Esters of p-hydroxybenzoic acid

Benzoic acid and esters of p-hydroxybenzoic acid Bronopol Chlorhexidine Formaldehyde Glutaraldehyde Halogens Hexachlorophane Mercurials... 

In emulsions, partitioning of the incorporated preservative can occur between the aqueous and the oil phase. A lipophilic preservative may pass into the oil phase so that a significant portion is removed from the aqueous phase. Since it is the latter in which microorganisms tend to grow, the use of water-soluble preservatives can be more effective, especially for O/W emulsions. For most emulsion systems, the esters of p-hydroxybenzoic acid (parabens) appear to be the most satisfactory. Since microorganisms can also reside within the oil phase, it is further recommended that a pair of preservatives having different oil and water solubilities be used in order to ensure appropriate concentrations in both phases. 

These esters of p-hydroxybenzoic acid have been used primarily to prevent growth of molds but in higher concentrations possess some weak antibacterial activity. Their effective use is limited by low aqueous solubility and by reports of stinging and burning sensations related to their use in the eye. They bind to a number of nonionic surfactants and polymers, thereby reducing their bioactivity. They are used in combination, with the methyl ester at 0.03-0.1% and the propyl ester at 0.01-0.02%. Parabens have also been shown to promote corneal absorption [140]. 

More compounds have been identified in the neutral oxygen methyl ester fraction, shown in Figure 9, than in either the alkaline or acidic oxygen reactions. In addition to the products found in the alkaline oxygen reaction, the methyl ether ester of p-hydroxybenzoic acid and the methyl ester of mellitic acid are present. The acidic oxygen methyl ester fraction, also shown in Figure 9, qualitatively resembles the alkaline oxygen reaction. All the acids found in this latter reaction, except benzoic, are found in this fraction. The methyl ether ester of dehydrodivanillic acid or the methyl... 

SYNS ABIOL ASEPTOFORM MASEPTOL METHYLBEN METHYL CHEMOSEPT METHYL ESTER of p-HYDROXYBENZOIC ACID METHYL p-HYDROXYBENZOATE METHYL p-OXYBENZOATE... 

METHYLENIUM CERULEUM see BJI250 METHYL-18-EPIRESERPATE METHYL ETHER HYDROCHLORIDE see MQR200 METHYL ESTER of p-HYDROXYBENZOIC ACID see... 

Allwood MC. The adsorption of esters of p-hydroxybenzoic acid by magnesium trisilicate. Int Pharm 1982 11 101-107. 

Parabens [Napp]. TM for the methyl, propyl, butyl, and ethyl esters of p-hydroxybenzoic acid. Antimicrobial agents for foods and pharmaceuticals. Approved by FDA as GRAS. 

Eklund, T. 1985. Inhibition of microbial growth at different pH levels by benzoic and propionic acids and esters of p-hydroxybenzoic acid. International Journal of Food Microbiology 2 159-167. 

In the 1960s the Carborundum Company introduced the homopolymer of p-hydroxybenzoic acid under the trade name Ekonol [35]. It is used in plasma coating. This wholly aromatic homopolyester is produced in practice by the self-ester exchange of the phenyl ester of p-hydroxybenzoic acid. 

History. In the beginning 1920 s T. Sabalitschka synthesised different alkyd and aryl esters of p-hydroxybenzoic acid with a view to discover a replacement for salicylic- and benzoic adds, which suffer from the drawback of being effective only in the highly acid pH-range. This is why esters of p-hydroxybenzoic acid are still mainly used in non food applications with pH-levels around the neutral, as pharmaceutieals or eosmetics. Due to their negative sensory properties, their toxicological behaviour and some other technical problems assodated with their use in foods, the consumption of parabens acid in food remains very small. Additionally esters of p-hydroxybenzoic acid are the most expensive of the available preservatives. Therefore an additional growth of the amount used currently in the food sector cannot be expected in the near future. 

Applications. In many countries the methyl, ethyl and n-propyl esters of p-hydroxybenzoic acid, as well as their sodium salts, are permitted for preservation of some foods. The maximum permissible quantity is around 0.1%. In the USA methyl- and propyl-paraben are considered GRAS. The maximum permissible quantity is 0.1%. Their main field of use is not food preservation but the preservation of pharmaceutical and cosmetic fat emulsions. The usage of the butyl ester is not allowed in food applications. 

The most common method for the preparation of PCPP is the macromolecular substitution of the polydichlorophosphazene precursor [37] (see Section 1.1). This requires the use of protecting group chemistry, as it is well known that free acid groups lead to skeletal breakdown reactions. Propylparaben, the n-propyl ester of p-hydroxybenzoic acid has been most widely used for this, with subsequent deprotection of the propyl ester by hydrolysis under basic conditions. The requirement for complete macromolecular reactions has been addressed and structurally homogeneous, fully deprotected PCPP can be attained with the correct structure on relatively large (2 kg) scales [21, 26, 38, 39]. 

The alkyl esters of p-hydroxybenzoic acid (PHB parabens) are quite stable. Their solubility in water decreases with increasing alkyl chain length (methyl butyl). The esters are mostly soluble in 5% NaOH. 

Food preservatives are used primarily to prevent or retard microbial growth. The most typical food preservatives are sorbic acid, benzoic acid, propionic acid, and methyl-, ethyl- and propyl-esters of p-hydroxybenzoic acid (PHB, parabens). 

Similar comments apply to the BF3-catalysed reaction of phytol with the methyl ester of p-hydroxybenzoic acid [31], and to substitutions at the reactive (2,5) positions in blocked thiophenes e.g. Fig. 1.20. 

Both aliphatic and aromatic isocyanates can be blocked by a variety of blocking agents. These include alcohols, phenols, oximes, lactams, j8-dicarbonyl compounds, bisulfite addition compounds, hydroxylamines and esters of p-hydroxybenzoic acid and salicylic acid. Perhaps the most widely used blocking agents at present are phenol, branched alcohols, 2-butanone oxime (methyl ethyl ketoxime) and 8-caprolactam. The use of blocked isocyanates in PU coatings has been comprehensively reviewed. ... 

Shampoos normally contain a blend of cleansing agents (usually two anionic surfactants or an amphoteric and an anionic), foam booster (also a surfactant, most often an alkanola-mide), conditioner, viscosity improver (inorganic salts or cellulose derivatives), opacifier, dye, perfume, biostats (often esters of p-hydroxybenzoic acid), and perhaps chelates (EDTA or citric acid). Propylene glycol or glycerin may be present to adjust solubility and viscosity. Depending on current fashion, other compounds will be present, such as vitamins and food or herb extracts (26,27). 

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