Tang Yu，Zhang Yuanming，Yang Jun
(Department of chemistry, Jinan University, Guangzhou, 510632, China)
Abstract 2-tert-butylhydroquinone (TBHQ) was prepared from 2,5-di-tert-butylhydroquinone (DTBHQ).The effects of p-toluenesulfonic acid amount, reaction time and water amount were discussed. The more the p-toluenesulfonic acid was used, and the longer the reaction proceeded, the lower the yield of TBHQ was. The best yield of TBHQ was only 44.6% without the use of water. The yield of TBHQ could be improved ,and the by-product could be reduced by adding water in this reaction. The good yield of TBHQ was 67.6% with DTBHQ conversion of 48.1%, when 20 g of DTBHQ was refluxed in 100mL of xylene containing 0.5mL of water for 2 hours catalyzed by 2 g of p-toluenesulfonic acid.
2-tert-butylhydroquinone (TBHQ) is an antioxidant widely used in foods, cosmetics and rubbers, especially in oils and fats.[1,2]It can be made from hydroquinone by alkylation with tert-butanol. The by-product 2,5-di-tert-butylhydroquinone (DTBHQ) is also produced in the preparation of TBHQ. So the yield of TBHQ is low to about 30%. The DTBHQ is also an antioxidant. We developed a simple purification method for it.But it has not been used widely, and can not be sold very well. If TBHQ can be synthesized from it, the more economy benefits can be reached. It was reported that derivatives of tert-butylphenol could be catalyzed dealkylation by HPA.In another paper, TBHQ could be produced with 60% yield from DTBHQ catalyzed by p-toluenesulfonic acid (PTSA).We tried to prepare TBHQ by the same method. Unfortunately, the result was found not good after the effects of p-toluenesulfonic acid amount and reaction time were discussed.
In this reaction, the dealkylation of DTBHQ (with two t-butyls) first produces TBHQ. Then the TBHQ (with one t-butyl) can also eliminate t-butyl to produce hydroquinone as shown in scheme 1. The more the p-toluenesulfonic acid was used ,and the longer the reaction proceeded, the more the hydroquinone and even some complicated, deep colored and oily by-products were produced, as well as the yield of TBHQ became lower. The yield of TBHQ was only 44.6% when DTBHQ was reacted under the relative proper condition (in refluxing xylene for 2 hours catalyzed by 2 grams of p-toluenesulfonic acid). It was observed that the yield could be improved obviously by addition of water. The good yield of 67.6% was gained by adding 0.5mL of water for 20 grams of DTBHQ.If too much water was added,the reaction could be stopped because the p-toluenesulfonic acid was dissolved in water, and the reaction occurred in two phases.
2.1 Materials and instruments
DTBHQ was an industrial product and purified by the method of literature 3.The other reagents were analytical or chemical grade.
WRS-1 digital melting point apparatus, EQUINOX 55 spectrometer for IR(KBr)
2.2 Preparation of TBHQ from DTBHQ
20 g of DTBHQ, 100 mL of xylene, some p-toluenesulfonic acid and water were placed together. The mixture was refluxed for some hours. After cooled to room temperature, the mixture was placed in refrigeratory for crystallization.The crystal was filtrated and washed with cool xylene. Then the dried solid was dissolved in 80 mL hot water. The hot solution was filtrated, and the unsoluble DTBHQ solid was collected. After the water solution cooled, the TBHQ crystal was filtrated and washed with cool water. The TBHQ was detected by comparing with IR and melting point of standard product.
3 RESULTS AND DISCUSSION
3.1 The effect of p-toluenesulfonic acid amount
The dealkylation of DTBHQ catalyzed by p-toluenesulfonic acid produced TBHQ and hydroquinone. Too much acid would produce more hydroquinone and make the yield of TBHQ low. Too less acid would make the reaction slow and less effective. So the amount of p-toluenesulfonic acid was discussed under the condition of reacting 2 hours without addition of water. The results were shown in Table 1.
Table 1 The effect of t-toluene sulfonic acid amount on the yield of TBHQ.*
|YDTBHQ of conv.(%)||61.0||66.8||69.8||84.8||85.9||83.2|
*Yield of TBHQ was calculated on the amount of consumed DTBHQ in reaction.Yield of DTBHQ conversion was calculated on the amount of DTBHQ charged in reaction.
By considering both yields of TBHQ and DTBHQ conversion, 2 g (0.012mol) of p-toluenesulfonic acid would be proper for the reaction. The more acid could change more DTBHQ to hydroquinone, so the yield of DTBHQ conversion became larger, but the yield of TBHQ became lower.
3.2 The effect of reaction time
Long reaction time would produce more hydroquinone and even some complicated, deep colored and oily by-products. The reaction time was discussed under the condition of 2 g of p-toluenesulfonic acid without addition of water. The results were shown in Table 2.
Table 2 The effect of reaction time on the yield of TBHQ.
|YDTBHQ of conv.(%)||62.0||66.8||67.3||85.4||83.5||97.5|
By considering both yields of TBHQ and DTBHQ conversion, 2 hours of reaction time would be proper for the reaction. When the reaction time changed from 3 hours to 4 hours the yield of DTBHQ conversion became larger, but the yield of TBHQ became lower obviously. When the reaction proceeded for 6 hours, almost all DTBHQ had reacted, but the yield of TBHQ was still low. After long time reaction some complicated, deep colored and oily by-products were also produced and could be seen in the hot solution of TBHQ, while the left unreactive DTBHQ became deep color and oily solid by mixing with these by-products.
3.3 The effect of water amount
Dehydration of tert-butyl alcohol with hydroquinone produces TBHQ and DTBHQ. Dealkylation of DTBHQ is the reverse reaction of dehydration. So we considered to add water in the reaction. The effect of water amount was discussed and shown in table 3.
Table 3 The effect of water amount on the yield of TBHQ.
|YDTBHQ of conv.(%)||60.2||54.2||50.4||50.5||48.1||46.3||32.4||19.5|
The better yield of TBHQ was 67.6% when 0.5 mL of water was added. It was much better than the results without water. The efficiency of reaction was still good with 48.1% of conversion of DTBHQ. When 0.8 mL of water was added, the yield of TBHQ became very low. This is because p-toluenesulfonic acid was dissolved in water and separated from DTBHQ in xylene. The two phase reaction could even be stopped when too much water was added. But the proper amount of water could be mixed with DTBHQ in xylene by p-toluenesulfonic acid and helped to improve the yield of TBHQ. After water was added, the by-product became less too.
The proper reaction condition for preparation of TBHQ from DTBHQ was that 20 g of DTBHQ was refluxed in 100mL of xylene containing 0.5mL of water for 2 hours catalyzed by 2 g of p-toluenesulfonic acid.The best yield of TBHQ was 67.6% with DTBHQ conversion of 48.1% in this paper.