Chemical Properties of Nitrogen
Valency of Nitrogen
|According to those fundamental conceptions of valency which are based on the numbers of elements which combine with one another to form stable compounds, the elements nitrogen, phosphorus, arsenic, antimony, and bismuth possess the well-defined valencies of 3 and 5, which have been already illustrated by typical compounds. The spatial directions in which these valencies are distributed around the central atom have been most fully elucidated in the case of nitrogen, which will therefore be taken as the prototype to which the other elements are already known to conform to some extent.|
Isomerism among compounds of tervalent nitrogen of the type will not be found if the valencies are in one plane, but only if they are bent; as, for example, along the edges of a tetrahedron.
Actually, substituted ammonia bases have not been prepared in isomeric forms (or stereo-isomeric optical antipodes) by any of the usual methods.
It does not follow that the valency directions are in the same plane, for the absence of isomerism would be equally well explained by an extreme lability or interchangeability of the occupied positions with the spare valency or " lone pair."
If the nitrogen atom is united to another atom by a double bond, which, as in the case of the ethylenic linkage between two carbon atoms, is incapable of free rotation, a geometrical isomerism becomes possible. The discovery of two isomers of benzildioxime, followed by that of a third form of this compound, was first explained by different configurations of the molecules round the singly linked carbon atoms, while the somewhat simpler example of the isomeric α- and β-benzaldioximes was explained at first as being due to differences in structure. These hypotheses, however, did not agree with further facts, and all such cases of isomerism are now attributed to the spatial configuration of the groups attached to the nitrogen atom.
|The only definite compounds of nitrogen with halogens which have been prepared are the chloride, NCl3, and the iodide, N2I3H3. Nitrogen fluoride was thought to be obtained by the electrolysis of ammonium fluoride, but this was shown later to be the chloride derived from the ammonium chloride as impurity. Nitrogen bromide is stated to be formed by adding potassium bromide to nitrogen chloride under water, when an explosive dark red oil is obtained.|
Stereochemistry of Nitrogen
|Many further proofs have been given of the hypothesis that the three valency directions of the doubly bound nitrogen atom are not coplanar. If we compare with one another the corresponding carbon and nitrogen atoms in acetylene and hydrogen cyanide, also benzene and pyridine, and assume that the three carbon bonds are directed to the angles of a tetrahedron, so also will be those of the nitrogen:|
If, now, one pair of bonds is severed, the nitrogen becomes doubly linked, and if the third valency retains its direction, two spatial arrangements result which may be represented by the diagrams or formulae
e.g. the chloro-benzo-phenone-oximes, by
These forms are known as syn- and anti- with respect to the groups "a," according to whether the group "c" is on the same side as, or on the opposite side to, "a." The isomers show differences in physical and chemical properties such as characterise the geometrical isomers derived from ethylene. Their configuration has been discovered by reactions, such as that of Beckmann. When treated with PCl5 in benzene or petroleum spirit they are converted into amides, and these with the acids formed by their hydrolysis should be, and are, different in the two cases:
Finally, it may be mentioned that the dioximes, such as benzildioxime, should exist in three isomeric forms, and this is found to be the case (see above):
A similar type of isomerism is found in the hydrazones, , and osazones. There is also considerable evidence that the diazo-compounds exist in syn- and anti-forms:
Full details may be obtained from text-books of organic chemistry.
Change of Valency There are many compounds, such as the ammonium bases, nitric acid, etc., in which it seems impossible or unnatural to retain the trivalency of nitrogen, and the simplest assumption, on the older ideas, is that of quinquevalent nitrogen. In compounds such as nitric acid, , nitryl fluoride, , the five negative valencies are the contravalencies of Abegg's system. Ammonium and substituted ammonium salts are clearly of a different type; of the two extra valencies, one is a positive or hydrogen valency, of the same kind as the first three, and the second is an electrovalency. A change in the valency of nitrogen is exhibited in the tautomerism of hydroxylamine and its derivatives:
H2=N-OH ⇔ H3≡N=O;
also in the formation of ammonium salts and the salts of ammonium bases:
R3N+R1 → R3NR1;
and in the tautomeric change of pseudo-bases into true bases or basoforms:
=C(OH)-NR2 → =C=NR2OH.
It is now no longer necessary, however, to postulate the existence of quinquevalent nitrogen, since all these changes are satisfactorily expressed by the new valency models based on the electronic structure of the atom. Indeed, these theories deny the possibility of quinque-valency of the organic type, i.e. quinque-covalency.
Stereochemistry of Ammonium Derivatives
|The resolution into optical isomers of compounds of the type NabcdX. had already been partly effected by Lebel, who, by acting on a solution of methyl-ethyl-propyl-ammonium chloride with the mould "penicillium," obtained a feebly laevo-rotatory effect on polarised light.|
By combining Wedekind's α-phenyl-benzyl-allyl-methyl-ammonium iodide with silver d-camphor sulphonate, Pope and Peachey obtained iodides having rotations [α]D of ±66.8 in chloroform solution.
The various models which have been used to represent the configuration of quinquevalent nitrogen compounds can now be replaced by the simple conception that the ammonium ion is quite analogous to the methane molecule, and the asymmetric nitrogen atom which it contains is dextro- or laevo-rotatory according to the arrangement of the groups
abc when viewed from the fourth angle d:
Ammonia, Amines, Ammonium Salts, and those of Quaternary Bases
|Ammonia and other tricovalent compounds may be represented as:|
in which the scroll divides the electrons which are contributed by each atom.
Nitrogen having completed its octet is now saturated. It still possesses, however, a "lone pair," and, by virtue of this, can attach another atom which lacks an electron, such as the hydrogen ion H+. This carries with it the positive charge, since the whole group, the ammonium ion, has one electron less than the sum of all those required to make all the atoms electrically neutral. Precisely the same reasoning applies to salts of substituted ammonium bases, since R4 in R4X must lose an electron to the halogen X before it can become attached to the nitrogen.
According to the rule of Langmuir, if p=number of duplets or bonds, n=number. of octet-forming elements, and He - the sum of electrons on all the atoms, then
and, and we have for ammonium chloride,
Thus there are no duplets uniting the main elements N and CI, which are only united by an electro valency.
It will be found that most, if not all, compounds containing the supposed quinquevalent nitrogen can be explained in this way. Thus a compound of the tetramethyl ammonium ion with triphenyl methyl is found to be an electrolyte, and may be regarded as a salt:
N(CH3)4+ . . . C(C6H5)3-
When ammonia combines with water which is only slightly dissociated, ammonium hydroxide, also slightly dissociated, is produced, whereas when it combines with hydrochloric acid which is highly dissociated, the highly dissociated ammonium chloride is produced. In both cases the NH3 acts as an "acceptor of hydrogen ions".
The tautomeric change of hydroxylamine into the tautomeric oxy-ammonia takes place, as in the older theory, by the migration of a hydrogen atom. The nitrogen, however, does not become quinquevalent, but quadrivalent, with one "mixed bond":
The compound on the left can form salts of hydroxylamine, while that on the right can only form "oxonium" salts.
In the substituted hydroxylamines and the amine oxides these two structures are fixed, and their isomerism then becomes evident by differences in properties. Isomerism is also shown by the compounds
[R3NOR+][OR1]- and [R3NOR1+][OR-].
A progressive weakening in the tendency to add H+ or CH3+, etc., and to give "-onium" compounds is shown by the series NH3, NH2OH, NCl3. It is due to the opposing pull on the electrons by the increasingly negative substituent, which thus leaves the nitrogen atom more positive and, therefore, with a less tendency to combine with positive groups. In NH3 the nitrogen atom exerts a stronger pull on the common electrons than does a hydrogen atom, as is shown by the fact that in NaNH2 the nitrogen has gained a negative electron with formation of a hydrogen ion, which then gains the electron again from the sodium:
NH2- . . . H++Na = NH2- . . . Na++½H2.
The potentially negative nature of the nitrogen atom thus revealed accounts for the ease with which NH4+, ammonium ion, is formed. In NH2OH the oxygen exerts an opposing pull on the common electrons, the nitrogen is less potentially negative, and consequently hydroxylamine salts are formed to a smaller extent, or are more hydrolysed, than ammonium salts.
In chloramine, NH2Cl, the strongly electro-negative chlorine completely destroys the tendency of the nitrogen to attach itself to positive ions.
Hydrides of Nitrogen
|The list of compounds or ions containing nitrogen and hydrogen alone is as follows:- |
NH•4 - Ammonium
NH3 - Ammonia
N2H•5 - Hydrazonium
N2H4 - Hydrazine
(N2H2) - Di-imide
N4H4 - Ammonium azide
N5H5 - Hydrazine azide
N3H - Azoimide
which can all be regarded as built up of the amino -NH2 and azide -N3 radicals in various combinations with hydrogen, hydrogen-ion H•, and one another.
One of the hydrogen atoms in the hydrides ammonia and possibly hydrazine, is of a sufficiently acidic character to be replaced by an alkali metal; but in spite of the electro-negative position of nitrogen in the periodic table, the enhancement of electro-negative character conferred by the azide ring is required in order that a hydrogen may be ionised in solution.
The reducing power or ease of oxidation reaches its maximum in the case of hydrazine, and is less in the case of the more highly hydrogen- ated ammonia, which resists atmospheric oxidation in the cold, as also the effect of mild oxidising agents in solution. The greater stability of azoimide towards oxidising agents is partly due to the stability of the cyclic structure, and is also intelligible on the grounds that it is already the most highly oxidised compound of nitrogen and hydrogen, as is experimentally proved by the products of its reduction with sodium amalgam, as also by the method of its preparation. Generally, the order of hydrogenation as shown above agrees with such reactions as the oxidation of ammonia to hydrazine by hypochlorites, and of hydrazine to azoimide by nitrous acid.
The three primary hydrides, NH3, N2H4, and N3H. all have a strong affinity for water, with which they mix in all proportions, and all three solutions probably contain hydrates.
Pure liquid ammonia and hydrazine have a strong resemblance to water in some physical properties, as well as in their properties as solvents. Hydrogen trinitride resembles hydrogen chloride in its salts, while in its reduction by nascent hydrogen, e.g. when it acts on metals, it resembles nitric acid.
Nitrogen and Sulphur
|Nitrogen combines with sulphur indirectly, usually by coupled reactions, giving a solid compound, S4N4, a liquid, S5N3, and possibly other compounds. These are probably nitrides of sulphur; the nitrogen is the electro-negative part, as is shown by the manner of formation and the reactions. They can be regarded as derivatives of ammonia, from which they are formed by some remarkable reactions.|