The Preparation and lsomerization David W. Emerson
University of Michigan Dearborn, Michigan 48128
of Allyl Thiocyanate A n organic chemistry experiment
Aside from a textbook description of the Claisen rearrangement of allyl phenyl ethers, the firstyear student in organic chemistry is little exposed to the topic of allylic rearrangements which are ccmmonly eucountered later in the practice of organic chemistry. The student is even less likely to he aware that displacement reactions a t atoms other than carbon often occur. The present experiment has been devised to focus attention on two displacement reactions, one at carbon and one a t sulfur (1)' and the facile allylic rearrangement of allyl thiocyanate (111) to allyl isothiocyanate, (IV) (4,5),which occurs during the distillation step (eqns. (1)-(3)).
marked differences (-S-C=N stretch 2160 em-' and -IT=C=S stretch at 2081 em-I).3 If desired, allyl isothiocyanate can be allowed to react with aniline to form the solid derivative N-ally-K'-phenylthiourca (VIII).
CHz=CHCH2Br I
To 74.5 g (0.3 mole) of sodium thiosulfate pentahydrate in a 500-ml round-bottom flask add 100 ml of 95% ethanol and 36.3 g (0.3 mole) of ally1 bromide., Fit the flask with a reflux condenser and heat on s. steam bath. Over R. period of 45 min add 100 ml of water through the top of the reflux condenser in approximately 25-ml portions. After 45 min, the liquid layers coalesce and heating should he continued for an additional 15 min. Pour the reaction mixture into a. 500-ml Erlenmeyer flask externally cooled with ice to promote rapid cooling. Stopper the fla5k.4
+ 2 Na+ + SsOal-
+ -
CHB=CHCH~SSO~- 2 Nu* I1 CHI=CHCHISSO~I1
+ Br-
(1)
+ 2 Ns+ + CN-
I1
CH2=CHCHxNHCNHCsHs VIII
Experimental Preporation of Sodium Allyl Thiosulfote
Preporofion of Allyl Thiocyonate
the^ isevidence to suggest that reaction (3) proceeds via a quasi six-membered ring transition state V
Prepare a. solution of 14.7 g (0.3 mole) of sodium cyanide in 150 ml of ice cold water and add it to the reaction mixture after the latter has been cooled to 20PC. Stir t h e mixture, stopper the flaskhnd allow it to stand in an ice bath with occa+mal swirling for 15 min. The separation of the oil begins almost at, once. Xxtract the reaction mixture with two 50-ml portions of ether. Combine the extracts and dry by swirling with 3 g of anhydrous magnesium sulfate until the solution becomes clear. Place approximately 1 ml of the ether solution on a clean watch glass and let the ether evapporrtte to leave a sample of ally1 thiocyanate for infrared a n d y ~ i s . ~ lsomerizotion of Allyl Thiocyanate to Allyl lsothiocyonote
with migration of the SCN group from Carbon 1 to Carbon 3 (6-8). The evidenceZconsists mainly of the observations that crotyl thiocyanate (VI) isomerizes to or-methylallyl isothiocyanate (VII) (6,s)
the low activation energy (23.8 kcal/mol) and large negative entropy of activation (-9.4 cal/deg/mole) in the kinetically first-order reaction in which allyl thiocyanate isomerizes to allyl isothiocyante (7); and the, insensitivity of the first-crder rate constants to solvent polarity (7,s). A dramatic change in odor accompanies the rearrangement, and the infrared spectra of I11 and IV display
Equip a 50-ml'round-bottom flask with a Claisen adapter distilling head, thermometer, condenser, adapter, and receiver. Add the ether solution to the distilling flssk through a dropping funnel while the flask is being simultaneously heeted on a. steam Far a review of nucleophilic substitution at sulfur, see reference ( 8 ) . Far an interesting application of the reaction see reference (8). For an excellent review of this reaction see reference (9). a The group frequency for the thiocyanate group of ally1 thiocyanate seems not to have been reported, probably because the compound is unstable. We found it to he 2160 cm-I (liquid film) (lo). 'Some people find the odor of organic sulfur compounds objectionable and the escape of odors can he minimized by keeping the containers closed ss much as possible. The odor of allyl tbiacyanste is reminiscent of garlio while ally1 iaathiocyaoate iu the compound which provides the odor and "bite" of freshlv eraund horseradish. The % % r e d spectrum should be obtained promptly because ally1 thiocyanste rearranges to the isothiocysnete even at room temperature. Volume 48, Number I , January 1971
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bath. After the ether is removed take out Claisen adapter and attach the boiling flask to the distilling head. Distill the ally1 isathiocyanrtte using an oil bath, and collect the fraction boiling a t 150-2°C. A yield of roughly 16 g can be expected. Analyse a sample of the product by infrared spectroscopy. Total elapsed time for the experiment is 3.5 hr.
Literature Cited (1) FOOTNEB, H.B.. A N D SMIGEB, S.,J . Chem. SOC.,127, 2887 (1925). (2) CIUFFARIN, E.,A N D FAT*,A , , P r w . Phys. O w . Chem., 6,81 (1068).
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(3) TONELATO. U.,ROBSETTO. 0..AND FAY*.A,, . I . Ow. Chcn.. 34, 4032 (1969). (4) GERL,CH, G,, Ann,, 178, (1875). c5) B ~ 0,. B",, ~8,462 (1875). ~ ~ ~ ~ ~ (6) MUMN. 0.. A N D Rlcmr~n,H., Ber.. 73, 843 (1940). (71 SNITX, P. A. S., AND EMERSON. D. W.. . I . Am. Chem. Soc.. 82. 3078 (19601. A,, FAT*,A,. A N D MAZZUCATO, U.,Tetrahedron Lelters, 11, 27 (8) ILICETO. (1960). (9) FA"*. A,, olgonic sulfur compound8. 4 7 3 (1966). (10) BELLANY.T.J.. "Advances in Infrared Group Frequencies," Methuen and Company. London. 1968, Chapter 3.
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