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Chapter

Cover Pharmaceutical Chemistry

The Carbonyl Group and its Chemistry  

Matthew Ingram

This chapter discusses the nature of the carbonyl group and the chemistry it can undergo. The chemistry of the carbonyl group can be found in lots of situations: in the body, in the environment, in manufacturing, and in pharmaceutical applications. At the heart of the chemistry of the carbonyl group is the carbonyl bond, a double bond comprising one σ and one π bond, which joins carbon and oxygen. The chapter points out the difference in electronegativity between carbon and oxygen, citing a dipole moment that exists between the two atoms. The carbonyl group is central to pharmaceutical chemistry and is present in many drug molecules and is present in many different types of compound.

Chapter

Cover Metal-Metal Bonded Carbonyl Dimers and Clusters

Terminal phosphine and bridging phosphido ligands  

This chapter discusses the substitution of one or more carbonyl groups for other ligands and mentions large numbers of phosphine and phosphite derivatives of metal carbonyl dimers and clusters. It explains how related compounds that contain arsine and stibine ligands may be prepared by replacing terminal carbonyl groups. It also describes the important roles that steric and electronic requirements play in determining the site preferences and solid state structures of derivatives. The chapter refers to the reduction of steric crowding with the use of a didentate ligand in place of two monodentate ligands. It talks about the replacement of a carbonyl by a phosphine or phosphate ligand which is carried out by photolyzing or heating the two reagents in a solution.

Chapter

Cover Organonitrogen Chemistry

Amides  

This chapter discusses enamines. It explains the process of achieving enamine through regaining neutrality by losing a proton adjacent to the iminium moiety. Additionally, these compounds rarely serve as nucleophiles or bases on nitrogen. The chapter discusses the formation of enamines from amine and aldehyde or ketone. It highlights how electrophiles are required to be quite reactive as enamines are moderately nucleophilic. The chapter adds the hydrolysis of enamine which is an effort to regenerate the original secondary amine and reveal the carbonyl group. Also, it explores the tautomerism of using imines as enamines and vice versa.

Chapter

Cover Core Carbonyl Chemistry

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

Cover Chemistry3

Aldehydes and ketones  

Nucleophilic addition and α-substitution reactions

This chapter concentrates on the chemistry of aldehydes (RCHO) and ketones (RCOR) and how they react in nucleophilic addition reactions. An aldehyde contains a C=O group bonded to at least one hydrogen atom, whereas a ketone has a carbonyl group bonded to two alkyl or aryl groups. The chapter illustrates general mechanisms for a nucleophilic addition reaction, an α-substitution reaction, and a carbonyl–carbonyl condensation reaction and mechanisms for nucleophilic addition reactions of aldehydes and ketones using reagents. It discusses keto–enol tautomerism and the factors that influence the stability of keto and enol forms. The structure of a product derived from a nucleophilic addition, an α-substitution, or a carbonyl–carbonyl condensation reaction of an aldehyde or ketone are also covered.

Chapter

Cover Stereoelectronic Effects

Rearrangements and fragmentations  

This chapter addresses two groups of reactions of synthetic importance and mechanistic interest, where the key interactions are through two or three bonds: fragmentations and rearrangements. Fragmentation reactions are observed when a strong electron donor interacts with a good leaving group three carbons away. The simplest case is the acid-catalysed fragmentation of a 1,3-diol to an alkene and a ketone or aldehyde. The addition of a nucleophile to a carbonyl group is a simple way to generate a donor oxyanion, and where the addition is sufficiently selective, clean reactions are observed. Meanwhile, rearrangement reactions may be observed even in some conformationally mobile systems, where it must compete with epoxide formation. Although various sorts of rearrangements are known, the most common and important are 1,2-shifts.

Chapter

Cover Organic Chemistry

Nucleophilic substitution at C=O with loss of carbonyl oxygen  

This chapter explores nucleophilic substitution at the carbonyl group with loss of carbonyl oxygen. It looks at two important examples: the carbonyl oxygen atom has been replaced by a nitrogen atom during imine formation and by two atoms of oxygen during acetal formation. When acetaldehyde is dissolved in methanol, a reaction takes place; the product is a hemiacetal. Like hydrates, most hemiacetals are unstable with respect to their parent aldehydes and alcohols. The chapter then considers how acetals are formed from aldehydes or ketones plus alcohols in the presence of acid. In the presence of acid, hemiacetals can undergo an elimination reaction, losing the oxygen atom that once belonged to the parent aldehyde’s carbonyl group.

Chapter

Cover Core Carbonyl Chemistry

Introduction: the main themes  

This introductory chapter provides an overview of the carbonyl group, which is the commonest reactive component of organic structures. In carbonyl compounds, both the carbon and oxygen atoms are sp 2 hybridized, with the σ-bonds to carbon and the oxygen lone pairs coplanar. As the electronegativity of oxygen is greater than that of carbon, the carbonyl group is polarized, with the carbon atom positively charged relative to the oxygen, so that the carbon atom is electron-demanding. All the reactions of carbonyl compounds discussed in this book involve electron-supply to the electron-demanding carbon as a key feature. Such electron-supply can be achieved by either nucleophilic attack or ionization at a contiguous centre.

Chapter

Cover Chemistry of the First-row Transition Metals

Compounds in lower oxidation states  

This chapter highlights the transition metal carbonyls which are useful and are easily available when starting materials for the synthesis of many other types of low-valent metal complexes. It examines the structures of the dinuclear and higher nuclearity species which feature bridging carbonyl groups and metal-metal bonds. It also discusses the number of electrons allocated to particular ligands for the purposes of electron counting. The chapter cites important exceptions to the 18-electron rule that are important in metal carbonyl and organometallic chemistry, such as the occurrence of stable compounds that have a formal 16-valence electron configuration. It reviews the thermodynamic stability of metal carbonyl derivatives that owes very little to the ability of CO to act as a Lewis base.

Chapter

Cover Amino Acid and Peptide Synthesis

α-Carboxy protection  

This chapter describes the amino groups of amino acids and peptides as more nucleophilic than their carbonyl groups when it comes to acylation. It explains how amino acids and peptides with unprotected carboxy groups can be used for peptide bond formation if separately preactivated carboxy components are employed. It also cites esterification as the usual means of carboxy protections, wherein the parent carbolic acid can be regenerated from the esters by acyl-oxygen or alkyl-oxygen fission. The chapter considers the fundamental alkyl-oxygen cleavage chemistry as qualitatively the same as that for alkoxycarboxyl groups, which are esters of a special kind. It notes that there is no significant distinction between methyl and ethyl esters.

Chapter

Cover Organic Chemistry

Chemoselectivity and protecting groups  

This chapter describes chemoselectivity and protecting groups. Most organic molecules contain more than one functional group, and most functional groups can react in more than one way, so organic chemists often have to predict which functional group will react, where it will react, and how it will react. These questions are what can be called selectivity. Selectivity comes in three sorts: chemoselectivity, regioselectivity, and stereoselectivity. The chapter focuses on chemoselectivity (which group reacts). It begins by looking at reductions of carbonyl compounds, introducing a few more specialized reducing agents. The chapter then considers the other type of reduction in the salmefamol synthesis: catalytic hydrogenation.

Chapter

Cover Organic Chemistry

Alkylation of enolates  

This chapter studies the alkylation of enolates. The alkylations in the chapter each consists of two steps. The first is the formation of a stabilized anion—usually (but not always) an enolate—by deprotonation with base. The second is a substitution reaction: attack of the nucleophilic anion on an electrophilic alkyl halide. All the factors controlling SN1 and SN2 reactions are applicable here. Problems that arise from the electrophilicity of the carbonyl group can be avoided by replacing C=O by functional groups that are much less electrophilic but are still able to stabilize an adjacent anion. The chapter then looks at the alkylation of nitriles and nitroalkanes.

Chapter

Cover Organic Chemistry

Nucleophilic addition to the carbonyl group  

This chapter details the simplest of all organic reactions: the addition of a nucleophile to a carbonyl group. Nucleophilic additions to carbonyl groups generally consist of two mechanistic steps: nucleophilic attack on the carbonyl group and protonation of the anion that results. Why does cyanide, in common with many other nucleophiles, attack the carbonyl group? And why does it attack the carbon atom of the carbonyl group? To answer these questions, the chapter looks in detail at the structure of carbonyl compounds in general and the orbitals of the C=O group in particular. The chapter then considers the attack of cyanide on aldehydes and ketones; the acid and base catalysis of hemiacetal and hydrate formation; and bisulfite addition compounds.

Chapter

Cover Organic Chemistry

Carbonyl chemistry  

This chapter studies carbonyl chemistry. In its simplest form, the carbonyl group is a carbon to oxygen double bond. A neutral carbon atom must have four bonds, therefore the nature of the other substituents at the central carbon atom will dictate what type of carbonyl group it is and also its reactivity profile. The chapter then looks at reactions with nucleophiles and with reducing agents. There are a range of methods to reduce a carbonyl group, but the simplest ones usually provide a source of hydride that acts as a nucleophile and adds to the central carbon atom. The chapter also considers carboxylic acids, acyl chlorides, esters, and amides.

Chapter

Cover Core Carbonyl Chemistry

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.

Chapter

Cover Organic Chemistry

Nucleophilic Addition to the Carbonyl Group in Aldehydes and Ketones  

This chapter discusses the polarity of the carbonyl bond and the catalysis of nucleophilic addition to carbonyl groups by acids and bases. It considers in detail the reactions of a functional group, and looks at nucleophilic additions to the carbonyl group of aldehydes and ketones. The chapter argues that these reactions provide clear examples of several fundamental principles of organic reaction mechanisms, and they also include some of the most useful reactions for organic synthesis. The carbonyl (>C=O) group is a functional group found in aldehydes and ketones as well as carboxylic acids and their derivatives, and is one of the most important in organic and biological chemistry. The chapter ends by explicating the acetal and dithioacetal formation as well as the Wittig reaction. It also investigates the formation of imines and enamines.

Chapter

Cover Oxidation and Reduction in Organic Synthesis

Oxidation of activated carbon–hydrogen bonds  

This chapter evaluates the oxidation of activated carbon–hydrogen bonds. During the course of an oxidation, hydrogen can be removed in three ways, as either H+, H*, or H-. Normally, it is beneficial to have a functional group which stabilises the carbon left behind by such removal of hydrogen. As such, the chapter considers the hydrogen atom that is lost, activated by the functional group. It begins by looking at oxidation adjacent to oxygen. This is the most useful and widespread area of oxidation, as it encompasses the alcohol-aldehyde-carboxylic acid sequence which is used so often in synthesis. The chapter then studies oxidation adjacent to a carbon–carbon multiple bond, a carbonyl group, and nitrogen, as well as the oxidation of phenols and the formation of quinones.

Chapter

Cover Organic Chemistry

Reactions of Carbonyl Compounds with Hydride Donors and Organometallic Reagents  

Tadashi Okusecyama and Howard Maskill

This chapter examines the nucleophilicity of metal–hydrogen and metal–carbon reagents. It states that carbonyl compounds are electrophilic and react with nucleophiles. The nucleophiles involved in the reactions discussed in the previous chapter all had lone pairs and, in each case, the lone pair was the nucleophilic centre. Metal hydrides and organometallic compounds are a different type of nucleophile in which the nucleophilic site is the σ bonding pair of electrons of the M–H or M–C bond (M = metal). The chapter highlights that they are important reagents for reducing carbonyl compounds to alcohols, and synthesizing alcohols with new C–C bonds from carbonyl compounds. The chapter focuses on these reactions of the carbonyl group. It then considers the aspects of designing an organic synthesis, and functional group protection.

Chapter

Cover Making the Transition to University Chemistry

Aldehydes and Ketones  

This chapter describes aldehydes and ketones. Aldehydes have one alkyl group and one hydrogen atom attached to the carbonyl carbon. Ketones have two alkyl groups and resist oxidation. Both aldehydes and ketones contain the carbonyl group which has a carbon atom doubly bonded to an oxygen atom. Fehling's solution and Tollens' reagent can also help determine the differences between aldehydes and ketones. Oxidation can also help the reduction of aldehydes and ketones to primary and secondary alcohols respectively. The chapter also explains nucleophilic addition, condensation reactions, and alpha carbon reaction of aldehydes and ketones.

Book

Cover Organometallics and Catalysis
Organometallics and Catalysis consists of three parts. Part I looks at the organometallic compounds of the main group elements. Part 2 focuses on the organometallic compounds of the transition metals. Part 3 examines homogeneous catalysis with organometallic transition metal complexes. There are also four appendices: the first one covers commonly used solvents and their properties; the second one, number and symmetry of infrared-active vibrations of metal-carbonyl complexes; and the other two appendices present answers to exercises and further readings.