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THEORY OF ORGANIC CHEMISTRY (by dr. SZÁNTAY Csaba) 3rd, Supplemented Edition MŰSZAKI KÖNYVKIADÓ, BUDAPEST, 1984 Dr. DEÁK Gyula, University Lecturer, PhD. of Chemical Sciences Contributed to the samples: Dr. MAJOROS Béla, Cert. Chem. Eng., Univ. Assist. Lecturer, Copyright: Dr. SZÁNTAY Csaba, Budapest, 1970 ISBN 963 10 0682 4 (3rd /revised/ edition) ISBN 963 10 5757 7 Editor in charge: BOROSS Anna, Cert. Chem. Eng.
The most the cybotactic region differs from the
pure solvent, the more difficult it is to draw
conclusions from the macroscopic properties of the
solvent regarding the part it plays in the course
of the reaction. Therefore, the solvent must, as a
rule, be considered as the complex of molecules.
From the point of view of a chemist involved in
organic chemistry solvents should be grouped into
b) apolar or less polar aprotic solvents,
Important solvents to be classified to fall in
Group “a” are for example, water, alcohol, amines
carbon cads, etc. These substances have nucleophil
and electrophil properties at the same time. By
their mobile hydrogen they solvate anions very
markedly, as a rule by the creation of a hydrogen
Thanks to the abilities as donor of their self-
standing electron pair, they are capable, at the
same time, of solvating cations, too., e.g.,
If this bi-directional solvating capability is
coupled by considerable polarity and a high
dielectric constant, then the solvents in Group
“a” can greatly contribute to spontaneous
ionization, that is, a reaction of SN1 type. This
substances, as a rule solvate, themselves, that
its, they are composed of associates bound by
Group “b” contains the majority of carburetted
hydrogen and chlorinated solvents. Within this
group we can make further distinctions in view of
the intensity of nucleophil property. Ether,
dioxin, tetra hydrofuran, (THF), etc. belong to
the category of comparably intensely nucelophil
The dipolar – aprotic solvants of Group “c” are an
extremely interesting and important group of
solvents. The hydrogen bound to a carbon atom in
them separates, as a rule, very difficultly: if
these solvents are to react with D2O, they do not
exchange it at all, or if yes, then very slowly
with deuterium. The difference in between them and
Group “b” is, that their dielectric constant is
bigger than appr. 15D. We make this voluntary
separation as the aggregations of ions in solvents
with smaller dielectric constants are so big, that
it is very difficult to examine the behaviour of
the individual ions separated in solvent shell.
The most important dipolar – aprotic solvent are
as follows: Dimethylformamid (DMF), Dymethyl-
acetamide (DMAC), Acetone, Nitromethene,
Nitrobenzene, Acetonitrile, Benzonitrile, Sulphur-
Dioxide, Prophylene-Carbonate, N-Methyl-2-
pirrolidone, and Tetramethyl-carbamide. Sulfolane
and Dimethylsulfon melting/dissolving above
ambient temperature are also used many times.
The main properties of some solvents are
The most frequently used dipolar – aprotic
solvents are DMF, DMAC, DMSO and Acetone. the
reason for their popularity is that they are cheap
We can make another distinction within Group “c”
(Meszmer, A.), on the basis whether a solvent is
intensely nucleophil (intensely alcaline) such as
for example DMSO or only slightly, as e.g., Aceto-
As a rule, SN2 type reactions, especially those
between anions and neutral molecules, are much
quicker in dipolar – aprotic solvents than in
rearrangments are slower than in the solvents
belonging to Group “c” or “a”. The reaction speed
independent of the solvent. These simple
observations, however, can be a help when we want
to achieve a poorer product with shorter reaction
time, better exploitation rate and the forcing of
In the light of our arguments regarding cybotactic
region, the dielectric constants of the various
solvents summarized in Table 5.1 do not in
themselves give any information as to what
reaction speed is to expect. Several reactions are
several million times quicker in a dipolar –
aprotic solvent then in a protic solvent having
similar dielectric constant. The polarity of
solvents have no exact definition as of yet (65),
but based on the assessment of the various
factors, protic solvents can, as a rule, be deemed
We can find intensely polar solvants in Group “c”
they, however, do not help SNI tpye reactions,
first of all, as they canot solvate and stabilize
the anion breaking lose by a strong hydrogen-
bridge. Simultaneously, they better solvate most
of the cations, such as Na+, and Ka+ than e.g.,
considerations can be used for understanding the
relationship between the change of polarity and
reaction speed within a given solvent type. If a
icreaseactivation energy (and reduce reaction
speed); if the transitory state is less polar than
the starting state of reactants, as the latter
gets better solvated in more polar solvent. If we
however take it the otherwise: if a solvent is
more polar, it shall reduce ctivation energy (in
other words: increase reaction speed if the
transitory phase is more polar than the starting
The effect exerted by the solvent on the SN2
reaction of molecules without polarity leading to
their getting ions is demonstrated by Table 5.3
Some physical constants of frequently used solvents Group “a” Group “b” Group “c”
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