Rank the given compounds based on their relative acidities. strongest acid HF HC=CH H,C=CH, NH, H,CCH, weakest acid None
The correct answer and explanation is:
Correct Ranking (from strongest to weakest acid):
- HF
- HC≡CH (ethyne)
- H2C=CH2 (ethene)
- NH3 (ammonia)
- H3CCH3 (ethane)
Explanation:
Acidity is determined by the compound’s ability to donate a proton (H⁺). This is influenced by the stability of the conjugate base formed after losing the proton. Here’s the rationale for the ranking:
- HF (hydrofluoric acid): HF is the strongest acid in this group. After losing a proton, it forms the fluoride ion (F⁻), which is highly stable due to fluorine’s high electronegativity. This stability greatly favors proton donation, making HF the most acidic compound.
- HC≡CH (ethyne): Ethyne is acidic due to the sp-hybridized carbon atom bonded to the hydrogen. Sp-hybridized carbons have a high percentage of s-character (50%), which makes the carbon atom more electronegative and stabilizes the negative charge on the conjugate base (acetylide ion, HC≡C⁻). This contributes to its relatively high acidity compared to other hydrocarbons.
- H2C=CH2 (ethene): Ethene is less acidic than ethyne because its sp²-hybridized carbon atoms have less s-character (33%) than sp-hybridized carbons. This makes the conjugate base (CH2=CH⁻) less stable, resulting in lower acidity.
- NH3 (ammonia): Ammonia is a weak acid because its conjugate base (NH2⁻) is not as stabilized as the conjugate bases of HF or hydrocarbons with sp-hybridized carbons. Nitrogen is less electronegative than fluorine and does not stabilize the negative charge as effectively.
- H3CCH3 (ethane): Ethane is the weakest acid because its sp³-hybridized carbons have the least s-character (25%), and the conjugate base (ethyl anion, H3CCH2⁻) is highly unstable. Hydrocarbons with sp³ carbons are generally non-acidic.
This ranking reflects the interplay between hybridization, electronegativity, and conjugate base stability, which collectively govern acidity.