In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent; the mixing process of a solution happens at a scale where the effects of chemical polarity are involved, resulting in interactions that are specific to solvation. The solution assumes the phase of the solvent when the solvent is the larger fraction of the mixture, as is the case; the concentration of a solute in a solution is the mass of that solute expressed as a percentage of the mass of the whole solution. The term aqueous solution is. A solution is a homogeneous mixture of two or more substances; the particles of solute in a solution cannot be seen by the naked eye. A solution does not allow beams of light to scatter. A solution is stable; the solute from a solution cannot be separated by filtration. It is composed of only one phase. Homogeneous means. Heterogeneous means; the properties of the mixture can be uniformly distributed through the volume but only in absence of diffusion phenomena or after their completion.
The substance present in the greatest amount is considered the solvent. Solvents can be liquids or solids. One or more components present in the solution other; the solution has the same physical state as the solvent. If the solvent is a gas, only gases are dissolved under a given set of conditions. An example of a gaseous solution is air. Since interactions between molecules play no role, dilute gases form rather trivial solutions. In part of the literature, they are not classified as solutions, but addressed as mixtures. If the solvent is a liquid almost all gases and solids can be dissolved. Here are some examples: Gas in liquid: Oxygen in water Carbon dioxide in water – a less simple example, because the solution is accompanied by a chemical reaction. Note that the visible bubbles in carbonated water are not the dissolved gas, but only an effervescence of carbon dioxide that has come out of solution. Liquid in liquid: The mixing of two or more substances of the same chemistry but different concentrations to form a constant.
Alcoholic beverages are solutions of ethanol in water. Solid in liquid: Sucrose in water Sodium chloride or any other salt in water, which forms an electrolyte: When dissolving, salt dissociates into ions. Solutions in water are common, are called aqueous solutions. Non-aqueous solutions are. Counter examples are provided by liquid mixtures that are not homogeneous: colloids, emulsions are not considered solutions. Body fluids are examples for complex liquid solutions. Many of these are electrolytes. Furthermore, they contain solute molecules like urea. Oxygen and carbon dioxide are essential components of blood chemistry, where significant changes in their concentrations may be a sign of severe illness or injury. If the solvent is a solid gases and solids can be dissolved. Gas in solids: Hydrogen dissolves rather well in metals in palladium. Liquid in solid: Mercury in gold, forming an amalgam Water in solid salt or sugar, forming moist solids Hexane in paraffin wax Solid in solid: Steel a solution of carbon atoms in a crystalline matrix of iron atoms Alloys like bronze and many others Polymers containing plasticizers The ability of one compound to dissolve in another compound is called solubility.
When a liquid can dissolve in another liquid the two liquids are miscible. Two substances that can never mix to form a solution are said to be immiscible. All solutions have a positive entropy of mixing; the interactions between different molecules or ions may be energetically favored or not. If interactions are unfavorable the free energy decreases with increasing solute concentration. At some point the energy loss outweighs the entropy gain, no more solute particles can be dissolved. However, the point at which a solution can become saturated can change with different environmental factors, such as temperature and contamination. For some solute-solvent combinations a supersaturated solution can be prepared by raising the solubility to dissolve more solute, lowering it; the greater the temperature of the solvent, the more of a given solid solute it can dissolve. However, most gases and some compounds exhibit solubilities that decrease with increased temperature; such behavior is a result of an exothermic enthalpy of solution.
Some surfactants exhibit this behaviour. The solubility of liquids in liquids is less temperature-sensitive than that of solids or gases; the physical properties of compounds such as melting point and boiling point change when other compounds are added. Together they are called colligative properties. There are several ways to quantify the amount of one compound dissolved in the other compounds collectively called concentration. Examples include molarity, volume fraction, mole fraction; the properties of ideal solutions can be calculated by the linear combination of the properties of
An acid–base reaction is a chemical reaction that occurs between an acid and a base, which can be used to determine pH. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems, their importance becomes apparent in analyzing acid–base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. The first of these concepts was provided by the French chemist Antoine Lavoisier, around 1776, it is important to think of the acid-base reaction models as theories. For example, the current Lewis model has the broadest definition of what an acid and base are, with the Bronsted-Lowry theory being a subset of what acids and bases are, the Arrhenius theory being the most restrictive; the first scientific concept of acids and bases was provided by Lavoisier in around 1776. Since Lavoisier's knowledge of strong acids was restricted to oxoacids, such as HNO3 and H2SO4, which tend to contain central atoms in high oxidation states surrounded by oxygen, since he was not aware of the true composition of the hydrohalic acids, he defined acids in terms of their containing oxygen, which in fact he named from Greek words meaning "acid-former".
The Lavoisier definition held for over 30 years, until the 1810 article and subsequent lectures by Sir Humphry Davy in which he proved the lack of oxygen in H2S, H2Te, the hydrohalic acids. However, Davy failed to develop a new theory, concluding that "acidity does not depend upon any particular elementary substance, but upon peculiar arrangement of various substances". One notable modification of oxygen theory was provided by Berzelius, who stated that acids are oxides of nonmetals while bases are oxides of metals. In 1838, Justus von Liebig proposed that an acid is a hydrogen-containing substance in which the hydrogen could be replaced by a metal; this redefinition was based on his extensive work on the chemical composition of organic acids, finishing the doctrinal shift from oxygen-based acids to hydrogen-based acids started by Davy. Liebig's definition, while empirical, remained in use for 50 years until the adoption of the Arrhenius definition; the first modern definition of acids and bases in molecular terms was devised by Svante Arrhenius.
A hydrogen theory of acids, it followed from his 1884 work with Friedrich Wilhelm Ostwald in establishing the presence of ions in aqueous solution and led to Arrhenius receiving the Nobel Prize in Chemistry in 1903. As defined by Arrhenius: an Arrhenius acid is a substance that dissociates in water to form hydrogen ions; this causes the creation of the hydronium ion. Thus, in modern times, the symbol H+ is interpreted as a shorthand for H3O+, because it is now known that a bare proton does not exist as a free species in aqueous solution. An Arrhenius base is a substance. Under this definition, pure H2SO4 and HCl dissolved in toluene are not acidic, molten NaOH and solutions of calcium amide in liquid ammonia are not alkaline; this led to the development of the Bronsted-Lowry theory and subsequent Lewis theory to account for these non-aqueous exceptions. Overall, to qualify as an Arrhenius acid, upon the introduction to water, the chemical must either cause, directly or otherwise: an increase in the aqueous hydronium concentration, or a decrease in the aqueous hydroxide concentration.
Conversely, to qualify as an Arrhenius base, upon the introduction to water, the chemical must either cause, directly or otherwise: a decrease in the aqueous hydronium concentration, or an increase in the aqueous hydroxide concentration. The reaction of an acid with a base is called a neutralization reaction; the products of this reaction are a water. Acid + base → salt + waterIn this traditional representation an acid–base neutralization reaction is formulated as a double-replacement reaction. For Example, the reaction of hydrochloric acid, HCl, with sodium hydroxide, NaOH, solutions produces a solution of sodium chloride, NaCl, some additional water molecules. HCl + NaOH → NaCl + H2OThe modifier in this equation was implied by Arrhenius, rather than included explicitly, it indicates. Though all three substances, HCl, NaOH and NaCl are capable of existing as pure compounds, in aqueous solutions they are dissociated into the aquated ions H+, Cl−, Na+ and OH−; the Brønsted–Lowry definition, formulated in 1923, independently by Johannes Nicolaus Brønsted in Denmark and Martin Lowry in England, is based upon the idea of protonation of bases through the de-protonation of acids – that is, the ability of acids to "donate" hydrogen ions —otherwise known as protons—to bases, which "accept" them.
An acid–base reaction is, the removal of a hydrogen ion from the acid and its addition to the base. The removal of a hydrogen ion from an acid produces its conjugate base, the acid with a hydrogen ion removed; the reception of a proton by a base produces its conjugate acid, the base with a hydrogen ion added. Unlike the previous definitions, the Brønsted–Lowry definition does not refer to the formation of salt and solvent, but instead to the formatio
An odor, or odour, is caused by one or more volatilized chemical compounds that are found in low concentrations that humans and animals can perceive by their sense of smell. An odor is called a "smell" or a "scent", which can refer to either a pleasant or an unpleasant odor. While "scent" can refer to pleasant and unpleasant odors, the terms "scent", "aroma", "fragrance" are reserved for pleasant-smelling odors and are used in the food and cosmetic industry to describe floral scents or to refer to perfumes. In the United Kingdom, "odour" refers to scents in general. An unpleasant odor can be described as "reeking" or called a "malodor", "stench", "pong", or "stink"; the perception of odors, or sense of smell, is mediated by the olfactory nerve. The olfactory receptor cells are neurons present in the olfactory epithelium, a small patch of tissue at the back of the nasal cavity. There are millions of olfactory receptor neurons; each neuron has cilia in direct contact with the air. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain.
When an electrical signal reaches a threshold, the neuron fires, which sends a signal traveling along the axon to the olfactory bulb, a part of the limbic system of the brain. Interpretation of the smell begins there, relating the smell to past experiences and in relation to the substance inhaled; the olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and forwarded to the central nervous system, which controls emotions and behavior as well as basic thought processes. Odor sensation depends on the concentration available to the olfactory receptors. A single odorant is recognized by many receptors. Different odorants are recognized by combinations of receptors; the patterns of neuron signals help to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix; this does not correspond to the intensity of any single constituent. Most odors consists of organic compounds, although some simple compounds not containing carbon, such as hydrogen sulfide and ammonia, are odorants.
The perception of an odor effect is a two-step process. First, there is the physiological part; this is the detection of stimuli by receptors in the nose. The stimuli are recognized by the region of the human brain; because of this, an objective and analytical measure of odor is impossible. While odor feelings are personal perceptions, individual reactions are related, they relate to things such as gender, state of health, personal history. The ability to identify odor varies among decreases with age. Studies show there are sex differences in odor discrimination, women outperform men. Pregnant women have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions; the ability to taste decreases with age as the sense of smell tends to dominate the sense of taste. Chronic smell problems are reported in small numbers for those in their mid-twenties, with numbers increasing with overall sensitivity beginning to decline in the second decade of life, deteriorating appreciably as age increases once over 70 years of age.
For most untrained people, the process of smelling gives little information concerning the specific ingredients of an odor. Their smell perception offers information related to the emotional impact. Experienced people, such as flavorists and perfumers, can pick out individual chemicals in complex mixtures through smell alone. Odor perception is a primal sense; the sense of smell enables pleasure, can subconsciously warn of danger, help locate mates, find food, or detect predators. Humans have a good sense of smell, correlated to an evolutionary decline in sense of smell. A human's sense of smell is just as good as many animals and can distinguish a diversity of odors—approximately 10,000 scents. Studies reported. Odors that a person is used to, such as their own body odor, are less noticeable than uncommon odors; this is due to habituation. After continuous odor exposure, the sense of smell is fatigued, but recovers if the stimulus is removed for a time. Odors can change due to environmental conditions: for example, odors tend to be more distinguishable in cool dry air.
Habituation affects the ability to distinguish odors after continuous exposure. The sensitivity and ability to discriminate odors diminishes with exposure, the brain tends to ignore continuous stimulus and focus on differences and changes in a particular sensation; when odorants are mixed, a habitual odorant is blocked. This depends on the strength of the odorants in the mixture, which can change the perception and processing of an odor; this process helps classify similar odors as well as adjust sensitivity to differences in complex stimuli. The primary gene sequences for thousands of olfactory receptors are known for the genomes of more than a dozen organisms, they are seven-helix-turn transmembrane proteins. But there are no known structures for any olfactory receptor. There is a conserved sequence in three quarters of all ORs; this is a tripodal metal-ion binding site, and
Lewis acid catalysis
In Lewis acid catalysis of organic reactions, a metal-based Lewis acid acts as an electron pair acceptor to increase the reactivity of a substrate. Common Lewis acid catalysts are based on main group metals such as aluminum, boron and tin, as well as many early and late d-block metals; the metal atom forms an adduct with a lone-pair bearing electronegative atom in the substrate, such as oxygen, nitrogen and halogens. The complexation has partial charge-transfer character and makes the lone-pair donor more electronegative, activating the substrate toward nucleophilic attack, heterolytic bond cleavage, or cycloaddition with 1,3-dienes and 1,3-dipoles. Many classical reactions involving carbon–carbon or carbon–heteroatom bond formation can be catalyzed by Lewis acids. Examples include the Friedel-Crafts reaction, the aldol reaction, various pericyclic processes that proceed at room temperature, such as the Diels-Alder reaction and the ene reaction. In addition to accelerating the reactions, Lewis acid catalysts are able to impose regioselectivity and stereoselectivity in many cases.
Early developments in Lewis acid reagents focused on available compounds such as TiCl4, BF3, SnCl4, AlCl3. The relative strengths of these Lewis acids may be estimated from NMR spectroscopy by the Childs method or the Gutmann-Beckett method. Over the years, versatile catalysts bearing ligands designed for specific applications have facilitated improvement in both reactivity and selectivity of Lewis acid-catalyzed reactions. More Lewis acid catalysts with chiral ligands have become an important class of tools for asymmetric catalysis. Challenges in the development of Lewis acid catalysis include inefficient catalyst turnover and the frequent requirement of two-point binding for stereoselectivity, which necessitates the use of auxiliary groups. In reactions with polar mechanisms, Lewis acid catalysis involves binding of the catalyst to Lewis basic heteroatoms and withdrawing electron density, which in turn facilitates heterolytic bond cleavage or directly activates the substrate toward nucleophilic attack.
The dichotomy can have important consequences in some reactions, as in the case of Lewis acid-promoted acetal substitution reactions, where the SN1 and SN2 mechanisms shown below may give different stereochemical outcomes. Studying the product ratio in a bicyclic system and colleagues showed that both mechanisms could be operative depending on the denticity of the Lewis acid and the identity of the R' group. In Diels-Alder and 1,3-dipolar cycloaddition reactions, Lewis acids lower the LUMO energy of the dienophile or dipolarphile making it more reactive toward the diene or the dipole. Among the types of reactions that can be catalyzed by Lewis acids, those with carbonyl-containing substrates have received the greatest amount of attention; the first major discovery in this area was in 1960, when Yates and Eaton reported the significant acceleration of the Diels-Alder reaction by AlCl3 when maleic anhydride is the dienophile. Early theoretical studies that depended on frontier orbital analysis established that Lewis acid catalysis operates via lowering of the dienophile's LUMO energy, still the accepted rationalization.
The concept of lowered LUMO energy is used to explain the enhanced electrophilic reactivity of carbonyl compounds towards mild nucleophilic reagents, as in the cases of the Mukaiyama aldol reaction and Sakurai reaction. In addition to rate acceleration, Lewis acid-catalyzed reactions sometimes exhibit enhanced stereoselectivity, which stimulated the development of stereoinduction models; the models have their roots in knowledge of the structures of Lewis acid-carbonyl complexes which, through decades of research in theoretical calculations, NMR spectroscopy, X-ray crystallography, were firmly established in the early 1990s: σ-Complexation: The complex in which the Lewis acid interacts with the carbonyl compound through a σ-bond with the oxygen lone pair is both thermodynamically favored and catalytically relevant. Bent geometry: The metal-oxygen-carbon bond angle is less than 180°, the metal is syn to the smaller substituent, unless influenced by a chelating group on the larger substituent.
An s-trans preference for α,β-unsaturated compounds. The Mukaiyama aldol reaction and the Sakurai reaction refer to the addition of silyl enol ethers and allylsilanes to carbonyl compounds, respectively. Only under Lewis acid catalysis do. Acyclic transition states are believed to be operating in both reactions for either 1,2- or 1,4- addition, steric factors control stereoselectivity; this is in contrast with the rigid Zimmerman-Traxler cyclic transition state, accepted for the aldol reaction with lithium and titanium enolates. As a consequence, the double bond geometry in the silyl enol ether or allylsilane does not translate well into product stereochemistry. A model for the Sakurai 1,2-addition, proposed by Kumada, is presented in the scheme below. A similar analysis by Heathcock explains the fact that, with simple substrates, there is no diastereoselectivity for the intermolecular Mukaiyama aldol reaction; the Lewis acid catalyst plays a role in stereoselectivity when the aldehyde can chelate onto the metal center and form a rigid cyclic intermediate.
The stereochemical outcome is consistent with approach of
In chemistry, pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature, pure water is neither acidic nor basic and has a pH of 7; the pH scale is logarithmic and approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in a solution. More it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic; the neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for strong acids and bases respectively; the pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode.
The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or a color-changing indicator. Measurements of pH are important in chemistry, medicine, water treatment, many other applications; the concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had the "H" as a subscript to the lowercase "p", as so: pH; the exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH stands for "power of hydrogen". It has been suggested that the "p" stands for the German Potenz, others refer to French puissance. Another suggestion is that the "p" stands for the Latin terms pondus hydrogenii, potentia hydrogenii, or potential hydrogen, it is suggested that Sørensen used the letters "p" and "q" to label the test solution and the reference solution.
In chemistry, the p stands for "decimal cologarithm of", is used in the term pKa, used for acid dissociation constants. Bacteriologist Alice C. Evans, famed for her work's influence on dairying and food safety, credited William Mansfield Clark and colleagues with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little and colleagues knew about Sørensen's work a few years prior, she said: In these studies Dr. Clark's attention was directed to the effect of acid on the growth of bacteria, he found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration; these methods replaced the inaccurate titration method of determining acid content in use in biologic laboratories throughout the world.
They were found to be applicable in many industrial and other processes in which they came into wide usage. The first electronic method for measuring pH was invented by Arnold Orville Beckman, a professor at California Institute of Technology in 1934, it was in response to local citrus grower Sunkist that wanted a better method for testing the pH of lemons they were picking from their nearby orchards. PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. PH = − log 10 = log 10 For example, for a solution with a hydrogen ion activity of 5×10−6 we get 1/ = 2×105, thus such a solution has a pH of log10 = 5.3. For a commonplace example based on the facts that the masses of a mole of water, a mole of hydrogen ions, a mole of hydroxide ions are 18 g, 1 g, 17 g, a quantity of 107 moles of pure water, or 180 tonnes, contains close to 1 g of dissociated hydrogen ions and 17 g of hydroxide ions. Note that pH depends on temperature. For instance at 0 °C the pH of pure water is 7.47.
At 25 °C it's 7.00, at 100 °C it's 6.14. This definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, electrode potential, E, follows the Nernst equation, for the hydrogen ion can be written as E = E 0 + R T F ln = E 0 − 2.303 R T F pH where E is a measured potential, E0 is the standard electrode potential, R is the gas const
An acid–base titration is a method of quantitative analysis for determining the concentration of an acid or base by neutralizing it with a standard solution of base or acid having known concentration. A pH indicator is used to monitor the progress of the acid–base reaction. If the acid dissociation constant of the acid or base dissociation constant of base in the analyte solution is known, its solution concentration can be determined. Alternately, the pKa can be determined if the analyte solution has a known solution concentration by constructing a titration curve. Alkalimetry and acidimetry are a kind of volumetric analysis in which the fundamental reaction is a neutralization reaction. Alkalimetry is the specialized analytic use of acid-base titration to determine the concentration of a basic substance. Acidimetry, sometimes spelled acidometry, is the same concept of specialized analytic acid-base titration, but for an acidic substance. A suitable pH indicator must be chosen in order to detect the end point of the titration.
The color change or other effect should occur close to the equivalence point of the reaction so that the experimenter can determine when that point is reached. The pH of the equivalence point can be estimated using the following rules: A strong acid will react with a strong base to form a neutral solution. A strong acid will react with a weak base to form an acidic solution. A weak acid will react with a strong base to form a basic solution; when a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength the equivalence pH will be neutral. However, weak acids are not titrated against weak bases because the colour change shown with the indicator is quick, therefore difficult for the observer to see the change of colour; the point at which the indicator changes colour is called the end point. A suitable indicator should be chosen, preferably one that will experience a change in colour close to the equivalence point of the reaction.
The pH of a weak acid solution being titrated with a strong base solution can be found at different points along the way. These points fall into one of four categories: initial pH pH before the equivalence point pH at the equivalence point pH after the equivalence point for more rigorous calculation, using a RICE chart is required. In fact the equations below are a simplification of the RICE chart. More a single formula that describes the titration of a weak acid with a strong base from start to finish is given below: ϕ = C b V b C a V a = α A − − − C a 1 + − C b α A − = K a + K a where " φ = fraction of completion of the titration C a, C b = the concentrations of the acid and base V a, V b = the volumes of the acid and base α A − = the fraction of the weak acid, ionized K a = the dissociation constant for the acid, = concentrations of the H+ and OH– ions respectivelyThis formula is somewhat cumbersome, but does describe the titration curve as a single equation; the titration process creates solutions with compositions ranging from pure acid to pure base.
Identifying the pH associated with any stage in the titration process is simple for monoprotic acids and bases. The presence of more than one acid or base group complicates these computations. Graphical methods, such as the equiligraph, have long been used to account for the interaction of coupled equilibria; these graphical solution methods are simple to implement, however they are infrequently used. Henderson–Hasselbalch equation Graphical method to solve acid-base problems, including titrations Graphic and numerical solver for general acid-base problems - Software Program for phone and tablets Simple analytical formulas for the titration of N-protic acids. Valid for any number of N: 1-monoprotic, 2-diprotic until N=6
The Gutmann–Beckett method is an experimental procedure used by chemists to assess the Lewis acidity of molecular species. Triethylphosphine oxide is used as a probe molecule and systems are evaluated by 31P NMR spectroscopy. Gutmann used 31P NMR spectroscopy to parameterize Lewis acidity of solvents by Acceptor Numbers. Beckett recognised its more utility and adapted the procedure so that it could be applied to molecular species when dissolved in weakly Lewis acidic solvents; the term Gutmann–Beckett method was first used in chemical literature in 2007. Prof. Dr Viktor Gutmann was an eminent Austrian chemist renowned for his work on non-aqueous solvents. Prof. Michael A. Beckett is a former Head of the School of Chemistry at Bangor University, UK; the 31P chemical shift of Et3PO is sensitive to chemical environment but can be found between +40 and +100 ppm. The O atom in Et3PO is a Lewis base, its interaction with Lewis acid sites causes deshielding of the adjacent P atom. Gutmann described an Acceptor Number scale for solvent Lewis acidity with two reference points relating to the 31P NMR chemical shift of Et3PO in the weakly Lewis acidic solvent hexane and in the Lewis acidic solvent SbCl5.
Acceptor numbers can be calculated from AN = 2.21 x and higher AN values indicate greater Lewis acidity. Boron trihalides are archetypal Lewis acids and have the following AN values: BF3 < BCl3 < BBr3 < BI3. The Lewis acidity of other molecules can be obtained in weakly Lewis acidic solvents by 31P NMR measurements of their Et3PO adducts; the Gutmann–Beckett method has been applied to Lewis acids derived fluoroarylboranes such as B3, borenium cations, its application to a variety of boron compounds has been reviewed. The Gutmann–Beckett method has been applied to alkaline earth metal complexes, p-block main group compounds and transition-metal compounds