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The influence of the position and nature of the substituent in the thiophene ring on the fragmentation of 3~alkyl-and 5-alkyl-2-thier~ylamines under electron impact was studied. The derivatives containing the ethyl group in position 3 decompose due to both /he elimination of the fragments from the substituents and thiophene ring rupture. In the case of the compounds with the tert-butyl group in position 5, the last process is not observed. The fragmentation of the 3-and 5-methyl-substituted derivatives occurs mainly from the isomeric thiopyran form of the molecular ion.Key words: C-alkyl-2-thienylamines, mass spectra, fragmentation, rearrangement, electron impact.We have previously z described the general regularities of the mass spectrometric fragmentation of secondary. and tertiary, 2-thienylamines containing no substituent in positions 3--5 of the thiophene ring. With the purpose of revealing the influence of the nature and position of an alkyl substituent in the thiophene ring on the decomposition of molecular ions of 2-thienylamines, in this report, we analyzed the mass spectra of secondara.' and tertiara,' 3-alkyi-and 5-alkyl-2-thienylamines 1--9. Deceased. strongest effect. In this case, fragmentation with the decomposition of the heterocyclic system does not occur, and the main process of the decomposition of molecular ions of compounds 7--9 is the elimination of the methyl radical to form ions stabilized due to conjngation, whose peaks are maximal in the mass spectra (Scheme I). The IM -Mel ~ and [bl -2 Me] +" ions are formed due to both the elimination of the methyl groups from the terr-butyl substituent (see Scheme 1) and N--Me bond cleavage, especially in the case of dimethylaminosubstituted derivative 9, whose mass spectrum exhibits an intensity of the peak of the {M -2 Me] +. ion almost fourlo[d higher than that for compound 8 containing the phenylamino group.In the mass spectrum ofdimethyl(3-ethyl-2-thienyl)-amine (5), along with the peaks corresponding to the M +', IM -Me] ~-, and [M -2 Me] +" ions, the peaks of the ions that appeared due to the fragmentation of the thiophene ring have a high intensity. These processes are identical to those observed by us previously g for alkyl(aryl)(2-thienyl)amines (R 3 = R 4 = H). The cleavage of the S(1)--C(2) and C(4)--C(5) bonds results (Scheme 2, /re1 (%) are given in parentheses) in ions with m/z 45 and t l0, and the cleavage of the S(I)--CI2) and C(2)--C(3) bonds in ion A affords ions with m/z 42 and 99. The thiopyrilium cation with m/z 97 is formed upon extension of the five-membered cycle. 3