Oxidation state of phosphorus in phosphine, $\ce{PH3},$ is $-3.$ $\ce{P-C}$ bonds are less polarised, i.e. almost purely covalent. In this concern, what is the oxidation state of a trialkylphosphine, i.e. $\ce{PR4},$ where $\ce{R} = \ce{C_nH_{2n+2}}?$

  • $\begingroup$ Based on your title you mean PR3. Google the electronegativities of P and C. $\endgroup$ – user55119 Jun 29 at 19:32
  • $\begingroup$ And ignore the results. I almost never see oxidation states used in organic chemistry. $\endgroup$ – Oscar Lanzi Jun 29 at 22:06
  • $\begingroup$ @Oscar Lanzi: In my experience oxidation states (I prefer the term levels) are a pedagogical tool for students so they can understand why the conversion of ethane to bromoethane is an oxidation and the formation of the Grignard reagent of the latter is a reduction. I am not a proponent of assigning charges to every atom but rather to focus on reacting bond polarity. I assume Olegant is a student and not an experienced chemist, which is a group that usually doesn't require the use of oxidation levels. $\endgroup$ – user55119 Jun 30 at 0:02
  • $\begingroup$ Yes, I also using polarisation of bonds as the main parameter of their potential reactivity, when analysing different chemical reactions pathways and byproducts formed. QC using DFT are very useful tools also. I have reposted this question, which I have received from a mechanical engineer, who would like to understand oxidation pathways of phosphorus containing additives in lubricants exposed to both oxygen and steel surfaces in the tribocontacts. Phosphorus in trialkylphosphines and tetraalkylphosphonium cations is readily oxidised by oxygen. What is the oxidation path? $\endgroup$ – Olegant Jun 30 at 6:07
  • $\begingroup$ I guess the oxidation number of phosphorus may assist in answering on that question: “0” in PR3, “+1” in [(HO)PR3]+, “+2” in O=PR3, “+3” in O=P(OH)R2, “+4” in O=P(OH)2R and, finally, “+5” in O=P(OH)3. Do you have any reference for the comparative oxidation reactivity (propensity) of trialkylphosphines for different alkyl chain lengths? $\endgroup$ – Olegant Jun 30 at 6:08

The correct formula should be PR3 or PR4+. Electronegativities of P and C on the Pauling’s scale are very similar, thus the P-C bond is the covalent bond with only a very slight electron density shift towards three (or four) carbons. The oxidation state of P is, therefore, “0” in PR3 and “+1” in PR4+. The oxidation states of carbons in the CH2 groups is “-2”, and “-3” in the CH3 groups, and “+1” for H.

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