Oxygen thioacid

Replacement of Oxygen by Other Chalcogens. Acids derived from oxoacids by replacement of oxygen by sulfur are called thioacids, and the number of replacements are given by prefixes di-, tri-, and so on. The affixes seleno- and telluro- are used analogously. 

Thioacids have a most disagreeable odour and slowly decompose in air. Their boiling points are lower than those of the coiTcsponding oxygen counterparts and they are less soluble in water, but soluble in most organic solvents. An important dithioacid is dithiocarbonic acid (HO—CS2H). Whilst the free acid is unknown, many derivatives have been prepared such as potassium xanthate giving a yellow precipitate of copper xanthate with copper salts ... 

Interest in the organotin derivatives of the acids and thioacids of phosphorus arises from their potential biological action and the wide variety of structures that have been identified by X-ray diffraction. These structures are often particularly complex when the compounds are derived from the partially hydrolyzed mono- or diorganotin compounds, and words such as cubes, drums, crowns, butterflies, clusters, oxygen-capped clusters, and extended clusters have been use to describe them. References to the early work are given in Ref 351, and a recent review is available.352... 

In the case of the sulfur analogues thioesters and thioacids, this delocalization is much less favourable. In the oxygen series, delocalization involves overlap between the oxygen sp orbital and the n system of the carbonyl, which is composed of 2p orbitals. Delocalization in the sulfur series would require... 

If all the oxygen ions in H2G03 are replaced by larger sulphur ions, the acidity should theoretically increase so little is known about the acidity of thioacids that it cannot be stated with certainty whether the predictions of the theory are correct here. 

The alkyl halides (halogenoalkanes), thiols and amines are at the same oxidation level as the alcohols, while acyl halides, thioacids and amides are similarly related to the carboxylic acids. Like oxygen, sulfur can be inserted into a chain to generate the equivalent of an ether such as the thioether... 

In his original paper (52) Saville enumerated a great number of cases to substantiate his viewpoint. His discussion of rule 1 extends to C—B bond cleavage, reactions of aldehydes, thioacids and esters, phosphoryl and sulfonyl compounds. For rule 2 he gives examples of C-O bond fission, substitution at oxygen and sulfur, etc. Some of these are reiterated here. 

In this section we focus on the analysis of the solvent effect and coumaryl tail on the absorption spectrum of some p-coumaric derivatives acid (pCA ), thioacid (pCTA ), methyl ester (pCMe ) and methyl thioester (pCTMe ), see Scheme 5.4. The comparison of the behavior of these systems permits to analyze the modifications introduced by the substitution of a sulfur by an oxygen atom and the influence of the methyl group. As we will show the presence of the sulfur modulates the solvent effect, as a consequence the first two excited states become practically degenerated for pCA- and pCMe- but moderately well separated for pCTA- and pCTMe-. 

Thioacids are acids derived by replacing oxygen by sulphur, and are named by adding thio- before the trivial name of the acid andseleno- and telluro-acids are treated similarly. Examples are H2S2O2 thio-sulphurous acid H2S2O3 thiosulphuric acid KSCN... 

A final complication, arising with sulfur, is that this Group VI element can take the place of oxygen in an oxoacid, so that there is the possibility of one or more S—S bonds, in what is then called a thioacid. We shall meet examples of this when we look at the chemistry of sulfur in detail. 

We considered oxoacids in some detail in Section 11.4. This Section will now give us the opportunity to revise those rules and apply them to the sulfur oxoacids. With sulfur, remember, we have the added complication that an oxygen atom can be substituted by sulfur, giving a range of thioacids, for example thiosulfuric acid (12.18), dithionic acid (12.19) and polythionic acid (12.20). 

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