. Centrifugal extraction of rare earths from wet-process phosphoric acid Now suppose that the distribution coefficient of A between ether and water is 4, which means: K = Concentration of A in ether / Concentration of A in water = 4. When extracting solvent is stirred with solution containing . Upon shaking, these separate into two layers. As you shake the sep funnel its normal for a gas to build up- for example, some solvent evaporates, or youre using sodium bicarbonate and carbon dioxide forms. Let's suppose that a solute A is present in 100 cc of water, and 100 cc of ether will be used for its extraction. For this purpose, to destroy the organic matter if present, acids such as Nitric acid, sulphuric acid, and aqua regia is added. Take the water layer from Step (3), lower the pH to a value of 1 using concentrated hydrochloric acid, shake against methylene chloride, and the neutral organic acids are now soluble in the methylene chloride (Solution 3: ORGANIC ACIDS IN METHYLENE CHLORIDE). Adjustment of the pH is often used to alter the selectivity of the extraction, thereby allowing different metal ions to be separated. Compare the weights of the alkaloid remaining in aqueous solution after 100 cc containing 1 gram has been shaken (a) with 100 cc of chloroform and (b) with two successive quantities of 50 cc of chloroform. While this method is generally the least accurate, the advantage is that it is the most general, being able to provide at least a rough estimate for a wide variety of molecules. The most common use of the distribution principle is in the extraction of substances by solvents, which are often employed in a laboratory or in large scale manufacturing. PDF SOLVENT EXTRACTION - Savitribai Phule Pune University The key requirement of solvent extraction for high efficiency is the conditions that lead to a higher distribution ratio of solute to be extracted. Because the numerator is large and the denominator is small, Kd should always be more than 1. Molecules | Free Full-Text | Comparison of the Determination of Some This website helped me pass! [34] Partition coefficient can also be used to predict the mobility of radionuclides in groundwater. Hydrophobic agrochemicals in general have longer half-lives and therefore display increased risk of adverse environmental impact. when the compounds are added they usually decrease the dielectric constant of an aqueous phase, which leads to the formation of ion association complexes. The distribution or partition coefficient, K d, measured at equilibrium, is a useful concept that expresses the relative affinity for a sorbate in solution to sorb to a particular solid. Later on, in 1940 this process get real importance because of its use in extracting rare earth metals. Extract with methylene chloride the neutrals and acids go into the methylene chloride, the bases stay in the water. The extraction is repeated two to three times, or perhaps more times if the compound has a low partition coefficient in the organic solvent. Solvent extraction is acknowledged as the most promising method for metal separation and purification especially in industrial production due to . The non-polar C18 group is hydrophobic and will be oriented into the organic phase. 106 lessons K The distribution coefficient w grams of a solute is extracted repeatedly from V1 mL of one solvent with successive portions of V2 mL of a second solvent, which is immiscible with the first. A solvent for extraction should have the following properties; The solvent must have the following properties, Some commonly used solvents are Trichloromethane, Methly benzene, and Diethlyether. The value is greater than one if a substance is more soluble in fat-like solvents such as n-octanol, and less than one if it is more soluble in water. The effectiveness of our approach was compared with the . 4: Extraction - Chemistry LibreTexts P How much hyoscyamine would be extracted with this method? We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. When using equal volumes, a \(K\) of \(\sim 6\) means there will be six times as many morphine molecules in the organic layer as there are in the water layer. expression for ionized solutes becomes simply an extension of this, into the range of values I > 0. Actinides Properties & Characteristics | What are Actinides? All other trademarks and copyrights are the property of their respective owners. If a second extraction (again, done with L cc solvent) is made, the quantity unextracted would be: x2 = x1 * KV / KV + L = x0 * KV / KV + L * KV / KV + L = x0 (KV / KV + L)2. We can use these facts to our advantage to purify the products of a reaction. It is nondestructive and best for volatile and unstable substances. Substances are separated by this method on the basis of their different solubilities in two immiscible liquids. Please enable JavaScript. 9YS1h Both ammonia solutions with different concentrations of ammonia are placed in a separating funnel. Its working mechanism is based upon Nernst distribution law. After shaking the sep funnel for a moment, compound A will dissolve in the organic layer and salts B and C will dissolve in the water layer. If a substance is present as several chemical species in the partition system due to association or dissociation, each species is assigned its own Kow value. The value of each log D is then determined as the logarithm of a ratioof the sum of the experimentally measured concentrations of the solute's various forms in one solvent, to the sum of such concentrations of its forms in the other solvent; it can be expressed as[10]:2758, In the above formula, the superscripts "ionized" each indicate the sum of concentrations of all ionized species in their respective phases. First extraction: in a sep funnel add 50.0 mL of the aqueous benzoic acid solution and 10.0 mL dichloromethane (DCM). The distribution coefficient represents the equilibrium constant for this process. Two main methods exist. CHEM 1305, CHEM 1405, CHEM 1413, CHEM 1411 (CHEM 1311, CHEM 1111), CHEM 2423 & CHEM 2425 Resources. Uncharged metal Chelate complex formation. How do you get it out? As before, we can assign the quantity of hyoscyamine extracted into the diethyl ether the value "\(x\)", which would leave "\(0.50 \: \text{g} - x\)" remaining in the aqueous layer of the first extraction. Solvent extraction is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. So if we add an aqueous solution of sodium bicarbonate to deprotonate the benzoic acid, the resulting sodium salt (the conjugate base of benzoic acid) will stay in the water layer. This is used extraction ion and purification of the analyte. {Hd85 j|p=mB[f"DSAm+q^diDQ}N72vVTNw4A4):9D*}5mQJ.Xt+l}>FD3KXqmME]yn>IT ~Jzxp(5) viwj(S,-1oGfB4:Mc=\zDghQEYE=]]rL*2p;7WvVfFzTg#n-)1!>.6:*2f 2 "8rGEZQRcjVW]..CYlTe+wVV3esNylslVk+T,T^[CK98. LLE is an extraction of a substance from one liquid into another liquid phase. [citation needed]. Most organic products arent, but its possible that a compound is so polar that its soluble in water. Partition coefficient - Wikipedia Devise a way to solubilize the organic anion shown below in the organic solvent of a two-phase system in which the second phase is water. Remember: salts are water soluble. After draining the organic layer from the first extraction, fresh solvent can be added to the aqueous layer remaining in the funnel to begin the second extraction (Figure 4.17b). I:1I]r'7DH N() "IEY2Tp f4tu6KH75 0N4QWZ.|cqR2aFiK5#RCzTkB;oCkS[]o[ WE \[\mathrm{D_C = \dfrac{[solute]_{org}}{[solute]_{aq}} = \dfrac{mol_{org}/V_{org}}{mol_{aq}/V_{aq}} = \dfrac{mol_{org}\times V_{aq}}{mol_{aq}\times V_{org}}}\]. Here are my sample calculations. It is to be noted that should be very large for the extraction of solute by solvent extraction. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Solvent extraction is a process in which a substance can be separated from its matrix. in these circumstances, the separation is calculated as the partition coefficient ratio of two solutes. \[4.07 = \dfrac{\left( \dfrac{x}{150 \: \text{mL ether}} \right)}{\left( \dfrac{0.50 \: \text{g} - x}{150 \: \text{mL water}} \right)}\]. BivL)`tU.g=&]kR|+/?Oo~3xzBu~mo#O G~mJ'A2 <0 It is possible to add a relatively hydrophobic ligand to the system. Second extraction: extract 50.0 mL aqueous benzoic acid with two 5.0 mL portions of DCM. The distribution coefficient represents the equilibrium constant for this process. To unlock this lesson you must be a Study.com Member. 0000008700 00000 n or estimated by calculation based on a variety of methods (fragment-based, atom-based, etc.). HS[o0~G8OyMd&V-euf$#1[kQbwizc9'@^B@qA-,`'Z>mb `-[s. OL.1"1D6c'XtM0!Zai=,TE Some features may be down while we improve a few things. PDF Solubility and Distribution Phenomena - Philadelphia University Now titrate the aqueous layer with NaOH to determine how much benzoic acid remained in the water. Otherwise your solvent might get sucked into the vacuum, which is bad for you and the equipment. The round bottom flask shouldnt be more than ~40% full of solvent or else you will get bumping (solvent slashing around- youll lose product this way). Youre going to use a 0.020 M solution of benzoic acid (BzOH) in water and a 0.020 M solution of sodium hydroxide (NaOH). 2.3: LIQUID-LIQUID EXTRACTION - Chemistry LibreTexts %PDF-1.4 % In multiple extractions, the organic layers are combined together,as the goal is to extract the compound into the organic solvent. 1. Percent efficiency is calculated for every used solvent and method to get the max possible yield. Q: Both benzoic acid and ethyl benzoate are soluble in organic solvents. Salting out is the method by which electrolytes are added to enhance the extractability of complexes. The purpose of this lab is to do the experiment and subsequent calculation to prove this fact. Lets say you did some reaction and got the following compounds as your products, and lets say A is the product we want. For example, the blood/gas partition coefficient of a general anesthetic measures how easily the anesthetic passes from gas to blood. Before examining chromatographic separations, it is useful to consider the separation process in a liquid-liquid extraction. [2] Most commonly, one of the solvents is water, while the second is hydrophobic, such as 1-octanol. According to distribution law if x1 is a concentration of solute in Phase 1 andx2 to the concentration of salute in phase 2 the distribution Coefficient is given by x1/x2. R ,dxay Water and organic solvents should not be miscible to each other. In metal extraction procedures, it is often difficult to separate the ion pairs. Metal ions cannot be separated in the organic phase so they are extracted through complexation. A molecular thermodynamic model for the extraction of CoCl2 from different chloride salts by 0.2 mol L-1 trioctylmethylammonium chloride in toluene using the OLI mixed-solvent electrolyte (OLI-MSE) framework is constructed and shows that the salting effects originate from indirect salt cation-solvent interactions that influence the availability of water in the aqueous and organic phases. The ion pair between the two effectively shields the two charged groups and allows the pair to dissolve in an organic solvent. Acetylation of Ferrocene | Mechanism, Equation & Major Product, SAT Subject Test Chemistry: Practice and Study Guide, CSET Science Subtest II Chemistry (218): Practice & Study Guide, NY Regents Exam - Chemistry: Help and Review, General Chemistry Syllabus Resource & Lesson Plans, Virginia SOL - Chemistry: Test Prep & Practice, NES Chemistry (306): Practice & Study Guide, BMAT (Biomedical Admissions Test): Practice & Preparation, WBJEEM (West Bengal Joint Entrance Exam): Test Prep & Syllabus, Organic & Inorganic Compounds Study Guide, GACE Special Education Mathematics & Science (088): Practice & Study Guide, Create an account to start this course today. 0000003948 00000 n Fractional Distillation Examples & Diagram | What is Fractional Distillation? Note that with equal volumes of organic and aqueous phases, the partition coefficient represents the ratio of particles in each layer (Figure 4.11a). The partitioning of the compound between the two layers caused the sample to be incompletely extracted. iOS8pe #H7//H|PYsHbc,"O W=t$Na \YN7Q4j$!'9xaURD*X@8J>F1;x7UBH!LXf[8o%uye=/FOB`? 9i PDF Isolation and Purification of Organic Compounds Extraction (Expt #2) It can be calculated for each solute and solvents used at a particular temperature. It can be used to separate minute quantities of almost every metal from its ores. [57][58], If the solubility, S, of an organic compound is known or predicted in both water and 1-octanol, then logP can be estimated as[46][59], There are a variety of approaches to predict solubilities, and so log S.[60][61], The partition coefficient between n-Octanol and water is known as the n-octanol-water partition coefficient , or Kow. A: By doing an extraction with an organic solvent (ether, DCM etc.) So 1/3 of the original amount is still retained in aqueous medium. QSAR equations, which in turn are based on calculated partition coefficients, can be used to provide toxicity estimates. of S in solvent A)/(conc. This ratio is therefore a comparison of the solubilities of the solute in these two liquids. Using this data you can calculate Kd. In other words, in two extractions using the same 100 cc ether we can separate (2/3 + 2/9), or 88.9%, of the original amount of the compound. It plays a key role in Organic synthesis. An aqueous sample contains a complex mixture of organic compounds, all of which are at trace concentrations. (It should be less than after one 10.0 mL extraction) Compare the actual amount of benzoic acid remaining with what you expect from the Kd calculation. [26] Likewise, it is used to calculate lipophilic efficiency in evaluating the quality of research compounds, where the efficiency for a compound is defined as its potency, via measured values of pIC50 or pEC50, minus its value of log P.[27]. (Imagine using 100 mL of organic solvent relative to a volume of water equal to that in an Olympic-sized swimming pool). For efficient transport, the drug must be hydrophobic enough to partition into the lipid bilayer, but not so hydrophobic, that once it is in the bilayer, it will not partition out again. Next we want to examine some general types of extraction procedures that are commonly used. <]>> Then the concentration of the substance in solvent = x0 - x1 / L , while the concentration of the substance in the water = x1 / V, So the distribution coefficient, K = x1 / V divided by x0 - x1 / L, or, x1 = KV(x0 - x1) / L = x0 (KV / KV + L). It requires simple equipment such as separating funnel, stopper, beakers and funnel stand. Using \(K\), the calculation is identical to the previous discussion, differing only in the smaller volume of the organic layer (\(50 \: \text{mL}\) instead of \(150 \: \text{mL}\)). The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. 0000001282 00000 n Solvent Extraction: Procedures and Applications - PSIBERG These two liquids, flowing in opposite directions, are brought into contact, mixed, and allowed to separate. "Extraction" refers to transference of compound (s) from a solid or liquid into a different solvent or phase. [33] For cases where a drug reaches its target locations through passive mechanisms (i.e., diffusion through membranes), the ideal distribution coefficient for the drug is typically intermediate in value (neither too lipophilic, nor too hydrophilic); in cases where molecules reach their targets otherwise, no such generalization applies. Take the tube with the cyclohexane solution, and add an equal volume of potassium iodide solution, without shaking. It has been shown that the log P of a compound can be determined by the sum of its non-overlapping molecular fragments (defined as one or more atoms covalently bound to each other within the molecule). The determination of the selected antihypertensive drugs in human plasma samples with the novel solvent front position extraction (SFPE) technique is presented. When equilibrium has established, the ratio of concentration of solute in each layer is constant for each system, and this can be represented by a value \(K\) (called the partition coefficient or distribution coefficient). Its like a teacher waved a magic wand and did the work for me. It is a critical parameter for purification using zone melting, and determines how effectively an impurity can be removed using directional solidification, described by the Scheil equation. In areas such as drug discoveryareas involving partition phenomena in biological systems such as the human bodythe log D at the physiologic pH= 7.4 is of particular interest. Create your account, 14 chapters | [46][47] Calculated partition coefficients are also widely used in drug discovery to optimize screening libraries[48][49] and to predict druglikeness of designed drug candidates before they are synthesized. Most applications of liquid-liquid extraction in the laboratory require only a few contacts (2), and the recovery rates for GPA (R 1) and IL (R 2) were defined by Eqs. In other words, if we added an organic cation that has a non-polar R group, this would form an ion pair with the organic anion. { "01_Liquid-Liquid_Extraction" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "02_Chromatography_\u2013_Background" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "03_Broadening_of_Chromatographic_Peaks" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "04_Fundamental_Resolution_Equation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "05_Liquid_Chromatographic_Separation_Methods" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "06_Gas_Chromatographic_Separation_Methods" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "07_Appendix_1:__Derivation_of_the_Fundamental_Resolution_Equation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { "01_In-class_Problems" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "02_Text" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "03_Learning_Objectives" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "04_Instructor\'s_Manual" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "05_Out-of-class_Problems" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "06_Laboratory_Projects" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "07_Specialty_Topics" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "08_Vignettes" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, [ "article:topic", "showtoc:no", "license:ccbyncsa", "licenseversion:40", "authorname:asdl", "author@Thomas Wenzel" ], https://chem.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fchem.libretexts.org%2FBookshelves%2FAnalytical_Chemistry%2FSupplemental_Modules_(Analytical_Chemistry)%2FAnalytical_Sciences_Digital_Library%2FCourseware%2FSeparation_Science%2F02_Text%2F01_Liquid-Liquid_Extraction, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), status page at https://status.libretexts.org. Why is methanol a good solvent for extraction? xbbr``b``3 1x4> s So now youre left with a solution of compound B in ether. Step 1: Lower the pH of the water using concentrated hydrochloric acid. Activity Coefficient Equation & Uses | What is an Activity Coefficient? +G# C h{{`T{w7M6lB^$$^lMA! @)A-E Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. The metal atom with positive charges aggregates themselves with negative charges to form neutral complexes. The distribution coefficient for GPA (K) was defined by Eq. In some cases the metabolites may be chemically reactive. Usually you wash more than once- so after draining off the aqueous layer, add some more water to the sep funnel and wash/rinse/repeat. 660 0 obj <> endobj Chromatography Phases, Types & Uses | What is Chromatography? The partition coefficient is the ratio of the moles of solute in the two phases, and is a more effective means of measuring whether you have achieved the desired goal. Some important factors are discussed as follows; It is considered the most important factor for the extraction of elements in designing a particular extraction procedure. You could have a very high concentration of the solute in the organic phase, but if we looked at the amount of solute in the organic phase relative to the amount still in the water, it might only be a small portion of the total solute in the system. Organic Chemistry Practice Problems and Problem Sets. The 's calculated using molarity and solubility values are not identical since different equilibria are involved. Organic compounds are then quite easily separated from the mixture with inorganic compounds in aqueous medium by adding benzene, chloroform, etc. Thus, a two-stage extraction is more efficient. SNDc%Aqw_|/ZX&lCJb|Q[lnl)6=acT*/7]g8 MendelSet works best with JavaScript enabled. However, caffeine is more soluble in chloroform than water, so chloroform would be the best choice of the solvents shown in terms of the maximum extraction of caffeine. 0000003429 00000 n Masking agents are metal complexing agents, introduced here to improve the separation factor. These liquids are usually water and an organic solvent. through the heating- condensation method. In addition, since log D is pH-dependent, the pH at which the log D was measured must be specified. This result means that \(0.12 \: \text{g}\) is extracted into the diethyl ether in the second extraction and \(0.09 \: \text{g}\) remains in the aqueous layer \(\left( 0.21 \: \text{g} - 0.12 \: \text{g} \right)\). The ratio is the comparison of the solute's solubilities in the two liquids. [3] Hence the partition coefficient measures how hydrophilic ("water-loving") or hydrophobic ("water-fearing") a chemical substance is. How to tell if a compound is a salt you ask? [10]:275ff[11]:6 The defined precedent is for the lipophilic and hydrophilic phase types to always be in the numerator and denominator respectively; for example, in a biphasic system of n-octanol (hereafter simply "octanol") and water: To a first approximation, the non-polar phase in such experiments is usually dominated by the un-ionized form of the solute, which is electrically neutral, though this may not be true for the aqueous phase. such organic products which have low boiling points or are thermally unstable are preferably separated or refined by this process, The principle of solvent extraction next line works on the principle of Nernst distribution law or partition law which states that when a solute particle is distributed in the two solvents is also distributed in a fixed ratio irrespective of the solute present. The only real exceptions you have to know about are halogenated solvents such as dichloromethane and chloroform- these are denser than water and so will always be on bottom. Immiscible liquids are ones that cannot get mixed up together and separate into layers when shaken together. It can be calculated, %E=100 x millimoles of solute extracted/ total millimoles of the aqueous phase. 0000051499 00000 n The distribution coefficient, log D, is the ratio of the sum of the concentrations of all forms of the compound (ionized plus un-ionized) in each of the two phases, one essentially always aqueous; as such, it depends on the pH of the aqueous phase, and log D = log P for non-ionizable compounds at any pH. o Before you turn on or turn off the vacuum open the hatch to allow air into the system. Extraction in Theory and Practice (Part I) - University of California A conventional method for predicting log P through this type of method is to parameterize the distribution coefficient contributions of various atoms to the overall molecular partition coefficient, which produces a parametric model. \[K = \dfrac{\text{Molarity in organic phase}}{\text{Molarity in aqueous phase}}\]. \[4.07 = \dfrac{\left( \dfrac{x}{50 \: \text{mL ether}} \right)}{\left( \dfrac{0.50 \: \text{g} - x}{150 \: \text{mL water}} \right)}\]. How to tell? Partition coefficients can be measured experimentally in various ways (by shake-flask, HPLC, etc.) Hn0l*!R)R*YlC` In addition, Hammett-type corrections are included to account of electronic and steric effects.
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