G e o r g e R. Newkome, J. W. Allen, and G. M. A n d e r s o n Louisiana State University Baton Rouge, 70803
The Preparation and Dehydration
of 1-Benzyltyd~alkanok
This is a sequential experiment utilizing instrumental analysis for the undergraduate organic laboratory. T h e dehydration of l-benzylcycloalkanols, which were prepared by action of benzylmagnesium chloride with cycloalkanones, was smoothly accomplished b y treatment with hot aqueous oxalic acid. This simple preparative sequence makes a n excellent organic laboratory experiment since: (a) the intermediary alcohols are easily crystallized from petroleum ether or hexane, and ( h ) the dehydration of these alcohols affords two major isomeric olefins.
I I1 Ill LY
(" = 2) (" = 3) (" = 4) (" =
51
Endo-V
Exo-V
T h e convenient glc analysis' of the olefin distribution in conjunction with the n m r integrated spectrum allows t h e structural assignments t o be made for the endo- and exoisomers. T h e endocyclic isomers have characteristically shorter glcl retention times t h a n the corresponding exoisomer, a s well a s show in the n m r spectra a pronounced singlet (6 3.2-3.4) for the isolated benzylic methylene group. On the other hand, the n m r d a t a for the conjugated exocyclic olefins show a signal a t 6 6.3 for the olefinic proton. T h e synthetic portion of this project normally takes about two 3-hr laboratory periods, followed by a n additional couple of hours for the structural analysis of the products. It may b e advantageous to divide the class into small groups, thus assigning the groups to study the olefin product ratios a s a factor of (a) different ring-size of t h e cycloalkanone, (h) various Grignard reagents, or (c) sundry dehydrating agents and conditions. T h e students are
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'Perkin-Elmer Model 900 gas chromatograph equipped with a flame ionization detector and ',&in. X 12 ft. Stainless Steel eolumns packed with 15% (w/w) Apiezon L an acid-washed, 80-100 mesh Chromosorb W. The chromatograph was operated with the column at 200°C, the injector and detector at 250°C with a nitrogen carrier gas flow of 100 ml/min. 2Carhontetrachloride with 1% tetramethylsilane can also he used.
enthusiastic about their individual research-oriented projects a s well as how their d a t a contributes t o the overall class summations. T h e reaction sequence sheds light on topics, such as: carbocation intermediates, steric effects, conjugation, elimination mechanisms, a s well a s t h e initial synthetic aspects. T h e theoretical emphasis is a factor of t h e specific assignment and can be readily molded t o the instructors preferences. Procedures Benzylrnognesiurn Chloride. Place 3.74 g (0.154 g atom) magnesium turnings into a dry 125-ml three-necked flask, equipped with an addition funnel, magnetic stirrer, and condenser with a calcium chloride drying tube. Anhydrous ether is added to cover the turnings. To initiate the reaction, add 5 ml of a solution of 17.74 g (0.140 mole) benzylchloride (bp 177-179'C) and 20 ml of anhydrous ether. After formation of bubbles at the surface of the turnings, add dropwise (at a rate to maintain a moderate reflux) the remainder of the benzyl chloride-ether solution. After the addition is complete, the mixture is gently heated with a warm water bath for 15-20 min. I-Ben~yleyelopentand(I). To the above etheral Grignard reagent, 9.77 g (0.120 mole) of cyclopentanone in 25-ml of anhydrous ether is added with constant agitation. The mixture is stirred at roam temperature for one hour, and then is poured into a cold aqueous ammonium chloride solution. The ether is decanted, washed with a saturated ammonium chloride sqlution, dried over anhydrous magnesium sulfate, and concentrated. After removal of the last traces of ether with the aid of a water aspirator, the residue, upon cooling, crystallizes. Recrystallization from petroleum ether (bp 30-60T) or hexane affords 11.1. g (63%) of the pure l-henzylcyelopentanol: mp 57-58'C [lit. mp 58-60°C (I)]; nmr (CDC131% TMS) 6 7.25 (s, C6H3-, 5H), 2.85 ( s , AICHP. 2H), 1.62 ( m , 8H), 1.43[s, -OH (eliminated with added DzO), lH]. Similar reactions of benzylmagnesium chloride with cyclohexanone, cycloheptanone, and cycloactanone afford (40-80%) 1henzylcyclohexanal [II; mp 58-59°C (needles, heaane), lit. mp 61-62°C (lo)]; l-benzylcyeloheptanol [In; mp 44-45'C (needles, heuane), lit. mp 46°C (la)]; and l-benzylcyclooctanol [IV; mp 32-34°C (needles, hexane), lit. mp 35°C ( l a ) ] ,respectively. Similar nmr spectra are recorded for II-IV: (CDC13-1%TMS) 8 7.25 ( s , C&-, 5H), 2.75 ( 8 , ArCH2-, 2H), 1.5 (s, HO-, lH, eliminated by addedDzO), and 1.3-1.8 (broads, -CHs-1. General Procedure for Dehydration. l-Benzylcyclopentanol (2g, 12.6 mmol) and aqueous oxalic acid (25 ml, 20%) are refluxed for Ihr. The mixture is extracted with ether (2 x 20 ml), washed consecutively with water, 5% aqueous sodium bicarbonate, and water and then dried with anhydrous magnesium sulfate. After filtration, concentration affords the crude olefin mixture. Product Analysis. For quantitative glc analysis, 1 r l of this reaction mixture is injected into the gas chromatograph' affarding: (a) the major component (76%), retention time 8.6 min, and
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Olefin Mixtures from Oxalic Acid Dehydrations of 1-Benzylcycloalkanols Endo Olsfin ale
E m Olefin
Alcohol
%
R.T. (mi")
%
6ArCH2
I I1 Ill
76 98 73 98
8.6 14.5 21.5 36.0
78 96 76 94
5.35(ql 3.231~1 3.24(81 3.26(51
IV
372 /Journal of Chemical Education
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SIC
nmr
J
5.35(q) 5.47iml 5.63(11 5.31i11
nmr
H
L
%
R.T. (rninl
%
24 2 27 2
14.0 20.0 30.0 455
22 2 24 6
r-
6 1\'
6.3jIql 6.25(11 6.27if) 627Ial
(h) the minor component (24%), retention time 14 min. Colleetions of analytical samples can be accomplished by standard procedures (2). A portion of the crude reaction mixture ( - 100 .I) is dissolved in 400 to 500 of CDCla2 containing tetramethylsilane as the internal standard. The spectra were obtained using standard nmr tubes on either a Varian A-60 or Perkin-Elmer R-12B nmr spectrometer. The integration of the singlet a t 6 3.4 (Ar-CH2) to 6 6.35 A~CH=C< indicated an endo to em ratio of 7822. Thus, the major isomer is 1-benzylcyclopentene (endo-V, n = 2) and the minor isomer is benzvlidenecvclo~entane(em-V. n = 2). ,~~ The table cites the glc and nmr data forother cyclic olefins prepared via this method. The preparation of these olefinic ~
mixtures from I i I , 3-61, I1 (5-7). and ods has been previously accomplished.
In (5,81 by diverse meth-
Literature Cited i l l (a1 S t d b , X., and Winter. W..Arzneimilfel-Furrth. 12, 149 (1962): C h e m Abstr, 57, 16453d (19621: lbl Schaeffer, H. J., and Collins. C. J.. J Arne,. Chem Soc.. 78, 124 (19561; (cl Smejks!. J., and Farkas, J.. Coil. C m c h Cham. Camm.. 28,
ko. A. I., Mstirher, A. F., and Karnitrksya. A. X.. Zh. Or8 f
X,? ,."",,.
(7) Chritol. H.,Lsunnt. A..andMousseron.M..Bu!i. C h i m Soc.. FT..23L3t19611 181 Simonyi.1.. Keka~y.I..sndToth.Z..Pharmazia 22.23119671.
Volume 50, Number 5, May 1973 / 373
