Sulfur trioxide trimer

Figure 15.27 Structure of the monomeric, trimeric and chain-polymeric forms of sulfur trioxide.

At normal temperatures, sulfur trioxide is a volatile liquid (its boiling point is 45°C), composed of trigonal planar S03 molecules. In the solid and to some extent in the liquid, these molecules form trimers (unions of three molecules) of composition S309 (14) as well as larger clusters. 

The gamma modification at ordinary temperatures can exist in sohd or liquid form. In sohd form it is a colloidal ice-hke mass melting at 16.8°C. In the hquid form it has a density of 1.9224 g/mL, boiling at 44.8°C. It has a vapor pressure of 433 torr at 25°C. The gamma phase consists of both cychc trimer and monomer molecules. When solid sulfur trioxide melts, it converts to its gamma phase which on sohdification changes to alpha modification. 

Solid sulfur trioxide exists in two well-defined modifications. Orthorhombic y-SOs consists of cyclic trimers (Figure 12.38a)/ which are converted by traces of water to the one-dimensional polymeric structure of monoclinic jl-SOi, (Figure 12.38b). In contrast, selenium trioxide forms a cyclic tetramer (Figure 12.38c). ... 

The structure of solid S03 is complex. At least three well-defined phases are known. y-Sulfur trioxide, formed by condensation of vapors at -80°C or below, is an icelike solid containing cyclic trimers with structure (12-XXV). 

SO3 Sulfur trioxide 0 0 0 mp = 16.9°C bp = 44.6°C formed from oxidation of SO2 SO2 + 2O2 SO3 in equilibrium with trimer S3O9 in liquid and gas phases reacts with water to form sulfuric acid... 

VM Fig. 15.16 The structures of solid state pol5unorphs of sulfur trioxide contains tetrahedral SO4 units (a) Y-SO3 consists of trimeric units and (b) a- and P-SO3 contain polymeric chains. Colour code S, yellow O, red. 

Properties. Sulfur trioxide polymerizes easily in a variety of ways at room temperature. The simplest polymer is the trimer (I), or y form, which has the properties listed in Table 7-1 and is the only form of sulfur trioxide marketed as an article of commerce. 

IT-ZI Sulfur trioxide can be prepared as the cyclic trimer, (803)3. Draw Lewis and VSEPR diagrams for such a structure. 

The reaction of olefins with SO3 is exothermic, and often only secondary reaction products are isolated. Attempts to control the cycloaddition reaction involve the use of dioxane- or pyridine-S03 complexes. However, to generate the monomer from the cyclic trimer, freshly distilled SO3 at low temperatures is more effective. Also, oleum seem to have an advantage over the pure sulfur trioxide in [2+2] cycloaddition reactions. ... 

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