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化学专业英语之晶体化学
CRYSTALLIZATION
General
methods of Crystallization. Since the days of the
earliest
alchemists, solids have been
purfied by crystallization from suitable
solvents.
Today
this
technique
still
stands
as
the
most
useful
method
for
the
purification of solid substances.
As
commonly
practiced,
purification
by
crystallization
depends
upon
the
fact
that most solids are more soluble in hot than in
cold solvents. The
solid
to
be
purified
is
dissolved
in
the
hot
solvent
at
its
boiling
point,
the
hot
mixture
is
filtered
to
remove
all insoluble
impurities
.and
then
crystallization
is
allowed
to
proceed
as
the
solution
cools.
In
the
ideal
case, ail of the desired substance
separates in nicely crystalline form
and all the soluble impurities remain
dissolved in the mother liquor.
Finally, the crystals are collected on
a filter and dried. If a single
crystallization operation does not
yield a pure substance, the process
may
be repeated with the same or another solvent.
The great beauty of crystallization as
a purification technique lies
in the
fact that the orientation of molecules in a
crystal lattice is an
extremely
delicate
and
selective
process.
Only
infrequently
do
different
substances crystallize in the same
lattice. At times, the desired solid
can
be
crystallized
selectively
from
a
solution
also
saturated
with
other
solid
impurities
simply
by
the
careful
introduction
of
a
tiny
seed
crystal.
In such cases, the
molecules of the desired compound leave the
solution
to
take
positions
in
the
crystal
lattice,
while
the
mother
liquor
remains
saturated, or even becomes
supersaturated , with respect to the foreign
materials.
A solid solute
may, of course. be crystallized by spontaneous
evaporation of solvent from a saturated
solution. Occasionally, this is
used
as
a
method
of
purification.
Evaporation
should
proceed
very
slowly
to
avoid
formation
of
a
crust
of
impure
solid
at
the
evaporating
surface.
In general, this method is less
effective than the classical
crystallization technique
Selection
of
Solvent.
Similia
sinilibus
solvunter
(like
dissolves
like)
was a watchword among
the alchemists and medieval iatrochemists. It is
still
perhaps
the
best
three-word
summary
of
solvent
behavior;
a
detailed
study
of the relationship between structure and solvent
action becomes
highly complex. In the
final analysis, the best way to find a suitable
recrystallization
solvent
for
a
given
substance
is
by
experimental
trial.
A few helpful and reasonably valid
generalizations may, however, speed
the
process.
No nonionic compound dissolves
appreciably in water unless its
molecules are ionized in water solution
or can co-associate with water
molecules
through
hydrogen
bonding.
Thus,
hydrocarbons
and
their
halogen
derivatives are virtually insoluble in
water. Compounds whose molecules
contain
functional
groups
[such
as
alcohol,
aldehyde,
ketone,
carboxylic
acid, and amide groups] ,which can form
hydrogen bonds with water, are
soluble
in water unless the ratio of the total number of
carbon atoms to
such
functional
groups
in
the
molecule
exceeds
4
or
5.
Then
the
solubility
falls off
rapidly. Thus , acetamide is soluble in water, but
caproamide
is not. In fact. it is a
very general rule that as any homologous series
is ascended, the solubilities and all
other physical properties of the
members tend to approach those of the
parent hydrocarbon.
The hydrogen bonds
or hydrogen bridges with which we are concerned in
dealing with the water solubilities of
organic compounds are almost
exclusively those in which hydrogen
links oxygen atoms to oxygen or
nitrogen atoms.