LETTER OF PROFESSOR STANISLAO CANNIZZARO

TO
PROFESSOR S. DE LUCA:
SKETCH OF A COURSE OF CHEMICAL PHILOSOPHY

Given in the Royal University of Genoa

Il Nuovo Cimento, vol. vii. (1858), pp. 321-366

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Direct Combination
Of the Monoatomic Metallic Radicals
with the Halogens.
Of the Biatomic Metallic Radicals
with the Halogens.
* H2 + Cl2 = 2 HCl Hg + Cl2 = HgCl2
1 molecule
of
hydrogen.
  1 molecule
of
chlorine
  2 molecules
of hydro-
chloric acid.
1 molecule
of
mercury.
  1 molecule
of
chlorine
  1 molecule
of bichloride
of mercury.
K2 + Cl2 = 2 KCl Zn + Cl2 = ZnCl2
1 molecule
of
potassium.
  1 molecule
of
chlorine
  2 molecules
of chloride
of potassium.
1 molecule
of
zinc.
  1 molecule
of
chlorine
  1 molecule
of chloride
of zinc.
**(CH3)2 + Cl2 = 2 CH3Cl C2H4 + Cl2 = C2H4Cl2
1 molecule
of
methyl.
  1 molecule
of
chlorine
  2 molecules
of chloride of
methyl.
1 molecule
of
ethylene.
  1 molecule
of
chlorine
  1 molecule
of chloride of
ethylene.
+ Cl2 = 2 + Cl2 =
Apparent direct combination, in reality molecular double decomposition, in virtue of which two molecules of different kinds give two of the same kind. True direct combination or union of two different entire molecules into a single molecule.

* The direct combination of hydrogen and chlorine is expressed by some as H + Cl = HCl ; in the equations used, by me I always employ molecules.
** It appears that in practice this direct combination succeeds with difficulty, the chlorine having an action on the hydrogen of the radical ; it has been indicated merely for comparison with that of ethylene.

From what precedes it may be observed that a complete molecule of chlorine, and thus of any halogen, always reacts with a complete molecule of a metallic radical ; if the latter is monatomic it makes two molecules, if it is biatomic it forms only one.

Substitution in Chlorides, Bromides, and Iodides
Of one Monoatomic Metallic Radical for another. Of one Biatomic Metallic Radical for a Monoatomic.
K2 + HCl
HCl
= H2 + KCl
KCl
Zn + HCl
HCl
= H2 +
1 molecule
of
potassium.
  2 molecules of
hydrochloric
acid.
  1 molecule
of
hydrogen.
  2 molecules of
chloride of
potassium.
1 molecule
of zinc.
  2 molecules of
hydrochloric
acid.
  1 molecule
of
hydrogen.
  1 molecule of
chloride of
zinc.
H2 + AgCl
AgCl
= Ag2 + HCl
HCl
Zn + AgCl
AgCl
= Ag2 +
1 molecule
of
hydrogen.
  2 molecules of
chloride of
silver.
  1 molecule
of
silver.
  2 molecules of
hydrochloric
acid.
1 molecule
of zinc.
  2 molecules of
chloride of
silver.
  1 molecule
of
silver.
  1 molecule of
chloride of
zinc.
Ag2 + HI
HI
= H2 + AgI
AgI
Hg + HI
HI
= H2 +
1 molecule
of
silver.
  2 molecules of
hydriodic
acid.
  1 molecule
of
hydrogen.
  2 molecules of
iodide of
silver.
1 molecule
of
mercury.
  2 molecules of
hydriodic
acid.
  1 molecule
of
hydrogen.
  1 molecule of
biniodide of
mercury.
+ = + + = +
Substitution without change in the number of molecules. Substitution with change in the number of molecules; 3 becomes 2.

From what is written in this table it is seen that two molecules of hydrochloric acid or of another analogous monochloride always react with a single molecule of metallic radical; if this is monatomic, they change into two molecules of monochloride, if it is biatomic into a single molecule of bichloride. The cause of the last difference consists in this : that the molecule of the monatomic radical is divisible into two ; that of the biatomic radical, not being capable of division, collects into a single molecule the residues of two molecules of monochloride or monoiodide.

The biatomic radicals behave similarly to the acids containing 1 atom of monatomic metallic radicals (H, Ag, K); collecting into a single molecule the residues of two molecules of acids or of salts, as may be seen in the following comparative table.

Substitution in the Acids HX, and in General in the Salts
Of a Monoatomic Metallic Radical, for another, Of a Biatomic Metallic Radical, for a Monoatomic,
K2 + HAzO3
HAzO3
= H2 + KAzO3
KAzO3
Zn + HAzO3
HAzO3
= H2 +
1 molecule
of
potassium.
  2 molecules of
hydrated
nitric acid.
  1 molecule
of
hydrogen.
  2 molecules of
nitrate of
potassium.
1 molecule
of zinc.
  2 molecules of
hydrated
nitric acid.
  1 molecule
of
hydrogen.
  1 molecule of
nitrate of
zinc.
Na2 + HC2H3O2
HC2H3O2
= H2 + NaC2H3O2
NaC2H3O2
Zn + HC2H3O2
HC2H3O2
= H2 +
1 molecule
of
sodium.
  2 molecules of
hydrated
acetic acid.
  1 molecule
of
hydrogen.
  2 molecules of
acetate of
sodium.
1 molecule
of zinc.
  2 molecules of
hydrated
acetic acid.
  1 molecule
of
hydrogen.
  1 molecule of
acetate of
zinc.
+ = + + = +

These examples are sufficient to show that the compounds containing a monatomic metallic radical behave like the monochlorides : two molecules of these react with a single molecule of metallic radical, changing into two molecules if the latter is monatomic, into a single molecule if it is biatomic. We can prove more easily that the biatomic metallic radicals bind in a single molecule the residues X of two molecules RX, by comparing the double decompositions or mutual substitutions of the chlorides of the monatomic and biatomic radicals with the compound RX.

I write in the following table some examples of these double decompositions.

Mutual Substitution of the Compounds Contains a Monoatomic Radical,
With the Chlorides of the Monoatomic Metallic Radical, With the Chlorides of the Biatomic Metallic Radical,
KCl + HAzO3
HAzO3
= HCl + KAzO3 + HAzO3
HAzO3
= HCl
HCl
+
1 molecule of
chloride of
potassium.
  1 molecule of
hydrated
nitric acid.
  1 molecule of
hydrochloric
acid.
  1 molecule of
nitrate of
potassium.
1 molecule of
bichloride of
mercury.
  2 molecules of
hydrated
nitric acid.
  2 molecule of
hydrochloric
acid.
  1 molecule of
mercuric
nitrate.
KCl + AgC2H3O2 = AgCl + KC2H3O2 + AgC2H3O2
AgC2H3O2
= AgCl
AgCl
+
1 molecule of
chloride of
potassium.
  1 molecule of
acetate of
silver.
  1 molecule of
chloride of
silver.
  1 molecules of
acetate of
potassium.
1 molecule of
bichloride of
mercury.
  2 molecules of
acetate of
silver.
  2 molecules of
chloride of
silver.
  1 molecule of
mercuric
acetate.
C2H5, Cl + AgC2H3O2 = AgCl + C2H5, C2H3O2 + AgC2H3O2
AgC2H3O2
= AgCl
AgCl
+
1 molecule of
chloride of
ethyl.
  1 molecule of
acetate of
silver.
  1 molecule of
chloride of
silver.
  1 molecules of
acetate of
ethyl.
1 molecule of
chloride of
ethylene.
  2 molecules of
acetate of
silver.
  2 molecules of
chloride of
silver.
  1 molecule of
acetate of
ethylene.
+ = + + = +

All the reactions indicated in this table may be, summed up as follows : -- Whatever is combined with one atom of hydrogen or any other equivalent radical =(X) replaces one atom of chlorine, and conversely is replaceable by the latter ; if an indivisible radical in the double decompositions is found combined in a single molecule with two atoms of chlorine, it will, if the chlorine is exchanged for X, remain combined in a single molecule with 2X.

That ethylene is combined with two atoms of chlorine in chloride of ethylene, and that the acetate of ethylene contains in one molecule twice C2H3O2, is shown by the comparison of the gaseous densities of these substances. From the vapour density and from the specific heats, it is further demonstrated that the molecule of corrosive sublimate, like that of chloride of ethylene, contains two atoms of chlorine. Hence the mercuric salts are constituted in a similar manner to those of ethylene, whilst the salts of potassium, sodium, and silver are formed like those of ethyl.

Having proved, then, as I think I have already sufficiently indicated, that the lower or only chlorides of iron, manganese, zinc, magnesium, calcium, barium, etc., are constituted like corrosive sublimate, that is, have the formula MCl2 , there can remain no further doubt that the salts which are obtained by means of these chlorides and of the monobasic acids, or of their salts, are all similar to those of ethylene, propylene, etc. These important conclusions may be summed up as follows: --

1°. Amongst the salts of monobasic acids only those of hydrogen, potassium, sodium, lithium, silver, together with mercurous and cuprous salts, are similar to those of methyl and ethyl, that is, to compounds of the alcohols containing a monatomic radical ; all the other salts, of the so-called protoxides, are similar to those of ethylene and propylene, that is, to the compound ethers of the alcohols with biatomic radicals.

2°. A single molecule of the first is not sufficient to form the anhydrous acid and the metallic oxide ; two molecules instead are required; but a single molecule of the second contains the components of the molecule of the anhydrous acid and of that of the protoxide. This becomes clear by bringing the following equations into comparison :

AgC2H3O2
AgC2H3O2
= + C4H6O3 = HgO + C4H6O3
2 molecules of
acetate of
silver.
  1 molecule of
oxide of
silver.
  1 molecule
of
anhydrous
acetic
acid.
1 molecule of
mercuric
acetate.
  1 molecule of
oxide of
mercury.
  1 molecule
of
anhydrous
acetic
acid.
C2H5, C2H3O2
C2H5, C2H3O2
= + C4H6O3 = C2H4O + C4H6O3
2 molecule of
acetate of
ethyl.
  1 molecule of
oxide of
ethyl.
  1 molecule
of
anhydrous
acetic
acid.
1 molecule of
acetate of
ethylene.
  1 molecule of
oxide of
ethylene.
  1 molecule
of
anhydrous
acetic
acid.
= + (2 X - O) = + (2 X - O)

The mercuric salts and the salts of zinc, etc., being similar to those of ethylene, it is probable that salts of this type exist containing the residues of two different monobasic acids. I indicate by what reactions they might be generated: --

+ AgC2H3O2
AgC2H3O2
= AgCl
AgCl
+
1 molecule of
bichloride of
mercury.
  2 molecules of
acetate of
silver.
  2 molecules of
chloride of
silver.
  1 molecule of
acetate of
mercury.
+ AgC2H3O2
AgC7H5O2
= AgCl
AgCl
+
1 molecule of
bichloride of
mercury.
  1 molecule of
acetate and 1 of
benzoate of silver.
  2 molecules of
chloride of
silver.
  1 molecule of
benzacetate of
mercury.
+ AgC2H3O2
AgC7H5O2
= AgCl
AgCl
+
1 molecule of
chloride of
ethylene.
  1 molecule of
acetate and 1 of
benzoate of silver.
  2 molecules of
chloride of
silver.
  1 molecule of
benzacetate of
ethylene.

Just as acetates are produced from anhydrous acetic acid and the oxides of biatomic metallic radicals, so from anhydrous benzacetic acid the benzacetates will be formed, as I indicate in the following equation : --

C4H6O3 + = C4H6O4 =
C9H8O3 + = C9H8O4 =

Having already proved that zinc is a biatomic radical, and that in consequence its atomic weight should be doubled, I stop to examine the reactions and the mode of formation of zinc ethyl, zinc methyl, etc. I show you by means of equations the method by which I interpret these reactions.

The vapour densities demonstrate the accuracy of the following formulae corresponding to equal volumes :-- C2H5Cl (chloride of ethyl) C2H5, H (hydride of ethyl) C2H5, C2H5 (free ethyl) C2H5, CH3 (methyl ethyl), Zn(2H5)2 = (zinc ethyl).

C2H5Cl + H2 = C2H5, H + HCl
C2H5Cl
C2H5Cl
+ Zn = (C2H5)2 +
C2H5Cl
C2H5Cl
+ = 2 (C2H5)2 +
C2H5Cl
C2H5Cl
+ 2 Zn = (C2H5)2 +
C2H5Cl
CH3Cl
+ 2 Zn = +

No one has yet demonstrated, as far as I know, the existence of the type of compound indicated in the last equation. But it being proved from the density of zinc ethyl vapour, and from its specific heat, that the complete molecule of zinc ethyl contains a single atom of zinc combined with two ethyl radicals, that is, with the molecule of the free radical, no one can deny that there will be prepared compounds containing a single atom of zinc combined with two different monatomic radicals. It may also be predicted that ethylene and propylene will form compounds in whose molecules an atom of zinc is combined with the biatomic radical.

I will give you later an account of some of my experiments directed to show the existence of the compounds just mentioned.

After having spoken of the mode of behaviour of the compounds containing monatomic or biatomic radicals with regard to monobasic acids, I examine the mode of behaviour with regard to those compounds which contain in each molecule two atoms of hydrogen, or, as they are called, the bibasic acids, to which I have given the general formula H2Y.

To predict the reactions, it is sufficient to bear in mind what follows : --

1°. The two atoms of hydrogen are united in a single molecule by the forces of all the other components which together we call Y, hence what is equivalent to H2 can enter into a single molecule. with Y.

2°. What is combined with H2 is equivalent to two atoms of chlorine Cl2 ; hence in double decomposition H2Y will act either on a single molecule of a bichloride (=) or on two molecules of a monochloride ; what is combined with two atoms of chlorine, whether in one or in two molecules, will combine with Y; and HI combining with Cl2 will always form two molecules of hydrochloric acid.

The examples of double decomposition which follow clearly show what I have just indicated.

Double Decomposition of Hydrated Sulphuric Acid, H2SO4
With the Monochlorides With the Bichlorides
NaCl
NaCl
+ H2SO4 = HCl
HCl
+ Na2SO4 + H2SO4 = HCl
HCl
+ HgSO4
NaCl
NaCl
+ Ag2SO4 = AgCl
AgCl
+ Na2SO4 + Ag2SO4 = AgCl
AgCl
+ HgSO4
C2H5Cl
C2H5Cl
+ Ag2SO4 = AgCl
AgCl
+ (C2H5)2SO4 + Ag2SO4 = AgCl
AgCl
+ C2H4SO4

In connection with this point I compare the formulae of the oxy-salts proposed by me with those of Berzelius and of Gerhardt, and discuss the causes of the differences and of the coincidences, which may be summed up as follows :--

1°. All the formulae given by Berzelius to the oxysalts of the biatomic metallic radicals are the same as those proposed by me, whether the acid is monobasic or bibasic ; all these oxy-salts contain in each molecule the elements of a complete molecule of oxide and of a complete molecule of anhydrous acid.

2°. There correspond also to the formulae proposed by me all those of Berzelius for sulphates and analogous salts, if we introduce the modification by Regnault, i.e., if we consider the quantity of metal contained in the molecules of potassic, argentic, mercurous, and cuprous sulphates equal to 2 atoms and those on the other hand of metal contained in the molecules of mercuric, cupric, plumbic, zincic, calcic, baric, etc., sulphates, equal to a single atom.

3°. The formulae proposed by me for the oxy-salts of potassium, sodium, silver, hydrogen, methyl, and all the other analogous monatomic radicals with a monobasic acid, are equal to half the formulae proposed by Berzelius and modified by Regnault, ie., each molecule of them contains the components of half a molecule of anhydrous acid and half a molecule of metallic oxide.

4°. The formulae of Gerhardt coincide with those proposed by me only for the salts of potassium, sodium, silver, hydrogen, methyl, and all the other monatomic radicals, but not for those of zinc, lead, calcium, barium, and the other metallic protoxides; Gerhardt having wished to consider all the metals analogous to hydrogen, which I have shown to be erroneous.

In the succeeding lectures I speak of the oxides with monatomic and biatomic radicals, afterwards I treat of the other classes of polyatomic radicals, examining comparatively the chlorides and the oxides ; lastly, I discuss the constitution of acids and of salts, returning with new proofs to demonstrate what I have just indicated.

But of all this I will give you an abstract in another letter.

GENOA, 12th March 1858.