Stereo Electronic Effects in Fuel Dehydrating Icing Inhibitors - Research Paper Example

Stereo Electronic Effects in Fuel Dehydrating Icing Inhibitors Introduction Dissolved water is a usual component of jet fuels. The increasing popularity of long range jet aircrafts has come with numerous problems related to jet fuel contamination with impurities such as dissolved water. Although the water component is normally at low concentrations (< 0.2%) and can easily be vaporized, free water is a major contaminant that can potentially cause serious damage to jet engines (Taylor, 2008, p.2401). Icing in the critical components of jet fuel system is particularly attributed to the fact that solubility normally decreases with decreasing temperatures. The solubility of water in most hydrocarbon jet fuels often decreases by roughly 2 ppm v/v per 1°C. In this regard, the dissolved water components of the jet fuel may freeze to form tiny ice crystals and blocking fuel feeds, enhance corrosion as well as supporting microbial growth. A number of plane crash incidences attributed to fuel starvation due to icing have been recorded. For example, during extended high latitude flights, fuel temperatures often fall to extremely low levels thereby causing the water components of the jet fuel to freeze. In most cases, the ice crystals thus formed may not only clog jet fuel filters but can also lodge into the fuel with potential implications such as engine malfunction or flameout (Repetto et al., 2013, p.556). However, there are currently a number of possibilities of effective elimination of the potential hazards associated with jet fuel water contamination. According to Trohalaki and Pachter (2009, p.79), although the application of organic molecules as potential dehydrating agent remains underutilized due to their complex chemical refining requirements, a number of organic molecules with efficient dehydrating properties such as ortho esters, ketals, hemiketals, acetals, hemiacetals are increasingly being seen as potential candidates for the development of novel and effective fuel dehydrating Icing Inhibitors (FDII). This research proposal investigates the feasibility of using stereo electronic effects of organic molecules such as ortho-esters in the management of both jet fuel water contamination as well as ice formation in jet engines during high latitude flights. Reaction Mechanism and Interaction of Orthoesters Stereoelectronic effects play a critical role in the hydrolytic processes of the organic water scavenging molecules such as ortho esters (Chiang et al., 2003, p.58). According to many experts, orthoesters are organic molecules that contain a functional group consisting of three alkoxy groups attached to one of the carbon atoms. Ortho esters generally work as water scavengers by getting rapidly hydrolysed when in contact with the free water thereby acting as an ice inhibitor. Stereo electronic effects are critically important in the reactivity of ester functional groups like ortho-esters. The primary stereo electronic effects of ortho esters are particularly caused by the delocalization of electrons between the carboxylic group and the ethereal oxygen during the interaction between ortho esters and free water from the jet fuel. Orthoesters are often readily hydrolyzed in mild acid solutions to form esters as the main product. For example: RC (OR’) 3 + H2O → RCO2R’ + 2 R’OH. This type of ester or the triethylorthoformate has a chemical formula of RC (OR’) 3. Esters are chemical compounds consisting of both a carbonyl adjacent to an esther linkage. Esters are derived from a reaction of an oxyacid together with hydroxyl compound such as a phenol or an alcohol. Esters are usually derived from either inorganic or organic acids through esterification process. In the esterification reaction at least one –OH (hydroxyl) group is substituted by an –o- alkyl group. This mechanism and interaction of esthers is commonly experienced from the reaction of alcohol and carboxylic acids. The ortho-esters are therefore formed by condensing an acid with an alcoholic substance. Ortho esters are often naturally occurring oils and fats. As earlier been noted, stereoelectronic effect plays a critical form a key element on the chemical reactivity of water scavengers. According to Chiang et al. (2003, p.62), this is particularly attributed to the fact that stereoelectronic factors normally tend to control the rate of chemical reactions as well as the nature of their products. For example, acetals normally take a conformation where each oxygen atom takes up an electron lone pair antiperiplanar to the carbon hydrogen (C-H) bond. The actual mechanism of the acid catalyzed ortho ester hydrolysis is usually comprised of three distinct reaction stages namely: i. Generation of dialkoxy carbonium ions, ii. Hydration of the generated dialkoxy carbonium ions to hydrogen ortho esters iii. Breakdown of hydrogen ortho-esters into carboxylic acid and alcohol products. Although the process is considered to be a reasonable reaction, the initial stage is rate determining and does not usually have a direct kinetic relationship with the rest of the reaction. The rate determining characteristic of stage 1 is particularly evident in basic or neutral solutions where relatively efficient catalysis of stage 3 makes it comparatively faster than stage 1 which does not normally undergo any catalysis by bases at all. However, some esters normally experience less effective acid catalysis of stage 3 than stage 1 during the hydrolysis. In such cases, the rate determining step moves from step one at high pH to step 3 at relatively low pH. For example, under certain specific conditions, the acid catalyzed hydrolysis of ortho esters may undergo change in the rate determining step and consequently diakoxy carbonium may eventually be detected as the reaction intermediates. During the fist reaction stage of acid catalyzed ortho ester hydrolysis, two important bonding changes usually occur. First and foremost, a proton is normally transferred from the catalyst to the ether oxygen of the reaction substrate. Secondly, the C-O bond that joins the oxygen to the pro-acyl carbon atom is then cleaved. Generally, the two bonding changes can either occur together or separately. In this regard, the mechanism of this reaction may be considered to be either concerted or stepwise. However, distinguishing concerted from stepwise mechanisms of ortho ester acid catalyzed hydrolysis is normally based on the actual strength of the oxygen atom receiving the proton during the first stage of the chemical reaction. An examination of the rate constants for various water scavengers such as acetals, ketals and ortho esters when in contact with water reveal a number of correlations between their barrier to reaction and molecular structures. For example, hydration rates of acetals significantly increase with the replacement a hydrogen atom with an alkyl group at the functional carbon atom. On the other hand, the rate increase that usually accompanies progression when ketals are used is up to ten times higher than that of acetals. Lastly, ortho esters have the highest rate increase among the molecular scavengers. This is particularly explained by the existence of a higher steric decompression factor of ortho esters as the tetrahedral intermediate often collapses to its trigonal counterpart during the acid catalyzed hydrolysis. In addition, the rates of reaction in ortho esters usually correspond to the stability of the corresponding dialkoxy carbenium intermediate formed. Structure, Geometry and Reactivity Pattern of Ortho Esters The interaction between the atomic and molecular orbitals often results in a stereo electronic effect on the molecular structures of many organic molecules including ortho esters. In most cases a typical stereoelectronic effect coupled with certain orbital overlaps may result in a specific molecular conformation and energy differentiation during some transition states that eventually lead to certain reaction selectivity. For example, the resultant electrostatic field produced by the interactions may significantly impact on their reactivity. Generally, the reactions normally involve a donor-acceptor interaction whereby the donor is a nonbonding and high-lying bonding orbital while the acceptor is an antibonding and low lying orbital. However, the stereoelectronic effect can only be favored if the donor-acceptor orbitals have a low energy gap and have retained antiperiplanar geometry in order to allow for a perfect interaction direction. In this regard, it can be argued that stereoelectronic effect is simple the kinetic and chemical consequences of orbital overlap during such reactions. The structure, bonding angles, bond length and torsion of organic structures is critically important in determining their potential stereoelectronic effects. This particularly arises from the fact that molecules and atoms normally occupy certain amounts of space. Consequently, bringing the atoms closer together may result in electronic energy due to overlapping. The resultant energy may be exploited by to prevent unwanted side reactions and change the reactivity pattern. During the reaction, the overall structure may also undergo significant changes depending on the conditions. Angles and atomic distances within organic molecules tend to vary from one group to another. i. Mtdxyp (2 methyl-4,5tetramethylene-1,3-dioxolan-2-ylium perchlorate ) ii. doxylp(methyl 1, 3 dioxolan -2-ylium perchlorate) iii. Oxlcli iv. Pexsam v. Pexsiu vi. Pexsoa vii. pexsug viii. pmdxlp The diverse molecular geometry of organic molecules significantly varies as shown in the above set of 8 structures. However, the geometry and the arrangement of atoms in a molecule critically determine a number of properties of a substance including its phase of reaction, reactivity and polarity among others. Generally, the actual position of an atom in the molecule is normally determined by the nature and type of the chemical bonds through which the atom is attached to the neighboring atoms. As a result, the molecular geometry usually based on the bond lengths or bond angles of any three interconnected atoms as well as torsion angles in the three consecutive bonds. The torsion angles which are also known as dihedral generally refer to the angle between the two planes of an atom. For example, the 1, 3 diaxial interactions of the structures should be taken into account when determining the potential stereoelectronic effects of the molecules including their anomeric effects. However, the conformation of the 1, 3 diaxial interactions are not often disfavored due to the potential steric congestion and the potential shift in equilibrium. On the other hand, the bond length or the averages distance between the nuclei of any two bonded atoms in an organic molecule. Generally, the bond length of a molecule is closely related to the border order in which more electrons may be able to participate in the formation of the bond often tend to get shorter. Additionally bond length is also critically associated with the bond dissociation energy and the shorter the bond the stronger it is. In the organic molecules such as ortho esters, the actual bond length between any two given atoms in molecule is normally dependent on a number of factors including the electronic nature of the substituents as well as the orbital hybridization. The potential effectiveness of the molecules presented above when used as FDIIs will be highly dependent on their selectivity during the reaction with jet fuel components. For example, a good FDII will be expected to only react with water with water acting as a weak nucleophile in order to initiate the hydrolysis reaction. How Ortho esters Act as Jet Fuel De-Icers Although there is still a lot to be uncovered about the mechanism of acid catalyzed hydrolysis of ortho esters ith respect to water scavenging properties, it is widely accepted that the entire process can be summarized into two major stages namely; the formation of water scavenging dialkoxy carbenium ion from the ortho ester as well as the subsequent hydration of the formed dialkoxy carbenium ion to form an ester. As earlier been noted, the initial protonation of the ortho ester is considered to be a slow process (rate determining) while the subsequent steps are relatively fast. The protonation as well as the subsequent formation of dialkoxy carbenium ion is often subject to two distinct mechanisms; namely the general mechanism involving undissociated HA and the specific mechanism through dissociated hydrogen ions. Fig. 2: Hydrolysis of a Ketal CH3 OCH3 CH3 C + H2O C═O +2CH3OH CH3 OCH3 CH3 The general acid catalysis is normally facilitated by undissociated weaker acids. On the other hand, specific acid catalysis is largely facilitated by stronger and fully dissociated acids. It is however it is worth noting that apart from the strongest acids, both the general and specific pathways are normally operative. Given the fact that the environment of operation of a jet fuel is normally mildly acidic, the ortho esters will react with the mild acids of the jet fuel and dissociate in the presence of water which is a polar solvent as shown in the figure below: Fig 3: Delocalization of electron between the carbonyl group and the etherneal oxygen It is widely assumed that the basic strengths of the ether oxygen atom during the reaction is usually inversely proportional to the acidity of the corresponding alcohols. The three atoms involved in the delocalization during the interaction are considered to be sp2 hybridized. This is particularly because esters are believed to be planar. On the other hand, the secondary electronic effect involves the interaction n→σ* that resemble anomeric effect. In this regard, the carboxylic oxygen in the ortho ester posses an electron pair orbital oriented antiperiplanar to the C-OR bond which is electron deficient to allow the interaction to take place. The use of organic molecules as the de-icing agents in jet fuels is one of the neglected approaches due to its complexity nature. The stereochemistry involved in developing and refining these systems involve the use of esters with dehydrating properties such as ortho-esters presents themselves as an outstanding starting-point for the optimization and development of the Novel Fuel Dehydrating Icing Inhibitors (FDII). According to recent researches, ortho esters present one of the best alternative jet fuel de-icing agents. This is particularly attributed to its unique features such as high reactivity even at relatively low temperatures, combustibility, atom economy as well as the fact that the organic molecule is cheap and can readily be synthesized. Generally, ortho esters are highly suited as jet fuel dehydrating icing inhibitors is their potential dual action as water scavengers. For example, upon hydrolysis with water, ortho esters often cleave out and create numerous sites for hydrogen bonding in order to enhance the effectiveness of the ice- inhibition process. On the other hand, ortho esters are also distinguished from most of the other water scavenging molecules due to their thermal stability that makes them particularly effective water scavengers even at lower temperatures. The use of ortho esters as an alternative additive for de-icing jet fuels will not only helps in the elimination of ice formation but will also remove any free water formed during the freezing of the jet fuel. When used as a fuel dehydrating icing inhibitor, the ortho esters additive should be thoroughly mixed with the fuel to ensure it is evenly distributed throughout the jet fuel. Consequently, as the aircraft gains latitude and dissolved water begins to separate from the fuel, the ortho esters water scavenging molecules will preferentially react with any available water through hydrolysis thereby preventing icing. On the other hand, the additive is also best suited for water removal due to its high efficiency as compared to most of the conventional fuel dehydrating icing inhibitors. These functions is particularly enhanced by the unique properties of ortho esters such as its high solubility in water, rapid reaction with free water from the fuel even in low temperatures and non corrosiveness to the jet fuel system components. Potential Benefits of using Ortho esters as De-ices in Jet Fuel Although dissolved water in the hydrocarbon fuels can be eliminated by a traditional number of techniques some of which include distillation, desiccant adsorption and dry gas stripping, most of these techniques are not only expensive but may also come with additional operational and maintenance costs. Moreover, most of these conventional techniques are not usually very effective and icing may still occur in the Aircraft engines due to breathing and condensation. Apart from improved ground fuel handling procedures and some of these straightforward techniques, jet fuel can be effectively protected from the hazardous risks of free water in the jet fuels using dehydrating icing inhibitors and water scavenging molecules such as ortho-esters. One of the obvious advantages of using ortho esters and other organic molecules as de-icers in jet fuels is that only a small quantity of the additive needs to be added to the fuel to ensure a trouble free operation. Modern jet engines require quality fuels to ensure their optimal performance and safety particularly during high latitude flights (Taylor, 2008, p.2404). The use of ortho-esters as fuel dehydrating icing inhibitor still remains one of the most promising methods. In jet fuels, these ortho-esters act as anti-icing agents as well as carburetor agents. These water scavenging molecules therefore acts as both the de-icing agents as well as the reducing the emulsion problems that may arise in the jet engine. References Alabugin, I. V. 2002. Stereoelectronic Effects and General Trends in Hyperconjugative Acceptor Ability of σ Bonds. Journal of the American Chemical Society 124 (12), pp.3175–3185. Carey, A. F. 2007. Advanced organic Chemistry Part A: Structure and Mechanism. New York: Springer. Chiang Y., Kresge A., Lahti, M. Weeks D. 2003. Hydrolysis of ortho esters: further investigation of the factors that control the rate-determining step. J. Am. Chem. Soc., 105 (23), pp 52–68. Potts, R.A. & Schaller, R.A. 1993. Kinetics of the Hydrolysis of Orthoesters: A General Acid-Catalyzed Reaction. Journal of Chemical Education70(5), pp.421-424. Repetto, S., Costello, J., De Lacy Costello, B., Ratcliffe, N. et al. 2013. The Development of Novel Fuel Dehydrating Icing Inhibitors. SAE Int. J. Fuels Lubr. 6(3), pp. 553-563. Taylor, S.E. 2008. Component Interactions in Jet Fuels: Fuel System Icing Inhibitor Additive. Energy & Fuels 22(4), pp. 2396-2404. Trohalaki, S. & Pachter, R. 2009. Modeling of Fuel System Icing Inhibitors. Energy & Fuels 13(5), pp.992-998. Wenthe, A.M. & Cordes, E.H., “Concerning the Mechanism of AcidCatalyzed Hydrolysis of Ketals, Ortho Esters, and Orthocarbonates. Journal of the American Chemical Society 87(14), pp.73-80. Read More