W. Preston Reeves, Mitchell R. White, and Deana Bier Texas Lutheran College Seguin. Texas 78155
II
One of the standard experiments found in most organic lahoratorv manuals is the nucleonhilic disnlacement reaction. Such experiments as the preparation of n.-hutyl bromide ( I ) and the Williamson ether svnthesis (. 2.) are amone those commonly encountered. In recent years the use of phase transfer catalysis for the generation of dichlomcarhene has been adapted to the undergraduate organic lahoratory (la. 3). Many examples of nucleophilic displacements hy phase transfer catalysis are now known ( 4 ) ;among these is the preparation of alkyl thiocyanates. We have found the preparation of alkyl thiocyanates to he an excellent and versatile experiment for first year organic students. RX
-
+ KSCN PTC R-SCN
This experiment has heen performed successfully when R = primary, allyl, or henzyl, and when X = CI, Br, or I. With
such variations students are able to compare reactivities of substrates and leaving groups, creating a research atmosphere in the laboratory. Generally quaternary ammonium or phosphonium salts have been used as catalysts for phase transfer systems. Recently we have reported the use of amines as effective catalysts for nucleophilic displacements ( 5 ) .This allows students still another option of using different catalysts and comparing results. When the distillation of product was conducted a t atmospheric pressure rather than at reduced pressure as suggested in the experimental section helow, an interesting rearrangemerit occurred, in the case of +,heallyl Gas chromatographic analysis indicated a mixture of allyl thiocvanate and allvl isothiocvanate. This allows the in. structor to introducebmbident kxleophiles and presents the student with a challenge to explain his two products. The infrared spectrum of this class of compounds is quite
56 / Journal of Chemical Education
Nucleophilic Substitution by Phase Transfer Catalysis characteristic and lends itself to analysis hy the introductory organic student. CAUTION!
Renzvl thiocvanate is a skin irritant and lacmator. Rubber gloves ihould be worn and the reaction run under a hood. Ally1 thincyanate and the low molecular weight thiocyanates are lacrymators and should he handled under a hood. The lacrymator properties decrease with increasing molecular weight and the hutyi and higher homalogues may he routinely handled in a properly ventilated lahoratory. In a 250-ml round-hottom flask place potassium thiocyanate (28 g, 0.288 mole), water (30 ml), n-butyl bromide (19 g, 0.139 mole), and tetrahutyl ammonium bromide catalyst (0.75g, 1.2 X 10-:'mole). Introducea stirring magnet, attach an efficient reflux condenser and reflux on a steam or oil bath with vigorous stirring for 76 min. Allow the mixture to cool, and transfer to a separator? funnel. Separate off the aqueous layer. Dry the organic layer with a small amount of MgSO* Separate the product and distill a t reduced pressure: 9.6-12 e (60-7616 yield)' h . ~ 85'/30 . mm. With mire reactive substrates, particularly allyl and benzyl halides. and with iodideas theleavineeroun heatinemavnot he necessary and the reaction may tike leis time.
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Literature Cited 11) (a) ~ i n e rL , P..and wi~lirmm,n.K. I. .. "or~anir ~(~perimenrr," 11.c Heath and CII. I.exinmm. ~ars..1975. ihi dam^. H.,and dl1hnmn. J. R..- ~ . a b ~~ ~~ p ~e tr ~i ~~ ~~ ~ t s in Oryrnir Chemisby''4th &The Marm#llanC~~.. New Yurt, ,919. I21 Danniey. R. L.. and (.rum. .I.11.."ExperimmnlO i ~ m i (.hemistrY."7h~ i Msemillan
cs.. N=W vwk. 1 % ~ . 1:" 1 h h . S . H . .l.rHEM. F:L)IIC.,Sn,fi7X1197:11.
161 U~~?kx,.l.,.Y?n,h~~%~~~, 441 ll97:YL 11ehmlc,w, E. V . . A n e m ~?hem. . lmtrme!. Ed. Em/., 13. 170 119741. 151 Reeves. W. P . m d While. M. R..Synlh?liiO~mm.. 6.199 119761.
' These represent the average yields when the reaction was c r m ducted hy undergraduate organic chem~strgstudents.
