Tang Yu£¬Zhang
Yuanming£¬Yang Jun 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. 1 INTRODUCTION
Scheme 1 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 EXPERIMENTAL 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 Table 1 The effect of t-toluene sulfonic acid amount on the yield of TBHQ.*
*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.
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. 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.
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. REFERENCE |