Table Of ContentHard and Soft Acids
and Bases Principle
in Organic Chemistry
TSE-LOK HO
Department of Chemistry
Case Western Reserve University
Cleveland, Ohio
ACADEMIC PRESS New York San Francisco London 1977
A Subsidiary of Harcourt Brace Jovanovich, Publishers
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Library of Congress Cataloging in Publication Data
Ho, Tse-Lok.
Hard and soft acids and bases principle in organic
chemistry
Includes bibliographical references and index.
1. Acids. 2. Bases (Chemistry) 3. Chemistry
Organic. 4. Chemical reactions. I. Title.
QD477.H6 547'.l'39 76-13939
ISBN 0-12-350050-8
PRINTED IN THE UNITED STATES OF AMERICA
To
Honor, Jocelyn,
and
my Parents
Preface
The formulation of a new concept always signifies major advances
in scientific endeavors, and such was the case when the hard and soft
acids and bases (HSAB) principle was introduced by Professor R.
G. Pearson more than a dozen years ago. Since that time a tremen
dous number of chemical phenomena related directly to or derived
from the HSAB principle have accumulated, most of which support
Pearson's original premise. As it stands, there is no denying the
value of the principle as a convenient yet powerful tool for assessing
and predicting chemical events without having to resort to lengthy
and cumbersome calculation. Because the HSAB principle offers
great assistance in and stimulation for both pedagogy and research,
it should be introduced to students of chemistry at an early stage.
Initially used to explain inorganic coordination chemistry, HSAB
has subsequently been applied to almost every chemical ramifica
tion with considerable success. Although pioneering applications to
the organic area by Hudson, Saville, and Pearson himself have
irrefutably demonstrated the relevance of HSAB to organic chemis
try, subject matter hitherto touched on can be likened to the exposed
tip of an iceberg. Pertinent findings are bound to surface as long as
organic research does not cease.
This volume represents an attempt to examine the many organic
facets within the HSAB context. It is descriptive in nature, and is
intended for the senior undergraduate or graduate chemistry major.
However, it can also serve as an introduction and reference source
for organic chemists who are not familiar with the HSAB concept.
Although data quoted in this monograph are established fact, a good
deal of it is related to the HSAB principle for the first time.
A number of chemists have been bothered by the terms "hard"
and "soft." The irritation of these critics may be somewhat soothed
now that physicists describe the supposedly elementary particles,
xi
xii Preface
quarks, in terms of "color," "flavor," and "charm" {S. L.
Glashow, Scientific American 233 [4], 38 (1975)}.
I am keenly aware of the fact that a survey of all aspects is an
insuperable task, thus personal predilection and ignorance became
factors dictating selection or omission of various topics. Where the
original researcher may not agree with the treatment presented, I
assume total responsibility for any misrepresentation. I hope that
some of the material presented will initiate further chemical re
search along these lines.
Tse-Lok Ho*
* Present address: Medical Research Department, Brookhaven National Laboratory, Upton,
New York.
1
Introduction
1.1. DEFINITIONS OF ACIDS AND BASES
Classically acids are defined as substances that give off protons and bases
are those that generate hydroxide ions when dissolved in water. These restric
tive definitions are no longer adequate and have been modified or supple
mented by others. Currently the most widely used acid-base definitions are
due to Bronsted and Lowry, and Lewis.
1.1.1. The Br0nsted-Lowry Definition
Bronsted and Lowry independently defined an acid as a proton donor. The
classical acids can also be considered Bronsted acids. Upon ionization the
Bronsted acids yield protons and conjugate bases. Conversely, Bronsted bases
accept protons to form conjugate acids.
It is important to note that Bronsted acids include both uncharged and ionic
species, e.g., HCl, HSOf, and NHf, because of their tendency to lose a
proton.
1.1.2. The Lewis Definition
The Lewis system defines acids as electron acceptors and bases as electron
donors. An acid-base reaction involves the transfer of an electron pair from
the base to the acid. Although the Lewis bases are largely the same as the
Bronsted bases, the range of acids differs markedly. For example, in the
Bronsted system the acids donate a proton, but in the Lewis sense the proton is
1
2 1. Introduction
the acid. The most important generalization of the Lewis theory is that many
aprotic compounds are included in the acid category.
Cationic entities of a nonmetallic nature, either stable or transient, are
important intermediates in organic reactions. They belong to the class of Lewis
acids. Organic groups with polarizable multiple bonds such as carbonyl,
cyano, and nitroso groups readily undergo addition with nucleophiles (Lewis
bases). These are generally regarded as Lewis acids.
Although the differences between the Bronsted and Lewis bases are minimal
compared with that between the acids, Bronsted bases constitute, in fact, a
small group of Lewis bases. Besides anions, Lewis bases consist of molecules
with lone-pair electrons as well as 7r-electron systems, π Complexes play signi
ficant roles in organic and organometallic chemistry. Many of these can be
detected spectroscopically or can be isolated.
1.2. THE HARD AND SOFT ACIDS AND
BASES (HSAB) CONCEPT: HISTORICAL
DEVELOPMENT
R. G. Pearson has given a lucid account of the historical background con
cerning the development of the hard and soft acids and bases concept in the
Introduction of his book (1) of collected papers.
The collaboration of Pearson and Edwards in 1961 in an incisive evaluation
of factors determining nucleophilic reactivities (2) sowed the seed which
eventually blossomed into a series of classic papers systematizing chemical
stabilities, specificities, and reaction rates (3-5) on the basis of the "Hard and
Soft Acid and Bases Principle."
The principle was first applied to inorganic coordination compounds and
then used to interpret the chemistry of organic molecules and even electrode (6)
and catalytic (7) phenomena. It appears that the principle can be employed in
every branch of descriptive chemistry.
Pearson envisages a chemical bond as being made up of an acid-base com
bination. One can always imaginarily dissect a compound into Lewis acid and
Lewis base moieties for analysis. The properties of the intact molecule, in
cluding the ease of its formation, can be inferred by considering its acid and
base components. The classes of acids and bases are named "hard" and
"soft"; the A or (a) metal ions belong to hard acids and the B or (b) metal ions
(8,9) are soft acids.
According to Pearson, the choice of these terms was influenced by S.
Winstein's remark that the iodide ion is a "soft, mushy base," and D. Busch's
description that the hydroxide ion is a "hard, tight base." Despite occasional
scorn from some chemists, the terms have been perpetuated since Pearson's
References 3
first paper (3) appeared in 1963. In that article, Pearson propounded that
"hard acids bind strongly to hard bases and soft acids bind strongly to soft
bases" This has become known as the HSAB principle.
The concept received immediate and enthusiastic acclaim from some
chemists and an international symposium on HSAB (pronounced "hassab")
was sponsored by Cyanamid European Research Institute in Geneva in May
1965. Two years later, the second symposium in London attracted a greater
number of scientists.
In 1967, another major paper by Pearson and Songstad (5), applying the
HSAB principle to organic chemistry, appeared. Saville published a full ac
count of his analysis of multicenter reactions (10) based on the HSAB prin
ciple. Efforts have been made to determine quantitatively the hardness
(softness) of some donors and acceptors (11). Klopman presented a perturba
tion treatment (12) of chemical reactivity and related HSAB to molecular or
bital theory.
Summaries of the HSAB principle have been given by Pearson (13-16) and
reviews dealing mainly with organic topics have been written in French (17)
and English (18). On the basis of the steadily increasing quotations of Pear
son's papers, it may be concluded that the HSAB principle has gained impor
tance in the thought of chemists.
REFERENCES
1. R. G. Pearson, "Hard and Soft Acids and Bases." Dowden, Hutchinson, & Ross, Inc.,
Stroudsburg, Pennsylvania, 1973.
2. J. O. Edwards and R. G. Pearson,/. Am. Chem. Soc. 84, 16 (1962).
3. R. G. Pearson,/. Am. Chem. Soc. 85, 3533 (1963).
4. R. G. Pearson and J. Songstad, /. Am. Chem. Soc. 89, 1827 (1967).
5. R. G. Pearson and J. Songstad, /. Org. Chem. 32, 2899 (1967).
6. D. J. Barclay, J. Electroanal. Chem. 19, 318 (1968); D. J. Barclay and J. Caja, Croat.
Chem. Acta 43, 221(1971).
7. R.Ugo, C/nm. Ind. (Milan) 51, 1319(1969).
8. G. Schwarzenbach, Adv. Inorg. Chem. Radiochem. 3, 257 (1961).
9. S. Ahrland, J. Chatt, and N. R. Davies, Q. Rev., Chem. Soc. 11, 265 (1958).
10. B. Saville, Angew. Chem. Inter. Ed. Engl. 6,928 (1967).
H.A. Yingst and D. H. McDaniel, Inorg. Chem. 6, 1067 (1967); S. Ahrland, Chem. Phys. Lett.
2,303(1968).
12. G. Klopman,/. Am. Chem. Soc. 90, 223 (1968).
13. R. G. Pearson, Science 151, 172 (1966).
14. R. G. Pearson, Chem. Br. 3, 103 (1967).
15. R. G. Pearson, /. Chem. Educ. 45, 581 and 643 (1968).
16. R. G. Pearson, Surv. Prog. Chem. 5, 1-52 (1969).
17. J. Seyden-Penne, Bull. Soc. Chim. Fr. p. 3871 (1968).
18. T.-L. Ho, Chem. Rev. 75, 1 (1975).
2
Classification of Hard and
Soft Acids and Bases
2.1. PEARSON'S GENERALIZATION
After collaborating with Edwards to analyze and systematize the rate data
(1) for nucleophilic displacement, Pearson focused his attention on a similar
treatment of the equilibrium constants (2) for the reaction
N + S-X ► N-S + X
It was concluded that there are two kinds of substrate acids. Members of one
class bind strongly to bases which have a high affinity for protons, while those
of the second class bind preferentially with highly polarizable bases. The
bases are called "hard" and "soft," respectively, as suggested to Pearson by D.
H. Busch. The acids with which these bases combine preferentially are the hard
and the soft acids. Metal ions of class A (3) or (a) (4) are hard acids, and the
class B or (b) ions are soft.
The hardness of an acid (acceptor) or a base (donor) is generally character
ized by a small atomic radius, a high effective nuclear charge, and a low
polarizability, whereas softness implies all the opposite properties. Further
more, the softness of a base can be associated with low electronegativity, easy
oxidizability, or empty low-lying orbitals.
The main criterion adopted by Pearson in his classification of Lewis acids is
more or less the same as that employed by Ahrland et ah (4). Hard acids will
form complexes whose stability sequences are
N > P > As > Sb > Bi
O > S > Se > Te
F > CI > Br > I
4
Pearson's Generalization 5
Conversely, the complex stability of soft acids with bases decreases as the base
varies according to the following pattern.
N«P>As>Sb>Bi
0< S<Se~Te
F < Cl< Br < I
A number of Lewis acids have been assigned to hard or soft groups ac
cording to this general scheme (see Table 2.1). Since no one can determine the
demarcation line between a hard and a soft species, there exist certain inter
mediate or borderline cases. In fact, the hardness (or softness) of a central
atom is influenced by its ligands.
TABLE 2.1 Classification of Hard and Soft Acids"
Hard acids
H+, Li+,Na+,K+(Rb+,Cs+)
Be2+, Be(CH), Mg2+, Ca2+, Sr2+ (Ba2+)
32
Sc3+, La3+, Ce4+, Gd3+, Lu3+, Th4+, U4+, VO\+, Pu4+
Ti4+, Zr4*, Hf4\ V02+, Cr3+, Cr6+, Mo03+, W04+, Mn2+, Mn7+, Fe3+, Co3+
BF, BCl, B(OR), Al3+, Al(CH), AlCl, AlH, Ga3+, In3+
3 3 3 33 3 3
C0 , RCO+, NC+, Si4+, Sn4+, CH Sn3+, (CH)Sn2+
2 3 3 2
N3+, RPO+, ROPO+, As3+
2
S0 , RSO+, ROSO+
ci33+,cr+,iJ+,r+ 2
HX (hydrogen-bonding molecules)
Borderline acids
Fe2+, Co2+,Ni2+, Cu2+,Zn2+
Rh3+, Ir3+, Ru3+, Os2+
B(CH,)„ GAH
3
R C+,C H+,Sn2+,Pb2+
3 6
NO+, Sb3+,Bi3+
so
2
Soft acids
Co(CN)3-, Pd2+, Pt2+,Pt4+
Cu\ Ag+, Au+, Cd2+, Hg+, Hg2+, CH Hg+
3
BH, Ca(CH), GaCI, GaBr, Gal , Tl+, Tl(CH)
3 33 3 3 3 33
CH, carbenes
2
7Γ acceptors: trinitrobenzene, chloroanil, quinones, tetracyanoethylene, etc.
HO+, RO+, RS+, RSe+, Te4+, RTe+
Br, Br\ I .I+, ICN, etc.
2 2
O, Cl, Br, I, N, RO ,R0
2
M° (metal atoms) and bulk metals
a Reprinted with permission from J. Chem. Educ. 45, 581 (1968). Copyright by the American
Chemical Society.
