (potassium carbonate or hydroxide) in the presence of a phase-transfer catalyst. 

As an illustration of this principle, a volatile polar molecule is a byproduct, eliminated as a result of the microwave heating (Eq. 4), and the equilibrium is shifted to completion. The second effect of irradiation is activation of the alkylation step itself (Eq. 5). All the reagents can be used in the theoretical stoichiometry. Some indicative results are given in Tab. 5.4 [9]. 

Z Time (min) Yield (%) Performed salt Salt in situ 

A striking example in this series is the alkylation of terephthalic acid (Eq. 6). The specific effect of the microwaves appears clearly in this example, because, other factors being equal, the yields are unambiguously much higher. 

4-hydroxybenzoates (228) has been developed during work directed towards Avermectin syntheses and consists of sequential addition-elimination of 1,3-diketone dianions (227) to ( )-3-bromopropenoate followed by mild base treatment overall 

Chan have discovered that the silyl crotonate (229) unexpectedly reacts with carbonyl electrophiles at the v-position leading to 0-hydroxybenzoates [e.g. (230)]. Yields are around 60%. Related chemistry has been used to prepare the acetoacetate derivative (231). Benzylic ethers (232) can be 

The analgesic properties of various phenylacetic acids continue to stimultae the development of new routes to compounds of this type. Aryl bromides can be converted directly into phenylacetates by palladium-catalysed coupling with ethyl a-(tri-n-butylstannyl)acetate in the presence of zinc bromide. Aryl iodides can also be directly coupled to the potassium 

Various substituted phenylacetates can be prepared by phase-transfer catalysed Michael additions of t-butyl phenylacetate to acceptors such as cinnamates or chalcone.  

The rearrangement of a-bromoacetophenones, or acetals thereof, by a [1,2]aryl shift to a-arylacetates is somewhat limited because of the requirement of toxic and expensive thallium or silver 

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A number of selected aromatic nitro compounds are collected in Table IV,16A, It will be noted that a few nitro aromatic esters have been included in the Table. These are given here because the nitro group may be the first functional group to be identified aromatic nitro esters should be treated as other esters and hydrolysed for final identification. 

Aromatic esters usually burn with a smoky flame, possess reasonably high boiling points, and are (particularly esters of phenols) sometimes crystalline solids. Phenyl esters usually give phenol upon distillation with soda hme (see Section IV,175 for general details). 

The experimental details already given for the detection and characterisation of aliphatic esters (determination of saponification equivalents h3 diolysis Section 111,106) apply equally to aromatic esters. A sfight modification in the procediu-e for isolating the products of hydrolysis is necessary for i)henolic (or phenyl) esters since the alkaline solution will contain hoth the alkali phenate and the alkali salt of the organic acid upon acidification, both the phenol and the acid will be hberated. Two methods may be used for separating the phenol and the acid ... 

Table IV,183 summarises the physical properties of a few selected aromatic esters.

By the reduction of aromatic esters with sodium and absolute ethyl alcohol, for example ... 

Kinetic studies of nitration using dilute solutions of dinitrogen pentoxide in organic solvents, chiefly carbon tetrachloride, have provided evidence for the operation, under certain circumstances of the molecular species as the electrophile. The reactions of benzene and toluene were inconveniently fast, and therefore a series of halogenobenzenes and aromatic esters was examined. 

Triflates of phenols are carbonylated to form aromatic esters by using PhjP[328]. The reaction is 500 times faster if dppp is used[329]. This reaction is a good preparative method for benzoates from phenols and naphthoates (473) from naphthols. Carbonylation of the bis-triflate of axially chiral 1,1 -binaphthyl-2,2 -diol (474) using dppp was claimed to give the monocarboxy-late 475(330]. However, the optically pure dicarboxylate 476 is obtained under similar conditions[331]. The use of 4.4 equiv. of a hindered amine (ethyldiisopropylamine) is crucial for the dicarbonylation. The use of more or less than 4.4 equiv. of the amine gives the monoester 475. 

Aromatic carbon Aromatic compounds Aromatic esters Aromatic ethers... 

In view of the ready availabiUty of optically pure lactic acid derivatives this reaction offers an attractive general method for the preparation of optically pure aromatic ester derivatives (41). Stereoselective alkylation (15—60% inversion) of ben2ene with optically active 1,2- 1,3- and 1,5-dihaloalkanes was also reported (42). 

Acetophenone can react with formaldehyde to yield light-resistant resins which are used as additives in nitrocellulose paints. It is also used as a photoinitiator, and in the pharmaceuticals, perfumery, and pesticide industries (344). It can be hydrogenated to 1-phenylethanol which is used for the production of aromatic ester fragrances (345). Technical-grade acetophenone is available at 2.29/kg perfume-grade acetophenone was 6.50/kg in October 1994. 

Donoi—acceptoi chromogens in solution are often strongly affected by the nature of the solvent or the resinous substrate in which they are dissolved. The more polar the solvent or resin, the longer the wavelength of the fluorescent light emitted. Progressing from less polar to more polar solvents, the bathochromic, or reddening, effect of the solvents on the dye increases in the order of aUphatics < aromatics < esters < alcohols < amides. 

The major components of camauba wax are aHphatic and aromatic esters of long-chain alcohols and acids, with smaller amounts of free fatty acids and alcohols, and resins. Camauba wax is very hard, with a penetration of 2 dmm at 25°C and only 3 dmm at 43.3°C. Camauba also has one of the higher melting points for the natural waxes at 84°C, with a viscosity of 3960 rare]/s at 98.9°C, an acid number of 8, and a saponification number of 80. 

The reaction is irreversible and can be used to synthesize aUphatic and aromatic esters. In addition, there are no complications involving water removal or azeotrope formation. Boron tribromide can be used ia place of boron trichloride, but the bromide has a stronger tendency to halogenate the alkyl group of the alcohol (26). Boron tritiuoride does not give the ester, but gives either a complex or dehydrated product. 

The carrier-active chemical is selected according to its effectiveness at various temperatures. Members of the phenoHc group (Table 2), considered to be stronger carriers, are employed for formulations to be used in open equipment at the boil. Weaker carriers, such as the members of the aromatic ester group, are utilized generally for high temperature dyeing. 

Special additives are often included in a carrier formulation to provide specific properties such as foam control, stabiUty, and fiber lubrication during dyeing. Most important are the solvents used to solubilize the soHd carrier-active chemicals. These often contribute to the general carrier activity of the finished product. For example, chlorinated benzenes and aromatic esters are good solvents for biphenyls and phenylphenols. Flammable compounds (flash point below 60°C) should be avoided. 

Chemically all the currently commercial materials are linear aromatic esters. Xydar (3) is a terpolymer of, -biphenol, -hydroxyben2oic acid, and terephthahc acid. Vectra (4) is a copolymer of -hydroxyben2oic acid and 6-hydroxy-2-naphthoic acid. 

H-Bond Acceptor (HBA) Acyl chlorides Acyl fluorides Hetero nitrogen aromatics Hetero oj gen aromatics Tertiary amides Tertiary amines Other nitriles Other nitros Isocyanates Peroxides Aldehydes Anhydrides Cyclo ketones Ahphatic ketones Esters Ethers Aromatic esters Aromatic nitriles Aromatic ethers Sulfones Sulfolanes... 

KOH, 18-crown-6, toluene, 100°, 5 h, 94% yield.These conditions were used to cleave the /-butyl ester from an aromatic ester they are probably too harsh to be used on more highly functionalized substrates. 

Aromatic Esters Ethyl benzoate Benzyl benzoate Methyl salicylate... 

Adjacent formyl groups facilitate the hydrolysis of certain aromatic esters, as illustrated by the examples below. Indicate a mechanism for this rate enhancement. ... 

Solubility. Generally, resins are soluble in most common organic solvents, especially aromatics, esters, and chlorinated solvents. They are insoluble in... 

The mechanism of this reaction has been studied by several groups [133,174-177]. The consensus is that interaction of ester with the phenolic resole leads to a quinone methide at relatively low temperature. The quinone methide then reacts rapidly leading to cure. Scheme 11 shows the mechanism that we believe is operative. This mechanism is also supported by the work of Lemon, Murray, and Conner. It is challenged by Pizzi et al. Murray has made the most complete study available in the literature [133]. Ester accelerators include cyclic esters (such as y-butyrolactone and propylene carbonate), aliphatic esters (especially methyl formate and triacetin), aromatic esters (phthalates) and phenolic-resin esters [178]. Carbamates give analogous results but may raise toxicity concerns not usually seen with esters. 

Phenyl-ethyl Acetate.—Phenyl-ethyl alcohol yields a series of highly aromatic esters. That of acetic acid has the formula... 

From the preceding discussion, it is easily understood that direct polyesterifications between dicarboxylic acids and aliphatic diols (Scheme 2.8, R3 = H) and polymerizations involving aliphatic or aromatic esters, acids, and alcohols (Scheme 2.8, R3 = alkyl group, and Scheme 2.9, R3 = H) are rather slow at room temperature. These reactions must be carried out in the melt at high temperature in the presence of catalysts, usually metal salts, metal oxides, or metal alkoxides. Vacuum is generally applied during the last steps of the reaction in order to eliminate the last traces of reaction by-product (water or low-molar-mass alcohol, diol, or carboxylic acid such as acetic acid) and to shift the reaction toward the... 

The silica gel surface is extremely polar and, as a result, must often be deactivated with a polar solvent such as ethyl acetate, propanol or even methanol. The bulk solvent is usually an n-alkane such as n-heptane and the moderators (the name given to the deactivating agents) are usually added at concentrations ranging from 0.5 to 5% v/v. Silica gel is very effective for separating polarizable materials such as the aromatic hydrocarbons, nitro hydrocarbons (aliphatic and aromatic), aliphatic ethers, aromatic esters, etc. When separating polarizable substances as opposed to substances with permanent dipoles, mixtures of an aliphatic hydrocarbon with a chlorinated hydrocarbon such as chlorobutane or methylene dichloride are often used as the mobile... 

Synthesis and characterization of ABA type copolymers containing polydimethyl-siloxane or poly(trifluoropropyl,methyl)siloxane middle blocks and aromatic ester based liquid crystalline end blocks were reported 252,253). These materials were synthesized in solution by the reaction of primary or secondary amine-terminated, di-... 

Esters of aromatic acids are not reduced by this procedure, so an aromatic COOH group can be reduced in the presence of a COOR group. However, it is also possible to reduce aromatic ester groups, by a variation of the trichlorosilane procedure. The o- and p-hydroxybenzoic acids and their esters have been reduced to cresols (HOC6H4CH3) with sodium bis(2-methoxyethoxy)aluminum hydride, NaAlH2(0C2H40Me)2 Red-Al). ... 

Esters of 2-(2-methylphenyl)hydrazinecarboxylic acids were metaUated with excess LDA, and the resulting polyanion intermediates were condensed with aromatic esters followed by acid cyclization to give 3-substituted l(2f/)-isoquinolones 76 <96SC(26)1763>. 

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