1 69 CHAPTER 5 DETECTION OF AMINES 5.1 Introduction The methods available for the identification of the functional groups involve chemical reactions that are characteristic of the" individual groups. However, within the past few years, numerous instruments have become available that provide considerable information regarding many functional groups. Most of these instrumental methods make use of some type of spectrascopy. The instrumental methods which are most useful for the qualitative analysis of organic compounds are ultraviolet and visible absorption spectroscopy, infrared spectroscopy, nuclear magnetic spectroscopy, and mass spectroscopy. All these methods require the use of complex instruments which are expensive, and requires considerable skill and time. Therefore, chemical methods of identification are still popular. Organic compounds are generally recognized by the detection of the functional group that is present in the molecule. for example, detection of the carbonyl
1 70 group indicates an aldehyde or a ketone; the presence of nitrogen with basic properties indicates the presence of amine; detection of hydroxyl groups indicates either alcohols or phenols. Simple amines are easily detected by the fact that they contain nitrogen, give bas~c reactions, and ere f soluble in acidic solutions. HoweverJmany of the substituted aromatic amines fail to dissolve in dilute acids and do not give positive tests with reagents that ere effective with simple amines. 5. 1.1 General tests for amine Amines that have a pka of 11 or less may be detected by the formation of white precipitate when the protonated amine is treated with a solution of tetraphenylborate. Slight precipitation may be caused by other functional groups. Dimethylformamide gives precipitates with the reagent.
1 71 (ii) Copper ion test 344 Low molecular weight amines that ere soluble in water including amino alcohols, may be detected by 10 percent copper sulphate solution. formation of a i. blue or blue-green coloration or precipitate with the reagent indicates the presenc~ of an amine. f (iii) fluorescein chloride test 345 When amine salt is fused with fluorescein chloride at 250-260 c in presence of anhydrous zinc chloride, it forms red water soluble rhodamine dye. Hydrazo compounds end some N-aryl amides also give positive test with the reagent. (iv) Quinhydrone test 346 347 Quinhydrone has been found to be an excellent reagent for the detection of certain amines. Quinhydrone reacts with aliphatic and aryl amine in aqueous or in ethanolic medium to give different colors depending upon the nature of amine i.e. primary, secondary or tertiary. Nitrosubstituted amines can not be detected by the reagent while phenylenediamine gives false colors.
172 (v} N-Halos~inimide test 348 N-Bromosuccinimide and N-iodosuccinimide are good detecting and differentiating reagents for amines. Primary and tertiary amines react with N-iodosuccinimide to give brown colors that do not fade on heating. Secondary amines produce brown colors that fade to yellow or colorless within few minut/~. Tertiary amines give an orange precipitate with N-brbmosuccinimide. This test is based on the fact that primary and secondary amines react with benzenesulfonyl halides to form N-substituted sulfonamides, whereas tertiary amines do not react with the reagent. further, the sulfonamides of primary amines are soluble in alkaline solution may be differentiated from the sulfonamides of secondary amines which are insoluble in alkali. (vii) Diazonium salt test Most of the aryl amine and phenols undergo coupling reactions with diazonium salts to form colored azo.compounds. Alkyl amine gives less intense yellow color than that of aryl amines. 4-Nitrobenzenediazonium fluoroborate 350 is found to be the most satisfactory reagent out of
1 73 various diazonium salts employed for the test. It does not react with the amines with both ortho and para positions substituted. (viii) Lignin test 351 Webster has proposed e very simple test for primary and secondary aryl am~nes. The test depends on the action of lignin in newspri~t paper. Ethanolic solution of amines applied on newsprint paper and treated with 6N hydrochloric acid. Primary and secondary aryl aminesproduce immediate yellow or orange color, while alkyl and alicyclic amine require hot solution for color development. (ix) Miscellaneous Chloranil test, in which primary and secondary arylamines react with chloranil to produce blue, red or brown color and 2,4-dinitrofluorobenzene 352 test are used for detection of amines. Para dimethylarninobenzal~ dehyde 353 is utilized for the detection of primary aliphatic, aromatic and heterocyclic amines. Water soluble primary alkyl amine is detected by Rimini test. 344 Primary aromatic amines are detected by diazotization,
174. 'd t t 344 1socyan1 e es, alde h y d e, 354 so d' ium or by the reaction with glutaconic 355 pentacyanoaquoferrate, 5-nitroso-p-hydroxyquinoline, furfural and other 356 357 reagents. ' Secondary amine is detected by nickle-dithiocarbamate358 test. Aconitic anh~dride 359 and citric acidt. h d. d 360 ace ic an y ri e test are f per f orme d for tertiary.. amines.. It is revealed from the literature survey that there is no specific test for the detection of primary amine which can be applied to all types of primary amines. Therefore, it was desirable to establish a general test for the detection of the primary amino group containing compounds.. 1 2 1 3 In Hantzsch reaction, ' the primary amines react with acetylacetone-formaldehyde reagent, to produce N-substituted-3,5-diacetyl-1 1 4-dihydrolutidine which gives yellow color in aqueous solution. Therefore, it was thought of interest to extend the application of the reaction to detect amines. In the present work, the reaction conditions are suitably modified to establish a method for the detection of the primary amine in aqueous media.
175 5.2 Experimental s.2.1 ~retus As described in Chapter 2. 5.2.2 Reagents Acetylacetone (SD'S); fofmaldehyde (BDH); Sodium acetate (BDH); Ethanol (BP); Acetic Acid (BDH) end doubled distilled water were used in the study. 5.2.2.1 Preparation of amine solution The solutions of amines were prepared by dissolving sufficient amount of amine in water or in a minimum quantity of ethanol and diluted with water to give the final concentration of 5 x 10-J mole/litre of amino nitrogen (Table XXIII). 5.2.2.2 PI;;eparati~~ the."reagent sol~~ion The reagent solution was prepared by mixing sodium acetate (16.4g), acetic acid (12 ml), formaldehyde (150 ml) and freshly distilled acetylacetone (78 ml). The final volume was adjusted to one litre with distilled water. The ph of the reagent was 4.7 ~ 0.1.
1 76 5.2.2.3 Procedure An aliquot (2 ml) of standard solution of amine was mixed with reagent (4 ml) in 100-ml conical flask. The reaction mixture was immersed in a boiling water beth for 15 min. It was cooled to room temperature. The contents of the flask were transferredjquantitatively to a 25 ml volumetric flask with the help of water. The volume was adjusted to the mark with water. The absorbance of the colored solution was measured at 412 nm against the reagent blank (Table XXllI). 5.3 Results and Discussion The presence of amino group has been detected by various tests. There is no specific test for all types of amines. Therefore, the Hantzsch reaction was modified to establish test for amines. The solution of amine was reacted with the acetylacetone-formaldehyde in presence of acetate buffer (ph 4.7), in boiling water bath for 15 minutes. The yellow colored product formed was measured at 412 nm (Table XXIII). The test is specific for primary amines. Secondary and tertiary amines do not give positive test.
1 77 TABLE XXIV Detection of Amines No. Name of compound Concentration moles/lit. Absorbance at 412 nm. 1 2. 3. 4. s. 6. 7. 8. 9. 1 0. 11 1 2. 1 3. 14. 15. 16. 1 7. 1 B. 19. 20. Ethylenediamine 0.00125 Aminophylline D.0010 p-aminobenzoic acid 0.0020 Procain hydrochloride f~.0050 p-toludine 0.0050 o<.-napthylamine D.0050 Sodium p-amino 0.0010 salicylate Anthranilic acid o.0050 Monoethenalamine 0.0050 Sulphanilamide 0.0050 Calcium pentothanate 0.0050 Streptomycin sulphate 0.0100 Butyl amine D.0050 0-Toludine 0.0050 p-chloroaniline 0.0050 Diethylamine 0.0100 Triethyl amine 0.0100 Diphenylamine 0.0100 Morpholine 0.0100 Triethanolamine 0.0100 0.314 0.11 3 o.347 0.328 0.182 D.274 0.310 0.119 D.357 o.1os 0.102 D.155 0.098 o.11s 0.237 o.ooo o.ooo o.ooo o.oos 0.006