Dipentene

C, b.p. 156 C. The most important of the terpene hydrocarbons. It is found in most essential oils derived from the Coniferae, and is the main constituent of turpentine oil. Contains two asymmetric carbon atoms. The (- -)-form is easily obtained in a pure state by fractionation of Greek turpentine oil, of which it constitutes 95%. Pinene may be separated from turpentine oil in the form of its crystalline nitrosochloride, CioHigClNO, from which the ( + )-form may be recovered by boiling with aniline in alcoholic solution. When heated under pressure at 250-270 C, a-pinene is converted into dipentene. It can be reduced by hydrogen in the presence of a catalyst to form... 

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 . 

Terpenes, specifically monoterpenes, are naturally occurring monomers that are usually obtained as by-products of the paper and citms industries. Monoterpenes that are typically employed in hydrocarbon resins are shown in Figure 2. Optically active tf-limonene is obtained from various natural oils, particularly citms oils (81). a and P-pinenes are obtained from sulfate turpentine produced in the kraft (sulfate) pulping process. Southeastern U.S. sulfate turpentine contains approximately 60—70 wt % a-pinene and 20—25 wt % P-pinene (see Terpenoids). Dipentene, which is a complex mixture of if,/-Hmonene, a- and P-pheUandrene, a- and y-terpinene, and terpinolene, is also obtained from the processing of sulfate Hquor (82). 

Boron tritiuoride etherate— -hexanol complexes have successfully been used to polymerize P-pinene, as well as dipentene, to yield resins with softening points >70° C (82). Limonene or dipentene sulfate has been polymerized with aluminum chloride in a mixed toluene/high boiling aUphatic naphtha to give high yields of light colored resins (96). For the polymerization of dipentene or limonene, 4—8 wt % of AlCl has been used. Polymerization of P-pinene typically requires lower levels of catalyst relative to limonene or dipentene. 

Terpene Copolymers. Terpenes are routinely polymerized with other terpenes or with nonterpene-type monomers (97—102). The AlCl catalyzed polymerization of P-pinene, dipentene, and terpene oligomers (oily dimers and trimers) has been found to yield resins with softening points ranging from 0—40°C (103). 

A process of polymerization of isomerized a-pinene or turpentine with vinylbenzenes has been disclosed (105). a-Pinene or turpentine is isomerized by flash pyrolysis at 518 5° C in a hot tube reactor to yield a mixture of predominantly dipentene and i7t-alloocimene... 

Alkylation of cyclohexane with isoprene can be carried out with alkyl radicals formed at 450°C and 20.3 MPa (200 atm) (73). 40% Pentenylcyclohexanes, 20% dipentenes (ie, substances having the general formula C qH ), and 40% higher boiling compounds are obtained using a 6.8 molar ratio of cyclohexane to isoprene and a space velocity of 2.5. Of the pentenylcyclohexanes, the head and tail products are in equal amounts. Even... 

Fig. 2. Acid treatment and thermal isomerization of a-pinene where R = CH. The dipentene (15) is 4 -lhnonene (18) is a-pyronene (19) is P-pyronene.

Thermal isomerization of a-pinene, usually at about 450°C, gives a mixture of equal amounts of dipentene (15) and aHoocimene (16) (49,50). Ocimene (17) is produced initially but is unstable and rearranges to aHoocimene, which is subject to cyclization at higher temperatures to produce a- and P-pyronenes (18 and 19). The pyrolysis conditions are usually optimized to give the maximum amount of aHoocimene. Ocimenes can be produced by a technique using shorter contact time and rapid quenching or steam dilution (51). 

Carene has also been isomerized over an S-alumina catalyst to a 50 50 mixture of dipentene (15) and carvestrene (30). The cmde mixture can be readily polymerized to a terpene resin or copolymerized with piperjiene (63,64). 

Alloocimene Manufacture. a-Pinene (8) is converted thermally first to cis-ocimene (17), which rearranges to give about 40—50 wt % of the two alloocimene isomers, ie, alloocimene [7216-56-0] (53) and 4-trans-(i-trans-2S[oo[Pg.417]

The principal constitaenta of liussian turpentine are, according to Sohindeltaeiser, fl-pinene, sylvestrene, and dipentene. A little deifro-ti-... 

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