What is the correct order of acidity among the following compounds?

Question

Accepted Answer

1. **Hybridisation and Electronegativity:** The acidity of hydrogen attached to a carbon atom depends on the s-character of the carbon's hybrid orbitals. Higher s-character means the electrons are closer to the nucleus, making the carbon more electronegative and the attached hydrogen more acidic .
   - Ethyne ($HC\equiv CH$): sp hybridised (50% s-character).
   - Ethene ($H_2C=CH_2$): sp² hybridised (33.3% s-character).
   - Ethane ($H_3C-CH_3$): sp³ hybridised (25% s-character).
   - Order based on hybridisation: Alkyne > Alkene > Alkane.

2. **Substituent Effect (+I Effect):** Comparing ethyne ($CH\equiv CH$) and propyne ($CH_3-C\equiv CH$), both contain sp-hybridised carbons with acidic hydrogens. However, the methyl group ($-CH_3$) in propyne exerts an electron-donating inductive effect (+I effect). This increases electron density on the carbon atom and destabilises the conjugate base (alkynide ion) formed after proton removal. Therefore, propyne is less acidic than ethyne .

3. **Final Order:** $CH\equiv CH$ (sp) > $CH_3-C\equiv CH$ (sp, +I effect) > $CH_2=CH_2$ (sp²) > $CH_3-CH_3$ (sp³).

Step-by-Step Solution

Practice All CHEMISTRY Questions