Fine Chemical Production - Essay Example

Fine Chemical Production: Citral Fine Chemical Production: Citral Introduction Citral (C10H16O), also commonly referred to as 3,7-dimethyl-2, 6-octadienal is a light yellow liquid with a distinctive lemon scent that naturally occurs in the essential oils of plants. With regard to its reaction profile, citral has a poor stability when exposed to alkalis, air and daylight. The chemical compound is less dense than water, insoluble in water but soluble in diethyl ether, ethanol and mineral oil. In addition, as an aldehyde, cytral is normally involved in polymerization and self-condensation reactions. These reactions are generally exothermic and may be catalyzed by an acid to give carboxylic acids (Chen, Viljoen, 2010). Chemically, citral consists of a mixture of a pair of aldehydes (terpenoids) having similar molecular formula but different structures. The two compounds are considered to be double bond isomers with the E-isomer being referred to as citral “a” or geranial while Z-isomer is known as citral “b” or neral. Although citral naturally occurs in compounds such as lemon grass and can be isolated from these citral containing oils, it is only available in sizable quantities. For effective synthetic manufacture of citral on a large scale basis for industrial purposes, it is increasingly becoming important to have a synthetic method of fine production from simple, inexpensive but readily available commercial chemicals such as isoprene and bata-pinene among others. Citral is widely used in the manufacture of perfumes, lotions, creams, shampoos and flavourings. In addition, the chemical compound is also an important basis for synthesizing a variety of popular fragrance compounds such as geraniol, eugenal, ionones and citronellol as well as in making carotinoids used for both vitamin A and E. Being one of the most important acyclic monoterpenes due to its numerous industrial applications, citral has many synthetic routes some of which are currently being used in commercial production of the chemical. This paper presents a fine chemical production of Citral including its chemistry, synthetic route selection, thermodynamics, kinetics and wastes issues. Fig. 1: Isomers of Citral Synthetic Route The current selected synthetic route for the production of citral involves the use of isobutene and formaldehyde as the primary feeding stocks with nitric acid as the preferred choice of catalysts. Using this method, citral can effectively be produced by from the reaction between isobutene and formaldehyde molecules under high temperatures and pressure and using a catalyst. The resultant chemical is then subjected to numerous special distillation processes before delivery. For example, the initial reaction between isobutene and formaldehyde produces 3-Methyl-3-buten-1-ol (H–[CH2CCH3=CHCH2] n–OH) which eventually isomerizes to form 3-methyl-2-buten-1-ol (Nissen, Axel Rebafka, Walter; Aquila and Werner, 1981). However, at the same time, 3-Methyl-3-buten-1-ol is also converted to 3-methyl-2-butenal by dehydrogenation as well as through the subsequent isomerization. The next step involves reacting the produced 3-methyl-2-buten-1-ol and 3-methyl-2-butenal under azeotropic conditions in the presence of a catalyst (nitric acid) to form an acetal which at higher temperatures eliminates a molecule of 3-methyl-2-buten-1-ol. Lastly, the intermediate enol ether is then subjected to Claisen rearrangement and finally by Cope rearrangement to make citral in relatively excellent yield. Fig 2: The selected synthetic route for the production of citral The current selected synthetic route is a remarkably sophisticated process. Although there are numerous production routes currently available, this route has been particularly chosen due to its greater environmental efficiency as well as minimal production of toxic/hazardous wastes as compared to the other commonly used industrial synthesis routes. However, the process conditions some of which necessitate the application of high pressure and temperatures in order to enhance and improve the conversions and selectivities may result in high capital costs thereby restricting the applicability of the process. Thermodynamic Calculations The reaction between isobutene and formaldehyde during the synthesis of citral involves a number of stages that can be categorized as endothermic or exothermic depending on whether energy is absorbed or released during the reaction process. For example, the initial reaction between the two feed stocks, isobutene and formaldehyde normally results in the formation of 3-Methyl-3-buten-1-ol (prenol) which eventually isomerizes to form 3-methyl-2-buten-1-ol (isoprenol). The oxidation reaction that occurs during the formation of 3-Methyl-3-buten-1-ol is a highly endothermic process that requires sufficient energy of activation. As a result, the reactants are often heated in the presence of high temperatures (between 125-150°C). The thermodynamic calculations of the reaction between the two feed stocks to produce citral are based on the equation below: First and foremost, Isobutene (2-methylpropene) is an unsaturated hydrocarbon with four carbon branches (Yaws, 2009). Thermodynamically, isobutene has a latent heat of fusion of 105.714 kJ/kg and melting point of -140.34°C. Thermodynamic properties of the reactants and their products Isobutene formaldehyde Methyl-3-buten-1-ol (prenol) ΔHf o (KJ/mol) 105.714 -108.6 234.15 ΔGf o (KJ/mol) 39.92 -120.3 383 cp (J/mol K) 81.46 218.95 192 The bond enthalpies are as shown below: H= 413KJ/ mol C= 346 kJ/mol C=O 740 KJ/mol O=O 497 KJ/mol O 360kJ/mol In the initial reaction, the heat energy required for breaking bond energy as well as bond forming can be given as follows: The above equation demonstrates that the first part of the chain of reaction is a highly endothermic process which wills sufficient energy of activation in form of heating. This explains why the reactants have to be heated between 125-150°C to guarantee optimum reaction. On the other hand, the spontaneity of the Gibb’s free energy reaction can demonstrated using the equation below: The negative value of Gibb’s free energy shown in the equation above is a likely indication that the reaction is spontaneous at a given standard temperature and pressure. Catalysts A number of catalysts are used during the synthesis of citral from the reaction of isobutene and formaldehyde. In the first reaction between the two feed stocks to give 3-methylbut-2-en-1-ol (prenol), palladium is used under a stream of hydrogen as a catalyst to speed up the conversion process. On the other hand, nitric acid was used as the preferred choice of catalyst for the other part of the reaction process. Nitric acid is preferred as a catalyst for the reaction between isobutene and formaldehyde due to a number of reasons. For example, the chemical is a weak acid and therefore an excellent catalyst for the reaction. In addition, nitric acid has a widespread use in different chemical reactions both at laboratory and industrial level. For example, during the process of synthesis of citral, the use of nitric acid is instrumental in improving the purity of citral using fractional distillation. The acid works by preventing the production of isocitrals that result from isomerization of citral while distillation takes place. The method entails reduction of the pH of some mixture which contains citral to impede the production of isocitrals from citral during heating, as it is the case during distillation. According to Kogan, Kaliya Froumin (2006), the commercial nitric acid happens to be an azeotrope that contains water which has a concentration of HNO3 tuning to 68%. Kinetics and Mechanism When synthesizing citral from isobutene and formaldehyde, 3-Methyl-3-buten-ol, got from formaldehyde and isobutene, will isomerizes and result into 3-methyl-2-buten-1-ol. Nonetheless, it is also changed to 3-methyl-2-buten-1 via dehydrogenation followed by isomeraization. In case of an azeotropic scenario, in the presence of nitric acid, the two compounds 3-methyl-2-butenal and -3-methyl-2-buten-1-ol will end up forming an acetal, which gets rid of a single 3-methyl-2-butenal molecule at heightened temperature levels (Bauer & Surburg, 1997, p.36). The intermediary enol ether then goes through Claisen arrangement and Cope rearrangement in order to form citral in perfect yield. The reaction between isobutene and formaldehyde during the synthesis of citral involves a chain of endothermic and endothermic reactions depending on the stage of the process. For example, the initial reaction between the two feed stocks, isobutene and formaldehyde normally results in the formation of 3-Methyl-3-buten-1-ol (prenol) which eventually isomerizes to form 3-methyl-2-buten-1-ol (isoprenol). The oxidation reaction that occurs during the formation of 3-Methyl-3-buten-1-ol is a highly endothermic process that requires sufficient energy of activation. As a result, the reactants are often heated in the presence of high temperatures (between 125-150°C). Fig 3: Endothermic reaction between isobutene and formaldehyde Although the thermodynamically prepared isomer known as isopropanol can not be directly formed during the reaction, it is subsequently yielded through isomerization to provide the desired product. The isomerization of 3-methylbut-2-en-1-ol (prenol) to 3-methylbut-3-en-1-ol (isoprenol) can effectively be catalyzed by catalysts which have the ability to form allyl complex without the substrate undergoing excessive hydrogenation. A good example of the preferred catalyst at this stage is palladium catalysts. Generally, the process can be undertaken in both a continuous mode as well as in a batchwise mode. At this stage, the main by product that is produced is 3-methylbut-2-en-1-al (prenal), which is normally formed from the oxidation of prenol under certain oxidative conditions (Nissen, Axel Rebafka, Walter; Aquila and Werner, 1981). However, prenol can easily me removed, recycled and used as a precursor of manufacturing citral in a separate chamber. Fig 4: Isomerisation of prenol into Isoprenol On the other hand, the pathway in which the the intermediate enol ether is converted to citral(5) normally involves a Claisen rearrangement, which is subsequently followed by Cope rearrangement in the same reaction chamber. The Claisen rearangement is an exothermic reaction (producing about 84 kJ mol−1). Claisen rearrangement of the Chemical Reaction O A Summary of reaction ² A pressure ² A metal catalyst OH OH A air B O Process Block Diagram References Bauer, K., Garbe, D., & Surburg, H. (1997). Common fragrance and flavor materials preparation, properties, and uses. Weinheim, Wiley-VCH. Chen, W., Viljoen, A. (2010). Geraniol: A review of a commercially important fragrance material. S. Afr. J. Bot. 76, 643–651. Kogan, S., Kaliya, M., Froumin, N. (2006). Liquid phase isomerization of isoprenol into prenol in hydrogen environment. Appl. Catal. A: Gen. 297, 2, 231–36. Nissen, Axel; Rebafka, Walter; Aquila, Werner(1981). Preparation of citral, United States Patent 4288636 (09/08/1981). Yaws, C. (2009). Chemical Properties Handbook. New York: McGraw-Hill.  Read More