Dimerization reactions, very high pressure

Eqs. (28) and (29) show that ion - indueed + permanent dipole association reactions at high pressures should be dominated by the permanent dipole at low temperatures while they are governed by the induced dipole at high temperatures. At very high temperatures, finally the valence part of the potential eomes into play 115]. In the following, we eonfront this expeetation with experimental observations [16] for the formation of the proton-bound dimer of ammonia... 

Both reactions could be carried out under mild conditions (100-110°C and atmospheric pressure). The very high selectivity to VCH in the former reaction is of particular interest since the only other butadiene dimerization catalysts (167) known at that time were homogeneous and in general gave, in addition to VCH, several other dimerization products (e.g., 1,4-cyclo-... 

Thermal cyclooligomerizations of olefins and alkynes require severe and often dangerous reaction conditions and the yields of cyclic products are usually very low. Acetylene ean be trimerized to benzene at 500 °C [1] and butadiene (BD) dimerizes at 270 °C and under high pressure to give small amounts of 1,5-cyclo-octadiene [2]. Reppe s discovery in 1940 that acetylene can be cyclotetramerized to cyclooctatetraene (COT) using a nickel catalyst [3] shows that transition metals can act as templates for the synthesis of cyclic hydrocarbons from acetylenic or olefinic building blocks (Scheme 1). 

The oxidation of nitric oxide to nitrogen peroxide involves heat generation and a decrease in the total number of moles. Therefore, the reaction is favored by a decrease in temperature and an increase in pressure. The dimerization reaction is very fast. The solid form (melting point — ll C boiling point 21.1 °C) consists entirely of dimer. The dimerization is favored by low temperature and high pressure, just as in the case of nitrogen peroxide formation. 

If, as will become evident with the dimerizations discussed in Section III.C, phase rebuildings are essential for the success of solid state photodimerizations, the nonreactivity of some topochemically allowed systems can be looked at in a different way. First, if there is no reaction, the crystal surface will stay unchanged upon irradiation and this can be probed with AFM very sensitively. Figure 2 shows a particularly highly structured natural surface of photostable tetraphenylethylene 1 prior and after 10 min irradiation in air with a Hg high-pressure lamp through Solidex from a distance... 

To eliminate possible secondary reactions, it is essential to prevent the formation and decomposition of dibenzyl in the hot zone, and this is achieved by using very short contact times and high flow rates. The prevention of dimerization is also aided by working at low pressures, high temperatures, and low fractions of decomposition. 

---