tailieunhanh - Ebook Advanced practical organic chemistry: Part 2

(BQ) Part 2 book "Advanced practical organic chemistry" has contents: Different elements, process of reaction, process of oxidation and reduction, reaction and mechanism. | Advanced Practical Organic Chemistry 125 6 Different Elements Chemistry Thiols Chemistr y Thiols can be prepared by the action of alkyl halides with an excess of KOH and hydrogen sulphide. It is an SN2 reaction and involves the generation of a hydrogen sulphide anion (HS– ) as nucleophile. In this reaction, there is the possibility of the product being ionised and reacting with a second molecule of alkyl halide to produce a thioether (RSR) as a by-product. An excess of hydrogen sulphide is normally used to avoid this problem. The formation of thioether can also be avoided by using an alternative procedure that involves thiourea. The thiourea acts as the nucleophile in an SN2 reaction to produce an S-alkylisothiouronium salt that is then hydrolysed with aqueous base to give the thiol. Thiols can also be obtained by reducing disulphides with zinc in the presence of acid. 126 Advanced Practical Organic Chemistry Fig. Synthesis of thiols. Properties Thiols form extremely weak hydrogen bonds—much weaker than alcohols — and so thiols have boiling points that are similar to comparable thioethers and which are lower than comparable alcohols, . ethanethiol boils at 37°C whereas ethanol boils at 78°C. Low molecular weight thiols are process disagreeable odours. Reactivity Thiols are the sulphur equivalent of alcohols (RSH). The sulphur atom is larger and more polarisable than oxygen which means that sulphur compounds as a whole are more powerful nucleophiles than the corresponding oxygen compounds. Thiolate ions (. CH3CH,S–) are stronger nucleophiles and weaker bases than corresponding alkoxides (CH3CH,O–). Conversely, thiols are stronger acids than corresponding alcohols. The relative size difference between sulphur and oxygen also shows that sulphur’s bonding orbitals are more diffuse than oxygen’s bonding orbitals. Due to this, there is a poorer bonding interaction between sulphur and hydrogen, than between oxygen and hydrogen. Because, the S–H bond .