Thioacyl chloride

In organic chemistry, a thioacyl chloride is an organic compound containing the functional group −C(=S)Cl. Their formula is usually written R−C(S)Cl, where R is a side chain. Thioacyl chlorides are analogous to acyl chlorides, but much rarer and less robust. Indeed, one of the simplest, thioacetyl chloride (c.f. acetyl chloride), remained unknown as of 1972.^[1]: 350

General structure of the thioacyl chloride functional group

Most thioacyl halides are "highly coloured, labile oils with pronounced pungent odours".^[1]: 358 Simple alkyl derivatives are volatile.^[1]: 359

The best studied thioacyl chloride is thiobenzoyl chloride, a purple oil first prepared by chlorination of dithiobenzoic acid with a combination of chlorine and thionyl chloride.^{[2][1]: 350–351} A more modern preparation employs phosgene as the chlorinating agent,^[3] by-producing carbonyl sulfide:

PhCS₂H + COCl₂ → PhC(S)Cl + HCl + COS

One example of a thioacyl chloride has been crystallized.^[4]

In general, thioacyl chlorides are produced from passing halocarbons through phosphorus pentasulfide or boiling sulfur. Such is the case for the simplest thioacyl chloride, thiophosgene.^{[1]: 351–352, 354} Thiocarbamyl and -carbonyl chlorides are prepared from the corresponding amines and alcohols by careful thiophosgenation.^{[1]: 356–358}

Reactions

Thioacyl chlorides are electrophilic at the carbon atom. Amines, alcohols, and thiols displace the chloride to form (respectively) thioamides, thionoesters, and dithioesters.^{[1]: 362–365} Friedel-Crafts reaction thioacylate arenes.^{[1]: 369–370}

Cyanide reacts with thioacyl chlorides to form thioacyl cyanides. These thioacyl cyanides are known to dimerize to dicyanoalkenes, eliminating sulfur.^[1]: 365 Likewise thioacyl azide cyclizes to a thiatriazole, and diazo compounds and tetrazoles react to thiadiazoles.^{[1]: 366–368}

Like other unstabilized thiocarbonyls, thioacyl chlorides undergo a variety of dimerization and cycloaddition reactions. "Simple thiocarbonyl halides all show a tendency to dimerize to some degree, especially fluoro­thio­acyl fluorides," the only exception being trifluorothioacetyl fluoride.^{[1]: 370–371} Carbenes attack the double bond to form episulfides, and the double-bond in general is an extremely active dienophile.^{[1]: 371–372}

Safety

Thioacyl chlorides are toxic.^[1]: 359