Reactions of Thiocyanate Ions with Acid A Laboratory Experiment Christopher Glidewell, James S. McKechnie, a n d Peter J. P o g o r z e l e c University of St. Andrews, St. Andrews, Fife KY16 9ST, U.K. T h e reaction of solid ionic thiocyanates, such as K N C S and NHdNCS, with concentrated sulfuric acid to produce carhonyl sulfide, OCS, along with COz, SO2 and traces of H C N has been known for many years as a convenient small-scale preparation of OCS ( I ) : of greater antiquity is t h e reaction of concentrated sulfuric acid with aoueous solutions of ionic thiocvanates t o s~, yield t h e solid predict xanthan hydride (2) c ~ H ~ N ~first DreDared hv Wohler (3) as long a m as 1821. Xanthan hvdride has'heen characterizedin t h e state by X-ray anal& (4) as 5-amino-1.2.4-dithiazol-3-thione. a n d this constitution . (I). , persists in s&tion also (5).
H SS J Y b yS 1
T h e mechanism of formation of xanthan hvdride from thiocyanate ions a n d acid has recently been elucidated by means of concurrent. a n d svnereistic. a n d theoretical " " . exoerimental . investigation; (5). T h e reaction which forms carhonvl sulfide a n d other gaseous products is widely used as a safe introduction t o vacuum line preparations in lat~t,r.~ti,ry courses in inorganic rherniatry ( 6 ) .\\'I, havt, dt~vt~lr,ped a more extended investigation of thc reilcrimi between ionic thiocyanawa and sulfuric arid. which nut only includes the usu.il \acuum line experiment, but also involves ihe elucidntion d r h e d r u c r u r r nf xanthan hsdride from a comhination of infrared, mass-spectral, and '3C a n d 15N N M R data, together with t h e testing of xanthan hydride a s a pwsihle intermediate in the formation of carhonyl sulfide. W e use this exercise in our third (i.e., penultimate) year lahoratory, where t h e emphasis throughout is on students learning t h e use of a wide range of techniques t o solve real chemical problems (7,8).This laboratory represents t h e final stage of structured experimental work before t h e students undertake research oroiects durine their final vear. T h e experiment fallsUintothreebarts. T h e students have n o prior knowledge of t h e nature of a n y of t h e reaction products, all of which must he identified: (a) reaction of solid KNCS with sulfuric acid (identification of the reaction products); (h) reaction of aqueous KNCS solution with 96% sulfuric acid (isolation and identification of the involatile product); (c) reaction of the solid product from (b) with 96% sulfuric acid (identification of volatile products). Experimental For parts (a) and (c) the apparatus is a simple vacuum system (9) consisting of a rotary oil pump and eold-trap; a manifold equipped with a cold finger. a take-off for an IR cell or molecular weight bulb, and a mercurykanometer; and finally two U-traps to which reaction vessels can be connected.
(a) Reaction of solid KNCS with sulfuric acid Attach a 250-ml two-necked flask equipped with an L-shaped sidearm to the end of the vacuum system, and place in it 5 ml of sulfuric acid: add 4 g of KNCS to the sidearm and evacuate the system. When the system is leak free, admit air to a pressure of 25 cm Hg to the apparatus, place a -78°C bath (ethanol1COa)around the U-trap nearer the reaction vessel and a -196Y! bath (liquid Nz) around the further U-tran. Place a bath of ice water around the reaction flask and r ;acid hy n,tnrmg thr ;~dmrm. gradually add the KNCS r l ~ t h %ulfum A t h rf l n o r ~ s n i cCompoundr," Prentice-Hall. Bnglewod r.,:',",>,, KT...,., .,,, ,*,.,. 7
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(101 Emol6us. H.L Haar.A..and Shsppard, N.. J. Chem Soe.. 3165 (19631. ( I l l Herr..l.H..andlilidewell,C., JMriISLiacture,RS,377 119821.
Volume 61 Number 1 January 1984
79
