Ester ve poliester sentezinde titan ve fosfor bileşiklerinin kullanılması

Abstract

Esterler genellikle karboksilik asit; asit klorürü veya asit anhidritlerin alkollerle reaksiyonu sonucunda sentezlenirler. Poliesterler endüstri ve tekstil alanlarında oldukça yaygın olarak kullanılmaktadırlar. Bu çalışmada, titan bileşiklerinin molekül yapılan nedeni ile su bağlama özelliğinden yararlanılarak doğrudan doğruya organik asit ve alkollerden ester elde edilmesine çalışıldı. Bu çalışmanın ilk bölümünde TiCU ve TPP'nin model bileşik olarak seçilen metil benzoat sentezi üzerindeki etkileri incelendi. Çalışmalar TiCl4 kullanılarak yapılan ester sentezlerinin, oksialkil titanyum bileşiği üzerinden yürüdüğü fikrini verdi. Reaksiyonlarda asit fazlası veya alkolle eşdeğer miktarda asit kullanıldığında ester oluşumu gözlenemedi. TiCU kullanımında hem çözücü olarak hemde oluşan HCPli tutması amacıyla, TiCU derişiminde piridin kullanıldığında piridinin etkili olduğu görüldü. TPP ve TİCI4/TPP kullanıldığında piridinin ve reaksiyon süresinin uzamasının yan ürünlere sebep olduğu, sadece TiCU kullanıldığında reaksiyon süresinin uzamasının, reaksiyon verimini arttırdığı belirlendi. Deneysel verilerden ester elde edilmesinde TPP veya TiCt/TPP kullanımının artı yarar sağlamadığı sonucuna varıldı. Literatürde, tersiyer alkollerin esterlerinin sentezlenmesi ile ilgili olarak sınırlı sayıda metoda rastlanması üzerine, asitlerden tersiyer alkol esteri sentezinde TiCU'ün etkili olabileceği düşünüldü. Model bileşik olan metil benzoat üzerinde saptanan uygun şartların kullanılmasıyla, tersiyer alkollerin esterleri düşük verimlerde sentezlenebildi. Son olarak, model reaksiyonlardan yola çıkılarak iki işlevsel grup içeren alifatik yapılı adipik asitle yine iki işlevsel grup içeren etilen glikol arasındaki polikondenzasyon reaksiyonunda TiCU'ün etkili olduğu, ancak düşük verimlerle polikondenzasyon ürünü oluşturduğu belirlendi.

Esters are polar compounds but locking a hydrogen attached to oxygen their molecules cannot form strong hydrogen bonds to each other. As a result, esters have boiling points that are lower than those of acids and alcohols of comparable molecular weight. Unlike the low-molecular-weight acids, esters usually have pleasant odors, some resembling those of fruits, and these are used in the manufacture of synthetic flavors. Esterification reactions are acid catalyzed. They proceed very slowly in the absence of strong acids, but reach equilibrium within a matter of a few hours when an acid and an alcohol are refluxed with a small amount of concentrated sulfuric acid or hydrogen chloride. Since the position of equilibrium controls the amount of the ester formed, the use of an excess of either the carboxylic acid or the alcohol increases the yield based on the limiting reagent. Just which component we choose to use in excess will depend on its availability and cost. The yield of an esterification reaction can also be increased by removing water from the reaction mixture as it is formed. In this study, synthesis of esters and polyesters were investigated by using titanium tetrachloride and triphenylphosphite. The optimum reaction conditions were determinated after the calculation of the yield and examination of the physical properties. In the first part of this study, the reaction of benzoic acid with methanol in the presence of titanium tetrachloride was investigated as a model reaction. Benzoic acid and methanol were placed in a three naked flask. Titanium tetrachloride was dropped into the mixture which was cooled in ice bath. At the end of the reaction, Ti02 and/or a titanium compound was formed as a by-product. This by-product was extracted from methyl benzoate by using water and CCI4. In organic phase (CCI4 phase), unreacted acid was removed by washing with sodium carbonate. Organic and water phases were separated. Organic phase was dried with MgS04. Methyl benzoate was purified by distillation. The residue was considered as a titanium compound and was analyzed to describe the reaction mechanism. To support the reaction mechanism, blank experiments were done. The residue, which was by-products of the model reaction, was analyzed with FTIR. Blank experiments and spectral analysis indicated that titanium compound was formed at the end of the reaction as below. IX OCH3 O H3C0-T1- o- c- ^y 0CH3 Reaction conditions and results are shown in Table 1 The methyl benzoate yield depends on the mole ratio of reactants and reaction period. Especially, when excess of methanol was used, ester was formed. However, when excess of acid was used, ester wasnt formed. Using pyridine, as a solvent in the same concentration of TiCU, increases the yield. The reaction mechanism is effected by the experimental conditions. Formed products were analyzed by FTIR/H NMR,13C NMR and GC. The spectral properties of methyl benzoate were: FTIR(CCU-sol), Vm^F 3065cm"1 and 3000cm" ^arom. C-H str), 2953cm"1,2941cm"1 and 2885 cm^aliph. C-H str),1724 cm"1 (ester C=0 str), 1601 cm^arom. C-C str), 1278 cm"1 (C-O-C str); *H NMR (CHC13- d): 6 8.06-8.03(o-arom.-H), 7.59-7.53(p-arom.-H), 7.47-7.41(m-arom.-H),3.91(0-CH3), 13C NMR (CHCI3- d): 8 166.98 (C=0), 132.80 (p-arom.-C), 130.06(ester C=0),129.46(o-arom.-C), 128.25 (m-arom.-C),51.96 (O-CH3); EI MS; m/z(%);136 ^(30), 105(100), 77(79), 51(25). Reaction mechanism is assumed as below: O ROH O xv II H,CO İ OCH3 ^v II l^J + CH3"0H -1^,\