Frederick G. Bordwell
and Keith M. Wellman' Northwestern University
Evanston, lllinois
I
I
A Rapid Test to Distinguish Tertiary from Primary or Secondary Alcohols
In comparing various preparative methods of oxidation of 4-phenylcyclohexanol to Pphenylcyclohexanone we were impressed by the superiority, in terms of reaction time and yield, of the procedure in which a solution of the alcohol in acetone is titrated with a solution of chromic anhydride in aqueous sulfuric acid (1). Since the oxidation is practically instantaneous, we were encouraged to investigate its usefulness as a qualitative test to distinguish tertiary alcohols from primary or secondary alcohols. It appears t o be ideally suited for this purpose. The oxidizing test solution is prepared by dissolving 25 g of chromic anhydride in 25 ml of concentrated sulfuric acid and diluting (cautiously) with 75 ml of distilled water. One drop of this test solution is then added with shaking to a solution prepared by dissolving one drop of liquid alcohol (or about 10 mg of solid Primary or secrmdary alcohol) in one ml of a c e t ~ n e . ~ alcohols react within two seconds to give a precipitate that causes the test solution to beeme opaque and to take on a g ~ e a i s heast. Tertiary alcohols do not appear to react within the time ~pecified.~
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The following primary and secondary alcohols give positive tests under the conditions specified: methanol ethanol I-propanol I-butanol I-pentanol cyclohexylmethsnol phenylmethrtnol 2-propanal 1,3-diphenyl-2-propanol 2-butrtnol 4methyl-2-pentanol 2-octanol
2,4dimethyl3-pentannl cyclopentanol cyclohexanol 2-isopropylcyclohexanol 2-t-butylcyclohexsnol 4phenyleyelohexanoI xsnthydrol diphenylmethanol 1,2,2-triphenylethanol cholesterol
The following tertiary alcohols give no apparent
' Esstman Kodak Fellow, 1961-62.
4 Acetone distilled from potassium permanganate is best. Ordinary commercial acetone gives cloudiness within 20 seconds, but doesn't otherwise interfere with the test. 8 With tertiary alcohols (or an acetone blank) the solution may become slightly cloudy after about 60 seconds, but the orange color does not change. If l(t20% of a. secondary alcohol is present as an impurity, the cloudiness develops more rapidly and a green color gradually appears.
reaction withim the test time specified: 2-methyl-2-butinol 2-methyl-2-pentsnol 2.3.3-trimethvl-2-hutanol
1,1,2-triphenylethanol 1,1,2,2-tetraphenyl-1,Z-ethanedin ethvl 2-hvdroxvisobutvrate
.
.
1,4d&th lcyelohexanol 2-henaamiJo-4,4-dimethyl-1-phenylcycloheanol 2-amino-I-phenylcycloheptanolhydrochloride 2-scetamida-1-phenylcycloheptenol
Primary and secondary alcohols respond much more rapidly to this test than they do to the permanganate in aqueous acetic acid test (8), as may be judged by the usefulness of methanol, ethanol, or 2-propanol as solvents for potassium permanganate in testing for C=C unsaturation (3). The chromic anhydride test is not only much more rapid but is also more decisive. For example, the secondary alcohol 1,2,2-triphenylethanol responds very slowly to the permanganate test; the only basis for judging that the test is positive is a gradual, almost imperceptible, darkening of the purple solution. In contrast, the phase change and color change in the chromic anhydride test is as abrupt and decisive with 1,2,2-triphenylethanol as with other secondary alcohols. The chromic anhydride test is also superior to the Lucas test (4), use of which is limited largely to watersoluble alcohols. The test should find wide application, particularly since infrared spectra are of only limited diagnostic value for distinguishing tertiary from secondary or primary alcohols (6). The presence of amine, ether, ketone, alkyne, or even alkene4functions in the molecule does not appear to interfere with the test. Aldehydes will, of course, respond positively. Phenols react, but those tested behaved atypically in that they gave dark test solutions, rather than the characteristic green color. Enols appear to give positive tests, judging from the behavior of pentane-2,4-dione and 4,4-dimethylcyclohexane-1,2-dione. Application to a Problem
The usefulness of the test may be illustrated by a problem encountered in this laboratory (6)-that of identifying the product formed from the reaction of chloroacetyl chloride with excess phenylmagnesium bromide. The reaction was formulated by the original investigators (7) as involving rearrangement of a hydrogen atom, the overall equation being:
The product isolated on hydrolysis was rcported to be the secondary alcohol 1,2,24riphenylethanol, mp 87.588.5". This assignment of structure was based primarily on the fact that the alcohol had a melting point comparable to that reported (87') for the alcohol formed by reduction of the ketone (C6H.&CHCOCeHs (8). This evidence seemed insufficient to us, particularly since the isomeric tertiary alcohol, C6H6CH2C(OH)(CeH&, is reported to melt at 88' (9). Furthermore, "mpure alkene samples may give positive tests, presumably due to the presence of hydroperoxides. For example, an impure sample of 4methylcyclohexene gave a positive test, but with a distilled sample the test solution became cloudy only after 25 seconds.
there is a reasonable path by which chloroacetyl chloride and excess phenylmagnesium bromide can react to form the tertiary alcohol, and this reaction path is simpler since it does not involve rearrangement. It was therefore of interest to establish firmly the structure of the product from this reaction. The reaction of chloroacetyl chloride with excess phenylmagnesinm bromide was repeated, and the properties of the resulting alcohol were compared with those of an authentic sample of the tertiary alcohol prepared by the action of phenylmagnesium bromide with benzyl methyl ketone (6). The melting points of the two alcohols were nearly the same, as anticipated (7-9). A melting point of a mixture of the two samples was not depressed appreciably, suggesting that they might be identical. Differences in the infrared spectra were apparent, however. The proton NMR spectra (Fig. 1)
Figure 1.
Proton NMR spectra of 1.1.2- and 1.2 2-triphenylethanols.6
showed that the products were indeed different, and gave unequivocal support to the structure assignments for the two alcohols that had been made by the original investigators (7,9). The NMR spectrum of 1,2,2-tripheuylethanol shows a singlet a t 7.777 for the hydroxyl proton6 and unsymmetrical doublets at 6 . 0 8 ~and 5.027 as expected for the AB system (10) represented by the two nonequivalent protons Hband Hc (the coupling constant Jk is 7.1 CPS). The areas under the two doublets and the singlet are in the ratio 1:1: 1, as required for this structure. The S M R spectrum of 1,1,2-triphenylethanol shows a singlet at 7.867 for the hydroxyl proton (H,), and a singlet a t 6 . 4 5 ~for the two equivalent hydrogen atoms (Hh). As can be imagined, the structure assignment arrived a t in this way required considerable time, as well as the The spectra were taken at 60 megaoyoles in carbon tetrachloride solution using tetrsmetbylsilane as an internal standard. , V. D., J. The line positions are given in r units [see T l ~ n s G. Phys. C h a . , 62, 1151 (1958)j. The appearance of a. singlet rather than a doublet in an dcohol containing a single a-hydrogen stom is not unexpected; note, for example, that the signal from the hydrogen atom attached to J. T., Phtl~.Rev., oxygen in ethanol is often a, singlet [see ARNOLD, 82, 443 (1951); or JACKMAN, L. M., in "Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," Pergamon Press, Ine., New York, 1959, p. 28.
Volume 39, Number 6 , June 1962
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use of costly instruments. Application of the qualitative test described herein would have given the same answer (see above) in a matter of a few minutes. Literature Cited (1) BOWDEN, K., ET AL.,J . Chem. Soc., 39 (1946). (2) RIWER. F. 0..J. CEEM. EDUC.. (1953): . 30.395 . . .. FUSON.R.
C., SHRINE;, R. L., AND CURTIN,D. Y., in "The S&ematic Identification of Organic Compounds," John Wiley & Sons, Ine., New York, 1956, p. 133. (3) IPATIEFF, Y. N., THOMPSON, W. W., AND PINES,H., J . Am. Chem. Soe., 70, 1658 (1948).
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(4) L u c ~ s H. , J.,
J . Am. Chem. Soe., 52,803 (1930). (5) BELLAMY, L. J., in "The Infrsrred Spectra af Complex Molecules," John Wiley & Sons, Ine., New York, 1960,
--.
n 96 r.
(6) BRANNEN, W. T., JR., unpublished result$. (7) BOYLE,J. S. W., MCKENZIE, A,, AND MITCEEL,W., Bw., 70, 2153 (1937). (8) SAINT-PIERRE. M. 0.. Rd1. Sac. h i m . (Francel. .131 . 5.. 292 (1891). (9) KLAGES, A,, AND HEILMANN, S., Rer., 37, 1455 (1904). (10) POPLE,J . A,, SCANEIDER, W. C., AND BERSTEIN,H. J., i n
"High Resolution Magnetio Resonrtnoe," &Craw-Hill Book Co., New York, 1959, pp. 119-23.
