Question

If the oxide \(P_{4} O_{6}\) is dissolved in an aqueous solution of sodium carbonate, compound A of formula \(\mathrm{Na}_{2} \mathrm{HPO}_{3}\) may be crystallized from solution. The IR spectrum of \(\mathbf{A}\) contains a band at \(2300 \mathrm{cm}^{-1}\). The corresponding band in the IR spectrum of \(\mathbf{B}\) (obtained by an analogous method from \(\mathrm{P}_{4} \mathrm{O}_{6}\) and \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) dissolved in \(\mathrm{D}_{2} \mathrm{O}\) ) is at \(1630 \mathrm{cm}^{-1}\) On recrystallization of A from \(\mathrm{D}_{2} \mathrm{O}\), however, its IR spectrum is not affected. Discuss the interpretation of these observations.

Short answer

The unchanged IR spectrum from \( D_2O \) recrystallization indicates stable P-H bonds in compound \( A \).

Step-by-step solution

Analyzing the Formation of Compound A

The oxide \( P_{4}O_{6} \), when dissolved in an aqueous solution of sodium carbonate (\( Na_2CO_3 \)), forms a compound \( A \) with the formula \( Na_2HPO_3 \). This indicates that the \( P_{4}O_{6} \) reacts with the \( Na_2CO_3 \) to form a phosphite species \( (HPO_3^{2-}) \), suggesting the transformation of the oxide to a phosphite salt.

Interpreting the IR Spectrum of Compound A

The IR spectrum of \( A \) shows a band at \( 2300 \mathrm{cm}^{-1} \), typical for a P-H stretching vibration. This corroborates the presence of a hydrogen in the phosphite ion \( (HPO_3^{2-}) \), as this frequency is characteristic of P-H bond vibrations.

Analyzing Compound B in Deuterium Oxide

When \( P_4O_6 \) is reacted with \( Na_2CO_3 \) in deuterium oxide (\( D_2O \)) to form compound \( B \), the IR spectrum shows a band at \( 1630 \mathrm{cm}^{-1} \). This band shift is due to the replacement of a hydrogen atom with deuterium, as the P-D bond has a lower stretching frequency than the P-H bond.

Understanding the Recrystallization from Deuterium Oxide

Recrystallizing \( A \) from \( D_2O \) does not change its IR spectrum. This suggests that during recrystallization, there is no exchange of the hydrogen in \( A \) with deuterium from \( D_2O \), indicating the P-H bond in \( A \) is stable and not easily exchanged under the conditions of recrystallization.

Key concepts

Sodium Carbonate Reaction

The reaction involving sodium carbonate ( _2CO_3) and the oxide _4O_6) results in the formation of a new compound with the formula _2HPO_3). This transformation highlights a typical interaction between an oxide and a carbonate. When _4O_6) dissolves in _2CO_3), it leads to a chemical reaction producing a phosphite species, specifically hpo_3^{2-}). This process is a noteworthy example of how different chemical species interact in aqueous environments.

• The sodium ions ( ^+) from the sodium carbonate remain as spectator ions in the reaction.
• The release and formation of the phosphite ion hpo_3^{2-}) signify a conversion process.
• This conversion is crucial as it forms the basis for recognizing a phosphite salt in this experimental setup.

Understanding such reactions helps comprehend how fundamental chemical processes govern changes in molecular structures.

Phosphite Ion

The phosphite ion hpo_3^{2-}) is a crucial product formed during the reaction between _2CO_3) and _4O_6). This ion is part of the class known as phosphorus oxyanions, involving phosphorus atoms covalently bonded with oxygen. Typically, these ions are recognized for:

• Their ability to form salts, such as the sodium phosphite seen here ( _2HPO_3)).
• Their structural configuration, in which hydrogen atoms are tightly bonded within the ion.

In this context, the phosphite ion plays a key role in understanding the reaction dynamics. It demonstrates how hydrogen, though less common in such ions, maintains significant involvement in the ion's chemical behaviors.

IR Spectrum Interpretation

Infrared (IR) spectroscopy provides valuable insight into molecular structures by identifying specific bond vibrations. For compound A ( _2HPO_3)), the IR spectrum reveals a notable band at 2300 cm^{-1}. This frequency is characteristic of the P-H stretching vibration, confirming the presence of a phosphorus-hydrogen bond. Conversely, when compound B is formed using deuterium oxide ( _2O)), the IR spectrum shifts to 1630 cm^{-1}. This change in frequency results from replacing hydrogen with deuterium, causing the P-D bond to exhibit a lower stretching frequency.

• The original IR spectrum's key band proves the existence of the P-H bond in the phosphite ion.
• A change from H to D leads to discernible spectral shifts, invaluable in identifying isotopic substitutions.

These IR observations underscore the stability of the P-H bond in various conditions, such as recrystallization, and serve as a potent tool in elucidating chemical compositions.