Para-alkoxy substitution

The effect of a para-alkoxy substitution of benzoic acid is also clear the pKa-values and Tmax.(exotherm)-values are increasing going from benzoic acid, to 4-methoxy-benzoic acid and to 4-ethoxy-benzoic acid. Besides, the dH-values of the last two mentioned acids are clearly lower than all the other dH-values. Such a difference in Tmax.(exotherm)-value is not present between benzoic acid and 3-methoxy-benzoic acid (meta-alkoxy substitution). The reason for the decreased epoxide reactivity due to para-alkoxy substitution might be the conjugated mesomeric structure which causes an extra negative charge on the carbonyl-group. 

The dH-values of eight of these systems seem to be more or less constant i.e. 80 5 kJ/mol. The dH-values of the para-alkoxy substituted benzoic acid systems and that of hydroxy-pivalic acid are significantly lower. The dH-value of the meta-alkoxy substituted benzoic acid system is significantly higher (99 kJ/mol). The reasons for these differences are not clear. 

Some Recent Studies Models for the para-Alkoxy Diaryl Substituted Polysilanes... 

Dihydroxy-4-alkoxy derivatives of benzophenone are also produced by allqrlation of 2,2, 4-trihydroxybenzophenone with the corresponding allqrl halide [29, 31] in this case the alkyl group is inserted primarily in the para-position. When reliable methods of synthesizing the initial 2,2, 4-trihydroxybenzophenone are available, this method is the most expedient for the industrial production of 2,2 -dihydroxy-4-alkoxy-substituted benzophenones. 

Polyaniline, one of the most studied conducting polymers (in general as well as for corrosion protection), is usually obtained by protonation of what is called the emeraldine base form, shown in Figure 2. The protonation reaction does not change the number of electrons in the polymer backbone. However, starting at the leucoemeraldine form of polyaniline, one would obtain the emeral e salt (conducting) form of polyaniline by an oxidation reaction. Protonic doping has also been observed in the case of alkoxy substituted poly(para-phenylene vinylene) (PPV) ... 

The alkyl, amido, and alkoxy groups are activating and ortho-para-dkeciing, but not as strongly so as hydroxyl or amino groups. Synthetically useful conditions for selective substitution are available for essentially all the electrophiles in Scheme... 

Merlic et al. were the first to predict that exposing a dienylcarbene complex 126 to photolysis would lead to an ort/zo-substituted phenolic product 129 [74a]. This photochemical benzannulation reaction, which provides products complementary to the classical para-substituted phenol as benzannulation product, can be applied to (alkoxy- and aminocarbene)pentacarbonyl complexes [74]. A mechanism proposed for this photochemical reaction is shown in Scheme 54. Photo activation promotes CO insertion resulting in the chromium ketene in-... 

For the synthesis of the target structures, it is absolutely necessary to introduce solubilizing substituents in the positions peripheral to the benzoyl substituents. The primary coupling product, 117, a poly(2,5-dibenzoyl-l,4-phenyl-ene) derivative - a poly(para-phenylene) with two benzoyl substituents in each structural unit - is, as expected, very poorly soluble. Highly substituted monomers (2,5-dibromo-l,4-bis(3,4-dihexyloxy-benzoyl)benzene), containing four solubilizing alkoxy groups per monomer unit, allow the synthesis of polymeric materials with M of about 12,000 and M, of about 22,000 [139]. 

Jin et al. [487] synthesized and studied the PL and EL properties of polymers 403 and 404 that differ by the position of the alkoxy substituent in the phenyl ring, expecting different distortion of the polymer main chain (and consequently conjugation length) due to different steric factors for para- and ort/zo-substitution (Chart 2.98). The absorption spectrum of the ortho-polymer 403 showed a substantial blue shift of 40 nm compared to para 404 and a decrease in EL turn-on voltage (4.5 and 6.5 V, respectively). Both polymers demonstrated nearly the same PL and EL maxima (Table 2.1). 

The regiochemistry for the substituted t-S with ammonia and DCNB was also studied (equation 43). When there is an alkoxy substituent on the para position, the reaction yields selectively l-amino-2-aryl-l-phenylamine (equation 44). 

The influence of temperature on the ortho effect has been evaluated in the alkaline hydrolysis in aqueous DMSO solutions of ortho-, meta- and para-substituted phenyl benzoates (26). The alcoholysis of phthalic anhydride (27) to monoalkyl phthalates (28) occurs through an A-2 mechanism via rate-determining attack of the alcohol on a carbonyl carbon of the anhydride (Scheme 4). Evidence adduced for this proposal included highly negative A 5 values and a p value of 4-2.1. In the same study, titanium tetra-n-butoxide and tri-n-butyltin ethanoxide were shown to act as effective catalysts of the half-ester formation from (27), the mechanism involving alkoxy ligand exchange at the metal as an initial step. ... 

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. 

Explain the fact that tn aryl alkyl ethers (i) the alkoxy group activates the benzene ring towards electrophilic substitution and (ii) it directs the incoming substituents to ortho and para positions in benzene ring. 

Numerous further examples of protonation of variously substituted hydroxy- and alkoxy-benzenes in several acid solvent mixtures have been reported more recently and show a bewildering variety of behaviour. Several hydroxy- or alkoxy-groups favour C-protonation. Thus, 1,3,5-trihydroxybenzene and its ethers have been known for some time to be C-protonated at moderate acid concentrations (35-65% perchloric acid) (Kresge et al., 1962). In partial methyl and ethyl ethers of 1,3,5-trihydroxybenzene, there is a preference for protonation para to the hydroxy- rather than the alkoxy-group (Kresge et al., 1971a). 1,3-Dihydroxybenzene and its methyl ethers are also C-protonated in perchloric and sulphuric acid, whereas C-protonation of anisole would be expected only in >70%... 

Chemically, the triarylmethane dyes are monomethine dyes with three terminal aryl systems of wind] one or more arc substituted with primary, secondary, or tertiary amino groups or hydroxyl groups in the para position to the methine carbon atom. Additional substituents such as carboxyl, sulfonic acid, halogen, alkyl, and alkoxy groups may be present on the aromatic rings. The number, nature, and position of these substituents determine both the hue or color of the dye and the application class to which the dye belongs. 

Our interest in silane coupling agents and in the preparation of silicates has led us to study the mechanisms involved in the various reactions. In a previous paper, we investigated the kinetics and mechanism of the alcoholysis of TMOS [13]. This present work studies the effects of changing the substituents on a para-substituted phenyl attached directly to the alkoxysilane. The alkoxysilane silanes used have only one alkoxy group present to eliminate complications from a competing second or third alcoholysis reaction. The resulting Hammett plot yields additional information on the mechanism of this reaction. 

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