Dimerization thermal

Properties Colorless, odorless, flammable gas, boiling point. 7°C (23.5°F). Dimerizes thermally and forms explosive peroxides on contact with oxygen. 

When cyclobutadiene is generated by oxidation of cyclobutadieneiron tricarbonyl, most of the product is a mixture of the dimers 1 and 2. Is this dimerization thermally allowed or forbidden, and which isomer is expected to predominate ... 

Thermal Dissociation of Metal Dimer Thermal Dissociation of Metal Carbide Thermal Dissociation of Gaseous Halide Thermal Dissociation of Gaseous Oxide Reduction of Metal Oxide... 

Arylbenzazetes II were produced by photolysis of 4-aryl-1,2,3-benzotriazincs 10. The benz-azetes II dimerized thermally to give the angular dimers 12 and, in the presence of Lewis acids, the linear dimers 13 -303,330 Photolysis of 4-ter/-butyl-l,2,3-benzotriazine (10, R = r-Bu X = H) also gave the dimer 12 (R = /-Bu X = H), but only in 14% yield.302... 

In the recoil system chemically stable products containing a single silylene unit predominate, since the low concentration of intermediates precludes dimerization. Thermally generated SiF2 is quite unreactive, with a 150-s half-life in the gas-phase (12), so nudeogenic SiF2 may also be reacting before it is thermally equilibrated. 

Azete 6 dimerizes thermally to the 1,3-diazetidine system 7, which can be isomerized to the 1,5-diazocine 7. Azete 6 shows numerous cycloaddition reactions, especially with activated acetylenes and phosphaalkines for its transformations to the pyridine valence tautomers see p. 364. 

Mixed ketene dimers are also obtained by generating haloketenes in the presence of dimethylketene, by mixing of solutions of two different ketenes, and by cogeneration of two different ketenes from the carboxylic acid precursors Bis(trifluoromethyl)ketene does not dimerize thermally, but it reacts with Me2C=C=0 to form cyclobutanone and /3-propiolactone-type dimers ". The cycloaddition always proceeds across the C=C bond in the dimethylketene. In the reaction with ketene and methylketene, only the jS-propiolactone-type mixed dimers are formed. 

In their first papers on this topic, the authors stressed the fact that disubstitution at the 4 position of the oxazolinone ring was necessary since, in the presence of a base, a hydrogen atom abstraction at the 4 position was expected to cause the oxazolinone rings to undergo dimerization. Thermal or cationic polymerizations have also been reported for these compounds. 

The most intriguing hydrocarbon of this molecular formula is named buUvalene, which is found in the mixture of products of the reaction given above. G. SchrOder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicais of cyciooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of buUvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,6(X) different combinations of the carbon atoms are possible. 

In the commercial Gorham process (2), the extremely reactive PX is conveniendy generated by the thermal cleavage of its stable dimer, Vo-di- -xyljIene (DPX), a [2.2]paracyclophane [1633-22-3] (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becorning substituents of the polymer in the coating. 

Acrolein is produced according to the specifications in Table 3. Acetaldehyde and acetone are the principal carbonyl impurities in freshly distilled acrolein. Acrolein dimer accumulates at 0.50% in 30 days at 25°C. Analysis by two gas chromatographic methods with thermal conductivity detectors can determine all significant impurities in acrolein. The analysis with Porapak Q, 175—300 p.m (50—80 mesh), programmed from 60 to 250°C at 10°C/min, does not separate acetone, propionaldehyde, and propylene oxide from acrolein. These separations are made with 20% Tergitol E-35 on 250—350 p.m (45—60 mesh) Chromosorb W, kept at 40°C until acrolein elutes and then programmed rapidly to 190°C to elute the remaining components. 

Cycloaliphatic Diene CPD—DCPD. Cycloatiphatic diene-based hydrocarbon resias are typically produced from the thermal or catalytic polymerization of cyclopeatadieae (CPD) and dicyclopentadiene (DCPD). Upon controlled heating, CPD may be dimerized to DCPD or cracked back to the monomer. The heat of cracking for DCPD is 24.6 kJ / mol (5.88 kcal/mol). In steam cracking processes, CPD is removed from C-5 and... 

Hydrocarbon resins based on CPD are used heavily in the adhesive and road marking industries derivatives of these resins are used in the production of printing inks. These resins may be produced catalyticaHy using typical carbocationic polymerization techniques, but the large majority of these resins are synthesized under thermal polymerization conditions. The rate constants for the Diels-Alder based dimerization of CPD to DCPD are weU known (49). The abiHty to polymerize without Lewis acid catalysis reduces the amount of aluminous water or other catalyst effluents/emissions that must be addressed from an environmental standpoint. Both thermal and catalyticaHy polymerized DCPD/CPD-based resins contain a high degree of unsaturation. Therefore, many of these resins are hydrogenated for certain appHcations. 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

In the absence of air or peroxides, only cycHc dimers are formed in the thermal dimerization of isoprene (33). Six cycHc dimers are formed in good yields four substituted cyclohexenes (3—6) and two dimethylcyclooctadienes (7—8). The latter two are, of course, not Diels-Alder dimers. There is some evidence that the isoprene dimerization mechanism differs from the usual Diels-Alder route. 

In the process of thermal dimerization at elevated temperatures, significant polymer is formed resulting in seriously decreased yields of dimer. Dinitrocresol has been shown to be one of the few effective inhibitors of this thermal polymerization. In the processing of streams, thermal dimerization to convert 1,3-cyclopentadiene to dicyclopentadiene is a common step. Isoprene undergoes significant dimerization and codimerization under the process conditions. 

The dimer of phosphonic acid, diphosphonic acid [36465-90-4] (pyrophosphoms acid), H4P2O3, is formed by the reaction of phosphoms trichloride and phosphonic acid in the ratio of 1 5. It is also formed by the thermal decomposition of phosphonic acid. Unlike the chemistry of phosphoric acid, thermal dehydration does not lead to polymers beyond the dimer extended dehydration leads to a disproportionation to condensed forms of phosphoric acid, such as [2466-09-3] and phosphine. 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

Molecular Nature of Steam. The molecular stmcture of steam is not as weU known as that of ice or water. During the water—steam phase change, rotation of molecules and vibration of atoms within the water molecules do not change considerably, but translation movement increases, accounting for the volume increase when water is evaporated at subcritical pressures. There are indications that even in the steam phase some H2O molecules are associated in small clusters of two or more molecules (4). Values for the dimerization enthalpy and entropy of water have been deterrnined from measurements of the pressure dependence of the thermal conductivity of water vapor at 358—386 K (85—112°C) and 13.3—133.3 kPa (100—1000 torr). These measurements yield the estimated upper limits of equiUbrium constants, for cluster formation in steam, where n is the number of molecules in a cluster. 

Myrcene Manufacture. An important commercial source for mycene is its manufacture by pyrolysis of p-piaene at 550—600°C (87). The thermal isomerization produces a mixture of about 75—77 wt % myrcene, 9% limonene, a small amount of T -limonene [499-97-8] and some decomposition products and dimers. The cmde mixture is usually used without purification for the production of the important alcohols nerol and geraniol. Myrcene may be purified by distillation but every precaution must be taken to prevent polymerization. The use of inhibitors and distillation at reduced pressures and moderate temperatures is recommended. Storage or shipment of myrcene in any purity should also include the addition of a polymerization inhibitor. 

Thermal stability is enhanced in chelates thus dimethyl-2-methy1pentane-2,4-dio1titanium [23916-35-0] (22) is much more stable than (CH2)3Ti(OCH(CH2)2)2 (68)- The stmcture of the former has been shown by x-ray diffraction to be dimeric and five-coordinate through oxygen bridges. The more highly substituted the six-membered ring, the mote thermally stable the compound. 

Minor Uses. Small amounts of benzene find use in production of benzene-sulfonic acid. y -Benzenedisulfonic acid is used to produce resorcinol [108-46-3] (1,3-dihydroxybenzene). Benzene is thermally dimerized to yield biphenyl [92-52-4] Benzene can also be converted... 

The thermally induced Diels-Alder dimerization reaction producing vinylcyclohexene is very difficult to prevent except by lowering the storage... 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Thermal Oligomerization. Commercial manufacture of dimer acids began ia 1948 with Emery Industries use of a thermal process involving steam pressure. Patents were issued ia 1949 (45) and 1953 (46) that describe this process. Earlier references to fatty acid oligomerization, antedating the USDA work of 1941—1948, occur ia patents ia 1918 and 1919 (47,48), and ia papers written ia 1929—1941 (49—51). There appears to still be some small use of this approach to making dimer products. 

Other Polymerization Methods. Although none has achieved commercial success, there are a number of experimental alternatives to clay-catalyzed or thermal oligomeriza tion of dimer acids. These iaclude the use of peroxides (69), hydrogen fluoride (70), a sulfonic acid ion-exchange resia (71), and corona discharge (72) (see Initiators). 

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