FRITZ FEIGL Laborat6ria da Producio Mineral, Rio de hneiro, Brazil (Translated by Ralph E. Oesper, University of Cincinnati)
IT
WAS shown in a series of papers1 that in many instances spot tests and drop reaction provide a valuahle teaching aid for a variety of chemical facts and principles. Certain laws and principles may be demonstrated a t the lecture table and, if need be. enlar~ed by projection apparatus. Such spot reaction demonstrations offer not merely a supplement to well-known Iarge-scale demonstrations. The simplicity, the slight consumption of material and time also make the repetition of such experiments feasible by the individual student, who thus is made acquainted with the technique of working with small quantities of materials. The examples described in the earlier papers dealt primarily with inorganic chemistry, in conformity with the fact that for a long time inorganic analysis was the preferred field for spot reactions. Recently there has been a marked development of organic spot test a n a l y s i ~ which ,~ was guided in large measure by the experiences and modes of thought that are included in the chemistry of specific, selective, and sensitive reaction^.^ The development of new spot tests for organic analysis led t o the discovery of reaction modes of organic compounds, which are of interest as spot tests for teaching purposes.
PYROLYSIS OF ORGANIC COMPOUNDS
The dry heating (pyrolysis) of organic compounds sometimes yields characteristic gaseous products whose detection or identification may give information of value in the so-called "preliminary examination" of Little is samples in organic known about the mechanism of pyrolytic cleavages. They may often involve various kinds of reactions, m.hich proceed simultaneously or in succession, depending on the nature of the particular organic com~ o u n d s(~resenceof certain grou~s),the conditions bf heating (temperature range and rate bf heating), and the presence of other organic or inorganic materials. Pyrolysis often involves also redox reactions and hydrolytic fissions. For example, heating an organic material with potassium nitrate yields nitrous acid, which can readily be detected in the gas phase by the familiar Griess reaction. This formation of nitrous acid may be accounted for
' FEIGL,F., J. CHEM.EDUC., 20, 137,174,240,294,300 (1043); 21. 347. 479 (1944):. 22.. 36,. 342. 554 (1945): FEIGL. . F... AND G. B.HEISIG,J..CHEM. EDITC., 29, iv2 (1652). FEIGL.F.. ' ' S ~ o Tesh t in Oreanic Analvsis." 5th ad.. Elsuvier. ?Jew'~&k.i956. a FEIGL,F., 'Chemi~tryof Specific, Snlectiva and Scnaitivc Reactions," Academio Press, New York, 1949. ' See footnote 2, p. 65. VOLUME 34, NO. 9, SEPTEMBER, 1957
as follows: The contact of carbonaceous pyrolysis products (solid, liquid, gaseous) with hot solid potassium nitrate gives rise to a topochemical solid-body reaction which can be represented schematically by the eouation 2KN03
+C
-
2KN01
+ C02
(Analogous redox reactions involving solid molybdic trioxide and potassium iodate are also used for the detection of carhon-bearing compounds?) The pyrolysis of organic compounds containing hydrogen and oxygen always leads to the release of water a t temperatures far above its boiling point. Under such conditions, the water is present as a sort of superheated steam and acts as such. Accordingly, hydrolysis may follow the nitrite-formation just shown: KNO,-
+. HqO -
-
KOH
+ HNO,
It is obvious that, at the prevailing temperature, the nitrous acid will be produced and then volatilized to a much greater extent than is possible in a similar hydrolysis in aqueous solution. Consequently, the generation of nitrous acid when organic compounds are heated with an alkali nitrate provides a clear proof that redox reactions and hydrolytic decompositions are possible during the pyrolysis of organic compounds. Procedul-e: A micro test tube is used. A few milligrams of a dry mixture of potassium nitrate and sugar are introduced and the mouth of the tuhe is covered with a strip of filter paper moistened with Griess reagent. The mixture is then heated to earbonizntian or caramelization over a. micro burner. A red ,bin appears on the reagent paper because of the generation of nitrous aeid. Reagent: A frefihly prepared mixture of equal volumes of a 1% mlution of sulfanilic aeid in 30% acetic acid and a 1% solution of a-naphthylamine in 30% acetic acid.
REDUCTION OF NITRIC ACID BY BENZOYL PEROXIDE
The familiar fact that nitric acid is reduced to nitrous acid by nascent hydrogen is often employed in analysis. Therefore, it is surprising that this reduction can also be accomplished by means of benzoyl peroxide, which is frequently used in organic preparation work as an oxidant. The reaction between nitric acid and henzoyl peroxide (if water is excluded) proceeds: HNO.
-
+ (C6H&O)202
(C8HjCO),O
+ Or + HNOs
There may he an initial production of pelnitric acid, which decomposes: HNOr 6
FEIGL, F.,
AND
HNCL
+ O*
D. GOLDSTEIN, Mikrochim. Acta, 1956, 1317.
457
I n this reduction of nitric to nitrous acid, benzoyl peroxide shows itself as an organic derivative of hydrogen peroxide, which is well known to function as a reducing agent. Typical instances are the reduction in acid surroundings of MnOl-, Fe(CNS)-3, and Au+% ions, as well as the reduction in alkaline surroundings of alkali hypohalogenites to alkali halides. Proeedu~e: A micro drop of conoentrated nitric acid or a. small amount of the nitrate of an aliphrttio base is mixed with 1 centigram of benzoyl peroxide in a micro teat tube. The mouth of the tube is covered with a disc of filter paper moistened with Griess reagent (compare above). The test tube is then placed in boiling water. A red stain due to nitrous acid appears almost a t once with nitric acid, and after 2 minutes a t most with s. solid nitrate of an aliphatic base.
PRODUCTION OF SULFUR DIOXIDE FROM THIOUREA BY BENZOYL PEROXIDE
If thiourea is heated with benzoyl peroxide (m.p. 106') there results a characteristic replacement of the sulfur with formation of sulfur dioxide: NH*
/
SC
N ' H,
-
NHs
+ 3(C~HaC0)20z 0d
+ 3(CeHsCO)~0+ SO,
'sH,
This redox reaction cannot be realized in the wet way with hydrogen peroxide and its derivatives. The following experiment accordingly clearly demonstrates a hitherto unknown reaction mode of fused benzoyl peroxide. Other characteristic oxidative decompositions which may be accomplished with henzoyl peroxide and which are useful in analysis are given in Chapter IV of "Spot Tests in Organic Analy~is."~ The sulfur dioxide formed from the thiourea can be sensitively detected by the conversion of green nickelous hydroxide into the black oxy-hydrate of nickel(II1) or nickel(1V) in the autoxidation of sulfur dioxide.=
' FEIGL,F.,A N D E. FRAENKEL, Bw.,65,545 (1932)
Other orgauic compounds containing X S (thioketo) groups give an analogous reaction. This has led to the development of a test for this functional group. Pmedure: A milligram or two of thiourea is treated in a micro test tube with a drop of a 10% solution of henzoylperoxidein benzene. The mixture is taken to dryness in a water bath. The mouth of the tube is covered with a. strip of filter paper impregnated with nickelous hydroxide. The tube is immersedin aglycerol bath that has prcviouslv been brought to 120". A black stain appears on the-green paper within 1-2 minutes. Reagent paper: Strips of filter paper are bathed in a 30% solution of NiSOa.6H90in coneentrst,ed ammonium hvdroxide. The strips are dried and then bathed far several minutes in 1 sodium hydroxide to obtain a homogeneous deposition of Ni(OH)2 in the pores of the paper. Tho paper should be washed with water, but it should not be allowed to dry. If stored aver moist cotton wool, it will keep for months. ~
~
~~
LOSS OF SULFUR DIOXIDE FROM THIOGLYCOLIC ACID BY REACTION WITH BENZOYL PEROXIDE
A singular mode of reaction of henzoyl peroxide was described above. Much more amazing is the finding that thioglycolic acid is not oxidized to disulfide by benzoyl peroxide (as is the rule with aliphatic and aromatic compounds containing SH-groups). The following equation can be written for the reaction which takes place at even as low a temperature as 100°C. H%CH2-COOH
+ (C6H,CO)rOs
-
(C,H&O),O
+ OCHCOOH + HIS
The resulting hydrogen sulfide may be readily detected by means of lead acetate paper. Other aliphatic thiol compounds behave analogously. This finding has led to the development of a. test for t,his class of compounds. P~oeedure: A micro drop of concentrated thioglycalic acid is treated with several milligrams of heneoylperoxide in a micro test tuhe. The open end of the tube is covered with a discof moist lead acetate paper. The test tuhe is dipped into hoiliig water. A black stain, due to the hydrogen sulfide formed, appears almost immediately.
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