Liquid Chromatography-Mass Spectrometry analysis of some early green dyes

Abstract

Five historical green dyes from the Historische Farbstoffsammlung, Technical University of Dresden, have been analysed by liquid chromatography – mass spectrometry (LC-MS). The dyes were determined to be formed by the alkylation of either crystal violet or rosaniline and contained both doubly and singly charged ions. One sample had a spread of higher molecular weight doubly charged ions reminiscent of the alkylation of a Hofmann violet. Malachite green was the dye present in one sample.

Introduction

The beginning of industrial organic synthesis was led by Perkin ¹ mauveine in 1856 then F-E Verguins fuchsin (or rosaniline) in 1859^2–6 followed by Hofmann’s ⁷ and Lauth’s^8,9 violet dyes in the 1860s. The dye and its transformations of interest in this paper is fuchsin or rosaniline since alkylation, by heating with methyl bromide or iodide, gives first a violet dye ¹⁰ but this can be also alkylated to a green dye.^11–14 According to the account given by Fox, ¹¹ Albert Chatelaine improved the work of the Read Holliday & Sons ¹⁵ patent and used pure methyl violet (crystal violet) as starting material in place of rosaniline to give methyl green. The Colour Index describes Holliday’s methyl green as ethylhexamethyl-para-rosaniline bromide. ¹¹ According to Perkin, in the preparation of Hofmann’s violet from the alkylation of rosaniline, a valuable green dye was isolated which was supplied to dyers as iodine green.^12,13 The process was improved by Keisser^12,13 in France. A number of marketed green dyes remain obscure. These include aniline green, ¹⁶ aldehyde green, ¹⁷ Paris green, or Lauth’s ¹⁸ green. Malachite green came later in 1878 and was made from 1,1,1-trichloromethylbenzene, N,N-dimethylaniline, and AlCl₃. ¹⁹ Perkin^12,13 green was described but no patent reference has been found. The historic green dyes studied in this paper were prepared from rosaniline^2–6 (Figure 1) or crystal violet^20,21 by alkylation with methyl or ethyl bromide or iodide either in the 19th century or early 20th century.

Figure 1.

Molecular structure of fuchsin or rosaniline homologues.

Discussion

Five methyl green dyes from the museum collection in Dresden were analysed by liquid chromatography – mass spectrometry (LC-MS). ²² Figure 2 shows the proposed molecular structures, chemical formulae, and molecular weights of doubly charged molecular ions. Figure 3 shows the proposed molecular structures, chemical formulae, and molecular weights of singly charged molecular ions. Some compounds occur repeatedly in more than one sample so the molecular structures are grouped together.

Figure 2.

Proposed doubly charged green dyes.

Figure 3.

Proposed singly charged violet dyes (11-15) and green dyes (16-17).

Sample 1: C.I. 42556; Jodgrün; Theodor Peters Chemnitz (derived from rosaniline)

The LC-MS chart of doubly charged ions is shown in Figure 4, and the data are summarised in Table 1. Three compounds were present, compounds 5, 6, and 7. Compounds 5 and 7 were minor chromophores and compound 6 was the major chromophore. In this sample, the main chromophore came from crystal violet 13 which was methylated to a doubly charged ion 6. Compound 7 would form by the ethylation of crystal violet 13. ²⁰ So the alkylation mixture was mainly MeI/MeBr over EtI/EtBr. The structure of crystal violet is shown in Figure 3, compound 13. The LC-MS chart of singly charged ions is shown in Figure 5, and the data are summarised in Table 2. The main chromophore is crystal violet 13 which has not been alkylated to the doubly charged ions 6 and 7. The alkylation chemistry has not gone to completion so the dye is a mixture of chromophores. The other proposed compounds, 11, 12, and 14, are partially alkylated (Figure 3). Compounds 11 and 12 would not be expected to form in the crystal violet synthesis.^20,21 However, the synthesis uses a Lewis acid, AlCl₃, which will complex to the basic nitrogen of the aniline or the dye and might cause some slow demethylation. Otherwise they might be unexpected impurities arising from a partially alkylated aniline, ⁹ or they are partially alkylated rosanilines added to the final product to modify the shade.

Figure 4.

Sample 1: UV at 580 nm, EIC of M²⁺ ions for compounds 5–7.

Table 1.

Mass spectral data for the compounds in Figure 4 which are doubly charged ions.

Compound	Formula	Theoretical mass	Measured mass
5	C24H28N32+	358.2277	358.2285
6	C26H33N32+	387.2664	193.6337
7	C27H35N32+	401.2820	200.6416

Figure 5.

Sample 1: UV at 580 nm, EIC of M⁺ ions for compounds 11–14.

Table 2.

Mass spectral data for the compounds in Figure 5 which are singly charged ions.

Compound	Formula	Theoretical mass	Measured mass
11	C23H26N3+	344.2121	344.2124
12	C24H28N3+	358.2277	358.2285
13	C25H30N3+	372.2434	372.2433
14	C26H32N3+	386.2591	386.2594

Sample 2: C.I. 42556; Jodgrün; unknown manufacturer (derived from rosaniline)

The LC-MS chart is shown in Figure 6, and the data are summarised in Table 3. This chart contains compound 16 and 17 (Figure 3). Compound 17, malachite green, ¹⁹ is the dominant compound. The minor product, compound 16, has demethylated. The malachite green synthesis may lead to this on a small scale by partial demethylation of N, N,-dimethylaniline with AlCl₃.

Figure 6.

Sample 2 UV at 580 nm, EIC of M⁺ ions for compounds 16 and 17.

Table 3.

Mass spectral data for the compounds in Figure 6 which are singly charged ions.

Compound	Formula	Theoretical mass	Measured mass
16	C22H23N2+	315.1856	315.1860
17	C23H25N2+	329.2012	329.2022

Sample 3: C.I. 42585; Methylgrün; Theodor Peters Chemnitz (derived from a Hofmann violet)

The LC-MS chart is shown in Figure 7, and the data are summarised in Table 4. The chart has compounds 5–10 which are all doubly charged ions. The distribution of ions suggests that they were made by the over-alkylation of rosaniline and not by the alkylation of pure crystal violet 13. Both MeI and EtI were used in the synthesis. The distribution of higher molecular weight doubly charged ions suggests that a Hofmann ⁷ violet may have been alkylated. Hofmann violets were made by the alkylation of rosaniline with MeI and EtI so different isomers to those shown are also possible but as yet they cannot be distinguished. They might contain –NMeEt or –NEt₂ groups. The original green dye made from Hofmann’s violet was called iodine green.^12,13 The singly charged ions, compounds 11–15, in Figure 8 and Table 5, also suggest that rosaniline was the starting material, especially the partially methylated precursor compounds 11 and 12.

Figure 7.

Sample 3: UV at 580 nm, EIC of M²⁺ ions for compounds 5–10.

Table 4.

Mass spectral data for the compounds in Figure 7 which are doubly charged ions.

Compound	Formula	Theoretical mass	Measured mass
5	C24H28N32+	358.2277	358.2285
6	C26H33N32+	387.2664	193.6337
7	C27H35N32+	401.2820	200.6416
8	C28H37N32+	415.2976	207.6495
9	C29H39N32+	429.3133	214.6572
10	C30H41N32+	443.3289	221.6653

Figure 8.

Sample 3: UV at 580 nm, EIC of M⁺ ions for compounds 11–15.

Table 5.

Mass spectral data for the compounds in Figure 8 which are singly charged ions.

Compound	Formula	Theoretical mass	Measured mass
11	C23H26N3+	344.2121	344.2124
12	C24H28N3+	358.2277	358.2285
13	C25H30N3+	372.2434	372.2433
14	C26H32N3+	386.2591	386.2594
15	C27H34N3+	400.2747	400.2763

Sample 4: C.I. 42585; Lichtgrün gelblich; Merck

Does not show any meaningful data.

Sample 5: C.I. 42585; Methylgrün; Timmersdorf (derived from crystal violet)

The LC-MS chart of doubly charged ions is shown in Figure 9, and the data are summarised in Table 6. Three compounds 5–7 are present. The chart is dominated by compound 6, as in Figure 4, Sample 1, but the manufacturer is different. It would have been made by the methylation of crystal violet 13 which contained a small amount of pentamethylrosaniline 5. The LC-MS chart of singly charged ions is shown in Figure 10, and the data are summarised in Table 7. The charts show compounds 11–13 to be present. Compound 13 is crystal violet and compound 12 is similar to compound 5. In agreement with the anticipated synthesis by which the doubly charged ions are formed, the singly charged ions are precursors to the next alkylation reaction step.

Figure 9.

Sample 5: UV at 580 nm, EIC of M²⁺ ions for compounds 5–7.

Table 6.

Mass spectral data for the compounds in Figure 9 which are doubly charged ions.

Compound	Formula	Theoretical mass	Measured mass
5	C24H28N32+	358.2277	358.2285
6	C26H33N32+	387.2664	193.6337
7	C27H35N32+	401.2820	200.6416

Figure 10.

Sample 5 UV at 580 nm, EIC of M⁺ ions for compounds 11–13.

Table 7.

Mass spectral data for the compounds in Figure 10 which are singly charged ions.

Compound	Formula	Theoretical mass	Measured mass
11	C23H26N3+	344.2121	344.2124
12	C24H28N3+	358.2277	358.2285
13	C25H30N3+	372.2434	372.2433

Conclusion

Five samples of green dyes from the Historische Farbstoffsammlung, Technische Universität Dresden, Dresden, were analysed by LC-MS. These green dyes were originally noted by Perkin^12,13 as a side product in vats for alkylating rosaniline to make violet dyes. The dyes were determined to come from the methylation or ethylation of either rosaniline or crystal violet. The use of crystal violet gives a purer product. Although crystal violet is a purer starting material than an alkylated rosaniline, the analysis of by-products with the main dye suggests that partial demethylation of crystal violet or N,N-dimethylaniline with AlCl₃ occurred in the synthesis. Sample 1 is methylhexamethyl-para-rosaniline. Sample 2 is malachite green. Sample 3 is methylhexamethyl-para-rosaniline, ethylhexamethyl-para-rosaniline and related homologues. A likely synthesis is the alkylation of rosaniline to a Hofmanmn violet then further alkylation because the product is a mixture of doubly charged ions. Sample 4 is not a soluble, organic green dye like these. Sample 5 is methylhexamethyl-para-rosaniline. In these studies, samples of Paris green ¹⁸ and Aldehyde green were not present.^16,17 However, sample 4 is green and pigmentary so might be a different like one of these whose structures are less clear and possibly polymeric.

Experimental

Infrared (IR) spectra were recorded on a diamond-attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectrometer. Ultraviolet (UV) spectra were recorded using a Perkin Elmer Lambda 25 UV-Vis spectrometer with EtOH as the solvent. High-resolution mass spectra were obtained at the University of Wales, Swansea, using an atmospheric solids analysis probe (ASAP) (positive mode) instrument: Xevo G2-S ASAP. Melting points were determined on a Cole-Palmer Stuart microscope.

Methyl-green dyes from museum collection in Dresden

• Sample 1: C.I. 42556; Jodgrün; Theodor Peters Chemnitz

• Sample 2: C.I. 42556; Jodgrün; unknown manufacturer

• Sample 3: C.I. 42585; Methylgrün; Theodor Peters Chemnitz

• Sample 4: C.I. 42585; Lichtgrün gelblich; Merck

• Sample 5: C.I. 42585; Methylgrün; Timmersdorf

LC-MS instrument

• Separation: 5F-phenyl (Kintex) 150 *4.6 mm

• Eluent A: 0.1% formic acid in water

• Eluent B: 0.1% formic acid in methanol

• Eluent C: 0.1% formic acid in acetonitrile

• Gradient: 100% A, in 15 min 60% B+ 40% C, hold 10 min, reset

• DAD-detection: 190 – 700 nm; 580 nm extracted

• MS: Agilent 6546 q-TOF, positive mode, 4 kV capillary voltage, fragmentor 110 V, skimmer 1 45 V, auto MS/MS (extraction window 1.3 m/z, exclusion after 2 spectra, CID: 10, 20,40), reference masses enabled