237
Fractional Composition
thiourea in the adduct than are the less compact molecules. The weight ratio decreases from 2.7 to
2.8 for aliphatic compounds and monocyclic naphthenes, to 2.5 for dicyclic naphthenes, to 2.2–2.3
for condensed ring systems; the weight ratio (or unsaturated compounds) is generally very similar
to that of the corresponding saturated structure.
9.5.2.4 Adduct Structure
X-ray diffraction patterns have been obtained for many different urea complexes, and it appears
that the hydrocarbon in the adduct does not make any contribution to the structure. In the adduct,
the urea molecules are held together by spirals of hydrogen bonds between an oxygen atom and a
nitrogen atom of adjacent molecules; these adjacent molecules are turned 120°C with respect to one
another. A distance of 3.7 Å separates adjacent non-hydrogen-bonded molecules along the axis of
the spiral, and the edge length of the unit cell is 4.8 Å. In this molecular arrangement there exists a
hexagonal channel, or canal, into which the reactant molecule must fit, which has been calculated to
be 4.9 Å in diameter. The diameter of an n-paraffin chain is of the order of 3.8 × 4.2 Å, thus allowing
the n-paraffins to fit into the channel.
The crystal structure of urea in the adduct is entirely different from that of the pure reagent.
There are six urea molecules in the hexagonal unit cell of the adduct, whereas pure urea is a tetragonal, close-packed crystal with no canal or available free space in which other molecules could be
enclosed. Thus the change in crystal structure from a tetragonal to a hexagonal system occurs at
some time during the adduction process, and it is considered likely that the urea molecules grow
in a spiral around the hydrocarbon. X-ray studies also indicate that many of the thiourea adducts
have crystalline structures similar to one another and quite analogous to those of the urea adducts.
Although pure thiourea crystals have an orthorhombic structure, the unit cell of the thiourea complexes is usually rhombohedral. A few cases are known in which the unit cell of the complex is
orthorhombic, and with the thiourea adducts different crystalline forms can apparently be precipitated under different reaction conditions. The channel concept also applies to the thiourea adducts
as it does to those of urea. However, the larger size of the sulfur atom in thiourea in relation to the
oxygen in urea results in a channel with a larger cross section. The unit cell constants of the thiourea
adducts apparently vary with the nature of the adducted molecule, and hence the channel crosssectional dimensions also vary and are presumed to be of the order of 5.8–6.8 Å.
It is generally accepted that, in the urea adducts, the urea molecules are connected into spirals by
hydrogen bonds between the oxygen and the amino groups of adjacent urea molecules, resulting in
a channel into which the adducting compound can fit. The size of the channel limits the molecules
that may adduct to those having cross-sectional dimensions equal to, or less than, those of the channel in the urea adduct, but in certain cases distortion of the urea lattice may occur and slightly larger
molecules can adduct. The structure of the thiourea adducts is similar to that of the urea adducts.
The larger size of the sulfur atom results in a channel of somewhat larger cross-sectional dimensions, hence allowing larger molecules to adduct with thiourea than are able to do so with urea. In
general, urea forms adducts with organic compounds containing a long unbranched chain, such as
the n-paraffins, whereas thiourea forms complexes with compounds that contain a moderate amount
of branching of cyclization. However, especially among the higher molecular weight hydrocarbons,
urea adducts can be formed from n-paraffins.
9.5.2.5 Adduct Properties
The amount of dissociation that occurs when an adduct is contacted with solvents is a function of the
solvent type. These types may be classified as (1) hydrocarbon solvents such as benzene, (2) urea or
thiourea using solvents such as water, and (3) hydrocarbon and urea (or thiourea) solvents (methanol
or benzene–methanol mixtures).
As expected, the last two types cause the greatest dissociation. Solvents in which urea or thiourea
is relatively insoluble, but in which the hydrocarbon is readily dissolved, exert relatively little influence. With any solvent, increasing temperature increases the amount of dissociation.
Fractional Composition
thiourea in the adduct than are the less compact molecules. The weight ratio decreases from 2.7 to
2.8 for aliphatic compounds and monocyclic naphthenes, to 2.5 for dicyclic naphthenes, to 2.2–2.3
for condensed ring systems; the weight ratio (or unsaturated compounds) is generally very similar
to that of the corresponding saturated structure.
9.5.2.4 Adduct Structure
X-ray diffraction patterns have been obtained for many different urea complexes, and it appears
that the hydrocarbon in the adduct does not make any contribution to the structure. In the adduct,
the urea molecules are held together by spirals of hydrogen bonds between an oxygen atom and a
nitrogen atom of adjacent molecules; these adjacent molecules are turned 120°C with respect to one
another. A distance of 3.7 Å separates adjacent non-hydrogen-bonded molecules along the axis of
the spiral, and the edge length of the unit cell is 4.8 Å. In this molecular arrangement there exists a
hexagonal channel, or canal, into which the reactant molecule must fit, which has been calculated to
be 4.9 Å in diameter. The diameter of an n-paraffin chain is of the order of 3.8 × 4.2 Å, thus allowing
the n-paraffins to fit into the channel.
The crystal structure of urea in the adduct is entirely different from that of the pure reagent.
There are six urea molecules in the hexagonal unit cell of the adduct, whereas pure urea is a tetragonal, close-packed crystal with no canal or available free space in which other molecules could be
enclosed. Thus the change in crystal structure from a tetragonal to a hexagonal system occurs at
some time during the adduction process, and it is considered likely that the urea molecules grow
in a spiral around the hydrocarbon. X-ray studies also indicate that many of the thiourea adducts
have crystalline structures similar to one another and quite analogous to those of the urea adducts.
Although pure thiourea crystals have an orthorhombic structure, the unit cell of the thiourea complexes is usually rhombohedral. A few cases are known in which the unit cell of the complex is
orthorhombic, and with the thiourea adducts different crystalline forms can apparently be precipitated under different reaction conditions. The channel concept also applies to the thiourea adducts
as it does to those of urea. However, the larger size of the sulfur atom in thiourea in relation to the
oxygen in urea results in a channel with a larger cross section. The unit cell constants of the thiourea
adducts apparently vary with the nature of the adducted molecule, and hence the channel crosssectional dimensions also vary and are presumed to be of the order of 5.8–6.8 Å.
It is generally accepted that, in the urea adducts, the urea molecules are connected into spirals by
hydrogen bonds between the oxygen and the amino groups of adjacent urea molecules, resulting in
a channel into which the adducting compound can fit. The size of the channel limits the molecules
that may adduct to those having cross-sectional dimensions equal to, or less than, those of the channel in the urea adduct, but in certain cases distortion of the urea lattice may occur and slightly larger
molecules can adduct. The structure of the thiourea adducts is similar to that of the urea adducts.
The larger size of the sulfur atom results in a channel of somewhat larger cross-sectional dimensions, hence allowing larger molecules to adduct with thiourea than are able to do so with urea. In
general, urea forms adducts with organic compounds containing a long unbranched chain, such as
the n-paraffins, whereas thiourea forms complexes with compounds that contain a moderate amount
of branching of cyclization. However, especially among the higher molecular weight hydrocarbons,
urea adducts can be formed from n-paraffins.
9.5.2.5 Adduct Properties
The amount of dissociation that occurs when an adduct is contacted with solvents is a function of the
solvent type. These types may be classified as (1) hydrocarbon solvents such as benzene, (2) urea or
thiourea using solvents such as water, and (3) hydrocarbon and urea (or thiourea) solvents (methanol
or benzene–methanol mixtures).
As expected, the last two types cause the greatest dissociation. Solvents in which urea or thiourea
is relatively insoluble, but in which the hydrocarbon is readily dissolved, exert relatively little influence. With any solvent, increasing temperature increases the amount of dissociation.