The evaluation of 2,4-di-tert-butylphenol reactivity in the sulfonation process by elemental sulfur

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УДК 547.56, 661.2, 678.048.4

Kazan National Research Technological University

The evaluation of 2,4-di-tert-butylphenol reactivity in the sulfonation process by elemental sulfur

Karaseva J.S.

Samuilov A.Ya.

The creation of polyfunctional antioxidants (AO), i.e. stabilizers with such structures that are capable to inhibit the oxidation process of organic matter by different mechanisms, is one of the modern area of additives' synthesis for polymers These AO are sulfides of spatially-hindered phenols (SHP-sulfides).

The most widely used method of substituted-phenols sulfides' creation is based on the interaction of 2,6-di-alkyl-phenol with sulfur monochloride or dichloride. The reaction product is bis(3,5-di-alkyl-4-hydroxyphenyl)sulfide (bis(SHP)sulfide). The reaction of sulfur with 2,4-di-alkyl-phenols go in а similar way [3]. The processes go with high enough yields. However, the input sulfur chlorides are inaccessible, making it difficult to implement the synthesis of bis(SHP)sulfides in an industrial scale.

Another known method of bis(SHP)sulfides' synthesis is the interaction of elemental sulfur with 2,6- di-tert-butylphenol (2,6-DTBP). The reaction requires the presence of equimolar amounts of either an alkaline catalyst (KOH, NaOH), or 5-10-fold sulfur excess related to 2,6-DTBP to shift the equilibrium towards the formation of the end product. These methods are not easy to produce due to long duration of the process. In addition, the use of an alkaline catalyst leads to formation of large amounts of water and organic wastes. In later works it was proposed to carry out the interaction of elemental sulfur with 2,6-DTBP in the presence of a catalyst 4-dimethylaminomethyl-2,6-di-tert-butylphenol (solid Mannich base (MBs)) in the medium of the polar aprotic solvent, which is capable to form hydrogen bonds with hydrogen atoms of phenol's hydroxyl groups, and thereby increase their nucleophilicity. The selected conditions made it possible to reduce the reaction time and to achieve 100 % conversion of 2,6-DTBP, but the problem of the sulfur excess in the process was not resolved. Published data's analysis showed that in the process of sulfonation of alkylphenols by elemental sulfur is used 2,6-DTBP as the input reagent and there is no information on the interaction of elemental sulfur with 2,4-DTBP. The purpose of that research was to study the reactivity of 2,4-DTBP in the reaction with elemental sulfur.

Results and Discussion

In accordance with the set purpose at the first stage of the research the comparison of the reactivity of 2,4-DTBP and 2,6-DTBP was carried out, using the methods of quantum-chemical calculations by the B3LYP/6-31G(d) with the software package GAUSSIAN-2003. Preliminary optimization was carried out using semiempirical method PM3. According to the study, 2,4- DTBP and 2,6-DTBP form self-associates with different structures: hydrogen bond between HO-groups in the self-associate of 2,6-DTBP has a length of 2.53Е, while the 2,4-DTBP form stronger hydrogen bond with a length of 1.92Е (Fig. 1).

Fig. 1. SHP molecules' association: a. 2,4-DTBP, b. 2,6-DTBP

This, therefore, leads to a decrease of the reaction center's nucleophilicity.

It is confirmed by quantum-chemical data, that is 2,4-DTBP and its self-associate have more energy of the highest occupied molecular orbital (HOMO) and less energy of the lowest unoccupied molecular orbital (LUMO) in comparison with 2,6-DTBP and its self-associate. Consequently, 2,4-DTBP and its self-associate participate easier in reactions of electrophilic nature, and 2,6-DTBP and its self-associate - in reactions of nucleophilic character (table 1).

Table 1. Energy value of HOMO and LUMO of 2,4-DTBP and 2,6-DTBP and its its self-associates (GAUSSIAN-2003, B3LYP/6-31G(d))

So as elemental sulfur because of the peculiarities of its electronic structure, preferably interacts with compounds of electron donor type, it is clear that in the process of sulfonation alkylphenols with substitution in different positions by elemental sulfur, 2,6-DTBP is more reactive in nucleophilic medium in comparison with 2,4-DTBP.

At the next stage by using the procedure [6] the reaction of 2,6-DTBP and 2,4-DTBP with elemental sulfur was carried out (table 2, experiments 1 and 2, respectively). However, in contrast to 2,6-DTBP, 2,4-DTBP didn't interact with elemental sulfur under 4-dimethylaminomethyl-2,6-di-tert-butylphenol catalysis' conditions, which was judged by the amount of sulfur remaining in the sediment.

Table 2. The composition of the reaction mixture and the reaction conditions of the interaction of SHP with sulfur (T = 140 ° C, DMFA)

The replacement of the catalyst MBs by stronger base KOH in other equal conditions did not lead to the formation of the expected bis(3,5-di-tert-butyl-2-hydroxyphenyl)polysulfide (table 2, experiments 3, 4). Only increase the amount of KOH to equimolar with respect to 2,4-DTBP and increase of the reaction time up to 10 hours made it possible to get bis(3,5-di-tert-butyl-2-hydroxyphenyl)polysulfide containing 2.16 % of sulfur (table 2, experiment 5). The product is a liquid tar caramel color.

The experimental part

The process of interaction of SHP with sulfur

In a four-necked round-bottomed flask equipped with a reflux condenser, a stirrer, a thermometer and pipe for a gas supply put SHP, sulfur, DMFA, and the catalyst (table 1). The reaction was carried out for 1.5 - 10 hours at 140 °C with blowing the reaction system by inert gas (nitrogen). The reaction was monitored with TLC every 30 minutes. After the end of reaction the reaction mixture was cooled to room temperature and left for a day waiting while unreacted sulfur would completely precipitate. Then sulfur filtered, the precipitate was washed by 30 ml of DMFA, which was distilled off under vacuum.

Total sulfur was determined by combustion, state standart 28644-90.

Quantum chemical calculations were carried out, using the methods of quantum-chemical calculations by the B3LYP/6-31G(d) with the software package GAUSSIAN-2003. Preliminary optimization was carried out using semiempirical method PM3. butylphenol sulfonation alkaline catalysis

Conclusions

The synthesis of bis(3,5-di-tert-butyl-2-hydroxyphenyl)polysulfide was carried out. It is shown that the interaction of 2,4-di-tert-butylphenol with sulfur goes under alkaline catalysis and needs long reaction time. The low reactivity of 2,4-di-tert-butylphenol in comparison with 2,6-di-tert-butylphenol can be explained by the peculiarities of its electronic structure. That was confirmed by quantum-chemical calculations.

Acknowledgements

The work was made as a part of the Federal Target Program "Scientific and scientific-pedagogical personnel of innovative Russia" during 2009-2013, state contract № 14.740.11.0383

Abstract

The 2,4-di-tert-butylphenol reactivity in the process of sulfonation by elemental sulfur was evaluated. It is shown that the process is carried out under alkaline catalysis conditions. The low reactivity of 2,4-di-tert-butylphenol in comparison with 2,6-di-tert-butylphenol can be explained by the peculiarities of its electronic structure. That was confirmed by quantum-chemical calculations.

Key words: hindered phenols, hindered phenols' sulfides.

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