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ALKYLATION REACTIONS OF ISOCYANATES
Khatamova Mukhabbat Sattarovna
Candidate of Chemical Sciences, Acting Professor
Omanov Behruzjon Shukhratovich
Ph.D. in Technical Sciences (PhD), Associate Professor
Saibova Fatima Sadullovna
Teacher
Navoi State University, Navoi, Uzbekistan
https://doi.org/10.5281/zenodo.15526753
Annotation:
In this article, the course of the reaction and the influence of the
isomer composition on the radical nature, branching and length of the molecule,
solvent, and the “hardness” or “softness” of the methylating agent were studied
by determining the reactivity of the N-H alkylation of carbamate and bis-
carbamate derivatives with alkyl halides.
Keywords:
carbamate, bis-carbamate, isocyanate, carbon-nitrogen bond,
N,N
’
–disodium derivative, methyl iodide, benzene.
Introduction
Currently, carbamate and bis-carbamate derivatives are widely used in the
chemical industry, catalysts, agrochemistry, medicine, pharmaceuticals and
various other fields, and are of great importance as pharmacologically active
substances in many drugs, in particular as sedatives, analgesics and
antipsychotics, in agrochemistry, in agriculture, in pest control, in the chemical
industry, in the production of plastics and other materials, in the food industry, in
improving the quality of products and preserving products. This is one of the
urgent tasks facing chemistry to meet the needs of the national economy in
bioactive substances and to search for new compounds, develop technologies for
their production and apply them in practice.
Carbamates and their derivatives are highly reactive synthons in the field of
basic organic synthesis of biologically active compounds
.
Level of study
A number of patents, articles and monographs have been published on the
chemistry of carbamates and their chemical properties. Also, scientists who have
contributed and are contributing to the study of the subject: S.G. Entelis, O.B.
Nesterov, H.H. Melnikov, Henkel Kgaa, Heinzce Michael, S.Yu. Vyazmin, S.E.
Berezina, L.A. Remizova, I.N. Domnin, R. Glyater, Mitsui Chemicals, Aso Shinji,
Noguchi Takeshi, Ogawa Shinji and other specialists from the CIS, European,
German and American countries have also contributed.
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Among the scientists from Uzbekistan: A.G. Maksumov, N. Madikhanov, U.A.
Baltaboyev, M.A. Atakhodzhayeva, N.A. Avinov, U. Tadjibayev, M.A. Talipova, A.D.
Zakirov, U.B. Ju’rayev, U.A. Aripov, O.V. Afanasyeva, H.U. Usmanov, Z.G.
Khaydarova, B.S. Sulaymanov, M.S. Khatamova and others are conducting
scientific research in this area.
Research methods
Mono- and diisocyanates are highly reactive substances, and a wide variety
of their compounds are known, which are of great interest both theoretically and
practically. Using these methods, the authors used to obtain a number of
isocyanates in high yields. The importance of isocyanates can be further enhanced
by obtaining urea derivatives, which are products of the reaction of isocyanates
with alcohols. When studying the chemical properties of isocyanates, it is
necessary to dwell on the structure of the -N=C=O– groups and the distribution of
electron clouds in isocyanate molecules in both static and dynamic states, since it
is these factors that often determine the nature of the reactions in which
isocyanates enter.
Isocyanate groups are a linearly arranged (cumulative) system in which π-
orbitals can be divided into two orthogonal systems πх and πу – orbitals. Due to
the energy splitting of orbitals, the interaction of electrons in this orbital with
electrons in the π-system is significantly greater than that of localized σ-
component electrons. The presence of such π,π-bonding indicates that the -N=C
bond in the O=C=N-R-N=C=O and -N=C=O molecules has a reduced length.
Therefore, a model is used in isocyanates that treats the orbital as a π-orbital
(Figure 1).
N
C
O
z
x
y
x
x
y
y
Figure 1. Distribution of electron clouds in the isocyanate molecule
The reactivity of the NCO group is determined by its electronic structure.
Some information on the electronic structure of the NCO group was obtained from
[1,2], and the results of the calculation of the distribution of the π-electron density
in methyl and phenyl isocyanates were performed using the coordinated field
method in the Pariser-Parra-Poplo approximation. Often, the carbon atoms in the
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NCO group have a large positive charge, which determines the ability of
isocyanates to bind nucleophilic reagents:
Calculations show that the reaction proceeds through a large weakening of
the -N=C< bond compared to the >C=O bond. The change in the electron
configuration is higher in the -N=C< bond than in the >C=O bond. When the
reacting particles approach, the negative charge on the nitrogen atom increases
significantly compared to the oxygen atom in the >C=O group. This factor
indicates that the polarizability of the -N=C bond is greater than that of the >C=O
bond and it is precisely the -N=C< bond that should be easily broken when the -
N=C=O group is attacked by nucleophilic reagents.
The carbon-nitrogen bond of the isocyanate group, due to the mobility of the
active hydrogen atom, enters into ionic addition reactions with functional
compounds such as water, alcohols, phenols, thiols, amines, amides, and
carboxylic acids.
Isocyanates undergo alkylation reactions to form three main cyclic
compounds: amides and ureas.
Without the presence of a catalyst, these reactions usually occur with
isocyanates and are coupled. It is assumed that they proceed with the formation
of an intermediate complex containing the compound. According to this
mechanism, the compound added to the intermediate complex is attached to the
second molecule. Therefore, the reaction is a first-order, active reaction with
respect to the isocyanate.
Alkylation of the N-N group in carbamates with alkyl halides is of great
interest for determining the reactivity of compounds containing N-H.
Alkylation reactions can be carried out by reacting the N,N
’
–disodium
derivatives of hexamethylene bis-[(alkyl)-carbamates] with methyl iodide in
R N C O
R N C O
R N C O
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benzene at room temperature at 25-260
0
C. The fact that alkylation reactions
proceed at the nitrogen atom in N,N is explained by the relatively easy
dissociation of the sodium atom due to the presence of a carbonyl group next to
this atom.
Studies of the methylation reactions of N,N'-disodium hexamethylene bis-
[(alkyl)-carbamates] show that the course of the reaction and the isomer
composition depend on the nature of the radical, the branching and length of the
molecule, the solvent, the "hard" or "soft" nature of the methylating agent, and
other factors.
The alkylation of the above compounds with alkyl halides CH3X is of
undoubted interest in determining the nature and degree of substitution of the
methylating agent in the halide "X" in the reagent. The physical properties of CH
3
X
are given in Table 1.
Table 1
Physical properties of CH
3
X
Halogen
compounds
Boiling temperature,
0
C
d
420
F
Cl
Br
I
F
Cl
Br
I
CH
3
X
-79
-24
4
42
--
0,991
(25
0
C)
1,732
( 0
0
C)
2,279
Alkanes are chlorinated under the influence of UV light or temperature. An
equimolar mixture of methane and chlorine can explode. Therefore, the
chlorination reaction is carried out in a reactor with a UV lamp, with an excess of
alkane. The reaction proceeds stepwise according to the free radical mechanism.
Hydrogen on the tertiary carbon atom exchanges more easily, hydrogen on
the secondary carbon atom more difficult. Halogenated methyls belonging to
primary compounds of normal structure can be easily identified by considering
their boiling points and relative densities. In CH
3
J, the relative densities and
boiling points are higher than those of the corresponding bromine-substituted
ones, and in turn, the densities and boiling points of bromine-substituted ones are
higher than those of chlorine-substituted ones.
In conclusion, the relative density of CH
3
X increases with increasing atomic
weight of X in the molecule. In addition, it also depends on the bond energy, size,
etc. (Table 2).
Table 2
General properties of bonds in C-X
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Connect
ion
Energy,
kkal/mol
Length, A
0
Polarity,
D
Polarizability,
cm
3
C-F
102
1,40
2,3
1,7
C-Cl
78
1,76
2,3
6,5
C-Br
65
1,91
2,2
9,6
C-I
57
2,12
2,0
14,6
Organic molecules are characterized by polarizability, that is, the ability to
increase the polarity of the bond when an attacking agent approaches. The more
mobile the electron cloud of the atoms forming the bond, the higher the
polarizability of the bond. The tendency to polarizability is best observed in the
series of C-X bonds.
Electron-withdrawing substituents increase the reactivity of isocyanates,
which occurs in the reaction with nucleophilic compounds, regardless of whether
the increase in positive charge on the carbon atom is calculated or due to the
stabilization of the transition state, while electron-donating substituents reduce
it.
Thus, the closer the polarity of the CH
3
-F,
CH3-
Cl, CH
3
-Br and CH
3
J bonds, the
greater the polarizability of the C-J bond compared to that of the C-F. In all
nucleophilic substitution (S
N
) reactions, C-J is maximally active relative to other
C-X, in strict accordance with the polarizability.
References:
1.
Kutepov, A. M. General chemical technology / A. M. Kutepov, T. I. Bondareva,
M. G. Berengarten. - M.: Higher. school, 2003. - 520 p.
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Abdusamatov A. Organik kimyo /-T.: “Talqin” nashriyoti, 2005.- 25 bet
3.
Entelis S. G., Nesterov O. V. Kinetics and mechanism of reactions of
isocyanates with compounds containing active hydrogen // Zhurnal uspehi
khimii.-Moscow. 2006.- v. XXXV, vyp.11.-P. 2178-2203.
4.
Velikorodov A. V., Bakov O. V., Mochalin V. B. / Synthesis of O-alkyl
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5.
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