Problem 8 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

White phosphorus on reaction with concentrated \(\mathrm{NaOH}\) solution in an inert atmosphere of \(\mathrm{CO}_{2}\) gives phosphine and compound \((X) .(X)\) on acidification with \(\mathrm{HCl}\) gives compound \((Y)\). The basicity of compound (Y) is: [Main Jan. 08, 2020 (II)] (a) 2 (b) 1 (c) 4 (d) 3

Step-by-Step Solution

Answer

The basicity of \(H_3PO_2\) is 1.

Step 1: Write the Reaction of White Phosphorus with NaOH

The chemical reaction of white phosphorus \((P_4)\) with concentrated NaOH in the presence of \(CO_2\) is given by:\[ P_4 + 3NaOH + 3H_2O \rightarrow PH_3 + 3NaH_2PO_2 \]Here, \(PH_3\) is phosphine, and \(NaH_2PO_2\) is the compound \((X)\).

Step 2: Acidification of Compound (X)

When compound \((X)\), which is \(NaH_2PO_2\), is acidified with \(HCl\), the reaction proceeds as follows:\[ NaH_2PO_2 + HCl \rightarrow H_3PO_2 + NaCl \]The resulting compound \((Y)\) is hypophosphorous acid \((H_3PO_2)\).

Step 3: Determine the Basicity of Compound (Y)

The basicity of an acid is determined by the number of replaceable hydrogen atoms that can be donated per molecule. In \(H_3PO_2\), only one hydrogen atom is replaceable by base (as a part of the molecule's structure), making its basicity equal to 1.

Key Concepts

White Phosphorus

White phosphorus is known for its unique tetrahedral structure, consisting of four phosphorus atoms forming a molecule \(P_4\). This form of phosphorus is highly reactive due to the significant strain in the bonds within the tetrahedron. It is usually handled in inert atmospheres such as \(CO_2\), to prevent unwanted reactions with oxygen which can lead to combustion.
White phosphorus commonly reacts with strong bases like \(NaOH\), especially in controlled conditions. In such reactions, it can yield phosphine \(PH_3\) and other compounds. Understanding the reactivity of white phosphorus is essential in chemical processes involving phosphorus derivatives.

Basicity

Basicity refers to an acid's ability to donate protons. It is an important concept when dealing with acids and their reactions. In the context of hypophosphorous acid \(H_3PO_2\), basicity is defined by the number of hydrogen atoms available for replacement by a base in a reaction.
In organic chemistry, basicity is specifically crucial for understanding the behavior of different acids. \(H_3PO_2\), resulting from the acidification of NaH\(_2\)PO\(_2\) with \(HCl\), is an example of an acid where not all hydrogens are replaceable. Its structure only allows for one hydrogen to be replaced by a base, giving it a basicity of 1.
This means in reactions where a base is needed, \(H_3PO_2\) can effectively donate only a single proton.

Phosphine Production

Phosphine \(PH_3\) is a colorless, flammable gas that is often associated with the smell of garlic or rotting fish. It is produced industrially through the reaction of white phosphorus with sodium hydroxide \(NaOH\) under a protective inert atmosphere like \(CO_2\). This reaction helps in controlling the creation of phosphine by preventing oxidation.
Since phosphine is volatile and reacts easily with oxygen, taking safety precautions during its production is important. It is typically utilized in the semiconductor industry and as a pesticide.

• Phosphine is an essential component in various industrial processes.
• It is a crucial intermediate for producing other phosphorus chemicals.

Understanding the methods of producing phosphine and the conditions required for its stability is vital in utilizing its chemical potential safely.