Of Limonene

By oxidation with permanganate it forms pinonic acid, C,oH,<503, a monobasic acid derived from cyclobutane. With strong sulphuric acid it forms a mixture of limonene, dipentene, terpinolene, terpinene, camphene and p-cymene. Hydrogen chloride reacts with turpentine oil to give CioHijCl, bomyl chloride, artificial camphor . 

Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

The two enantiomers of limonene have completely different tastes. One has the taste of lemon (as the name implies) and the other of orange. Assign R or S forms tc the two enantiomers of limonene. 

To 0.165 mole of BMB (prepared as in the preceding experiment) maintained at 0°, is added 20.4 g (0.15 mole) of /-limonene over a period of 5 minutes. The reaction mixture is allowed to stand at room temperature for approximately 3 hours. It is then oxidized by the addition of 50 ml of 3 A sodium hydroxide followed by 50 ml of 30% hydrogen peroxide. The alcohol is worked up in the usual manner. Upon distillation, the primary terpineol is obtained, bp 115-116710 mm. 

The fact that the amount of total linalol decreases whilst the richness in linalyl acetate increases, proves that linalol appears in the plant at an earlier period than its acetic ester. Further, the free acetic acid acting on the linalol esterifies a portion of it, whilst another portion of this terpene alcohol is dehydrated, with the production of limonene and. dipentene, which are the usual resultants of linalol in presence of certain dehydrating agents. This view is corroborated by the fact that the quantity of the mixed terpenes increases during the esterification, without the slightest variation being observed in the ratio between the... 

The following are the characters of the most highly purified specimens of limonene which have been prepared —... 

A specimen obtained by tbe reduction of limonene tetrabromide by Godlewski and Eoshanowitsch had a specific rotation + 125° 36 which is practically equal to an observed rotation of + 106° 30, thus confirming the purity of the specimens prepared by fractional distillation. 

For the identification of limonene, one of the most useful compounds is the crystalline tetrabromide, Cj(,HjgBr. This body is best prepared as follows the fraction of the oil containing much limonene is mixed with four times its volume of glacial acetic acid, and the mixture cooled in ice. Bromine is then added, drop by drop, so long as it becomes decolorised at once. The mixture is then allowed to stand until crystals separate. These are filtered off, pressed between porous paper, and recrystallised from acetic ether. Limonene tetrabromide melts at 104 5° and is optically active, its specific rotation being + 73 3°. The inactive, or dipeutene, tetrabromide melts at 124° to 125°. In the preparation of the tetrabromide traces of moisture are advisable, as the use of absolutely anhydrous material renders the compound very diflftcult to crystallise. 

Tilden and Leech have prepared nitrosocyanides of limonene by the action of potassium cyanide on the nitroso compounds. The table on opposite page gives tbe melting-points and optical rotation of the principal of these and other limonene compounds. 

The nitroanilides are very characteristic crystalline compounds of limonene. These have the constitution—... 

Limonene forms a monohydrochloride, which is a liquid, and which exists in both optically active modifications. By the action of moist hydrochloric acid on the acetic or alcoholic solution of limonene, no optically active dihydrochloride is formed, the terpene becoming inactive and dipentene dihydrochloride, C (,H,g2HCl, results. This body is prepared when limonene is mixed with half its volume of glacial acetic acid, and a current of hydrochloric acid gas is passed over (not into) the well-cooled liquid, with frequent shaking. The resulting mass is pressed on a porous plate, dissolved in alcohol, and precipitated with water. It melts sharply at 50°. 

Dipentene, the inactive form of limonene, not only occurs naturally in essential oils, but results by the action of heat on several other ter-penes, and to a considerable extent by the action of sulphuric acid on pinene. 

Its constitution is closely related to that of limonene, since its chloride passes at once, on reduction, into this terpene as shown in the following formulae —... 

Terpineol (that is a-terpineol) has been prepared synthetically by Perkin and his pupils, his method being described under the synthesis of limonene. 

These results show that the structure of perillic aldehyde is similar to that of limonene, and that, consequently, the reducible double bond is next to the aldehydic group. 

Although the different enantiomers of a chiral molecule have the same physical properties, they usually have different biological properties. For example, the (+) enantiomer of limonene has the odor of oranges, but the (-) enantiomer has the odor of pine trees. 

Problem 11.14 j Review the mechanism of geraniol biosynthesis shown in Figure 11.15, and then propose a mechanism for the biosynthesis of limonene from linalyl diphosphate. 

Numerous examples of the different biological effects of enantiomers are available. One of the enantiomers of limonene smells of lemons, the other of oranges one of carvone smells of caraway, the other of spearmint These differences obviously have important... 

Cyclical monoterpenes such as limonene have also been used as substrate for the production of valuable products. A good example is the conversion of limonene to a-terpineol by Cladosporium sp. Thus,... 

Peroxides, hydroperoxides [1, 2] e.g. (photo)-axidation products of limonene [3]... 

Used tires create serious environmental problems, because they do not degrade easily and they release noxious contaminants when they bum. Recently, a process has been developed that breaks tires down into a polyisoprene oil that can then be further decomposed into limonene. The yield of limonene is only a few percent, but if this process can be made more efficient it may become possible to turn ugly, smelly old tires into fragrant oil of lemon. 

Cold-pressed essential oils from the peel are some of the most important by-products recovered during the processing of Citrus fruits. The presence of limonene in the aqueous discharges, with its antimicrobial activity [1], decreases the effectiveness of the waste treatment system and increases the time necessary for the biological breakdown of the organic matter produced in the peel oil recovery system [2,3]. Additional recovery of essential oils from waste water would increase industry s returns and reduce the pollution problems associated with the disposal of waste water [4,5]. Several methods for reducing the levels of residual essential oils in the aqueous effluent have been developed over the years [6-11]. 

Both the (+)- and (-)- enantiomers of limonene were transformed by larvae of the cutworm Spodop-tera litura (Miyazawa et al. 1998). For both of them the reactions involved are (a) dihydroxylation between C-8 and C-9 and (b) oxidation of the C-1 methyl group to carboxyl. These transformations were not dependent on the intestinal microflora in contrast to the transformation of a-terpinene to p-mentha-l,3-dien-7-ol and p-cymene whose formation could be attributed to the intestinal flora. 

Hydroxylation and Baeyer-Villiger reactions carried out by monooxygenation are important in the degradation of a range of terpenoids and steroids. The aerobic degradation of limonene can take place by a number of reactions several of which involve hydroxylation at allylic positions... 

The liquid phase alkoxylation of limonene (3) with C4-C4 alcohols to 1-methyl-4-[a-alkoxy-isopropyl]-l-cyclohexene (5) was carried out both in batch and continuous fixed-bed reactor at 60 °C on various acidic catalysts (Scheme 3.1) [16]. The best yields were obtained in batch (85%) or continuous reactor (81%) using a /1-type zeolite with Si02/Al203 = 25. 

Heteropoly acids such as H3PW12O40 (PW) are good catalysts for the hydration of limonene and other monoterpenes. PWs can be used as homogeneous catalysts in solution or supported on, for example, silica or MCM-41 materials. In aqueous acetic acid limonene gives, in the presence of PW, mainly a-terpineol (7) and a-terpinyl acetate (8) [17]. 

A tandem hydroformylation/carbonyl ene reaction can be observed in cases, in which substrates with at least two isolated oleftnic bonds are hydro-formylated at only one double bond selectively. Thus hydroformylation of limonene with PtCkCPPlH /SnCk/PPlH or PtCl2(diphosphine)/SnCl2/PPh3 gives a mixture of two diastereomeric alcohols upon carbonyl ene reaction of the intermediate aldehyde, (Scheme 36). Best results are achieved with a PtC Cdppb) complex. The mechanism of the final intramolecular cycli-zation step resembles an acid catalyzed carbonyl ene reaction [89]. 

Limonene, one of the most prominent natural monoterpenes (cf Section VII), represents a particular derivative of 4-vinylcyclohexene since it has been studied with respect to the pronounced energy dependence of its fragmentation behaviour (Scheme 7). Counterintuitively, and in contrast to 4-vinylcyclohexene, the radical cations of limonene (27) do not undergo the retro-Diels-Alder reaction if the internal energy of the ions is low. As... 

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