Sabel / Robinson Basic Techniques of Preparative Organic Chemistry

1. Auflage 2013
ISBN: 978-1-4832-1377-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 206 Seiten

Basic Techniques of Preparative Organic Chemistry covers a detailed guide for carrying out the procedures commonly needed in preparative organic chemistry. The book discusses the nature of organic reactions; the basic principles of preparative organic chemistry; unit operations; and good laboratory practice. The text then provides a review of apparatus and equipment and describes the potential hazards involved in a chemical operation, such as toxicity, bodily injuries, smoking, fire, explosion, and implosion. Techniques and unit operations for carrying out a reaction and for isolating and purifying a reaction product; and the criteria for and methods of assessing purity are also considered. The book further tackles packing and storing products and samples and making reports and communications. Students taking organic chemistry courses will find the text useful.

Autoren/Hrsg.

Sabel, William

Robinson, Robert

Leseproben

CHAPTER 1

GENERAL INTRODUCTION

Publisher Summary

This chapter describes the nature of organic reactions. The materials normally encountered have physical properties associated with the covalent bond, and are usually gases, volatile liquids, or low-melting-point solids soluble in covalent, non-ionic liquids, in contrast to the inorganic compounds, which, because of their ionic character, are usually high-melting-point solids that dissolve in polar (ionic) solvents. The volatility and low-melting-point characteristics of covalent compounds are all explicable on the basis that in these substances the individual units are discrete molecules, held together by relatively weak van der Waals forces. By contrast, ionic materials contain electrically charged species (ions), which are held together by much stronger electrostatic forces. Organic reactions are usually slower than ionic ones. In ionic reactions, the necessary energy is built in by virtue of the existing electrostatic charges, but for organic reactions, the electron shifts and resulting bond rupture effects needed as a preliminary to the formation of new bonds are slow processes, requiring the input of energy (usually as heat) for a relatively long period of time, which may range from seconds to weeks. It is very often necessary to impose temperature limitations on an organic reaction; a suitable choice of solvent can facilitate this and also help to dissipate heat liberated in an exothermic reaction. In any chemical reaction, the yield is limited by the stoichiometry and can be demonstrated by reference to the formation of ethyl acetate.

The Nature of Organic Reactions

Although the line of demarcation between organic and inorganic reactions is not always entirely clear, organic chemistry can nevertheless be treated as the chemistry of the covalent bond. Ionic species are not frequently involved, and when they are, no special manipulative problems arise.

The classification of compounds as covalent or ionic must be treated with some reserve. There is no such thing as a purely covalent or purely ionic bond between two atoms of different elements; all that can be said is that the bonds in a molecule such as methane are predominantly covalent, while the sodium chloride crystal comprises an aggregation, not of sodium chloride molecules, but of sodium ions and chloride ions, although even here the bonding forces between the sodium and chloride entities are no more than predominantly ionic; there is still some covalent character.

There are some features characteristic of all organic preparations. The materials normally encountered have physical properties associated with the covalent bond, and are usually gases, volatile liquids or low melting-point solids soluble in covalent, non-ionic liquids, in contrast to the inorganic compounds, which, because of their ionic character, are usually high melting-point solids which dissolve in polar (ionic) solvents.

The volatility and low melting-point characteristics of covalent compounds are all explicable on the basis that in these substances the individual units are discrete molecules, held together by relatively weak van der Waals forces. In contrast to this, ionic materials contain electrically charged species (ions), which are held together by much stronger electrostatic forces. In all cases the physical form of a substance is a measure of the “randomness” of its constituent molecules or ions. The conversion of solid to liquid, and liquid to gas, requires energy input because these successive changes of state involve an increasing separation of the component units, whether they are molecules or ions, and this necessitates overcoming the inter-molecular or inter-ionic binding energies.

Organic reactions are usually slower than ionic ones. This is because most inorganic reactions merely involve the formation of ion pairs by mutual electrostatic attraction of oppositely charged particles, a process which, because of the mobility of the ions in solution, is virtually instantaneous. Although a variety of different mechanisms are possible, organic reactions can all be regarded as resulting essentially from electron shifts induced by the reaction environment, leading to the breakage of covalent linkages. This introduces certain reaction characteristics. In ionic reactions the necessary energy is “built-in” by virtue of the existing electrostatic charges, but for organic reactions the electron shifts and resulting bond rupture effects needed as a preliminary to the formation of new bonds are slow processes, requiring the input of energy (usually as heat) for a relatively long period of time, which may range from seconds to weeks. Another characteristic follows from this; for an organic reaction to occur it is usually necessary not only to supply energy in the form of heat, but also to provide special environmental conditions, such as a source of protons added, for example, as sulphuric acid. In the main, because of the rather complex electron shifts involved in organic reactions and their associated energy requirements, there is the possibility of several different routes being followed, all requiring somewhat similar environmental conditions. The result of this is that organic reactions can, and often do, give a multiplicity of products. Also, for similar reasons, equilibrium reactions are frequently encountered, so that again it is impossible to obtain a quantitative yield of the desired product.

In a reaction represented by the equation + = , the formation of each molecule of AB must be preceded by the collision of with , but, of course, not every collision will result in a reaction. It is obviously essential therefore to provide an environment for the reaction that makes and sufficiently mobile to enhance the possibility of collision between them. The conditions prevailing in a solid substance represent a minimum of mobility of the constituent species, and are therefore least conducive to the collisions required before reaction can occur. Thus, reactions do not normally occur easily in the solid state. For an organic reaction, it would appear to be possible to meet the difficulty by applying heat to melt the solid reactants; this is sometimes done, but usually a solvent is used to provide the necessary liquid phase. Gas phase reactions are also quite feasible, but are relatively uncommon in elementary preparations.

The choice of the type and quantity of solvent used in a reaction depends upon many factors, including its chemical compatibility with the other materials present, and ease of separation of the reaction product. In some cases, the solvent may be chosen to provide certain chemical characteristics, such as acidity or basicity.

It is very often necessary to impose temperature limitations on an organic reaction; a suitable choice of solvent can facilitate this and help also to dissipate heat liberated in an exothermic reaction. Thus, if the desired reaction temperature is 80°C, this can easily be achieved by using a solvent such as benzene which boils at that level; the temperature cannot then rise above the boiling point, and any heat liberated in the reaction will be absorbed as latent heat of evaporation of the solvent. In some cases the use of the appropriate solvent in suitable quantity can affect the course of a reaction and possibly avoid the formation of unwanted by-products.

Even under optimum conditions, in the majority of cases the yield of the desired compound is less than 100 per cent of the theoretical quantity; the reaction may not go to completion and/or side reactions may occur, resulting in the loss either of reactants or the required reaction product. Thus, at the end of the reaction, the isolation of the desired product necessitates its separation from what may be a large number of other compounds. Many of the techniques of organic chemistry are related to that problem.

Basic Principles of Preparative Organic Chemistry

It cannot be emphasized too strongly that all preparative organic chemistry involves two main problems:

(1) How is the product to be made?

(2) How is the product to be isolated in a pure condition from its reaction mixture?

In the early stages of organic chemistry students are apt to concentrate on the first of these, but the second is frequently the major problem, demanding the most skill.

The problem of how to deal with a reaction mixture to extract the maximum amount of the desired product in the highest degree of purity requires considerable thought before starting the reaction. This is a particular illustration of a general principle; successful work in practical organic chemistry always requires the ability to think ahead, not only to the next stage but to the operations beyond that as well. Consideration in advance of how a reaction mixture is going to be treated in order to extract the reaction product, can affect decisions about the way in which the preparation is to be carried out, and the materials to be used for it.

It is sometimes convenient to consider the problem of separation in two stages—the isolation of the main product in a reasonable degree of purity, and the final task of purifying this crude material. In the majority of elementary work in practical organic chemistry, separation operations are the most exacting part of the job, involving many physical techniques and some chemical methods. Physical...