$H_3PO_3$

Definition

$H_3PO_3$, also known as phosphorous acid, is a chemical compound composed of one phosphorus atom and three hydrogen atoms bonded to three oxygen atoms. It is an important intermediate in the chemistry of acid halides, specifically in the context of 21.4 Chemistry of Acid Halides.

5 Must Know Facts For Your Next Test

1. The oxidation state of phosphorus in $H_3PO_3$ is +3, indicating that the phosphorus atom is in a lower oxidation state compared to phosphoric acid ($H_3PO_4$), where the oxidation state of phosphorus is +5.
2. Phosphorous acid can be prepared by the hydrolysis of phosphorus trichloride ($PCl_3$) or by the reduction of phosphoric acid ($H_3PO_4$).
3. Phosphorous acid is a reducing agent and can be oxidized to phosphoric acid ($H_3PO_4$) by various oxidizing agents.
4. In the context of 21.4 Chemistry of Acid Halides, $H_3PO_3$ can be used as a nucleophile to react with acid halides, forming esters or other derivatives.
5. Phosphorous acid is a weaker acid compared to phosphoric acid, with a pKa of 1.3 for the first dissociation and 6.7 for the second dissociation.

Review Questions

• Explain the role of $H_3PO_3$ in the chemistry of acid halides.
  • In the context of 21.4 Chemistry of Acid Halides, $H_3PO_3$ can act as a nucleophile and react with acid halides, such as acyl chlorides, to form esters or other derivatives. The lower oxidation state of phosphorus in $H_3PO_3$ compared to $H_3PO_4$ allows it to participate in these nucleophilic addition reactions with acid halides, which are important in the synthesis of various organic compounds.
• Describe the differences in the acid-base properties between $H_3PO_3$ and $H_3PO_4$.
  • Phosphorous acid ($H_3PO_3$) is a weaker acid compared to phosphoric acid ($H_3PO_4$). The pKa values for the first dissociation of $H_3PO_3$ and $H_3PO_4$ are 1.3 and 2.1, respectively, indicating that $H_3PO_3$ is a weaker acid. This difference in acidity is due to the lower oxidation state of phosphorus in $H_3PO_3$ (+3) compared to $H_3PO_4$ (+5), which affects the stability of the conjugate base and the ease of proton dissociation.
• Analyze the potential synthetic applications of $H_3PO_3$ in the context of 21.4 Chemistry of Acid Halides.
  • Given its nucleophilic nature and lower oxidation state, $H_3PO_3$ can be utilized in the synthesis of various organic compounds within the context of 21.4 Chemistry of Acid Halides. For example, $H_3PO_3$ can undergo nucleophilic addition reactions with acid halides, such as acyl chlorides, to form esters or other derivatives. This reactivity allows for the incorporation of the phosphorus-containing moiety into the final product, which can be useful in the synthesis of phosphorus-containing organic compounds or in the functionalization of molecules. The versatility of $H_3PO_3$ in these types of reactions highlights its importance in the chemistry of acid halides.

Related terms

Acid Halides: Acid halides, also called acyl halides, are a class of organic compounds with the general formula $R-C(=O)-X$, where $R$ is an organic group and $X$ is a halogen atom (F, Cl, Br, or I).

Phosphorus Oxoacids: Phosphorus oxoacids are a group of compounds containing phosphorus, oxygen, and hydrogen, including phosphoric acid ($H_3PO_4$), phosphorous acid ($H_3PO_3$), and hypophosphorous acid ($H_3PO_2$).

Oxidation State: The oxidation state of an atom is a measure of the degree of oxidation of an atom in a chemical compound, represented by a number that can be positive, negative, or zero.