A Hydration of an Alkyne Illustrating Steam and Vacuum J. P. Wasacz and V. G. Badding Manhattan College, Bronx, NY 10471 In most schools, organic chemistrv lahoratorv is taken siIrrturt~and it is often multaneously with orinnir diifirult to find experiments relatinr to rarlv I t ~ t ~ lmaterial. re In search of s u c h m experiment to demonstrate steam and vacuum distillation while also introducing an organic preparation, we adapted a procedure that one of us had previously used in his research ( I ) . We report here on the conversion of 2,5-dimethylhexyn-2,5-diol(1)to 2,2,5,5-tetramethyltetrahydrofuran-3-one(I1) using aqueous mercuric sulfate without the use of acid.
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During the past ten years, this experiment has been performed with consistently good results by over 1200 students during the middle of our first semester Organic Chemistry ~ahoritory. A survey of current organic chemistry laboratory manuals reveals that very few have experiments involving alkynes. Brewster, VanderWerf, and McEwen (2) include the preparation and properties of acetylene; however, this is a dangerous and generally impractical experiment. Fieser and Williamson (3)present an experiment on the oxidative coupling of a terminal alkyne. Pasto and Johnson (4) suggest a very general procedure for hydration of an alkyne. Yip and Dalton ( 5 ) have an experiment on the hydration of 3-methylbutyn-3-01 to 3-methvl-3-hvdroxv-2-butanone (originally reported by Rose (6)), b i t thi; procedure uses mercuric oxide, concentrated sulfuric acid and an extraction with ether (5) (or benzene (I?)), which requires subsequent solvent distillation. An additional disadvantage we have noted in this latter procedure is the formation of elemental mercury in the steam distillate when it is stored overnight before salting out is performed. Furthermore, in the latter procedure, steam distillation is not readily apparent because only one layer is observed in the receiver flask. Discussion The conversion of 2,5-dimethyl-3-hexyn-2,5-diolt o 2.2.5.5-tetramethvltetrahvdrofuran-3-one demonstrates all of thk following pknciples and techniques: Hydration of an alkyne Steam distillation Vacuum distillation (optional) Drying of an organic liquid
694
Journal of Chemical Education
Stereochemistry Synthesis of a ketone Keto-enol tautomerism Synthesis of a cyclic ether, and Salting out techniques (optional). Since this reaction involves several functional groups, this experiment could also he utilized at other times in the lahoratorv ~, course ~ ~ ~ ~~~~-~~ A maior advantage of this reaction is that the mercuric sulfntp mixture after steam distillation may b~sawd nnd used in future distillat;ons, thus reducing both cusr and pusaihle pollution. Other advantages inherent in this procedure are: Inexpensive starting materials Concentrated sulfuric acid is not used High yield Mercurv was not detected in the steam distillate Poor d$ng and distillation technique by the student is readily apparent (water in final distillate) Product is not commercially available, and The experiment can be completed within four hours. Prior to performing this experiment in our first-semester sequence, students have synthesized an alkene by dehydration of an alcohol. During that experiment, formation of an ether by-product is emphasized. Our experiment again demonstrates ether formation through alcohol dehydration and provides an excellent contrast between the competing reactions of elimination to form alkenes and substitution to form ethers. The students are told that hydration of the alkyne must precede dehydration to the cyclic ether (7) because the reverse sequence would necessitate a highly strained cyclic alkyne(II1):
%Xrn CH,
CH,
m T o extend the synthetic possibilities, other homologs were tested. However, for the preparation of 2,5-diethyl-2,5-dimethyltetrahydrofuran-3-one, the steam distillation took much too long for a 4-hr laboratory and for the preparation of 7oxa-dispiro[5.1.5.2]pentadecan-14-one,steam distillation was not observed. Experimental Procedure This method was adapted from H. Riehet (8). In a 250-ml distillating flask place 0.15 mole of 2,5-dimethyl-3-
hexyne-2,s-diol (Note I), 100 m l of water, and 1.0 g of mercuric sulfate (handle with caution). The mixture is steam distilled using either external or in situ steamgeneration (Notes 2,3,4). After 2 or 3 min, two layers are evident in both the distilling and receiving flasks. Terminate steam distillation when only water is condensing (Note 5). Place the residue from steam distillation in the bottle provided. Do not discard. The layers are separated (Note 6 ) and the 2,2,5,5tetramethyltetrahydrofuran-3-one is dried with anhydrous sodium sulfate for 10-15 mi". Distillation usually yields 14-17 g of 2,2,5,5tetramethyltetrahydrofuran-3-one (h.p. 7&75'C/60 Torr or 1501516C/760Torr, d P = 0.925) (Note 7). Acknowledgment this Our thanks t o all the students who h e b e d us to d e v e l o ~ experiment, especially: James Arena, Maureen Casey, T o m D'Arrieo. Marearetta Diemer. R a v McCarthv, James Moore, T o m go&k, a i d Gerald ~ h e p h e i d . Literature Cited (1) Hagens.G., Wssarz, J. P., Joulli&,M.,andYates, P.,J. Or#. Chem.,35,3682 (1970). (2) Breunter. R. Q., VsnderWerf. C. A , and McEwen, W. E., "Unitized Experimene in Organic Chemistry: 4th Ed.. D. Van Nostrand Campany, New York, 1977, p. 136. (3) Fiaser, L. F.. and Williamson, K.L.. "Organic Experiments: 4th Ed.,D.C.Heath and Company. Lexington, Massachusetts,1979, p. 292. (4) Psstn, D. Land John8on.C R.,"Labomto'yText for O'ganiiChhiit'y,"hentitie-Hall, EnglewoodCliffs,N.J., 1979, p. 359.
(51 Yio. M. T..and Dalton. D. R.,"Or~anicChemlstm in the Laboratory," D. Van Ncetrand, N.W Y&k, 1979, p. 62. (6) Rose, N. C., J. CHEM. EDUC., 43,324 (1966). (7) Froning, J. F.,and Hennian. G. F.,J.Amer Chem. Soe., 62,653 (1940) (8) Richet,H.,Ann. Chim., 3,317 (1948).
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Obtained from Air Products and Chemicals. Allentown, PA i.-n i n 5 if the mercuric sulfate residues from previous steam distillations are saved for recycling. 100 ml of me residue mixture is used with 0.15 mole of the solid 2.5dimethyl-3-haxyn-2,5d1ol. To keep the concentration of the mercuric sulfate in the residue mixture in the correct range, in situ steam generation must be assigned to about 50% of the students while the other students use external steam generation. 'If the recyclmg procedure described above 1s not Lseo, the 2.5d!memyl-3-hexyn-2,5-01olmay oc anspensea easily as 100-ml ponlons of 1 5 Maqueous SO Luon 10 which the student must ado 1 0 g of mercuric sulfate. The use of sulfuric acid leads to by-product formation. The most obvious indications of completion of steam distillation in this reaction are an increase in distillation temperature from 98" to 100°C as well as foaming in the distilling flask. For increased yields, the steam distillate should be saturated with potassium carbonate sesquihydrate. If vacuum distillation is used, the product may not condense properly below 60 Twr, therefore the pressure must be controlled using a bleed valve.
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Volume 59
Number 8
August 1982
695
