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Essential oils: General aspects
Published in Anton C. de Groot, Monographs in Contact Allergy, 2021
Sometimes, essential oils undergo one or more physical processes to alter their chemical composition; these are called ‘post-treatment essential oils’. One such treatment is fractional distillation, resulting in ‘rectified’ oils. The objective of post-treatments can be to eliminate certain individual chemicals (e.g., phototoxic furocoumarins [psoralens] from bergamot oil, methyl eugenol from rose oils), lower the concentration of certain fractions, such as the terpenes or sesquiterpenes, to concentrate one or more chemicals or change the color of the essential oil (1,3).
Chemistry of Essential Oils
Published in K. Hüsnü Can Başer, Gerhard Buchbauer, Handbook of Essential Oils, 2020
The synthesis of l-menthol (94) provides an interesting example of different routes operating in economic balance. The three production routes in current use are shown in Figure 6.40. The oldest and simplest route is extraction from plants of the Mentha genus and M. arvensis (corn mint) in particular. This is achieved by freezing the oil to force the l-menthol to crystallize out. Diethylamine can be added to myrcene (70) in the presence of base and rearrangement of the resultant allyl amine (224) using the optically active catalyst ruthenium (S)-BINAP perchlorate gives the homochiral enamine (225). This can then be hydrolyzed to d-citronellol (209). The chiral center in this molecule ensures that, on acid-catalyzed cyclization, the two new stereocenters formed possess the correct stereochemistry for conversion, by hydrogenation, to give l-menthol as the final product. Starting from the petrochemically sourced m-cresol (226), propenylation gives thymol (97), which can be hydrogenated to give a mixture of all eight stereoisomers of menthol (227). Fractional distillation of this mixture gives racemic menthol. Resolution was originally carried out by fractional crystallization, but recent advances include methods for the enzymic resolution of the racemate to give l-menthol.
Reactivities of Amino Acids and Proteins with Iodine
Published in Erwin Regoeczi, Iodine-Labeled Plasma Proteins, 2019
A brief outline of the principle of the method is as follows. In analogy to the number of plates used in a column for fractional distillation, the efficiency of a chromatographic column can be described in terms of a “height equivalent to a theoretical plate” (HETP). The spreading of a sample during chromatography in a two-phase (liquid-liquid) system increases with plate height.496 Since the HETP is proportional to the square of the diameter of the particles used to hold the stationary liquid phase,497 a vast improvement in chromatographic resolution (the opposite to spreading) was achieved by the introduction of closely controlled microparticulate (3 to 10 μm) packing materials, mostly based on porous silica.498 The problem thus created for flow was overcome by high-pressure pumps. (There is an optimal flow rate in any chromatography, because diffusion from plate to plate gains in importance as the rate of flow decreases;497 slow flow therefore means an increase in HETP.)
Human biomonitoring of low-level benzene exposures
Published in Critical Reviews in Toxicology, 2022
Benzene is a small and volatile single-ring aromatic hydrocarbon that occurs naturally in crude oil and natural gas and is formed in almost any combustion process of organic matter. Benzene was first isolated as a discrete substance in the early nineteenth century and halfway through the nineteenth century the first industrial production came about through fractional distillation of coal tar (Mansfield 1849). Initially, benzene was used in cosmetics and after-shave because of its pleasant smell, and it was even used as therapeutic agent to cure several types of leukaemia (Rolleston and Boyd 1914; Ross 1914; Weber 1914; Stolkind 1931). The use of benzene increased dramatically when it was recognised as an excellent solvent for many (organic) materials. Benzene was widely used in adhesives and in the rubber industry as it proved the ideal solvent for latex, but also in a wide variety of other applications, such as degreasing of metal surfaces and decaffeinating coffee. The use of benzene in industrial applications increased over time and until the second world war its production was almost entirely based on coal tar distillation. After the second world war benzene became more important as a starting material to produce other chemicals and the production from crude oil increased sharply. Nowadays, more than half of all benzene produced annually serves as building block for ethylbenzene to produce styrene and subsequently polystyrene. Other uses comprise the production of phenol, aniline and detergent feedstocks.
The natural plant compound carvacrol as an antimicrobial and anti-biofilm agent: mechanisms, synergies and bio-inspired anti-infective materials
Published in Biofouling, 2018
Anna Marchese, Carla Renata Arciola, Erika Coppo, Ramona Barbieri, Davide Barreca, Salima Chebaibi, Eduardo Sobarzo-Sánchez, Seyed Fazel Nabavi, Seyed Mohammad Nabavi, Maria Daglia
CAR (2-methyl-5-(1-methylethyl)-phenol) (C10H14O) is a monoterpenic phenol, isomeric with thymol (Figure 2). The biologically active isomer, carvone, is a monoterpene ketone, traditionally isolated by fractional distillation of caraway or spearmint oils. S-(+)-carvone is the main component of caraway oil and dill, with an odor resembling that of these herbs. The other isomer, R-(−)-carvone, occurs at high concentrations (70-80%) in spearmint oil and is also the major component responsible for its aroma. While thymol is crystalline at room temperature, CAR and carvone are liquid. Carvone is practically insoluble in water but easily soluble in ethanol and ether (Rappoport 2004). In CAR the -OH group is placed in -ortho with respect to the methyl group present in the ring. The position and the presence of the substituents are essential for its chemical properties and biological activities.