Q.Which of the following compounds is most Acidic?

Acidic character of a hydrocarbon C-H bond rises with s-character of the carbon: sp (50% s) > sp2 (33% s) > sp3 (25% s). Ethyne (C2H2), with sp-hybridised carbons, is the most acidic among C2H4 (sp2, ethylene), C2H6 (sp3, ethane), and C2H2 (sp, acetylene).

Greater s-character means the bonding orbital holds electron density closer to the carbon nucleus, so the C-H bonding pair is more tightly held and the C atom is more electronegative -- making it easier for the molecule to release H+ (more acidic).

• C2H6 (ethane): sp3 carbons, least acidic.
• C2H4 (ethylene): sp2 carbons, intermediate.
• C2H2 (acetylene/ethyne): sp carbons, most acidic among these -- terminal alkynes can even be deprotonated by strong bases like NaNH2. …

Acetylene forming a precipitate with ammoniacal AgNO3 demonstrates the acidic nature of its terminal (triple-bond) hydrogen atoms.

Terminal alkynes (like acetylene/ethyne, HC=CH) have hydrogen atoms directly attached to an sp-hybridised carbon of the triple bond. Because sp carbons hold their bonding electrons closer to the nucleus (higher effective electronegativity than sp2 or sp3 carbons), the C-H bond in a terminal alkyne is more polarized, and that hydrogen is weakly acidic and can be replaced by a metal.

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Ammoniacal silver nitrate distinguishes propyne (forms a white ppt of silver propynide) from propene (no reaction), because only propyne has an acidic terminal alkyne hydrogen.

Propyne (CH3-C=CH) is a terminal alkyne — it has a hydrogen atom directly attached to the sp-hybridised carbon of the triple bond, which is weakly acidic and can be replaced by a metal ion.

When propyne gas is passed through ammoniacal silver nitrate solution ([Ag(NH3)2]+ ), this acidic hydrogen is replaced by Ag+, giving a white/pale-yellow precipitate of silver propynide: CH3-C=CH + [Ag(NH3)2]+ -> CH3-C=C-Ag (ppt) + NH4+ + NH3.

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Terminal-alkyne (sp) C–H is acidic; ethyne has acidic hydrogen — option (a).

From NCERT Class 11 Chemistry (Hydrocarbons): acidity of a C–H bond increases with the s-character of the carbon: sp (50%) > sp² (33%) > sp³ (25%). The sp carbon holds the electron pair closer, stabilising the anion. …

Terminal alkyne CH₃–C≡CH shows acidic character — option (a).

From NCERT Class 11 Chemistry (Hydrocarbons): a hydrogen on an sp-hybridised terminal alkyne carbon is weakly acidic and can be removed by strong bases / reactive metals.

• CH₃–C≡CH (propyne): terminal ≡C–H → acidic (gives an acetylide with Na/ammoniacal AgNO₃). …

Ethyne is more acidic than ethene because the sp-hybridised carbon of ethyne has a higher percentage of s-character than the sp2-hybridised carbon of ethene, making its C-H bond more polar and its acidic hydrogen easier to release as H+.

Acidity of a C-H bond in hydrocarbons depends on the hybridisation of the carbon atom bonded to that hydrogen. The greater the s-character of the hybrid orbital, the closer the electron density is held to the carbon nucleus, the more electronegative that carbon behaves, and the more easily the attached hydrogen is released as a proton.

• In ethyne (HC≡CH), the carbon atoms are sp hybridised (50% s-character).
• In ethene (H2C=CH2), the carbon atoms are sp2 hybridised (33% s-character).
• In ethane (H3C-CH3), the carbon atoms are sp3 hybridised (25% s-character). …

Acetylene is more acidic than ethylene because its sp-hybridised carbon (50% s-character) is more electronegative than ethylene's sp2 carbon (33% s-character), so the C-H bond in acetylene is more polarised and releases H+ more readily.

Acidity of a C-H bond depends on how tightly the carbon atom holds the bonding electron pair - the more electronegative (s-rich) the carbon, the more it pulls electron density towards itself, weakening/polarising the C-H bond and making it easier to lose H+.

• In ethylene (H2C=CH2), the carbons are sp2 hybridised: 33% s-character.
• In acetylene (HC≡CH), the carbons are sp hybridised: 50% s-character, making sp carbon more electronegative than sp2 carbon (which in turn is more electronegative than sp3 carbon). …