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Chapter

Cover Physical Chemistry: Quanta, Matter, and Change

Ab initio methods  

Contents Configuration interaction 251 Brief illustration 29.1: Configuration interaction 252 Example 29.1: Finding the energy lowering due to CI 252 Many-body perturbation theory 253 Example 29.2: Setting up Møller–Plesset perturbation theory 254 Checklist of concepts 254 Checklist of equations 255 Why do...

Chapter

Cover Elements of Physical Chemistry

Absolute entropy  

This chapter studies the molecular interpretation of entropy and how it relates to the third law of thermodynamics. The third law of thermodynamics states that the entropies of all perfectly crystalline substances are the same at T = 0. By convention (and as justified statistically), S(0) = 0 for all perfectly ordered crystalline materials. The standard molar entropy is the molar entropy of a substance in its standard state (pure, at 1 bar) at the temperature of interest. Meanwhile, the statistical entropy is the entropy calculated from the Boltzmann formula, as the logarithm of the weight of a configuration. The chapter then looks at the residual entropy of a solid, which is the contribution to the entropy at T = 0 from positional disorder that is frozen in.

Chapter

Cover Pharmaceutical Chemistry

Absorption, Distribution, Metabolism and Excretion  

Chris Rostron

This chapter discusses the effectiveness of a drug on the physiological processes within the body, which is determined by its ability to enter the body, reach its intended target, and remain in the body for as long as is necessary for it to achieve its desired effect. It refers to drugs that are xenobiotics, which prevents them from entering the body and removes them when they do get inside the body. The chapter examines a vital part of drug design and development which is dedicated to overcoming or exploiting these mechanisms, known as the science of pharmacokinetics. This is commonly divided into absorption, distribution, metabolism, and excretion. The chapter provides an overview of the science of pharmacokinetics in order to explain the basic concepts.

Chapter

Cover Core Carbonyl Chemistry

Acetals and ketals  

This chapter examines acetals and ketals. Aldehydes and ketones react reversibly 1:1 with alcohols under general acid or general base catalysis, to give hemiacetals and hemiketals, respectively. With excess alcohol and a catalytic amount of a strong acid, further reversible reaction takes place to replace the OH group and give an acetal (from an aldehyde) or ketal (from a ketone). If arrangements are made to remove the water from the continuously equilibrating mixture, by distillation or other means, then the aldehyde or ketone is quantitatively converted to an acetal or ketal, as the case may be. The reaction proceeds more easily with aldehydes than with ketones, and is practically limited to primary alcohols. The chapter then looks at dithioacetals, dithioketals, and orthoesters. It also considers the protecting group principle.

Chapter

Cover Applied Organometallic Chemistry and Catalysis

Acetic acid and acetyl chemicals  

This chapter outlines the acetyl chemicals family which includes acetic acid, acetaldehyde, methyl acetate, acetic anhydride, and vinyl acetate. It traces the manufacture of acetic acid and its use in a wide range of end-products, such as vinyl acetate, acetate esters, as a solvent specifically for the oxidation of p-xylene to terephthalic acid, and in food and pharmaceutical applications. It also talks about processes used for the production of acetic acid and the acetyl chemicals which involve homogeneous catalysis by transition metal ions or complexes. The chapter describes the reaction of acetaldehyde with acetic anhydride. This affords ethylidene diacetate, which can be thermally de-acetylated to form vinyl acetate with the elimination of acetic acid. The chapter finally refers to alkane oxidation, which is a radical chain process and main propagation that does not need to involve the metal ion.

Chapter

Cover Polar Rearrangements

Acid catalysed aromatic rearrangements  

This chapter reviews arbitrary distinction to create the classification of aromatic rearrangements. It considers reactions that are initiated by the generation of an arenium ion and involve migration around the aromatic ring or from a side chain onto the aromatic nucleus. It also points out that the migration terminus positioned on an aromantic ring is initiated by protonation or by a Lewis acid, which is commonly a consequence of equilibration to form thermodynamically stable products. The chapter highlights intramolecular migration, which occurs via a π-complex that leads to equilibration between arenium ion species. It talks about the mechanism of arenium ion generation which bears a close resemblance to the mechanism for electrophilic aromatic substitution.

Chapter

Cover How to Succeed in Organic Chemistry

Acid Catalysis in Organic Reactions Part 1  

Introduction We are going to have to do this one in two stages. We don’t know enough (yet) to give a full treatment of the problem, but it’s too important to not make a start! Acid catalysis is widely used to increase the rates of...

Chapter

Cover How to Succeed in Organic Chemistry

Acid Catalysis in Organic Reactions Part 2  

Answer 1 Here is the equilibrium again. [image] We know that...

Chapter

Cover How to Succeed in Organic Chemistry

Acid Catalysis in Organic Reactions Part 2  

Introduction We started looking at this problem in Basics 22. Now we have looked at the thermodynamics of reactions involving charged species (Basics 35), we can consider the implications of acid catalysis with a strong acid and with a weak acid.... This...

Chapter

Cover Elements of Physical Chemistry

Acid–base equilibria of salts in water  

This chapter studies the acid–base equilibria of salt solutions. The ions that a dissolved salt provide are themselves either acids or bases, sometimes both. Acidity constants can be used to predict the pH of solutions, and that information in turn can be used to account for the variation of pH during the course of a titration. That information is also helpful as a guide to the selection of solutes that stabilize the pH of solutions. The chapter looks at acid–base titrations, explaining how the pH of a mixed solution of a weak acid and its conjugate base is given by the Henderson–Hasselbalch equation. It then considers the buffer action, examining the buffer solution and differentiating between an acid buffer and a base buffer.

Chapter

Cover Making the Transition to University Chemistry

Acid–base Equilibrium  

This chapter explains the acid-base equilibrium. This involves the transfer of protons in line with the Brønsted–Lowry theory. A strong acid or strong base is fully ionized in an aqueous solution, while weak acids or bases are only partially ionized in an aqueous solution. Acid-base titrations measure the unknown concentration of one solution by reaction with another standard solution with a familiar concentration. The chapter also notes how indicators are typically water-soluble weak organic acids with varying colours at different pH values. It explores the alternative theory of acid-base reactions proposed by Gilbert Lewis: a Lewis acid is an electron-pair acceptor, while a Lewis base is an electron-pair donor.

Chapter

Cover Protecting Group Chemistry

Acid-labile protecting groups  

This chapter discusses acid-labile protecting groups. In the protected forms of potentially nucleophilic functional groups—alcohols, amines, and thiols—the original heteroatom is usually present and capable of reacting with electrophiles unless steric encumbrance or conjugation is completely effective. Whether or not this leads to deprotection depends on the groups attached to the heteroatom. Reversible protonation or activation with a Lewis acid polarises the electrons in the attached bonds leading to electron deficiency at adjacent centres. If one of the attached groups is capable of supporting a full positive charge then rapid cleavage is likely to follow. This behaviour forms the basis of a large range of acid-labile protecting groups that cleave by forming a stabilised cationic intermediate on addition of either a protic or Lewis acid reagent. The chapter then considers SN1-like deprotection, nucleophile-assisted deprotection, and deprotection following activation by alkylation.

Chapter

Cover Aquatic Environmental Chemistry

The acidity of water  

This chapter examines the acidity of natural water, which depends on the nature of material dissolved in it and its interaction with other materials such as the rocks, the organisms living in it, and added pollutants. Rivers and lakes in granite areas, for example, are unable to neutralize any added acidity and are therefore highly susceptible to the effects of acid rain. Significant increases in the acidity of these water bodies produces an environment which is not well suited to life for a number of reasons. Firstly, most organisms are not well adapted to acidic conditions or changes in acidity. In addition, acidification leads to the dissolution of a number of toxic elements which can poison flora and fauna. In limestone areas, however, the water can neutralize moderate quantities of added acid. The chapter then considers the chemical nature of water, before studying polyprotic acids and the solubility of gases.

Chapter

Cover Organic Chemistry

Acidity, basicity, and pK a  

This chapter assesses acidity, basicity, and pK a. An acid is a species having a tendency to lose a proton, while a base is a species having a tendency to accept a proton. The measure of acidity or basicity is called pK a; the value of pK a tells us how acidic (or not) a given hydrogen atom in a compound is. Knowing about pK a is useful because many reactions proceed through protonation or deprotonation of one of the reactants, and it is obviously useful to know what strength acid or base is needed. The chapter then considers nitrogen compounds as acids and bases, and looks at carbon acids, the development of the drug cimetidine, and Lewis acids and bases.

Chapter

Cover Organic Chemistry

Acids and Bases  

This chapter begins by outlining definitions of acids and bases based on Johannes N. Brønsted and Gilbert N. Lewis. Substances that taste sour have long been known as acids. Bases, on the other hand, are compounds which counteract or neutralize acids. The chapter then reviews the dissociation of a Brønsted acid in aqueous solution. It also presents the two important properties of buffer solutions. First, they allow us to prepare an aqueous solution of a desired pH using a weak acid and its conjugate base (a salt of the acid). The second important property of a buffer solution is that its pH will remain approximately constant if relatively small amounts of a further acid or base are added. Next, the chapter focuses on the factors which affect acid and base strengths. It also analyzes the basicity of organic compounds and the solvent effects on acid-base reactions.

Chapter

Cover Chemistry3

Acids and bases  

This chapter describes two broader definitions of acids and bases and introduces the Brønsted–Lowry theory, in which an acid is defined as a proton donor and a base is defined as a proton acceptor. It also reviews the Lewis theory, in which an acid is defined as an electron pair acceptor and a base is defined as an electron pair donor. This is considered a wide definition as all Brønsted–Lowry acids are Lewis acids, but not all Lewis acids are Brønsted–Lowry acids. The chapter identifies the conjugate acid and conjugate base for an acid–base reaction. It details how to calculate the pH of a solution of a strong or weak acid and a strong or weak base and determine the position of an acid–base equilibrium using Ka and Kb.

Chapter

Cover Inorganic Chemistry

Acids and bases  

This chapter focuses on the wide variety of species that are classified as acids and bases. It first explains the Brønsted -definition, in which an acid is a proton donor and a base is a proton acceptor. The discussion on the characteristics of Brønsted acids covers the periodic trends in aqua acid strength, simple oxoacids, anhydrous oxides, and polyoxo compound formation. The chapter also introduces the Lewis definition of acids and bases which deals with reactions involving electron-pair sharing between a donor (the base) and an acceptor (the acid). This broadening enables the extension of the discussion on acids and bases to include species that do not contain protons and reactions in nonprotic media. Furthermore, the chapter introduces a molecular orbital description of hydrogen bonding. Lastly, it discusses the nonaqueous solvents and -describes some of the most important applications of acid–base chemistry.

Chapter

Cover Periodicity and the s- and p-Block Elements

Acids and bases  

This chapter focuses on acids and bases. The broadest definition of acids and bases is the Lewis definition. In this scheme, a Lewis acid is an electron-pair acceptor while a Lewis base is an electron-pair donor. Meanwhile, the Brønsted–Lowry concept is probably the second most commonly employed description of acids and bases, although in aqueous solution it is by far the most widely used. In this scheme, an acid is defined as a proton donor and a base as a proton acceptor. It is important to note that the concept of Lewis acidity and basicity incorporates the Brønsted–Lowry approach as a special case. The chapter then looks at element oxides and hydroxides, as well as the Lewis acidity of the heavier p-block elements. It also considers hard and soft acids and bases.

Chapter

Cover Foundations of Organic Chemistry: Worked Examples

Acids and bases  

This chapter discusses equilibria and factors effecting acidity and basicity of organic compounds and other areas of chemistry, such as electronegativity of the atom that bear the charge. It examines delocalization, resonance, inductive effects, indicators, and interactions between molecules and ions. It also deals with the protonation on the carbonyl oxygen atom that gives the more stable, delocalized protonated form of the COOH group. The chapter mentions the inductive electron-withdrawing effect of the C=O that helps to weaken the adjacent O-H bond and carboxylic acids that are more acidic than alcohols, whose anions are not stabilized by delocalisation. It refers to Tenormin, which is a drug used in the treatment of high blood pressure, angina, and abnormal heart rhythms.

Chapter

Cover Foundations of Organic Chemistry

Acids and bases  

This chapter examines acids and bases. The Brønsted–Lowry theory states that acids are proton donors, and bases are proton acceptors. Acid/base reactions are largely equilibria and are therefore under thermodynamic control. Many organic acids, such as ethanoic acid, are weak acids. The equilibrium constants are small, much less than 1, and remarkably little of the acid donates its proton to water in aqueous solution. Moreover, many organic acids and bases are largely insoluble in water. The chapter then considers the reactivity of bases as leaving groups and nucleophiles, before comparing acid strengths and base strengths. It also looks at amino acids, which are the building blocks of proteins; they are compounds which have major structural and catalytic roles in all living organisms.