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
Acetals and ketals
Chapter
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
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
Acid Catalysis in Organic Reactions Part 2
Answer 1
Here is the equilibrium again.
[image]
We know that...
Chapter
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
Acid-labile protecting groups
This chapter discusses acid-labile protecting groups. In the protected forms of potentially nucleophilic functional groupsalcohols, amines, and thiolsthe 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
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
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
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.
Chapter
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
The addition of nucleophilic reagents to aldehydes and ketones
This chapter discusses the addition of nucleophilic reagents to aldehydes and ketones. In the series formaldehyde–acetaldehyde–acetone, reactivity to nucleophiles decreases in that order. The electronic factor here is deactivating, but slight, because the additional Me groups are not attached directly to the carbonyl carbon. They are largely insulated from it by saturated carbon atoms. The chapter then considers high degrees of hydration at equilibrium, which generally result when the carbonyl group has strong electron-withdrawing substituents attached, and in extreme cases it is the hydrate which is the familiar form of the carbonyl compound. It also looks at hemiacetal and hemiketal formation; cyanohydrin formation; bisulfate addition; the Meerwein–Ponndorf–Verley reaction; the Cannizzaro reaction; and reaction with Grignard reagents.
Chapter
Addition Reactions of Alkenes and Alkynes
This chapter begins with a discussion on the functional groups of alkenes and alkynes: carbon-carbon double and triple bonds. It highlights that the simplest alkene is ethene (ethylene) which acts as a plant hormone controlling the maturation of flowers and fruit. The chapter then reviews the regioselectivity and stereochemistry of electrophilic addition to alkenes. It argues that the characteristic reactions of alkenes and alkynes are addition reactions which may be initiated in various ways depending on the nature of the alkene (or alkyne) and the reagent. The chapter then presents the most common reaction of simple alkenes: electrophilic addition. It also looks at the electrophilic attack at 1,3-dienes which leads to both 1,2- and 1,4-addition products. Finally, it concludes by explaining the Diels-Alder reactions and the epoxidation of alkenes. It also considers the cyclopropanation of alkenes with carbenes and carbenoids and the hydrogenation of alkenes and alkynes.
Chapter
Additions and eliminations
This chapter assesses a large number of important reactions, which have in common the interconversion of sp2 and sp3-hybridized centres. In some cases, the overall reaction is substitution rather than addition or elimination. The more closely one looks at the orbital interactions involved, the clearer become the underlying similarities between different mechanisms. Stereoelectronic effects on some E2 reactions show marked similarities to stereoelectronic effects on SN1 reactions. Addition reactions going by ionic mechanisms involve the addition of both a nucleophile and an electrophile, almost always in separate steps. The usual classification into nucleophilic and electrophilic addition is based on the main HOMO/LUMO interaction in the rate determining step. For additions to ordinary alkenes, this involves the interaction of the π-bonding orbital with the LUMO of the electrophile.
Chapter
Additions to C–C double
This chapter focuses on additions to C–C double bonds. There are two particularly important types of additions to C–C double bonds. The reaction can be concerted, in which a new bond is formed to each end of the double bond more or less simultaneously. At the other extreme, the two bond forming events are effectively independent of each other, except that for mechanistic reasons one must precede the other. The chapter considers some of the more important types of additions, including hydroboration, Diels–Alder cycloaddition, electrophilic addition, and conjugate addition. An important general type of electrophilic addition which is useful in stereoselective organic synthesis involves cyclization. There are two possible modes of cyclization: endo and exo ring closures. The chapter also looks at cyclopropanation.
Chapter
Additions to carbon-carbon multiple bonds
This chapter studies the additions to carbon–carbon multiple bonds. In particular, it deals with additions of XY to double and triple carbon–carbon bonds to give products in which X and Y are separately bonded to the adjacent carbons of the original multiple bond. The simplest are reactions of diatomic molecules HX or X2 where X is a halogen, and these almost invariably have step-wise mechanisms. Such additions to alkenes or alkynes are the reverse of elimination reactions and two of the mechanisms are the E1 and E1cB in reverse. The chapter considers the mechanisms of addition in turn according to whether the initial step of the reaction of the unsaturated organic compound is with an electrophile, a nucleophile, or a radical. In electrophilic addition, the overall process is completed by the addition of a nucleophile and, in nucleophilic addition, completion involves addition of an electrophile.
Chapter
Additions to carbonyl
This chapter examines additions to carbonyl compounds, which is one of the most important reactions in organic synthesis. Nucleophilic additions to carbonyl groups can be divided into those which give enantiomeric products, and those whose products are diastereoisomers. In the latter case, the carbonyl compound is often chiral. The chapter then looks at additions to chiral carbonyl compounds and prochiral carbonyl compounds. There have been several models advanced to account for the trends observed in the diastereoselectivity of nucleophilic additions to ketones and aldehydes. Such models need to take into account the various factors which might influence such additions. These include conformational effects, steric interactions, electronic factors, and stereoelectronic effects.
Chapter
Afterword
So, you’ve got to the end. Hopefully you have enjoyed reading this book. More importantly, I hope you have understood all of the explanations and learned quite a lot of organic chemistry.
Did you ‘just’ read this book?
If you did, then that’s okay. The...
Chapter
Alcohols 10 Ethers
This chapter describes a reaction concerned with sodium periodate and lead tetraacetate, which are useful reagents that have similar oxidizing properties. It demonstrates how salt NaIO4 is dissolved using an aqueous medium and how a covalent compound Pb(OAc)4 is soluble in some organic solvents and is decomposed by water. It highlights that the choice of reagent is determined by the solubility of the substrate. The chapter explains the difference in rates between cis cyclopentane-1,2-diol, which is the easy ring formation, and the trans isomer, which is the difficult ring formation. It covers reagents that cause fission of several other systems that have 1,2-oxygen-containing groups.
Chapter
Aldehydes and ketones
This chapter talks about the carbon-oxygen double bond or carbonyl group, which is the most common group in organic chemistry. It examines keto and enol forms from the carbonyl group, the oxidation and reduction of carbonyl compounds, reactions of keto and enol forms and enolate anions, and aldol and related condensations. It also describes a polarization that can be expressed in terms of electronic effects or of resonance in which the outcome is the same. The chapter refers to carbonyl compounds that exhibit tautomerism, which is a tautomeric compound that consists of an equilibrium mixture of two or more structural isomers under stable conditions. It emphasizes that simple carbonyl compounds contain only very small amounts of the enolic forms, elaborating that the interconversion between keto and enol forms is slow in a solution free from impurities.
Chapter
Aldol condensations and related reactions
This chapter explores aldol condensations and related reactions. The reactions of enols and enolates with electrophiles are not confined to the simple α substitutions so far discussed. The electrophile can also be a carbonyl compound, and, as with the attack of simpler nucleophiles on carbonyl groups, the formation of a tetrahedral adduct can be followed by protonation, dehydration, or loss of a leaving group. Aldol condensations, whether carried out with acid or base catalysis, are often followed by spontaneous dehydration. Dehydration is practically the norm for acid-catalyzed conditions. The chapter then looks at crossed aldol condensations; enolate acylation reactions; the Thorpe–Ziegler cyclization; and αβ unsaturated carbonyl compounds.
