Question

Which one is the correct order of acidity ?

• A. $CH \equiv CH > CH_3 - C \equiv CH > CH_2 = CH_2 > CH_3 - CH_3$
• B. $CH \equiv CH > CH_2 = CH_2 > CH_3 - C \equiv CH > CH_3 - CH_3$
• C. $CH_3 - CH_3 > CH_2 = CH_2 > CH_3 - C \equiv CH > CH \equiv CH$
• D. $CH_2 = CH_2 > CH_3 - CH = CH_2 > CH_3 - C \equiv CH > CH \equiv CH$

Answer 1

The Correct Option is A

 To determine the correct order of acidity among the given compounds, we need to consider the acidity of hydrocarbons which primarily depends on the ability to release a hydrogen ion (H⁺). This ability is influenced by the type of hybridization of the carbon atom to which the hydrogen is attached.

Here are the key principles to remember:

• Alkynes are more acidic than alkenes, which are more acidic than alkanes. This is because the acidic hydrogen is attached to a carbon atom, and the s-character influences the acidity. Greater s-character increases acidity.
• s-character order: sp (alkyne) > sp² (alkene) > sp³ (alkane).
• Thus, CH≡CH (acetylene) is the most acidic due to its highest s-character (50%), followed by CH3–C≡CH (propyne), due to the presence of a more acidic terminal hydrogen attached to an sp-hybridized carbon.
• Following acetylene and propyne, CH2=CH2 (ethylene) is less acidic because the hydrogen is attached to an sp² hybridized carbon.
• CH3-CH3 (ethane) is the least acidic due to its sp³ hybridized carbon with only 25% s-character.

Given these rules, the correct order of acidity is:

\(CH \equiv CH > CH_3 - C \equiv CH > CH_2 = CH_2 > CH_3 - CH_3\)

Hence, the correct answer is \(CH \equiv CH > CH_3 - C \equiv CH > CH_2 = CH_2 > CH_3 - CH_3\) as indicated in the options.

This observation aligns with the general order of acidity in hydrocarbons:

• Acetylene (alkyne) being the most acidic.
• Followed by less substituted alkynes (propyne).
• Alkenes being less acidic than alkynes.
• Alkanes being the least acidic.