Acetylenic hydrogens are acidic because

sigma electron density of C–H bond in acetylene is nearer to carbon which has 50% s-character

The correct answer is option 1. sigma electron density of C–H bond in acetylene is nearer to carbon which has 50% s-character.

Acetylenic hydrogens in acetylene \((C_2H_2)\) are considered acidic primarily due to reason (1) Sigma electron density of C–H bond in acetylene is nearer to carbon which has 50% s-character.

Acidity of Acetylenic Hydrogens:

Electron Density and Bond Character:

In acetylene \((C_2H_2)\), each carbon atom is sp hybridized, meaning it has 50% s-character. The \(C-H\) bonds in acetylene are formed by overlap of \(sp\) hybrid orbitals of carbon with 1s orbital of hydrogen. The electron density in these \(C-H\) bonds is closer to the carbon atoms due to their high \(s\)-character (50%), making the hydrogen atoms more electron-deficient and the bond more polarized.

Acidity Explanation:

Acidity refers to the ability to donate a proton (\(H^+\) ion). In acetylene, the hydrogen atoms attached to the \(sp\)-hybridized carbons (acetylenic hydrogens) have a relatively weak bond due to the electron density being closer to the carbon atom. This makes the hydrogen atoms more acidic compared to typical \(sp^3\) hybridized \(C-H\) bonds found in alkanes, where the electron density is more evenly distributed between carbon and hydrogen.

Why the Other Options are Not Correct:

Option (2): Acetylene indeed has only one hydrogen on each carbon, but this alone does not explain the acidity of acetylenic hydrogens.

Option (3): Acetylene does have the least number of hydrogens among aliphatic hydrocarbons with two carbons, but this does not directly relate to its acidity.

Option (4): Acetylene does not resemble acetic acid structurally or chemically, except for the presence of carbon and hydrogen atoms in their molecules.

Therefore, the correct reason why acetylenic hydrogens are acidic is because the sigma electron density of the \(C-H\) bond in acetylene is nearer to the carbon due to its high \(50\%\) \(s\)-character in the \(sp\) hybridization of the carbon atoms. This results in a more polarized \(C-H\) bond and hence, increased acidity of the acetylenic hydrogens.