Alcohols compared with thiols

Of the crystalline derivatives of thiols, those formed with 3 5-dinitro-benzoyl chloride are not very satisfactory since they have, in general lower melting points than those of the corresponding alcohols (compare Section 111,27,7) and do not diflfer widely from ethyl to n-heptyl. The best results are obtained with 2 4-dinitrochlorobenzene. 

In a recent example metalloporphyrins are used as the sensing dyes for a wide variety of odorants, including alcohols, amines, arenes, ethers, halocarbons, ketones, phosphines, thioethers and thiols. An array of four different metalloporphyrins are placed on a substrate and exposed to the vapours for 30 s. The various vapour molecules coordinate onto the central metal atoms of the porphyrins, causing them to change colour and producing a unique four-colour array. The resulting colour array is compared with a library of known chemicals or mixtures and identification is achieved. 

The bond strengths and heats of formation of a variety of simple thiols and thioethers were calculated from appearance potential measurements (excess energy terms being disregarded) and were compared with the corresponding values for simple alcohols and ethers (Harrison et al., 1966). Using deuterium labelling where necessary. 

The lower reactivity of the secondary thiols when compared with that of the primary thiols is similar to the behavior of the corresponding alcohols. 

The oxidation of thiols follows a completely different course as compared with the oxidation of alcohols, because the capacity of the sulfur atom to form hypervalent compounds allows it to become the site of oxidation. Thiols are readily oxidised to disulfides by mild oxidants such as atmospheric oxygen, halogens or iron(III) salts (Scheme 6). This type of reaction is unique to thiols and is not undergone by alcohols, it is a consequence of the lower bond strength of the S-H as compared with the O-H bond, so that thiols are oxidised at the weaker S-H bonds, whereas alcohols are preferentially oxidised at the weaker C-H bonds (Scheme 7). The mechanism of oxidation of thiols may be either radical or polar or both (Scheme 6). The polar mechanism probably involves transient sulfenic acid intermediates like (7) and (8). In contrast, thiols react with more powerful oxidants, like potassium permanganate, concentrated nitric acid or hydrogen peroxide, to yield the corresponding sulfonic acids (10). This oxidation probably proceeds via the relatively unstable sulfenic (7) and sulfinic acids (9), which are too susceptible to further oxidation to be isolated (Scheme 8). 

Zinc bismuthate Zn(Bi03)2, readily obtained as a light brown solid from sodium bismuthate and zinc chloride, has been successfully employed for the oxidation of alcohols, thiols, thioethers and oximes (Scheme 5.8) [92BCJ1131, 94SC489]. The yields are comparable with or better than those of the barium permanganate and pyridinium chlorochromate oxidations. 

Besides the unusual regioselectivity of alkane oxidation (tertiary C-H bonds are less reactive than secondary C-H bonds ) another interesting feature of the reactive species in Gif-reactions is the unusual relative reactivity of alkane C-H bonds compared with some other readily oxidizable compounds such as alcohols, thiols and triphenylphosphine The explanation of such behavior is not clear so far. 

Alkyne-based MCPs are rarely reported, despite the rich chemistry of alkynes. Compared with traditional polymers, polymers prepared from alkyne monomers generally contain a conjugated polymer backbone and hence possess optoelectronic properties, enabling advanced functionalities of the polymer materials. However, their rigid structure can lead to poor solubility and limit the exploration of efficient polymerization methods [20]. In this review, the most up-to-date progress in the development of alkyne-based MCPs is introduced, including MCPs of alkynes, aldehydes, and amines MCPs of alkynes, azides, and amines/alcohols and multicomponent tandem polymerization of alkynes, carbonyl chloride, and thiols. 

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