Alkylphosphines

On account of the instability of the phosphines and phosphonium salts, the hydrogen valency of phosphorus is more clearly displayed in their alkyl substitution products which also, as is usual, possess a more pronounced basigenic character than the hydrogen compounds themselves. The methods of preparation of these compounds, and their properties, closely resemble those of the alkylamines.

The tertiary alkylphosphines were discovered by Thenard in 1845, and the primary and secondary by Hofmann in 1871. All may be prepared by general methods—by the action of phosphonium iodide on alkyl iodides or on alcohols, and by the action of zinc alkyls on phosphorus trichloride. The primary and secondary alkylphosphines are obtained as crystalline double salts with zinc iodide when the alkyl iodides are heated to 150° C. with phosphonium iodide in the presence of zinc oxide. The primary base is first liberated as a gas (PH₂CH₃) or as a volatile liquid (PH₂C₂H₅) on the addition of water, and is distilled off and condensed. The secondary bases P(CH₃)₂H and P(C₂H₅)₂H can be set free by the addition of potassium hydroxide to their hydriodides. The tertiary bases are prepared (as hydriodides) by heating phosphonium iodide with alcohols (CH₃OH, C₂H₅OH) at 150° to 180° C., and also by the action of zinc alkyls on phosphorus trichloride.

Alkyliodides of the quaternary phosphonium bases are made by heating (to over 150° C.) the theoretical proportions of the alcohols with phosphonium iodide, thus:—

4CH₃OH + PH₄I = P(CH₃)₄I + 4H₂O

They are well-crystallised salts, easily soluble in water, and highly dissociated even at dilutions of 32 litres to the mol. The bases, liberated by the action of silver oxide on the iodides, are also highly dissociated, and therefore behave as strong alkalies. Thus, in the case of P(CH₃)₄OH:—

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The mobility of the ion P(CH₃)₄⁺ is 42.3, that of P(C₂H₅)₄⁺ 32.7, the mobility diminishing with increasing addition of -CH₂- to about 23 in the case of P(C₅H₁₁)(C₆H₅)₃⁺ (isoamyltriphenylphosphonium ion). Alkylphosphines differ from the corresponding ammonia derivatives by the great ease with which they are oxidised even by atmospheric oxygen, either with spontaneous inflammation, as in the case of some of the less alkylated members, or with the formation of phosphine oxides, as in the case of trialkylphosphines. Thus oxygen at ordinary temperatures combines with P(C₂H₅)₃, giving P(C₂H₅)₃O, which may be distilled with steam.

Triethylphosphine Oxide may be made in quantity by heating 1 part of phosphorus with 13 parts of ethyl iodide in a sealed tube at 175°-180° C. The product is distilled, first with ethyl alcohol to remove excess ethyl iodide, and then with concentrated potash, which removes the iodine and oxidises the compound. Thus:—

P(C₂H₅)₄I + KOH = P(C₂H₅)₃O + KI + C₂H₆

Triethylphosphine also combines with sulphur with evolution of heat to give the corresponding triethylphosphine sulphide, P(C₂H₅)₃S, a very stable compound, which can also be made by oxidation, or by hydrolysis, of the curious addition compound P(C₂H₅)₃.CS₂, a red crystalline solid which is itself made by direct union of P(C₂H₅)₃ and CS₂. The oxidation is effected by silver oxide, according to the equation

P(C₂H₅)₃CS₂ +2Ag₂O = Ag₂S +2Ag + CO₂ +P(C₂H₅)₃S

The hydrolysis occurs at 100° C. and yields the oxide as well as the sulphide:—

4P(C₂H₅)₃CS₂ +2H₂O = 2P(C₂H₅)₃S + P(C₂H₅)₃O + P(C₂H₅)₃CH₃OH + 3CS₂

The structural formulae of these compounds, whether written with the usual bonds or with valency electrons, are clearly quite analogous to those of the alkyl derivatives of ammonia. Thus the presence of even one methyl group enables the phosphorus to accept a hydrogen ion from hydrogen iodide, giving a crystalline product which is not dissociated under ordinary conditions:—



In the alkylphosphine oxide (and sulphide) the lone pair of electrons on the phosphorus atom completes the octet of the oxygen (or sulphur), as in the case of amine oxides:—



Oxidation of the primary and secondary alkylphosphines gives alkyl acids, and chlorination of these with PCl₅ gives the corresponding acid chlorides. These substances are of importance in elucidating the structure of the phosphorous acids, etc. (q.v.), in one tautomeric form of which the phosphorus is not trivalent, but has the same valency as exhibited in phosphoric acid or quaternary phosphonium bases. This form is fixed in the compounds obtained as follows:—

When methylphosphine is passed into concentrated nitric acid it gives a crystalline dibasic acid (methylphosphinic acid), P(CH₃)O(OH)₂, and the action of PCl₅ on this yields P(CH₃)OCl₂. Oxidation of P(CH₃)₂H with nitric acid gives the monobasic dimethylphosphinic acid. Corresponding ethyl derivatives are also known.

V	16	64	256 litres per mol
λ	214	221	233 litres per mol
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