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Chapter

Cover Carbohydrate Chemistry

Open chain and ring structure of monosaccharides  

This chapter analyzes the empirical formula CH2O, which is the formula of a carbohydrate. This molecule has an oxygen atom attached to each carbon. The chapter states that aldose is the most common type of structure which consists of a linear carbon chain with an aldehyde CHO group at C-1. A varying number of carbon atoms are secondary alcohols CHOH, and there is a primary alcohol at the other end of the chain. The chapter also mentions glyceraldehyde as an example of aldose consisting of several carbons. The chapter explains the Fischer convention which is commonly used for structures that have several stereogenic centres. It highlights the carbon atoms of aldoses that are numbered so that the aldehyde terminus is labelled as C-1.

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 Organic Chemistry

Reactions of enolates with carbonyl compounds: the aldol and Claisen reactions  

This chapter highlights reactions of enolates with carbonyl compounds: the aldol and Claisen reactions. The simplest enolizable aldehyde is acetaldehyde. What happens if we add a small amount of base, say NaOH, to this aldehyde? Some of it will form the enolate ion. Each molecule of enolate is surrounded by molecules of the aldehyde that are not enolized and so still have the electrophilic carbonyl group intact. The enolate ion will attack one of these aldehydes to form an alkoxide ion, which will be protonated by the water molecule formed in the first step. The product is an aldehyde with a hydroxy (ol) group and it has the trivial name aldol. This reaction is so important because of the carbon–carbon bond formed when the nucleophilic enolate attacks the electrophilic aldehyde.

Book

Cover Core Carbonyl Chemistry
Core Carbonyl Chemistry starts with an introduction to the main themes in this topic. The first chapter covers nucleophilic reagents, aldehydes, and kenotes. There follows a chapter on acetals and ketals. The text also covers reactions of amino compounds with aldehydes and ketones, reactions of nucleophiles with carboxylic acid esters, and reactions of nucleophiles with other carboxylic acid derivates. There are also chapters on enols and enolates, and enolate allylations. Finally, the text looks at aldol condensations and related reactions.

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.

Chapter

Cover Organic Chemistry

Nucleophilic substitution at the carbonyl group  

This chapter describes nucleophilic substitution at the carbonyl group. Most of the starting materials for, and products of, these substitutions will be carboxylic acid derivatives. Acetyl chloride will react with an alcohol in the presence of a base to give an acetate ester. The first step of the reaction is addition of the nucleophilic alcohol to the electrophilic carbonyl group. The base is important because it removes the proton from the alcohol once it attacks the carbonyl group. A base commonly used for this is pyridine. If the electrophile had been an aldehyde or a ketone, we would get an unstable hemiacetal, which would collapse back to starting materials by eliminating the alcohol. With an acyl chloride, the alkoxide intermediate we get is also unstable. It collapses again by an elimination reaction, this time losing chloride ion, to form the ester.

Chapter

Cover Foundations of Organic Chemistry

Reactions with nucleophiles  

This chapter focuses on organic reactions with nucleophiles. Nucleophiles are 'electron-rich' and have either non-bonded pairs of electrons or π-bonds; they can be anions or neutral molecules. Nucleophiles can react with species which have either positive charge or low electron density. This is the basis of many reactions, which begin with the transfer of electron density from the more electron-rich atom (in the nucleophile) to the more electron-deficient atom (in the electrophile). The chapter then looks at nucleophilic substitution reactions of haloalkanes; substitution reactions of alcohols; polymerization of cyclic ethers; the carbocation intermediate; and the competition between nucleophilic substitution and elimination. It also considers the reactions of nucleophiles with aldehydes and ketones; the reactions of nucleophiles with esters, carboxylic acids, and derivatives; the comparison of acid derivatives with aldehydes and ketones; and the comparison of the reactivities of the different types of halocompound with nucleophiles.

Chapter

Cover Organic Synthesis

Allylboranes and boron enolates  

This chapter considers allylboranes in organic synthesis. Focusing on the stability of allylboranes, the chapter displays how allyldialkylboranes can be used in further reactions if either they are prepared and used directly at low temperature, or the predominant isomer at equilibrium is the required isomer. The chapter also discusses the reaction of allylboranes with carbonyl compounds. It examines how triallylboranes react with aldehydes and ketones to give, on hydrolysis, homoallylic alcohols. The chapter then investigates the synthesis of homochiral homoallylic alcohols containing one chiral centre, and the synthesis of homoallylic alcohols containing two chiral centres. It ends with a discussion of boron enolates in organic synthesis.

Chapter

Cover Chemistry for the Biosciences

Functional groups: adding function to the framework of life  

This chapter discusses functional groups, which enhance the physical and chemical properties of organic compounds. Members of the same family of organic compound possess the same functional groups, and exhibit similar physical and chemical properties. Functional groups may increase the water solubility of the molecules to which they are attached and the extent of molecular interactions they experience. The chapter notes how alkyl groups above a certain size override the effect of an attached functional group, causing a relative decrease in solubility and melting and boiling points. The chapter then looks at organic compounds with oxygen-based functional groups, including the hydroxyl group, alkoxy group, carbonyl group, carboxyl group, and ester group. It also considers organic compounds with nitrogen-based functional groups, including the amino group and the amide group. Finally, the chapter studies the sulfur-based functional group known as the thiol group.

Chapter

Cover Mechanisms of Organic Reactions

Reactions of nucleophiles with carbonyl compounds  

This chapter assesses reactions of nucleophiles with carbonyl compounds. In the context of organic synthesis, reactions of carbonyl compounds are probably the most useful of all; they are also amongst the most varied. This is because carbonyl is both the functional group of simple aldehydes and ketones, and also a principal component of a range of the more complex functional groups of carboxylic acid derivatives. Functional groups which include a carbonyl are very widespread in the molecules of nature. An understanding, therefore, of how the carbonyl group reacts is essential for the study of biological chemistry as well as organic synthesis. These reactions are invariably with nucleophiles at the carbon atom, as expected of a functional group with the polarity represented by the partial structure, although complexation of an electrophile by a lone pair on the oxygen sometimes leads to electrophile catalysis.

Book

Cover Functional Groups
Functional Groups introduces reactions of functional groups in a concise and systematic form. It covers the characteristic properties of functional groups and the methods for interconverting them, which constitute important foundations of organic chemistry. Individual chapters cover organic halides, organometallic compounds, alkanes, alkenes, alkynes, aldehydes and ketons, carboxylic acids and derivatives, ethers, and amines.

Chapter

Cover Functional Groups

Organometallic compounds  

This chapter covers the most useful organometallic compounds R-M with M = Li, Mg, or Cd, including Zn compounds and lithium cuprates. It illustrates the main features of R-Mg-Hal as reagents in synthetic work and reactions that produce Mg derivatives from which the products are liberated by a treatment with dilute acid. It also discusses the reactions with esters that result in ketones or aldehydes, which cannot be isolated under standard conditions. The chapter refers to the instability of intermediates, which is are not readily predicted by organic theory and stems from the decrease of free energy associated with their decompositions. It highlights the formation of the tertiary alcohol in high yield by R-Li, which provides a sharp contrast and exemplifies the modern trends towards the use of reagents.

Chapter

Cover Bifunctional Compounds

Selective protection of bifunctional compounds  

This chapter addresses a problem which frequently arises with a molecule that contains two reactive functional groups: how can one carry out a selective reaction on one group without affecting the other? It clarifies how acetals can be used to protect aldehyde or ketone groups while transformations are carried out elsewhere in a molecule. It also implies that acetals can be used to protect diols and simple alcohols, noting that they are often preferable to simple ether protecting groups as they can be removed under very mild conditions. The chapter points out that tetrahydropyranyl and methoxymethyl ethers are actually acetals that can be easily removed via treatment with aqueous acid. The chapter also covers the selective protection and deprotection of carbohydrates and amino acids.

Chapter

Cover Bifunctional Compounds

Reactions of hydroxy- and aminocarbonyl compounds  

This chapter examines hydroxyaldehydes and ketones that react intramolecularly to form cyclic hemiacetals. It highlights the production of five- or six-membered rings, wherein the cyclizationcyclisation becomes so facile that it occurs even under neutral conditions. It also talks about the equilibrium betweenSince the hydroxycarbonyl compound and the cyclic hemiacetalhemiciacetal are in a solution, compounds which do not formally display a carbonyl group that may undergo the reactions of a carbonyl compound. The chapter cites glucose as an example of a hydroxyaldehyde that exists mainly in the form of its hemiacetal, both in the solid state and in solution. It refers to the open chain aldehyde and the six-membered cyclic hemiacetal that are in equilibrium in solution, causing glucose to react as an aldehyde.

Chapter

Cover Bifunctional Compounds

Reactions of diols  

This chapter describes 1,2-diols that undergo acid-catalysed rearrangement to afford ketones, which is a general reaction that involves the generation of a carbocation intermediate. It refers to the driving force of the general reaction of 1,2-diols that is provided by the resonance stabilization of the resulting carbocation. It also explains how diols react with aldehydes and ketones under anhydrous conditions to give acetals and ketals, which is a reaction that is acid-catalysed and proceeds most readily when the heterocyclic ring formed is five- or six-membered. The chapter discusses a transformation that can be used to protect aldehydes, ketones, and diols, while other reactions that involve base treatment or reduction are carried out elsewhere in the molecule. It demonstrates a reaction wherein diols are cyclized to form cyclic ethers; this can be carried out under acidic conditions.

Chapter

Cover Carbohydrate Chemistry

Introduction  

This chapter provides a background on the term carbohydrate. The initial understanding of the term originated in the nineteenth century. It comes from the French for the family of compounds that possess an empirical formula. The chapter discusses how the term carbohydrate has been since extended to encompass other materials. It also describes carbohydrates to be polyhydroxylated aldehydes that contain a number of carbon atoms which vary in number between 3 and 9. The chapter considers glucose as the most abundant organic molecule on the planet. An example is cellulose. This is a polymer of glucose and DNA and RNA. The chapter illustrates the importance of the varied roles of sugars and shows that a consideration of sugar chemistry should go hand-in-hand with a knowledge of the biological importance of sugars.

Chapter

Cover Carbohydrate Chemistry

Reactions of hydroxyl groups Part II: cyclic acetals  

This chapter deals with the reactions of carbohydrate hydroxyl groups with other aldehydes and ketones to form cyclic acetals, which are utilized as a means of protecting particular sugar hydroxyl groups. It explains why the reaction of carbohydrates with acetone and acid produces cyclic 5-ring acetonides, whereas benzaldehyde and acid produce 6-ring benzylidenes. It also looks at the reaction of glucose with acetone and acid. This produces the pyranose diacetonide with only the 6-hydroxyl group free. The chapter then discusses the reaction of glucose with acetone and acid. This produces furanose diacetonide with 3-hydroxyl group free. It explains the protection of carbohydrates as butane diacetals. This results in selective protection of vicinal diequatorial diol groups.

Chapter

Cover Carbohydrate Chemistry

Reactions of the anomeric centre Part II  

This chapter focuses on reactions that are particular to the anomeric centre of a free sugar which is hemiacetal and explains how the mechanisms by which nucleophilic substitution at the anomeric centre may occur. It describes the problem of control of stereochemistry at the anomeric centre and the use of neighbouring group participation. This allows for the formation of 1,2-trans glycosides. It also clarifies how to form anomeric bromides and illustrates their substitution reaction chemistry. The chapter discusses nucleophilic addition reactions to sugars which occur via the trapping of the free aldehyde and displacement of the ring opening equilibrium. It demonstrates the methods for extending or shortening the carbon chain of a sugar by one unit.

Chapter

Cover Bifunctional Compounds

Enamines, enol ethers, and enolates  

This chapter deals with enamines, enol ethers, and enolates. Enamines are nucleophilic reagents that undergo alkylation, acylation, and conjugation addition reactions. The chapter elaborates how enamines afford an indirect route for preparing substituted carbonyl compounds, noting that they are prepared from aldehydes and ketones and their salts are hydrolysed by aqueous acid to regenerate the carbonyl group. It explains that for aldehydes and ketones, the enol tautomer is less stable than the keto form and only very little enol is present. The chapter highlights that the enol tautomer predominates for some carbonyl compounds due to extra stabilization afforded by intramolecular hydrogen bonding. It explains that the interconversion of the keto and enol forms is catalysed by acid or base.

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.