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Chapter

Cover Chemistry3

Halogenoalkanes  

Substitution and elimination reactions

This chapter begins with an overview of the structure and reactivity of halogenoalkanes, followed by a discussion of the most important methods of preparing these compounds. Halogenoalkanes are particularly useful intermediate compounds in organic synthesis as they react with nucleophiles and as bases to form valuable products and bases in elimination reactions, which is an important way of making alkenes. The chapter explains why different carbon–halogen bonds in halogenoalkanes react differently from one another. It details how halogenoalkanes react in SN1, SN2, E1, E2 reactions, including factors that affect SN2 versus SN1 reactions and factors that affect E2 versus E1 reactions. The factors that influence substitution versus elimination reactions are also covered.

Chapter

Cover Organonitrogen Chemistry

Ammonium compounds  

This chapter focuses on amines. Saturated amines are regarded as the simplest organonitrogen compound. The chapter highlights the importance of amines as they occur widely in nature and are used often as building blocks for more complex compounds and co-reagents in numerous organic reactions. The chapter notes nitrogen acting as nucleophile or base to facilitate the reactions on the nitrogen lone pair. Additionally, the chapter discusses the interplay between nucleophilicity and basicity alongside the reactions of amines. It concludes amines as bases that are readily protonated. Next, amines are also powerful nucleophiles following their reactivity with alkyl halides, carboxylic acid derivatives, aldehydes, ketones, and nitrous acid.

Chapter

Cover Chemistry3

Carboxylic acids and derivatives  

Nucleophilic acyl substitution and α-substitution reactions

This chapter focuses on carboxylic acids and derivatives, which are a large family of compounds that have similar structures as they all contain a C=O bond joined to an electronegative atom, typically oxygen, nitrogen, or a halogen atom. Esters, acid anhydrides, amides, and acyl chlorides are called carboxylic acid derivatives because they can be prepared from carboxylic acids. The chapter describes a general mechanism for a nucleophilic acyl substitution reaction and explains why carboxylic acid derivatives react with nucleophiles in substitution reactions rather than addition reactions. It discusses keto–enol tautomerism in carboxylic acid derivatives and the factors that influence the stability of keto and enol forms. It also shows how to predict the structure of a product derived from a nucleophilic acyl substitution, an α-substitution, or a carbonyl–carbonyl condensation reaction of a carboxylic acid derivative.

Chapter

Cover Making the Transition to University Chemistry

Carboxylic Acids and Their Derivatives  

This chapter discusses carboxylic acids and their derivatives. It notes RCOOH as the general formula for carboxylic acids. Thus, the weak acids neutralize alkalis and react with carbonates. Additionally, acyl chlorides typically react with water, alcohols, ammonia, and amines. In esters, the presence of a concentrated sulfuric acid catalyst forms an equilibrium mixture as the option of leaving the tetrahedral intermediate becomes available. On the other hand, acylation reactions are triggered when an acyl group replaces a hydrogen atom in the nucleophile. The chapter also covers the nucleophilic additional-elimination reactions of acid derivatives. It lists the general formula of esters and acyl chlorides as well.

Chapter

Cover Pericyclic Reactions

The nature of pericyclic reactions  

This chapter describes pericyclic reactions. These reactions are a third distinct class and have cyclic transition structures in which all bond-forming and bond-breaking takes place in concert. The chapter discusses the three classifications of organic reaction—ionic, radical, and pericyclic—and analyses ionic reactions which involve pairs of electrons that move in one direction. It also mentions the unimolecular reaction, wherein the carbon–halogen bond cleaves with both electrons going to the chloride ion and leaves an electron deficiency behind on the carbocation. The chapter describes the nucleophile, which provides both electrons for a new bond, in contrast to the electrophile, which receives them. It covers radical reactions that involve the correlated movement of single electrons..

Chapter

Cover Functional Groups

Alkynes  

This chapter focuses on the differences between alkynes and alkenes, particularly those that affect synthetic work. It explains that one difference concerns the CC double bond, which is usually created from saturated intermediates by elimination but is formed in a different way in alkynes. In these, ethene is widely used in building up the higher members and other compounds containing the triple bond. The chapter observes that alkynes are less reactive than alkenes, but with nucleophiles, the relative reactivity is reversed as simple alkenes do not undergo nucleophilic addition while simple alkynes undergo few additions. It discusses the hydrogenation of an alkyne with a poisoned catalyst that stops at the alkene stage.

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.

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 Organic Chemistry

Elimination reactions  

This chapter investigates elimination reactions. The elimination reaction of t-butyl bromide happens because the nucleophile is basic. The hydroxide is behaving as a base because it is attacking the hydrogen atom, instead of the carbon atom it would attack in a substitution reaction. The hydrogen atom is not acidic, but proton removal can occur because bromide is a good leaving group. As the hydroxide attacks, the bromide is forced to leave, taking with it the negative charge. Two molecules—t-butyl bromide and hydroxide—are involved in the rate-determining step of the reaction. This means that the concentrations of both appear in the rate equation, which is therefore second-order and this mechanism for elimination is termed E2. The other mechanism for elimination is termed E1. The difference between the two reactions is the strength of the base.

Chapter

Cover Organic Chemistry

Elimination reactions  

This chapter evaluates elimination reactions. In elimination reactions, a base (nucleophile) removes a hydrogen nucleus (proton), which results in the formation of an alkene with loss of a leaving group. This process can occur by three mechanisms: E2, E1, and E1cB. The mechanism of elimination is dependent on factors including the solvent, the nature of the nucleophile, and the nature of the eliminating species. Elimination reactions often compete with substitution reactions. Bulky nucleophiles, strong bases, and high temperatures favour elimination over substitution, as does increasing the concentration of base. The chapter looks at the major products of elimination reactions, but in practice some of these experiments would yield several impurities, including substitution products.

Chapter

Cover Organic Chemistry

Aromatic chemistry  

This chapter explores aromatic chemistry. It begins by looking at electrophilic aromatic substitution (SEAr), where a functional group on an aromatic ring, usually a proton, is replaced by an electrophile. In order to undergo reaction with the electrophile, the aromatic ring must be electron-rich. This is because the mechanism requires the cloud of electrons on the benzene ring to attack the electrophile; if the ring were electron-deficient, it would not be able to do this as readily. The mechanism proceeds via a carbocation intermediate, also known as the Wheland intermediate. The chapter then considers nucleophilic aromatic substitution (SNAr), where a group on an aromatic ring is replaced by a nucleophile. It also discusses azo coupling, which is when an aromatic diazonium-containing compound is joined to another aromatic compound.

Chapter

Cover Organic Chemistry

Nucleophilic substitution  

This chapter examines nucleophilic substitution. It begins by defining electrophiles and nucleophiles. An electrophile is a neutral or positively charged species with an empty orbital (or an energetically accessible anti-bonding orbital) which can accept electrons. Lewis acids can also be considered electrophiles as they have an empty orbital that can accept an electron pair. Meanwhile, a nucleophile contains a pair of electrons that can be used to form a new chemical bond. Nucleophiles thus act as electron donors. The chapter then looks at Lewis acids/bases.. The chapter then looks at Lewis acids/bases. It also considers SN1 and SN2, which are two classes of reaction that describe the nucleophilic substitution, or replacement, of one functional group at a saturated carbon centre by another. Finally, the chapter studies the impact of pK a on leaving group ability.

Chapter

Cover Making the Transition to University Chemistry

Halogenoalkanes  

This chapter explains the concept of halogenoalkanes which have carbon-halogen bonds which are significantly polar. However, the carbon atom is partially positively charged and susceptible to a nucleophilic attack. The chapter explains that carbon-halogen bond enthalpy determines the relative rate of reaction. The three common nucleophilic substitution reactions of halogenoalkanes involve sodium hydroxide, potassium cyanide, and ammonia. Elimination is the second vital characteristic type of reaction of halogenoalkanes as it produces an alkene. The hydroxide ion is acting as the base of the compound rather than a nucleophile. Elimination is favoured over substitution where we see nucleophiles with strong bases, bulky nucleophiles, and high temperatures.

Chapter

Cover Organic Chemistry

Nucleophilic substitution at saturated carbon  

This chapter discusses nucleophilic substitution at saturated carbon. In the first step, the nucleophile attacks the C=O π bond. It is immediately obvious that the first step is no longer possible at a saturated carbon atom. The electrons cannot be added to a π bond as the CH2 group is fully saturated. In fact there is no way for the nucleophile to add before the leaving group departs because this would give an impossible five-valent carbon atom. Instead, two new and different mechanisms become possible. Either the leaving group goes first, and the nucleophile comes in later, or the two events happen at the same time. The first of these possibilities is called the SN1 mechanism. The second mechanism, which shows how the neutral carbon atom can accept electrons provided it loses some at the same time, is called the SN2 mechanism.

Chapter

Cover Organic Chemistry

Retrosynthetic analysis  

This chapter examines retrosynthetic analysis. The best synthetic route to a molecule cannot be predicted with certainty. Retrosynthetic analysis allows us to suggest several different strategies for any given target molecule, and thorough literature searching plus experimentation in the laboratory will allow us to whittle the possibilities down to the most likely to succeed. Thinking like this underpins the design of syntheses of molecules, from the relatively simple molecules forming the next generation of drugs or agrochemicals to the most complex molecules known. Retrosynthetic thinking also reinforces the concept that the combination of electrophile and nucleophile is the basis for the understanding of organic reactions.

Chapter

Cover Foundations of Organic Chemistry: Worked Examples

Reactions with nucleophiles  

This chapter explores the nature and distinction of bases and nucleophiles, including interconversions and synthesis. It discusses reactions of halogenoalkanes by bimolecular processes. Here it covers the mechanism, transitions state, energy profile, stereochemistry, and competition with elimination. It also talks about the mechanism of addition reactions of the carbonyl group and various reagents, such as H-and CN-addition. The chapter deals with the intermediates of the addition-elimination reactions of the carbonyl group, ester hydrolysis, and other addition-eliminations on carboxylic acids and their derivatives. It analyses the transition states, energy profile, stereochemistry, and competition with elimination of the reactions of halogenoalkanes by unimolecular processes.

Chapter

Cover Organonitrogen Chemistry

Enamines  

This chapter looks into imines. It enumerates the key features of imines: oxidation level two, susceptibility to nucleophilic attack on carbon, and easy protonation in relation to producing reactive iminium ions. Imines are prepared from the reaction of an aldehyde or a ketone with a primary amine. Thus, the reaction is driven primarily through the removal of water. Imine will be more stable than simple aliphatic imines if it is conjugated by an aromatic ring. The chapter discusses the reactions of imines through attack by carbon nucleophiles, the Vilsmeyer reaction, the Mannich reaction, reduction, and hydrolysis. Additionally, the chapter shows that the reduction to amines is readily achieved via hydrogenation of hydride reducing conditions.

Book

Cover Foundations of Organic Chemistry: Worked Examples
Foundations of Organic Chemistry provides problems, with answers and tutorial guidance, on organic chemistry. The first three chapters cover basic physical organic chemistry, setting the scene for the mechanistic organic chemistry covered later. Chapters look at molecules, mechanisms, acids and bases, reactions with nucleophiles, reactions with electrophiles, and reactions with radical intermediates.

Chapter

Cover Foundations of Organic Chemistry

Mechanisms  

This chapter discusses organic reaction mechanisms, which are shorthand descriptions of how starting compounds are made into products. Molecules with the same functional groups usually react using similar mechanisms, and we can use mechanisms for known reactions to predict how other molecules will behave under similar conditions. Mechanisms are thus a connecting thread between similar reactions and allow us to rationalize the products of a known reaction and to predict the reactivity of other organic molecules. The chapter considers the mechanisms of three types of reaction: substitution, addition, and elimination. It then looks at nucleophiles, electrophiles, and radicals. The chapter also studies the process of drawing mechanisms using dot diagrams and curly arrows.

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.