Question

Arrange benzene, n-hexane and ethyne in decreasing order of their acidic behaviour

• A. Benzene > n-hexane > ethyne
• B. n-hexane > Benzene > ethyne
• C. ethyne > n-hexane > Benzene
• D. ethyne > Benzene > n-hexane

Answer 1

The Correct Option is D

1. Understand Acidity and Hybridization
The acidity of a C-H bond is related to the stability of the resulting carbanion after deprotonation (removal of the proton). The stability of the carbanion is, in turn, influenced by the hybridization of the carbon atom bearing the negative charge. A higher s-character in the hybrid orbital leads to greater electronegativity of the carbon, thus stabilizing the carbanion.

- sp hybridization: 50% s-character
- sp² hybridization: 33.3% s-character
- sp³ hybridization: 25% s-character

Therefore, the order of acidity based on hybridization is: sp > sp² > sp³.

2. Determine the hybridization of carbon atoms in the given compounds

- Ethyne (Acetylene, HC≡CH): The carbon atoms are sp hybridized. The hydrogen atoms are directly attached to sp hybridized carbons, making it acidic.

- Benzene (C₆H₆): The carbon atoms are sp² hybridized. Benzene is less acidic than ethyne because sp² hybridized carbon is less electronegative than sp hybridized carbon.

- n-Hexane (C₆H₁₄): The carbon atoms are sp³ hybridized. n-Hexane is the least acidic among the three because the sp³ hybridized carbon is the least electronegative.

3. Arrange in decreasing order of acidic behavior
Based on the hybridization, the decreasing order of acidity is:

Ethyne > Benzene > n-Hexane

4. Compare with the given options
Comparing the derived order with the options, we find that it matches option (D).

Final Answer:
(D) ethyne > Benzene > n-hexane

Answer 2

To compare the acidic behavior of benzene, n-hexane, and ethyne, we look at their ability to donate protons (H⁺):

• Ethyne (C₂H₂) has an sp-hybridized carbon, which is more electronegative and has a higher tendency to donate a proton (H⁺). Therefore, ethyne is the most acidic.
• Benzene (C₆H₆) has sp² hybridized carbons and is a weaker acid than ethyne, but more acidic than n-hexane.
• n-Hexane (C₆H₁₄) is a saturated hydrocarbon with sp³ hybridized carbons. It does not easily lose a proton and is the least acidic.

The correct answer is (D) : ethyne > Benzene > n-hexane.