Question

The hydrocarbon that forms disodium salt with excess metallic sodium is

• A. ethane
• B. ethene
• C. ethyne
• D. propene
• E. propane

Hint:

Terminal alkynes (with a -C≡CH group) have acidic hydrogens and can react with strong bases like Na or NaNH\(_2\).

Answer 1

The Correct Option is C

Step 1: Understanding Acidity of Hydrocarbons:
Certain hydrocarbons can act as weak acids. Specifically, the hydrogen atom attached to an sp-hybridized carbon atom (as in a terminal alkyne) is acidic enough to be removed by a very strong base, such as sodium metal (Na) or sodamide (NaNH\(_{2}\)). This reaction forms a sodium salt (an acetylide). The question asks which hydrocarbon can form a disodium salt, implying it must have two such acidic hydrogens.
Step 2: Analysis of Options:
Let's analyze the acidity of the C-H bonds in each option:
(A) Ethane (CH\(_{3}\)-CH\(_{3}\)): All hydrogens are attached to sp\(^3\)-hybridized carbons. These are not acidic.
(B) Ethene (CH\(_{2}\)=CH\(_{2}\)) and (C) Benzene (C\(_{6}\)H\(_{6}\)): All hydrogens are attached to sp\(^2\)-hybridized carbons. While more acidic than sp\(^3\) C-H bonds, they are not acidic enough to react with sodium metal.
(D) Propyne (CH\(_{3}\)-C\(\equiv\)CH): This is a terminal alkyne. It has one acidic hydrogen attached to the sp-hybridized carbon at the end of the triple bond. It will react with sodium to form a monosodium salt (monosodium propynide).
\[ 2\text{CH}_3\text{C}\equiv\text{CH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{C}\equiv\text{C}^-\text{Na}^+ + \text{H}_2 \] (E) Ethyne (H-C\(\equiv\)C-H), also known as acetylene: This is also a terminal alkyne. Crucially, it has two acidic hydrogens, one on each sp-hybridized carbon. Therefore, it can react with excess sodium metal in two steps to form a disodium salt (disodium acetylide).
\[ \text{H-C}\equiv\text{C-H} + \text{Na} \rightarrow \text{H-C}\equiv\text{C}^-\text{Na}^+ + \frac{1}{2}\text{H}_2 \] \[ \text{H-C}\equiv\text{C}^-\text{Na}^+ + \text{Na} \rightarrow ^-\text{Na}^+\text{C}\equiv\text{C}^-\text{Na}^+ + \frac{1}{2}\text{H}_2 \] Step 3: Final Answer:
Ethyne is the only hydrocarbon listed with two acidic hydrogens capable of reacting with excess sodium to form a disodium salt. This corresponds to option (E).