[THEMECHANISMOF
THE OXIDATION OF NAPHTHALENE.
I.]
THE OXIDATION O F NAPHTHALENE TO PHTHALONIC ACID BY ALKALINE SOLUTIONS OF PERMANGANATE BY R. ARTHUR DALY
In 1894 Tcherniacl showed that naphthalene may readily be oxidized to phthalonic acid by the action of a boiling solution of potassium permanganate ; and this reaction, which gives a yield of about 80 percent of the theoretical amount, has ever since been used as the best method of preparing that acid.2 The molecule of phthalonic acid contains but nine of the ten carbon atoms present in naphthalene, the tenth (marked with a star in the formula), being oxidized to CO, by the permanganate : C H CH
A/\ CH C CH I II I C H C *CH \A/ CH CH
Naphthalene
C H CO
giving
/\/\ C H C COOH I II CH C
\/\
C H COOH
Phthalonic acid
It is natural to suppose, however, that the phthalonic acid, although the product of the oxidation under the conditions of the method of preparation, is only the last of a number of compounds which have been formed and oxidized in turn. This supposition is supported by the fact that both a-and a-naphthol on treatment with potassium permanganate are converted into phthalonic acid;S and further by the existence of a large number of substances intermediate in composition between naphthalene and phthalonic acid, some of which-like a-naphtho-quinone4-may be prepared direct German patent 79693,March 23, 1894. Graebe und Triimpy : Ber. chem. Ges., Berlin, $-Henriques: Ibid., 21, 1608 (1888). Groves : Jour. Chem. SOC.,26, 209 (1873).
31, 369
(1898).
from the naphthalene by treatment with suitable oxidizing agents. The experiments described in the following pages were undertaken in order to ascertiin, if possible, which of the numerous intermediate compounds whose formulas suggest themselves may be regarded as actual stages in the formation of the phthalonic acid from the naphthalene, and which may not. As phthalonic acid is a ketone, it seemed proper to begin the investigation with the corresponding secondary alcohol, and hydrocarbon. These are known as carboxymandelic acid (existing in the free form as the inner anhydride, phthalid-carbonic acid), and homophthalic acid, respectively. CO.COOH CtlH4 COOH Phthalonic acid.
CHOH.COOH C,H,
COOH
Carboxy-mandelic acid.
CH,.COOH CBH,
COOH
Homophthalic acid.
Neither of these has been detected in the product of the action of permanganate on naphthalene-it would indeed be difficult if not impossible to detect small quantities of thembut this is no proof that they were not first formed and then oxidized further. Measurements of the rates at which naphthalene, homophthalic acid, and carboxy-mandelic acid are oxidized by potassium permanganate were accordingly undertaken, the idea being that if the two last named were really intermediate stages in the formation of phthalonic acid, they must be much more quickly oxidized than naphthalene itself, under the conditions which lead t o a good yield of phthalonic acid. The formation of phthalonic acid from naphthalene and potassium permanganate may be represented by the following gross chemical equation : CI,H,
+
I 6/3
KMnO, = C,H,O,
+ CO, + H,O + I 6 / 3 MIIO, + 8/3K,O.
As will be seen from this equation, the carbon dioxide formed in the reaction is not enough to neutralize the potash, so that the oxidation takes place in a solution which gradually
Oxidation of Naphthalene to Phthaloizic Acid
95
becomes more alkaline as the reaction proceeds, and whose alkalinity is apt t o be affected by the formation of carbon dioxide in subsidiary reactions, for as noted above, the yield of phthalonic acid is only about 80 percent of the theoretical amount. It was necessary, therefore, t o see whether the degree of alkalinity affects the rate of oxidation of the naphthalene ; and preliminary experiments were undertaken with this object in view. Five grams of naphthalene and 2 5 cc of a decinormal solution of potassium permanganate were placed in each of four glass-stoppered bottles, which contained, respectively, a 40 cc of decinormal potassium hydrate b 40 cc decinormal sodium bicarbonate (8.4 grams per liter) c 40 cc water d 40 cc decinormal sulphuric acid
All the solutions were a t the temperature of the room; the bottles were shaken for five hours and a half, and then the residual permanganate was determined. The analyses showed that in a 14.9cc permanganate had been reduced
b 11.5 cc c 5 . 7 cc " d all t h e permanganate
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