Fundamental Aliphatic Chemistry: Organic Chemistry for General Degree Students

By P. W. G. Smith and A. R. Tatchell

Organic Chemistry for General Degree Students is written to meet the requirements of the London General Internal examination and degree examinations of a similar standing. It will also provide for the needs of students taking the Part 1 examination for Graduate Membership of the Royal Institute of Chemistry, or the Higher National Certificate, whilst the treatment is such that Ordinary National Certificate courses can be based on the first two volumes

Within the limits broadly defined by the syllabus, the aim of this first volume is to provide a concise summary of the important general methods of preparation and properties of the main classes of aliphatic compounds. Due attention is paid to practical considerations with particular reference to important industrial processes. At the same time, the fundamental theoretical principles of organic chemistry are illustrated by the discussion of a selection of the more important reaction mechanisms. Questions and problems are included, designed to test the student’s appreciation of the subject and his ability to apply the principles embodied therein. A selection of questions set in the relevant examinations is also included.

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 24, 2014
• ISBN: 9781483139067

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Preface

Our object in writing a new textbook of organic chemistry has been to meet the particular needs of students reading for the B.Sc. General degree. The choice of material included in this first volume, which is devoted to the essential chemistry of aliphatic compounds, has been based broadly on the requirements for the Pt. I examination of the London General Internal degree. Two projected volumes will deal with aromatic and heterocyclic compounds, and with polyfunctional aliphatic compounds and selected additional mechanistic topics. These will provide a complete coverage of the material necessary not only for the final B.Sc. General examination, but also for the Pt. I examination for Graduate Membership of the Royal Institute of Chemistry, for courses leading to Higher National Certificates in Chemistry and for examinations of a similar standard.

The organization of the subject-matter in this volume into the main functional classes is largely conventional but we have attempted to provide a comprehensive yet concise treatment of the principal general methods of preparation and reactions of the main aliphatic classes together with adequate practical detail, particularly concerning preparative methods of industrial importance. At the same time the necessary balance between practice and theory is maintained by the introduction of basic theoretical principles from the beginning and by as full a discussion as is possible, within the limits of a work of this size, of the more important reaction mechanisms. To avoid undue repetition, we have adopted a fairly extensive system of cross reference and have attempted to give practical detail concerning a particular reaction under ‘general preparations’ while providing the necessary theoretical discussion under ‘general reactions’, although it has not always been desirable to adhere rigidly to this system.

A little elementary knowledge of the properties of simple aliphatic compounds, acquired for example through G.C.E. courses, is assumed, and no attempt has been made to deal exhaustively with the properties or reactions of individual compounds, the emphasis being placed upon the chemistry of functional groups. We believe that, with the obvious omission of the more advanced mechanistic discussions, the treatment adopted will make the first two volumes a suitable basis for a course for Ordinary National Certificate students who intend to proceed to H.N.C. and higher qualifications.

In the short selection of questions and problems provided we have included some chosen from the relevant examinations to give an indication of the standard required at various levels. Some of the problems are designed to enable the student to extend his knowledge of a particular topic beyond the actual examples given in the text. We are indebted to the University of London and to the Royal Institute of Chemistry for permission to reproduce selected examination questions.

We gratefully acknowledge the interest shown in this project by Dr. A. I. Vogel and express our sincere thanks to Mrs. G. E. Tatchell for her forbearance in deciphering and typing the manuscript.

P.W.G.S. and A.R.T.,     Woolwich Polytechnic, London, S.E.18

I

Introduction

Publisher Summary

Organic chemistry is the chemistry of carbon compounds, excluding compounds such as the carbonates, bicarbonates, carbon monoxide, and the metallic carbonyls. It was clearly realized by the early organic chemists that no attempt could be made on the structural elucidation of a particular compound until both the nature and relative proportions of the elements present in its molecule had been determined. This approach led to the establishment of the principal methods of the qualitative and quantitative elemental analysis of organic substances. The organic substances that contain metallic elements leave behind an incombustible residue after ignition that may be submitted to the usual methods of inorganic qualitative analysis. The detection of the elements nitrogen, sulfur, and the halogens in an organic compound is most conveniently carried out by fusion with sodium, which is known as Lassaigne method. The next step in determining the nature of an organic compound is the quantitative analysis of those elements found by the qualitative tests. This enables the relative atomic proportions of the molecule to be calculated and from the molecular weight of the substance, the absolute number of atoms of each element present in one molecule of the organic compound is ascertained.

Organic chemistry is the chemistry of carbon compounds (excluding such compounds as the carbonates, bicarbonates, carbon monoxide and the metallic carbonyls). This broad definition derives from studies carried out at the beginning of the nineteenth century on compounds isolated from animal and vegetable materials (i.e. of organic origin) as distinct from those isolated from mineral sources (i.e. of inorganic origin). In the initial classification of organic compounds it became convenient to distinguish those which from their structure and reactivity were closely related to the compound benzene, as distinct from those which were structurally related to the naturally occurring fatty acids. The former, from their wide distribution in the pleasant-smelling plant resins, gums and oils were termed aromatic compounds, whilst the latter were designated as aliphatic compounds.

Early studies in organic chemistry were frequently stimulated by the observation that certain plant and animal extracts possessed medicinal, nutritional or colouring (dyeing) properties. Work was therefore directed initially to an examination of the means of handling such extracts in order to isolate the ‘active principle’ (substantially free from the other numerous constituents) which was responsible for these specific characteristics. It was then possible to embark upon studies directed towards the elucidation of the manner in which the individual atoms in a molecule of the pure compound were linked together (i.e. the determination of the structure of the molecule).

Organic substances were always found to give carbon dioxide and water upon burning in oxygen, showing the presence of the elements carbon and hydrogen. As the number of such isolated substances increased it became apparent that other elements were often also present, those most commonly found being oxygen, nitrogen, the halogens and sulphur. It was clearly realized by the early organic chemists that no attempt could be made on the structural elucidation of a particular compound until both the nature and relative proportions of the elements present in its molecule had been determined. This approach led to the establishment of the principal methods of qualitative and quantitative elemental analysis of organic substances. Since this analytical information is still the vital first step in any structural investigation, and since all the pure organic compounds ever isolated from natural sources or synthesized in the laboratory have been submitted to this process, it is pertinent to consider briefly an outline of the methods which are now available. The full practical details are to be found in any practical organic chemistry book.

Qualitative Analysis

Those organic substances which contain metallic elements leave behind an incombustible residue after ignition which may be submitted to the usual methods of inorganic qualitative analysis.

The detection of the elements nitrogen, sulphur and the halogens in an organic compound is most conveniently carried out by fusion with sodium (the Lassaigne method). A convenient technique is to drop some of the compound to be examined on to sodium pre-heated in a Pyrex test-tube. During the subsequent vigorous reaction sodium cyanide, sulphide or halide is formed if the organic compound contains nitrogen, sulphur or halogen respectively. Methanol is added to the cooled tube to decompose unreacted sodium and the residue extracted with boiling distilled water to dissolve the sodium salts.

The cyanide ion is detected by adding aqueous ferrous sulphate solution to a portion of the extract, boiling to achieve some aerial oxidation of ferrous ions to ferric ions, and acidifying with sulphuric acid. A blue precipitate of ferric ferrocyanide (Prussian blue) indicates the presence of nitrogen in the original substance.

The halide ion is detected by acidifying a portion of the fusion extract with nitric acid and boiling to expel hydrogen cyanide (or sulphide) if these are present. Aqueous silver nitrate solution is then added to precipitate any silver halide. The nature of the halogen may be deduced in the usual way.

The sulphide ion is detected in the aqueous extract by the addition of a solution of sodium nitroprusside, when an unmistakable violet coloration is produced if sulphide ions are present. Alternatively the addition of sodium plumbite solution gives a black precipitate of lead sulphide.

Quantitative Analysis

The next step in determining the nature of an organic compound is the quantitative analysis of those elements found by the qualitative tests above. This enables the relative atomic proportions of the molecule to be calculated (the empirical formula), and thence from the molecular weight of the substance, the absolute number of atoms of each element present in one molecule of the organic compound is ascertained (the molecular formula).

For this analysis the compound must be rigorously purified by either careful and repeated distillations or by several recrystallizations.

The micro-analytical techniques which are available at the present time enable a complete quantitative analysis to be performed on as little as 5–15 mg of material. Details of methods used are to be found in suitable textbooks on practical organic chemistry, but the principles of these procedures are outlined below.

The basic principle of the carbon:hydrogen determination is that an organic compound when pyrolysed in oxygen gives quantitatively carbon dioxide and water, both of which may be collected and weighed in a suitable trapping system, which requires some modification if elements other than carbon, hydrogen or oxygen are present.

The nitrogen content of an organic compound is commonly determined by measuring the volume of nitrogen gas evolved (corrected to S.T.P.) when an organic substance is heated with copper oxide (Dumas method). The halogen is determined as silver halide which is produced when the substance is heated in a sealed tube with silver nitrate and nitric acid at 200° (Carius method). The sulphur present in an organic compound is converted into sulphuric acid by heating it in a sealed tube with nitric acid at 200°, and estimated gravimetrically in the usual way as barium sulphate. The oxygen content of an organic compound is not usually estimated directly but is calculated by difference.

Empirical and Molecular Formulae

The following will serve as an illustration of the method of calculating empirical and molecular formulae from basic analytical information.

Example: An organic compound was shown by qualitative analysis to contain nitrogen and bromine. A sample (4·835 mg) on combustion gave carbon dioxide (7·960 mg) and water (1·630 mg). A Carius analysis on a further sample (5·420 mg) gave silver bromide (4·760 mg). By the Dumas method, another sample (3·250 mg) gave nitrogen (0·17 ml after correction to S.T.P.). Calculate the percentage composition and the empirical formula of the compound.

The composition of the compound is therefore:

Dividing each value by the atomic weight of the element:

or

Dividing each value by 0·465:

These figures may now be rounded off to the nearest whole numbers to give the empirical formula, i.e. C8H8ONBr.

Such minor approximations are permissible as in practice deviations from whole numbers inevitably arise owing to the limitations of the experimental methods. When an empirical formula is derived from analytical data a necessary check is to calculate the percentage composition on the basis of this formula and to obtain satisfactory agreement (± 0·4 per cent) between the calculated and experimentally determined values for each element.

For the determination of the structure of an organic molecule it is necessary to know the total number of atoms of each element present in a molecule of the substance. The molecular formula must therefore be determined from a knowledge of the empirical formula and the molecular weight of the compound, from which it will be readily apparent whether the molecular formula is the same as the empirical formula or some simple multiple thereof. Most of the standard procedures for the determination of molecular weights, descriptions of which may be found in textbooks of physical chemistry, may be applied to organic compounds. The method based on the measurement of the depression of the melting point of camphor is particularly convenient and may be used on a semi-micro scale (the Rast method).

The Elucidation of a Structural Formula

Although the sequence for the determination of the molecular formula of most organic compounds follows that indicated above, and although the molecular formula provides the starting point from which it is possible to deduce the way in which the atoms are linked together (i.e. the structural formula), there is no general predetermined sequence by which this may be done. In fact four simultaneous thought-processes are likely to be adopted:

(a) a postulation of possible structural formulae,

(b) a consideration of the chemical reactivity of the compound,

(c) a knowledge of how the compound has been prepared (with a natural product this information is not, of course, available, and in this case an attempt is made to confirm the assigned structure by synthesis), and

(d) a knowledge of the nature of the products obtained as a result of its further reactions.

In addition to that derived from the chemical procedures, much valuable additional structural information can often be obtained from physical measurements, and in particular from an interpretation of the absorption spectra of the organic molecule.

Postulation of Possible Structural Formulae

Little progress was made in structural organic chemistry until Kekulé postulated (1857) firstly that carbon was capable of being strongly linked (bonded) to itself or to certain other atoms and secondly that the total number of bonds attached to one carbon atom was four, i.e. carbon was quadrivalent. The other atoms were similarly assigned fixed combining powers or valencies, e.g. hydrogen and the halogens represent monovalent atoms. The structures of some of the simplest organic compounds CH4 (methane), CH3Cl (methyl chloride), CH2Cl2 (methylene chloride), CHCl3 (chloroform) and CCl4 (carbon tetrachloride) may therefore be written in a diagrammatic fashion which clearly shows the number of bonds involved and illustrates the series of compounds which is obtained by successive substitution of hydrogen in methane by chlorine.

The structures of the compounds C2H6 (ethane) and C3H8 (propane) must of necessity contain carbon–carbon bonds if the valency requirements of the atoms are to be satisfied, and these may be represented in a similar diagrammatic fashion.

The fact that organic compounds are so numerous arises from this unique ability of carbon atoms to combine with one another to form stable molecules containing large numbers of carbon–carbon bonds.

and sulphur divalent (—S—). Compounds with the molecular formulae CH4O, CH5N, CH4S, for example, are written diagrammatically as:

In each of these simple cases there is only one possible structural formula which does not violate the valency considerations. With all but these simplest examples, however, it is usually possible to write down more than one structural formula. Thus the molecule C2H6O can be written in two possible ways:

These represent the structures of different compounds (ethyl alcohol and dimethyl ether respectively) which have the same molecular formula and are referred to as isomers. The careful examination of their reactions, synthesis and degradation is necessary to assign unambiguously to each isomer its structural formula.

With a more complex molecular formula it becomes difficult to write down per se all the possible structural formulae and it is usually needless to do so, without first considering all the relevant chemical information which may greatly limit the number of possible structures. This may be further illustrated by considering the greater number of possible isomeric forms of the molecule C3H6O, which are written out in full together with the customary way in which such structural formulae may be abbreviated.

are examples of triple bonds. Compounds containing carbon–carbon multiple bonding are called unsaturated to distinguish them from those saturated compounds in which all the carbon–carbon bonds are single.

Reactivity of Organic Compounds: Functional Groups

The effect of Kekulé’s postulates on the structure of organic molecules was to bring about the rationalization of their diverse chemical reactivity and to allow the classification of organic compounds on the basis of their functional groups. The functional group of a compound may be defined as that atom or grouping of atoms which gives rise to characteristic chemical behaviour. Some of the more important functional groups together with the names of the classes of compounds to which they give rise are listed below.

The assignment of the correct structural formula to a particular compound whose molecular formula has been determined, and for which a number of possible structural formulae have been suggested (as in examples I-IX) will depend on the recognition of the nature of the functional groups present from a knowledge of the reactivity of the compound. In this connection it may be noted that structures III and IX both contain hydroxyl groups attached to a saturated carbon atom, they are both therefore classified as alcohols and would be expected to behave in a similar manner towards those reagents which specifically attack the hydroxyl group. Additionally III contains an olefinic double bond and may be further classified as a bifunctional compound, i.e. an unsaturated alcohol; with reagents that attack the carbon-carbon double bond its reactivity would therefore be found to be different from IX. Similar considerations apply to the compounds VI, VII and VIII, all of which contain the ether grouping; in addition the unsaturated linkage in VI would impart to the molecule aspects of reactivity which would not be shown by a saturated ether, e.g. CH3·CH2·O·CH3. The cyclic ethers VII and VIII would also be expected to show additional reactivity due to the presence of the ring system. The pairs of compounds I-IV and II-V deserve special mention; both I and II possess a carbonyl group and are representative of ketones and aldehydes respectively, being distinguishable by characteristic reactions. However, the companion structures IV and V differ from I and II only in respect to the position of a hydrogen and the arrangement of the bonds; these pairs of compounds are referred to as tautomers and their especial significance will be discussed later (p.