Back to Directory
NEET CHEMISTRYMedium

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

A

CH≡CH>CH3−C≡CH>CH2=CH2>CH3−CH3CH \equiv CH > CH_3-C \equiv CH > CH_2=CH_2 > CH_3-CH_3

B

CH≡CH>CH2=CH2>CH3−C≡CH>CH3−CH3CH \equiv CH > CH_2=CH_2 > CH_3-C \equiv CH > CH_3-CH_3

C

CH3−CH3>CH2=CH2>CH3−C≡CH>CH≡CHCH_3-CH_3 > CH_2=CH_2 > CH_3-C \equiv CH > CH \equiv CH

D

CH2=CH2>CH3−CH3>CH3−C≡CH>CH≡CHCH_2=CH_2 > CH_3-CH_3 > CH_3-C \equiv CH > CH \equiv CH

Step-by-Step Solution

  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≡CHHC\equiv CH): sp hybridised (50% s-character).
  • Ethene (H2C=CH2H_2C=CH_2): sp² hybridised (33.3% s-character).
  • Ethane (H3C−CH3H_3C-CH_3): sp³ hybridised (25% s-character).
  • Order based on hybridisation: Alkyne > Alkene > Alkane.

  1. Substituent Effect (+I Effect): Comparing ethyne (CH≡CHCH\equiv CH) and propyne (CH3−C≡CHCH_3-C\equiv CH), both contain sp-hybridised carbons with acidic hydrogens. However, the methyl group (−CH3-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 .

  2. Final Order: CH≡CHCH\equiv CH (sp) > CH3−C≡CHCH_3-C\equiv CH (sp, +I effect) > CH2=CH2CH_2=CH_2 (sp²) > CH3−CH3CH_3-CH_3 (sp³).

Practice Mode Available

Master this Topic on Sushrut

Join thousands of students and practice with AI-generated mock tests.

Get Started
Solved: CHEMISTRY Question for NEET | Sushrut