GUEST
AUTHOR
James 5. Swinehart Americon University Woshington, D. C.
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Textbook Errors, 52
The Decolorization of Baeyer's Reagent by Primary and Secondary Alkanols
Several textbooks and laboratory manuals' state that primary and secondary saturated alcohols decolorize neutral dilute aqueous potassium permanganate solution (Baeyer's reagent) a t room temperature. They also state or imply that decolorization occurs as rapidly or almost as rapidly as the decolorization by alkenes and alkynes. No example of the decolorization of neutral potassium permanganate within 5 min by pure primary or secondary alkanols could be found in the literature. In fact, some investigators recommend the use of ethanol (a primary alkanol) as a solvent for alkenes and alkynes in the Baeyer test.%, One investigator3 reported that methanol, ethanol, and 2-propanol do not decolorize neutral dilute permanganate within 5 min at room temperature. Since the three alcohols mentioned are hardly an exhaustive list and since not much else could be found in the literature to substantiate or refute the textbook statements, a decision was made to test a large number of primary and secondary alcohols with Baeyer's reagent. These results are summarized in the table. The last three compounds in the table are unsaturated. These were included so that facile comparison of the reactions of alkanols and unsaturat,ed compounds could be made. The tests were done by shaking 0.2 ml of compound with a drop of aqueous neutral 2% potassium permanganate solution. If the drop was decolorized, more permanganate solution was added a drop a t a time until no more permanganate was decolorized. Becauseof the formation of manganese dioxide it could not be determined whether or not more than five or six drops of dilute permanganate were decolorized. The alcohols were selected a t random from the stockroom and, except for ethanol, were deliberately Suggestions of material suitable for this column and guest columns suitable for publication directly are eagerly solicited. They should be sent with as many details as possible, and particularly with referenoes to modern textbooks to Karol J. Mysels, Department of Chemistry, IJniversity of Southern Cdifornia, Los Angeles, California 90007. 1 Since the pnrphse of thls column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errore discussed mll not be cited. In order to be p resented an error must occur in at least two indenendent recent standmd bdbooks. SERINER, RALPH L., FUSON,REYNOLD C., AND CURTIN, DAVIDC., "Systematic Identification of Organic Campoundtr," 4th ed., John Wiley and Sons, New York, 1956, p. 133. 3 IPATIEFF, V. N., THOMPSON, W. w., AND PINES,HERMAN, J . A m . ChemSoc., 70,1658(1948).
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first tested without purification. The names of the alcohols and their original sources are in the first column of the table. The amount of time required to decolorize each of the drops is shown in the table. If the drop was decolorized immediately, an I is recorded. If more than 40 min were required to decolorize a drop, the infinity sign is recorded. All the alkanols that decolorized permanganate solution immediately or decolorized more than one drop before 5 min were distilled through a short Vigreaux column (maximum number of theoretical plates, 21/2) into three equal frmtions. The tests were repeated on the middle fraction, and the results are recorded in the lower part of the table. The three unsaturated substances, cyclooctene, methyl acrylate, and ally1 alcohol, gave the same results whether or not they were similarly purified. The table shows that none of the alcohols gave positive Baeyer tests after sufficient purification. For every alcohol except 1-heptanol distillation through a small Vigreaux column was sufficient to remove any permanganate-reducing impurities. This sample of 1-heptanol showed a significant infrared absorption a t 5.82 which is the absorption frequency of aldehydes. Distillation of the 1-heptanol through a twenty plate column gave a large fraction (bp 175-6'C) that had no carbonyl-infrared absorption and gave a negative Baeyer test. In order to be certain that the absence of reaction by the alcohols was not due to poor mixing, 0.2 g of alcohol was dissolved in 2 ml of ethanol, and the tests described above were repeated on some of the alcohols in the table. This procedure was also used for the solid alcohols shown in the table. The reactions with permanganate in ethanol were essentially the same as the reactions without the ethanol. That is, when dissolved in ethanol, the three liquid olefinic substances and some unpurified alcohols very rapidly decolorize permanganate, and some unpurified alcohols and all the purified alcohols decolorized permanganate very slowly or not a t all. I n no case did the use of ethanol cause a liquid alcohol to react more rapidly with neutral permanganate solution. The error that primary and secondary alkanols give uositive Baeyer's tests has a t least two probable origins. The first i s t h a t primary and secondary alkauol~~vill reduce aqueous permanganate under a variety of Conditions; such as when heated or when the solution is alkaline or acidic. The reduction of permal~ganate with alcohols in glacial acetic acid has been proposed
Reactions of Alcohols with Dilute Neutral Potassium Permanganate a t 27'C
Compound
1 drop
2 drops
3 drops
4 drops
5 drops
Methanolo Ethanol6 1-Propanolc %Propanold l-Butanoln 2-Butanol* 2-Methyl-l-propanole tert-Butyl alcoholJ 1-Pentanolg ZPentanola 3-Pentanolh 3-Methyl-l-hutanol' 2-Methyl-l-hut,anolJ l-Hexanolh %Hexilnoli l-Heptanoli 4Heptanoli 1-Octanola l-Oetanoli 2-Ethyl-l-hexanoli l-Dodecanolj Cyclohexanol" Benzyl qlcoholc p-Anisvl alc~holi Purified Alcoholsk %Butan01
15 26 5 5 10 1 12 30 I 2 5 4
.m
6 drops
15
35
m
I 15
I
I
I
5
35
m
I
2
3
4
5
5 19
6 32
20
35
I
I
2
6
I 20
I 32
I
I
m
-
m m m
--5
I 5
m
m
I 8 2 5 30 10 I 15 30 I 1
m
4 10
m
m
I m m
I 2
1-~e~tanol 2-Ethyl-l-hexand Benzyl alcohal p-Anisyl alcohol l-Heptanoll Olefinic Compounds Allyl almhol' Methyl acrylateh Cvelooctenem Fisher, certified reagent,. USI, distilled through a 20-plate column. Mathison, Coleman, and Bell. Merck, reagent. ' Student preparation. J Fisher, "highest purity." 0 Mathison, Coleman, and Bell, Practical.
a
as a test to distinguish among primary, secondary, and tertiary alcohol^.^ Another probable cause for the error is that many alcohols, especially primary alcohols (see table) contain impurities which do reduce cold Baeyer's reagent. Since the reduction of aqueous permanganate often produces hydroxide ion,
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the presence of enough of most permanganate-oxidizable impurities would produce sufficient base to catalyze the Rwmn, FRANK O., J. CHEM.EDUC.,30,395 (1953).
Eaatman, yellow lahel. source unknown. i Eastman, white hbel. k Samnle from ton Dart of tahle distilled through - Vimeaux column.' 8 Eastman, white label, distilled through a 20-plate fractionating column. Cities Service Research and Development,. i Relaheled,
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alkaline permanganate oxidation of primary and secondary alcohols. Other oxidation tests can be used to differentiate primary and secondary alcohols from tertiary alcohols. These include the use of permanganate in acetic acid mentioned above, bromine and N-bromosucci~~imide,~ and chromic oxide in sulfuric acid.6 The author finds that the last test is the one most easily mastered by beginning organic students and is the least sensitive to impurities. KRUSE,P. F., GRIST, K. L., A N D MCCOY,T. A,, Anal. Chenz., 26,1319(1954). 6 BOBDWELL, F. G., AND WELLMAN, K. M., J. CHEM.EDUC., 39,308 (1962).
Volume 41, Number 7, July 1964
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