Terpenes open-chain 327

Perkin pointed out that open chain compounds, which are analogous in structure to a terpene, show a certain similarity in behaviour thus the addition of an ethyl group to 2-methyl 1 5-hexadiene by converting it into 2-methyl 3-ethyl 1 5-hexadiene changes the unpleasant acrid odour into a pleasant one reminding of lemon and peppermint. 

Chemically, plant essential oils consist largely of mixtures of compounds known as terpenoids —small organic molecules with an immense diversity of structure. More than 35,000 different terpenoids are known. Some are open-chain molecules, and others contain rings some are hydrocarbons, and others contain oxygen. Hydrocarbon terpenoids, in particular, are known as terpenes, and all contain double bonds. For example ... 

Terpenes are a diverse class of small organic molecules. Some are open-chain molecules, some contain rings, some are purely hydrocarbons (classical terpenes), and some... 

Table 5.1. Structures and 13C Chemical Shifts (<5C in ppm) of Selected Open-Chain Terpenes. ...

A wide variety of cyclic terpene hydrocarbons are known and, as multiples of CsH8, these have fewer double bonds than the open-chain terpenes. Because it is time consuming to show all the carbon and hydrogen atoms of such substances, the structures often are drawn in a convenient shorthand notation wherein the carbon-carbon bonds are represented by lines, carbon atoms being understood at the junctions or the ends of lines. By this notation, myrcene can be represented by formulas such as the following ... 

The left semicyclic structural formula is useful to show relationships with the open-chain (acyclic) and cyclic terpene hydrocarbons. 

The chemical interrelationships among these seemingly widely different types of compounds justify the recognized segregation of the terpenes from other fields of organic chemistry. The structural significance of the isoprene residue ( C-C-C) to the different types of terpene carbon skeletons has already been indicated. The simplest type of terpene is an open-chain olefinic structure formed by the union of two isoprene units. Such compounds are called the acyclic terpenes. From two units of isoprene or from the simple acyclic structures, under proper conditions, simple... 

Monoterpenes of interest in flavourings can be open-chained or cyclic molecules. They can be more or less unsaturated, and they can contain various oxygen functions. Sometimes they may also be bound to non-terpene molecule parts. Among the compounds and their 5-values compiled in Table 6.7, typical representatives will be discussed in detail. Nature-identical isoprenoids are in this context compounds obtained from another natural isoprenoid precursor, because from a commercial point of view, a really synthetic product would not be of interest. 

The many compounds of each group are differentiated by the type of cyclisation (open-chain, monocyclic, bicyclic etc.), the number and position of double bonds, the centres of asymmetry and the nature and number of functional groups. The individual terpene derivatives are most conveniently treated in the order of increasing polarity hydrocarbons first, then esters and lactones, carbonyl compounds, alcohols and finally phenols and acids. 

The number of unsaturated bonds can be determined by a titration with a standard solution of bromine dissolved in glacial acetic acid. The sample is also dissolved in glacial acetic acid to which sodium and ammonium acetate are added, so as to give a final concentration of 0-5 and OT M respectively. The cathodic current of bromine is measured with a short circuit system using a rotating platinum indicator, and a calomel or chloranil reference electrode at the potential of the reference electrode. Several unsaturated open chain compounds and terpenes have been titrated in this way.( ) Other modifications of this technique have been used to determine the amount of unsaturation in fats in methanolic solutions. The standard solution of bromine is prepared in methanol, saturated with sodium bromide, and the methylester of stearic acid is added to suppress the maxima. If the methanolic solution is acidified, OT M potassium bromate with OT M sodium bromide can be used instead of bromine. The bromine generated in homogeneous solution can be used to analyse xylenol and cresol fractions of coal-tars. Other phenolic compounds may be similarly determined. The method was used for the determination of small concentrations of styrene... 

Figure 5.16 Monocyclic, bicyclic, and open-chain terpenes investigated as substrates in the hydroformylation-acetalization reaction.

Mono- and sesquiterpenes are most commonly associated with the flavor of citrus products, spices, and herbs. They are hydrocarbons based upon the five carbon isoprene unit (2-methyl-1,3-butadiene) with structures that may be open chain, closed chain, saturated, or unsaturated, and may contain O, N, or S. While a host of terpenes have been found to be produced by microorganisms, there are no fermented food products commercially available that derive their characteristic flavor from terpenes. 

Paint and varnish manufacturing Resin manufacturing closed reaction vessel Varnish cooldng-open or closed vessels Solvent thinning Acrolein, other aldehydes and fatty acids (odors), phthalic anhydride (sublimed) Ketones, fatty acids, formic acids, acetic acid, glycerine, acrolein, other aldehydes, phenols and terpenes from tall oils, hydrogen sulfide, alkyl sulfide, butyl mercaptan, and thiofen (odors) Olefins, branched-chain aromatics and ketones (odors), solvents Exhaust systems with scrubbers and fume burners Exhaust system with scrubbers and fume burners close-fitting hoods required for open kettles Exhaust system with fume burners... 

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