Valency of Carbon
|The study of the compounds of carbon played a very important part in the development of the doctrine of valency. Consequent upon his researches on fulminate of mercury, Kekule added to the types of Gerhardt another type, that of marsh gas, which at first he wrote C2H4 (C = 6), but afterwards recognised that in this compound 1 atom of carbon (C = 12) combines with four atoms of hydrogen. Thus, in extending the doctrine of valency which had been brought forward by Frankland, he declared the carbon atom to be quadrivalent. " If we look at the simplest compounds of this element, CH4, CH3Cl, CCl4, CHCl3, COCl2, CO2, CS2, and CHN, we are struck by the fact that the quantity of carbon which is considered by chemists as the smallest amount capable of existence - the atom - always binds four atoms of a monatomic or two of a diatomic element, so that the sum of the chemical units of the elements combined with one atom of carbon is always equal to four. We are thus led to the opinion that carbon is tetratomic." Kolbe and Frankland arrived at similar ideas, as well as Couper, to whom we owe the expression of graphic formulae by bonds. |
Kekule was an advocate of constant valency. According to him the "atomicity of the elements is a fundamental property of the atoms, quite as unalterable as their atomic weights." Therefore Kekule could not admit the valency of carbon ever to be less than four.
On the other hand, the valency of many elements is known to be variable, and to rise to a maximum corresponding to the group of the periodic system to which the element in question belongs. Whether or not carbon is ever less than quadrivalent may therefore be discussed.
In carbon monoxide the carbon atom is usually considered to be bivalent, thus: C=O; and Kekule and Kolbe reconciled this belief with the theory of constant valency, by assuming that the two remaining carbon valencies satisfied each other. Since it was discovered, however, that oxygen may be quadrivalent, it has been assumed that the four carbon valencies are satisfied thus: , though it does not appear how these four valencies, which are supposed to be directed through the angular points of a tetrahedron, can be so far " strained " as to meet in a single atom of oxygen.
Friend modifies this view by his theory of latent valencies, and assumes that the four carbon valencies are satisfied with two free and two latent oxygen valencies.
The validity of these theories is contingent upon our knowledge of the nature of valency itself. Meanwhile it is pertinent to ask whether anv real chemical difference is indicated between the formulae C=O and .
Fulminic acid may be regarded as the oxime of carbon monoxide, and so may contain bivalent carbon, thus: C=NOH. Similar iso- nitriles and isocyanides, C=NR, may contain bivalent carbon; also hydrocyanic acid itself if, as is possible, it has the constitution H-N=C.
If, however, nitrogen is assumed to be quinquevalent, carbon may still be quadrivalent in these compounds.
In the year 1900 Gomberg discovered "triphenylmethyl," to which he attributed the formula C(C6H5)3, suggesting that carbon is tervalent in this compound. Tschitschibabin regards the compound as hexaphenylethane (C6H5)3C-C(C6H5)3, and this view is supported by the researches of Piccard and of Schlenk and his co-workers, from which it appears fairly certain that in the solid condition the compound corresponds to the double formula, whilst in solution in organic liquids it dissociates into triphenylmethyl.
A number of carbon compounds exist, in each of which an atom of carbon is attached to fewer than four other atoms or groups - one of which, however, is another carbon atom. These compounds are all unsaturated; that is to say, they easily combine with other atoms or groups to produce compounds in wrhich each of their carbon atoms has four separate points of attachment. It is not usual to regard the valency of carbon in these compounds as less than four, double or triple bonds being employed to satisfy the valencies.
The simplest examples of such compounds are ethylene, C2H4, and acetylene, C2H2, which are usually represented thus:
Now it is not justifiable to introduce double and triple bonds in order to satisfy a preconceived opinion that carbon must needs be quadrivalent. Hinrichsen, indeed, objects to the use of double bonds; and if valency is regarded simply as the measure of the number of other atoms or groups with which an atom is combined in a particular compound his objection is valid, and carbon becomes tervalent in ethylene and bivalent in acetylene; and it might even be urged that both carbon and oxygen are univalent in carbon monoxide.
To discredit this view, therefore, it is necessary to show that double and triple bonds have more than a pictorial significance, and this can easily be done.
Thus the potential valencies of the two carbon atoms which remain inoperative in ethylene, for example, are not independent of each other; one cannot be saturated without the other, so that no such compound as CH2-CH2Cl exists. It may be that the saturation of both carbon atoms simultaneously is necessary for molecular stability, but if so, the fact is better expressed by the double bond than by representing each carbon atom as separately tervalent. Moreover, when the elements of a halogen hydracid are removed from an alkyl halide to produce an unsaturated hydrocarbon, it is always two adjacent carbon atoms which yield these elements. This would not appear necessary on the assumption that any carbon atom in a carbon chain might become tervalent, but is necessary if loss of halogen hydracid involves double linkage between two adjacent atoms. Propylene, for example, is formed by the elimination of HI from either normal or iso-propyl iodide, CH3CH2CH2I or CH3-CHICH3; consequently propylene must contain a methyl (CH3) group, and cannot be CH2-CH2-CH2, but is necessarily CH3CHCH2. Thus it is two adjacent carbon atoms that remain unsaturated in propylene, and this fact may therefore be expressed by the double bond, and the formula for propylene be written CH3-CH=CH2.
That the mode of linkage between two adjacent carbon atoms which are unsaturated is really different from that between carbon atoms which are saturated is shown by the fact that when oxidation which involves rupture takes place, this rupture always occurs at the double bond. The relative weakness of the double bond is accounted for by Baeyer's tension or strain theory, to which reference will shortly be made.
If carbon is admitted to be quadrivalent in unsaturated compounds such as the above, then, with the few exceptions already considered, carbon is always quadrivalent; for it does not appear that the valency of this element ever exceeds four.
Nevertheless Thiele has introduced a theory of partial valencies to account for the reactivity of atoms joined by double bonds, and especially of such carbon atoms in chain and ring compounds.
An example of the reduction of a chain compound will illustrate Thiele's theory.
yields on partial reduction:
and the problem is to account for the remaining double bond taking the central position in the chain.
According to Thiele the carbon atoms possess partial valencies over and above their quadrivalency, which are represented thus:
The partial valencies of adjacent carbon atoms are, however, conjugated, and the above system may be represented thus:
So the way is prepared for the appearance on reduction of a double bond between the central carbon atoms, thus:
Thiele has also applied his theory to the constitution of benzene, which according to Kekule's formula -
shows a difference between the 2 and 6 positions on account of the arrangement of the double bonds, unless these are supposed to oscillate. Thiele introduces partial valencies to conjugate the carbon atoms united by single bonds, and thence derives a symmetrical formula, thus:
This, however, appears to amount to making carbon quinquevalent. Tschitschibabin has discussed the various explanations which have been given of the unsaturated character of the carbon atom in different classes of organic compounds; and concludes that the numerical value of the valency of an atom is simply expressed by the number of other atoms with which it combines. Thus, for example, carbon becomes tervalent in ethylene and bivalent in acetylene.
The question of the equality of the four carbon valencies may be briefly mentioned. It has been investigated by Popoff and Geuther, and by Henry, who proved the equality of the four carbon valencies in nitromethane by preparing this substance by four distinct processes, so arranged that a different hydrogen atom should be replaced in each process.
The qualitative aspect of valency may here be considered, so far as it applies to carbon.
It is well known that many elements display reciprocal powers of combination with oxygen or halogen on the one hand and hydrogen on the other, the sum of the valencies in the two classes of compound being equal to 8. Thus there are the following compounds:
; ; ; Cl2O7:ClH.
Having regard to these relationships, Abegg introduced the theory that every element possesses a maximum valency of 8, made up of positive and negative or normal and contra valencies, as in the following series:
In the first three elements of this series the contra valencies are latent, in the last three they correspond to the powers of combination of the elements with hydrogen. Silicon, occupying a central position in the series, has normal and contra valencies numerically equal, as shown, for example, in SiCl4 and SiH4. Nevertheless, judging by the relative stabilities of the compounds, the + valencies of silicon, as manifested towards chlorine, are stronger than the - valencies, as manifested towards hydrogen. Now as regards carbon the + and - valencies appear to be, not only numerically equal, but also equal in strength. It was this fact which lay at the basis of Dumas' demonstration of substitution, by which it was shown that chlorine could displace hydrogen from a carbon compound without disturbing the stability of the compound or fundamentally altering its nature. And not only can carbon form stable compounds in which its atoms are linked with hydrogen or chlorine or both, but also stable compounds containing oxygen, nitrogen, sulphur, and other elements, including metals joined to carbon.
More fundamental, perhaps, than the power possessed by carbon of forming stable compounds with such a variety of elements is that qualitative property of the carbon valencies which enables the atoms of this element to combine with one another in chains and rings. This property, which may perhaps be regarded as a manifestation of neutral rather than + or - valency, has been shown by Martin to be deducible from a study of the affinities of the elements of the series which contains carbon.
By collecting the thermal and other data regarding the combination of each of the elements with as many other elements as possible, and erecting perpendiculars representing affinities, as shown by heats of formation, stabilities, etc., from the loci of these other elements on the periodic diagram, Martin has produced an "affinity surface" for each element, a glance at which shows the elements with which a particular element most readily combines. Thus in the series Li-F the affinity surface of lithium rises to a maximum altitude over fluorine and the other halogens, but on traversing the series to the right the affinity peak is seen to travel to the left, till in the case of fluorine it rests over the alkali metals. The affinity surface for carbon shows a steep peak over carbon itself, and a lesser altitude over oxygen. Thus it appears from the affinity surface for carbon, and consequently from the experimental facts on which it is constructed, that this element manifests a greater affinity for itself than for any other element. In this respect carbon is quite alone; and thus is furnished a striking commentary upon the combining capacity of carbon atoms for each other, on which property more than any other the facts of organic chemistry are based.
There remains to be considered one other aspect of the valency of the carbon atom, i.e. the direction in space in which the units of valency are exercised. That they do not all lie in one plane - being directed, for example, towards the corners of a square - is made evident by the non-existence of two di-derivatives of methane, which might be formulated thus:
Kekule, in 1867, suggested that the four valencies end in the faces of a tetrahedron; Paterno, in 1869, explained the isomerism of the compounds C2H4Br2 by assuming that the carbon valencies are directed towards the corners of a tetrahedron, and in the same year J. WislicenuS spoke of "Chemistry in Space"; whilst in 1874 Le Bel and van't Hoff, the former a disciple of Pasteur, the latter of Kekule, established the stereochemical theory of the carbon atom by-employing it to explain optical isomerism.
According to van't Hoff's presentation of the theory, the carbon atom appears to be a point at the centre of a regular tetrahedron, towards the angular points of which the valencies act as four equal forces. It has been shown, however, by Auwers that this view is mechanically inadequate, for two of the forces acting from each carbon atom thus:
would be resolved along a straight line joining the carbon atoms when union took place, thus:
This, however, is not the case, for the double bond between two carbon atoms cannot be represented as acting along a single line.
Le Bel has conceived that each of four atoms approaching a carbon atom is bounded in its influence by a sphere, and that these spheres meet each other around the carbon atom just as four equal spheres may lie against each other, so that lines joining their centres form a regular tetrahedron. Thus the valencies act as before towards the angular points of a tetrahedron; but the carbon atom at its centre is not necessarily a point.
The Strain or Tension Theory of Baeyer, which has been of much value in the study of cyclic carbon compounds, is expressed as follows: " The four valencies of a carbon atom act parallel to lines joining the corners of a tetrahedron with its centre, making an angle of 109°28' with one another. The direction of the valencies can be altered, but any such alteration produces a strain whose amount is proportional to the angle through which the valencies are diverted."
The above brief account of the stereochemistry of carbon suffices to show the lines on which the theory has developed. For the application of the theory to the constitution and properties of carbon compounds, books on organic chemistry must be consulted.