# What is “activated red phosphorus”? What made it so long to be discovered?

[WP = white phosphorus ; RP = red phosphorus ; ARP = activated red phosphorus]

Tom from Extraction & Ire did a video two years ago(2018) on converting WP to RP by keeping the former in sunlight for several days. He chose chloroform solvent to dissolve WP in a glass vial. The solution started to go deep orange to red on slight heating and then keeping in sunlight for 5 days. After 5 days, he tested the solution with sodium hydroxide solution to confirm that it is RP (RP doesn't react with $$\ce{NaOH}$$, WP does). On dumping it to the $$\ce{NaOH}$$ solution, it immediately started reacting (phosphine was evolving which had a dreadful smell). After 15 minutes on adding, the orange-red color vanished, that means all of the phosphorus has reacted. Some white crystals formed in the solution which is presumed to be sodium formate ($$\ce{HCOONa}$$) due to the chloroform (isn't it soluble in water?).

$$\ce{P4_{(?)} + 3NaOH + 3H2O -> PH3 ^ + 3NaH2PO2}$$

$$\ce{2NaH2PO2 -> PH3 ^ + Na2HPO4}$$

$$\ce{CHCl3 + 4NaOH -> HCOONa (\downarrow ?) + 3NaCl + 2H2O}$$

So, the conversion failed. He apparently named the species "ARP". He made the following conclusion:

$$\begin{array}{c|c|c|c} \mathbf{~} & \mathbf{WP} & \mathbf{RP} & \mathbf{ARP} \\\hline \text{color} & \text{white} & \text{red} & \text{red}\\ \text{soluble in CHCl3} & \text{Yes} & \text{No} & \text{No}\\ \text{Reactive towards NaOH} & \text{Yes} & \text{No} & \text{Yes}\end{array}$$

So, apparently WP converted to an unknown species(allotrope?) which he assumed to be ARP. He doesn't know its composition nor did he find convincing literature. Google didn't give any relevant hits except two papers. First one says it is polymerized organophosphorus chains and the second one says it is series of polyphosphides*.

Questions:

1. What is the actual composition of ARP?
2. Is it an allotrope? If so, other allotropes were discovered centuries ago, why was it unknown until recently? What made it so long to be discovered?

References

1. Sukhov, Boris & Malysheva, S. & Kuimov, Vladimir & Smetannikov, Yu & Tarasova, N. & Lupanov, A. & Gusarova, N. & Trofimov, BA. (2004). Reaction of Activated Red Phosphorus with Allyl Bromide under Phase-Transfer Catalysis. Russian Journal of General Chemistry - RUSS J GEN CHEM. 74. 1128-1129. 10.1023/B:RUGC.0000045878.42411.31.
2. Nucleophilic Activation of Red Phosphorus for Controlled Synthesis of Polyphosphides, Minyoung Jo, Alina Dragulescu-Andrasi, L. Zane Miller, Chongin Pak, and Michael Shatruk,Inorganic Chemistry 2020 59 (8), 5483-5489 DOI: 10.1021/acs.inorgchem.0c00108

The 2020 paper says "RP is readily activated by refluxing it with different potassium alkoxide to give soluble polyphosphide ions". Now, polyphosphides is known and has been studied a lot (first paper I could find dates way back in 1967). So, what does "readily activation of red phosphorus" mean in this context? Could it mean ARP?

• Another fancier allotrope of P is black phosphorous. Since nitrogen and P are of the same group, scientists have been able to make black nitrogen as well. There is a violet allotrope of P as well. – M. Farooq Sep 28 '20 at 6:18
• @M.Farooq Yes indeed, all those allotropes were discovered way back (violet - 1865, black -1914). Also, this question discusses about allotropes of grp 15 elements: chemistry.stackexchange.com/questions/28124/… – Nilay Ghosh Sep 28 '20 at 6:51
• – Nilay Ghosh Sep 28 '20 at 7:06
• Well if ARP is "a series of polyphosphide ions and polymerized organophosphorus chains" it isn't an allotrope of phosphorous – Ian Bush Sep 28 '20 at 7:49
• I can't answer that. But I will comment that the complex allotropy of phosphorous always amazes me, particular in the bizarre set of experimental conditions required to make this subtly different allotrope from that one. I mean recrystallise from molten lead - why did Hittorf think of that? So maybe these guys were simply the first to "wake up this morning, got the dissolve it in chloroform and irradiate for 5 days blues" As for the composition, well from what you have presented that looks like an active research question. – Ian Bush Sep 28 '20 at 9:29