The step-like alternations in melting points of both series
of the aldehyde thiosemicarbazones, containing an even and an
odd number of carbon atoms, are on average 10° for higher members
of the homologous series. This lends itself well to an easy identification
of higher aliphatic aldehydes

M. Munk-Weinert

M. Proštenik

N. Ž. Stanačev

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C R 0 AT I CA C HE MI C A . A CT A 34 (1962) 1 CCA-237 547.281 :547.288.3.04 Identification of Normal Higher Aliphatic Aldehydes by MeallS of the Melting · Point Data of Their Thiosemicarbazones M. Prostenik, N. 2. Stanacev, and M. Munk-Weinert Department of Chemistry, Faculty of Medicine, University of Zagreb, Zagreb, Croatia, Yugoslavia. Received December 1, 1961 The step-like alternations in melting points of both series of the aldehyde thiosemicarbazones, containing an even and an odd number of carbon atoms, are on average 10° for higher members of the homologous series. This lends itself well to an easy identifi-cation of higher aliphatic aldehydes. The aldehydes are usually identified by conversion into crystalline deri-vatives and determination of the physical constants. The derivatives commonly used for this purpose are 2,4-dinitrophenylhydrazones, oximes and semicarba-zones. However, care is required in their use since this way of identification when extended to higher aliphatic aldehydes can lead to erroneous conclusions. Malkin and Tranter1 revised the melting points of 2,4-dinitrophenylhydrazon~ of n-pentanal up to n-tridecanal and found that they lie within too narrow a range of only 1.5° to be readily distjnguished. However, long X-ray spacings -which are dependent on the number of carbon atoms - are found to be suitable for this purpose. No X-ray data are recorded for the members from tridecanal upwards. There also exist chromatographic methods for the separation and identification of 2,4-dinitrophenylhydrazones.2-7 In the course of our investigation of the aldehydes <fotmed by the periodate oxidations of yeast cerebrin bases, sphingolipide bases and similar compounds we were unable to identify them by virtue of the melting point data of the above mentioned derivatives. We have found that step-like alternations in melting points are shown by the thiosemicarbazones of the higher lipide aldehydes of both series, con-taining an even and an odd number of carbon atoms. It has been found that these alternations in melting points - the fact being attributed to the difference in crystal structure8 - are on average 10° for higher members of the homologous series. Therefore, we consider the thiosemicarbazones as particularly suitable for the identification of the fatty aldehydes. On basis of the melting point determination it can be established without difficulty whether an aldehyde belongs to the even or odd series. To our experience; the identification by means of oximes, semicarbazones and 2,4-dinitrophenylhydrazones is entirely unsuitable for following reasons: 1. The melting points of the neighbouring derivatives are to·o close to each other. -For example, the semicarbazones C1 3 to C1 8 melt over a range ·Of only 30 (106~109°), the six oiXimes melt over a range· of 8° (81-89°) and . the cor-responding 2,4-dinitropheilylhydrazones melt over a range· of only 2° (105-1070). 2 M. PROSTENIK, N . Z. STANACEV, AND M. MUNK~WEINERT 2. There is no alternation in melting potnts of both even and odd series. 3. The melting point depression of the mixed neighbouring derivatives is either minute (2,4-dinitrophenylhydrazones) or does not occur at all (semicarbazones). Sinoe the majority of the homologous thiosemicarbazones up · to· C18 were unknown, we prepared them from the conesponding aldehydes in the usual manner. The aldehyde!S have been obtained either by heating 2-methoxy P-....o 70 I I /..J 60 I I ,,j 145 I so I I 135 'i\ I I " /JJ I I I 125 ~'I 30 ~ I 'ti .!::: I Ill I ~ ., 11 c: I 20 2 .i;; 0 0 t· -e ,, r'-d Ill 0 '• :5' u u E 105 /1 10 <t ., 0 I I .?:" Ill .9 I I Ji ;:: I- I I I ~ -95 I I 0 -0 ,o .,, I 111 c: I Yi c if I & I C> 85 I ,, -10 ·.§ I '1 C> c: a:; I I I Q; L , , I/ I ·L 75 y I -20 I I I I , , 65 2 4 6 8 10 --40 12 14 16 18 Number of Carbon Atoms li1ig. 1. Melting Points of Aldehyde. Thiosemicarbazones and Fatty Acids fatty acids to the boiling point according to Darzeris and Levy9 - for odd members - or by the Rosenmund reduction of the acid chlorides10 - for even members. The melting points and nitrogen analyses of normal fatty a~dehyde thiosemicarbazones are listed in Table L Fig. 1. represents the NORMAL HIGHER ALIPHATIC ALDEHYDES 3 melting point alternations of th_e thiosemicarbazones. For comparison, the melting points o.f normal fatty acids are also given . and are represented by the discontinuous curve. The curves in Fig. 2. represent the melting points of the binary mixtures of some higher aldehyde thiosemicarbazones. From these data it becomes evident, that the melting point and the mixed melting point determination of a thiosemicarbazone with an authentl.c sample, combined occassionally with the carbon - hydrogen elemental analysis, offers a ready means of identification of higher aliphatic aldehydes. 11•12 Other reported derivatives are of little va,lue for identification purposes. C,, 105 100 100 C,s C,1 95 95 90 90 u 0 . !; C,s + C11 U1 85 85 .5 cP C,s O'> . ~ = .. L "O .. . ~ L Cis c1, + c,s 0 25 so 7S 100 % 100 75 so 2S ·o % Percentage of Thi osem1carba zone Fig. 2. Melting Points of Binary Mixtures of Aldehyde Thiooemicarbazones 4 M. PROSTENIK, N. :Z. STANACEV, AND M. MUNK-WEINERT TABLE I Melting Points and Analyses of the Normal Fatty Aldehyde Thiosemicarbazones Aldehyde M.p. in oc Analysis I Methanal Ethanal Prop anal Butanal P entanal Hex anal Heptanal Octanal Nonanal Decanal Undecanal Dodecanal Tridecanal Tetradecanal P entadecanal Hexadecanal Heptadecanal Octadecanal 155-156 154-156 155 108-109 65 92~93 78--79 97 79 101 87 105 94 104 98 109 99 111 24,25 20.87 19.52 18.32 17.27 16.32 15.48 14.72 12.37 24.02 20.78 19.86 18.60 16.98 16.15 15.84 14.89 12.58 Reference 14 15 16 15 17 14 18 19 19 The nitrogen analyses are given preferentially for the hitherto undescribed substances. Exceptions are octanal and decanal, which have been prepared by Benezet, Favro, Igolen, and Pallaud.13 Their melting points are somewhat lower: 94-95° for octanal and 99-99.5° for decanal, respectively. EXPEHIMENTAL The melting points reported are uncorrected values. The normal fatty aldehydes with an odd number of carbon atoms were prepared - with the exception of n-nonanal - by thermal decomposition of 2-methoxy fatty acids according to the m ethod of Darzens and Levy.9 The aldehydes with an even number of carbon atoms were obtained starting with the fatty acid chlorides following the procedure of Rosenmund.10 Preparation of 2-Methoxy Fatty Acids To a mixture of 2-bromo fatty acid methyl ester in absolute methanol a solution of sodium methoxide in absolute methanol (in 10-150/o excess) was added dropwise. The reaction mixture was then refluxed for 3 hrs. After addition of aqueous sodium hydroxide and refluxing the mixture for an additional 1 hr., the hydrolysis was complete. Upon cooling, the solution was poured into 100/o aqueous sulphuric acid, the precipitated methoxy acid filtered by suction, washed with water, dried and recrystallized either from petroleum ether or from 950/o ethanol. The yield of the pure acid was 75-80°/o. In this manner following substances were prepared: a) 2-Methoxydodecanoic acid, m.p. 55-560 (ref. m.p. 520) .9 b) 2-Methoxytetradecanoic acid, m.p. 63-640. Anal. C1sHsoOa (258.39) calc'd.: C 69.72; H 11.700/o found : C 69.79; H 11.860/o NORMAL HIGHER ALIPHATIC ALDEHYDES 5 c) 2- Methoxyhexadecanoic acid, m .p. 73-740 (ref. m.p. 73-740).9 d) 2-Methoxyoctadecanoic acid, m .p. 77-780 (ref. m .p. 75-760).9 Preparation of Aldehydes by Thermal Decomposition of 2-Methoxy Fatty Acids A sample of 2-methoxy fatty acid was heated in a modified Hickmann flask to the boiling point for 30 min., using a free flame. The content of the flask was then distilled slowly at atmospheric pressure. The aldehyde which contained one carbon atom less than the starting methoxy acid was redistilled to give a colourless, viscous oil which crystallized soon. Thus, the following normal aldehydes were prepared: undecanal, b.p. 2720, tridecanal, b.p. 2800, pentadecanal, b.p. 2860, and heptadecanal, b.p. 3180. Preparation of Aldehydes by the Rosenmund Reduction of Fatty Acid Chlorides The acid chlorides_ were prepared in the usual manner by heating the acids with thionyl chloride at 700 for 1 hr. and distillation of the crude product in vacuo. The reduction of the latter with hydrogen at 1500 and in the presence of palladium on barium sulphate catalyst gave the fol1owing normal aldehydes: hexanal, octanal, decanal, dodecanal, tetradecanal, and hexadecanal. The crude products were con-verted into thiosemicarbazones without further purifica tion. C>ther Methods of Preparation Nonanal has been prepared by the oxidation of 9,10-dihydroxyoctadecanoic acid with periodic acid.20 Preparation of Thiosemicarbazones A solution of aldehyde and thiosemicarbazide (in 200/o excess) in a mixture ethanol: water (1:1) was refluxed for 1 hr. After cooling the separated crystals were filtered off and recrystallized from methanol with addition of some water. The thiosemicarbazones crystallized in colourless platelets. REFERENCES 1. T. M a I kin and T. C. Tr a n t er, J. Chem.. Soc. 1951, 1178. 2. J. D. Rob er ts and C. Green, Ind. Eng. Chem., Anal. Ed. 18 (1946) 335. 3. J. W. White, Jr., Anal. Chem. 20 (1948) 726. 4. B. E. Gordon, Fred. Wop at, Jr. , H. D. B urn h am, and L. C. Jones, Jr., Anal. Chem. 23 (1951) 1754. 5. P. J . G. Kr a m er and H. van D u in, Rec. Trav. Chim. 73 (1954) 63. G. J . A. El vi d g e and M. Who 11 e y, Chem & Ind. 1955, 589. 7. E. L . Pippen, E . J. Eyring, and M. Nonaka, Anal. Chem. 29 (1957) 1305. 8. T . Ma I kin, in Progress in the Chemistry of Fats and Other Lipids, Vol. I, Pergamon Press, London, 195·2. p . 1. 9. G. D a r z en s and A. Levy, Compt. rend. 196 (1933) 348. 10. K. W. Ro s en mun d, Ber. 51 (1918) 585. 11. M. Prost en i k and N. 2. St an ace v, Ber. 91 (1958) 961. 12. M. Pro st en i k , M. Munk - Weinert, and N. 2. St an ace v, Suppl. Int . Abstr. Biol. Sci. 1958, 5. 13. L . B en e z et, L. Fa v r o, G. Igo le n, and R. Pa 11 au d, Bull. Soc. Chim. France, Mem. [5] 1955, 37. 14. P . Ch ab r i er and E. Catt e 1 a in, Bull. Soc. Chim. France 1950, 48. 15. L. K. Ev a ns a.nd A. E . G i 11 am, J. Chem. Soc. 1943, 565. 16. P . Gramm at i c a k is, Bull. Soc. Chim. France 1950, 504. 17. C. Neu b erg and W. Neiman n , Ber. 35 (1902) 2049. 18. S. Land a, Bull. Soc. Chim. France, [4] 37 (1925) 1235. 19. R. Fe u I g en, K. Im ha us er, and M. B eh re n s, Z. physiol. Chem. 180 (1929) 161. 20. G. Kin g, J. Chem. Soc. 1938, 1826. 6 M . PROSTENIK, N . Z. STANACEV, AND M. MUNK-WEINERT IZVOD Jdentifikacija normalnih visih alifatskih aldehida s pomocu odredivanja talista tiosemikarbazona M. Prdstenik, N. Z. Stanacev i M . Munk-Weinert , Visi masni aldehidi mogu se lagano identificirati na temelju alternacija taliSta, koje pokazuju tiosemikarbazoni s parnim i neparnim brojem C-atoma. Te su alter-nacije relativno visoke - u prosjeku oko 100 kod viSih clanova homolognog niza. TaliSta tiosemikarbazona aldehida C1-C18, od kojih je veci broj sada prvi put opisan, prikazana su u tablici I i slici 1. Graficki su prikazana i talista binarnih smjesa, koje tvore tiosemikarbazoni aldehida C14-C17 (sl. 2.). Identifikacija viSih masnih aldehida s pomocu opcenito upotrebljavanih derivata oksima i 2,4-dinitro-fenil-hidrazona, sasvim je nepouzdana. ZA VOD ZA KEMIJU · MEDICINSKI FAKULTET ZAGREB Primljeno 1. prosinca 1961. 

Identification of Normal Higher Aliphatic Aldehydes by Means of the Melting · Point Data of Their Thiosemicarbazones

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