Selenium oxoacids

With concentrated nitric acid, selenium and tellurium form only their +4 oxoacids, H2Se03 and H2Te03 respectively, indicating a tendency for the higher oxidation states to become less stable as the atomic number of the element is increased (cf. Group V, Chapter 9). 

Although the oxoacids of selenium and sulfur have comparable acid strengths (pITai 2.6 vs. p fa 1-9, respectively, for the quadrivalent species pA a 3 for both the hexavalent species), the hydride H2Se is much more acidic than H2S (pATai 3.9 vs. 7.0). Thus, while thiols such as cysteine are mainly protonated at physiological pH, selenols such as selenocysteine are predominantly dissociated under the same conditions. 

Sulfur and selenium form typical acidic oxides, Ch02 and Ch03, which form oxoacids with water. The oxides are also redox-active. Sulfur dioxide acts as both an oxidant and a reductant selenium dioxide is an important oxidant in organic chemistry. 

The trioxides, SeOg and TeOg, are also known, but they are not obtained by oxidation of the dioxide as is the case with SO3. The selenium and tellurium trioxides are usually prepared by dehydration of their respective oxoacids, H2Se04 and Te(OH)6 or (HgTeOe). 

Structures of selected sulfur and selenium oxides and oxoacids. (o) the molecular structure of sulfur dioxide,... 

Table 17.2 shows the formulas of the catenated sulfur oxoacids and one resonance structure for each corresponding anion. Note from the table that not all these acids exist in the free state, but all the anions are well-known. None of these acids or anions have selenium or tellurium analogs in which aU the sulfur atoms are replaced by a heavier congener. 

These trends are general ones, observed with other oxoacids of the nonmetals. Recall, for example, that nitric acid, HNO3 (oxid. no. N = +5), is a strong acid, completely ionized in water. In contrast, nitrous acid, HNO2 (oxid. no. N = +3),isaweakacid(JCa = 6.0 X lO"" ). The electronegativity effect shows up with the strengths of the oxoacids of sulfur and selenium ... 

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