Table Of ContentTHE CRYSTAL STRUCTURES
OP THE
ACID SAITS OP SOME AROMATIC ACIDS*
^ THESIS
Presented for tne Degree of
Doctor of Philosophy
in the
U niversity of Glasgow
by
John MaePhail Skinner, B. Sc.
University or Glasgow.
Jtagttst, 1950.
ProQuest Number: 13850818
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PREFACE
The resu lts of the investigation of the structure
of potassium hydrogen bis-p-hydroxybenzoate, described in
Part I of th is th esis, are being published conjointly
with Dr. J. C. Speakman.
I wish to express my sincere thanks to my supervisors,
Professor J. M. Robertson and Dr. J. C. Speakman, for
suggesting the problem for research, and for constant
encouragement and advice during the course of the work.
I am indebted to Dr. Speakman for samples of various
acid sa lts. I also wish to thank the Department of
S cien tific and Industrial Research for a Maintenance
Allowance.
J. M. S.
U niversity of Glasgow.
August, 1950.
SUMMARY
The crystal structures of the acid potassium sa lts of
p-hydroxybenzoic, benzoic and anisic acid have been examined
by the X-ray diffraction method.
It has been shown that potassium hydrogen bis-p-hydroxy-
benzoate is a monohydrate and that its crystal structure is
closely sim ilar to that of potassium hydrogen bisphenylacetate.
The principal common feature is in the linking of the carboxyl
groups by a short, and apparently symmetrical, hydrogen bond.
It is thought that the crystal structures of the acid sa lts
of most monocarboxylic acids may generally conform to th is
plan.
Further evidence for th is suggestion has also been
obtained from a preliminary analysis of the structure of
potassium hydrogen bisbenzoate which appears to be b uilt
on the same plan. The crystal structure of potassium
hydrogen bisanisate has also been examined but, as yet,
lit t le progress has been made with the analysis.
iii.
CONTENTS.
Page.
Preface ........................................................................................ i
Summary................................................................................................i i
I n tr o d u c tio n .......................................................... . . 1
Part I
Potassium hydrogen bis-p-hydr oxybenzoate
Crystal D a t a ................................................................ 6
E x p erim en ta l........................................................... 6
Analysis of the S tr u c tu r e .............................. 9
Discussion ........................................................... 27
Appendix 45
Part II
Potassium hydrogen bisbenzoate
Crystal Data. . . . . . . . . 53
E x p erim en ta l....................................................... 53
Analysis of the S tr u c tu r e ......................................56
Potassium hydrogen bisanisate
Crystal D a t a ............................................ . 68
Experimental .......................................68
Analysis of the Structure • 69
R eferen ces.........................................................................................74
1
Introduction
Acid sa lts of monobasic acids, in which a molecule of
normal salt is combined with one or more molecules of the
parent acid, are formed by most acids and most m etals, (For a
detailed survey see Smith, 1949) Many of the acid sodium and
potassium sa lts of mcnocarboxylic acids are mentioned in the
literatureo Of particularly frequent occurrence here is tne
1:1 compound i.e . where one molecule of acid (HX) is combined
with one molecule of normal salt (KX) to give the acid
sa lt (KHX2)o While the sodium salts are often hydrated, the
potassium sa lts are, in general, anhydrous. This much has
been known for some considerable time, but lit t le is known of
the physical properties of these compounds. Thermochemical
measurements (de Focrand, 1883; R ivals, 1897) clearly indicated
however that there was a d efin ite linkage between acid and sa lt.
Occasionally sane authors have suggested possible
structural formulae for these compounds. Farmer (1903), for
example, related them to oxonium sa lts and proposed structures
such as:
R.CO K
0S
E.COg H
On the other hand P feiffer (1914) suggested two possible
structures,
a) R.C=0 H-O-C-B b) R.C-0 M-O-C-E
I II I II
0M 0 OH 0
Structure (b), in which there is co-ordination to a central
2
metal atom, was preferred since compounds more complex than
the 1:1 type can also he formulated in th is manner. More
recently (Ross and Morrison, 1933) a sim ilar structure has
been proposed, v iz.
-© -1
R-C=0 -----M---- 0-C-R
1 II
OH 0
V itale (1936), however, prefers a cyclic structure
OH
NaO 0—C-R
y p
R 'O-C-OH
R
because a study of the Raman spectra of such compounds has
shown the absence of a carbonyl frequency. There is lit t le
p ositive evidence available to enable a decision to be made
between these possible formulae.
Recently (Speakman, 1949) an X-ray investigation of the
structure of potassium hydrogen bis-phenylacetate KH( c8h o2)2
has been carried out. It has been shown to be b u ilt up of
in fin ite layers of potassium and hydrogen atoms (or ions)
between pairs of layers of phenylacetate residues. Adjacent
carboxyl groups are linked by a very short hydrogen bond
reminiscent of the strong hydrogen bonds found in inorganic acid
sa lts. Of particular interest is the fact that th is bond
appears to be symmetrical since the oxygen atoms in question are
situated about a centre of symmetry.
5.
The position of tne proton taking part in hydrogen bond
formation has been the subject of considerable speculation.
Direct evidence on tn is point is d ifficu lt to obtain since it
is rarely possible to establish the positions of hydrogen atoms
by the X-ray method. Bernal and Megaw (1935) have suggested
that in the short hydrogen bonds, such as occur in inorganic
acid sa lts, the hydrogen atom may be symmetrically located.
The longer bonds (2.70A. or more), found in hydroxy compounds,
are considered to be unsymmetrical and called "hydroxyl" bonds.
However, spectroscopic investigation of a large number of
compounds containing hydrogen bonds has always revealed the
presence of an CH absorption band wnich would be expected to
be absent in a symmetrical 0-H. .0 arrangement. This d is
symmetry in the hydrogen bond also explains successfully the
occurrence of the "residual entropy" in many compounds such as
ice and inorganic acid sa lts. A survey of the evidence on
th is question has been given by Davies (1946). On tne other
hand, Ubbelchde (1949) maintains that protonic resonance makes
a certain contribution to the attractive forces which are
operative in hydrogen bond formation, and he has suggested
that in certain circumstances the proton may be situated
midway between tne two oxygen atoms.
In view of the interesting features found by Speakman in
the structure of potassium hydrogen bis-phenylacetate it seemed
worthwhile to extend the examination to the potassium sa lts of
4.
other monobasic acids. Part I. of th is Thesis describes an
investigation of tne structure of potassium hydrogen
p-hydroxybenzoate. This compound was prepared by a method
analogous to that outlined by Parmer (loc. c it.) who claimed
that it was anhydrous* As the analysis proceeded it became
clear that tne salt was hydrated. Confirmatory evidence on
th is point is given in an Appendix to Part I 0 In part II.
d etails are given of a less complete investigation of the struc
tures of potassium hydrogen benzoate and potassium hydrogen
anisate.
PART
