Fundamental Aliphatic Chemistry. Organic Chemistry for General Degree Students

295 Pages·1965·3.943 MB·English

by  P. W. G. Smith and A. R. Tatchell (Auth.)

Organic Chemistry
for General Degree Students
Vol. 1 Fundamental Aliphatic Chemistry
P. W. G. SMITH
and
A. R. TATCHELL
Senior Lecturers in Organic Chemistry, Woolwich Polytechnic
Pergamon Press
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Copyright © 1965 PERGAMON PRESS LTD.
First Edition 1965
Library of Congress Catalog Card No. 64-66138
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Set in 10 on 12 Times New Roman LONDON AND COLCHESTER
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 com-
pounds, and with polyfunctional aliphatic compounds and
selected additional mechanistic topics. These will provide a com-
plete 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 import-
ance. 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 theo-
retical 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 ali-
phatic compounds, acquired for example through G.C.E. courses,
vii
Vlll PREFACE
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.
Woolwich Polytechnic P. W. G. S.
London, S.E.18 A. R. T.
I
Introduction
ORGANIC chemistry is the chemistry of carbon compounds
(excluding such compounds as the carbonates, bicarbonates,
carbon monoxide and the metallic carbonyls). This broad defini-
tion 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 com-
pounds.
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
1
2 ORGANIC CHEMISTRY FOR GENERAL DEGREE STUDENTS
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 mole-
cule 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 sub-
mitted 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 tech-
nique 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.
INTRODUCTION 3
6NaCN+FeS0 > Na [Fe(CN) ]+Na S04
4 4 6 2
3Na[Fe(CN)]+2Fe(S04)3 ► Fe[Fe(CN)]3+6NaS04
4 6 2 4 6 2
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 addi-
tion of a solution of sodium nitroprusside, when an unmistakable
violet coloration is produced if sulphide ions are present. Alterna-
tively 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 qualita-
tive 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 com-
pound is ascertained (the molecular formula).
For this analysis the compound must be rigorously purified by
either careful and repeated distillations or by several re-
crystallizations.
The micro-analytical techniques which are available at the
present time enable a complete quantitative analysis to be per-
formed on as little as 5-15 mg of material. Details of methods used
are to be found in suitable textbooks on practical organic chemi-
stry, 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 quanti-
tatively 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.
4 ORGANIC CHEMISTRY FOR GENERAL DEGREE STUDENTS
The nitrogen content of an organic compound is commonly
determined by measuring the volume of nitrogen gas evolved (cor-
rected 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 sul-
phuric 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 esti-
mated directly but is calculated by difference.
Empirical and Molecular Formulae
The following will serve as an illustration of the method of cal-
culating 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.
7Ί.-Ο9£6ί\0 vx1102 11Λ0Λ0
Carbon
44 X 4-835
=44-9%
1-630x2 100
Hydrogen=
" 18 X 4-835
= 3-75%
4-760x80 100
Bromine =
187-9 X 5-420
= 37-4%
0-17x28 100
N itrosen = „ = 6-5°/
A = ———■— x
22-4 3-250 /o
Oxygen =100-(44-9+ 3-75+ 37-4+ 6-5)= 7-45%
INTRODUCTION 5
The composition of the compound is therefore:
C, 44-9; H, 3-75; O, 7-45; N, 6-5; Br, 37-4%
Dividing each value by the atomic weight of the element:
C449 : Η^ : Ο745 : NOJ : Bn74
12 1 16 14 80
or
Q-74 : H3.75 : Ο ·465 : N -464 : Br0*467
0 0
Dividing each value by 0-465:
Q-03 : H . 5 : O : N -996 : B^.04
8 0 x 0
These figures may now be rounded off to the nearest whole
numbers to give the empirical formula, i.e. C H ONBr.
8 8
Such minor approximations are permissible as in practice devia-
tions 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 mole-
cular 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
6 ORGANIC CHEMISTRY FOR GENERAL DEGREE STUDENTS
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, avail-
able, 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 interpreta-
tion of the absorption spectra of the organic molecule.
Postulation of Possible Structural Formulae
Little progress was made in structural organic chemistry until
Kekule 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. hydro-
gen and the halogens represent monovalent atoms. The structures
of some of the simplest organic compounds CH (methane),
4
CH3CI (methyl chloride), CH C1 (methylene chloride), CHC1
2 2 3
(chloroform) and CC1 (carbon tetrachloride) may therefore be
4
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.
H H Cl Cl Cl
H—C—H H—C—Cl H—C—Cl H—C—Cl Cl—C—Cl
I I I I I
H H H Cl Cl