Question

Arrange benzene, \(n\) -hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.

Short answer

The order is: Ethyne > Benzene > n-hexane due to decreasing s-character.

Step-by-step solution

Understanding Acidity

Acidity refers to the tendency of a compound to donate a proton ( H^+ ). In general, compounds with more electronegative atoms or with greater s-character in their bonding tend to be more acidic.

Comparing s-character

Benzene, -hexane, and ethyne have different hybridizations, which affect their acidity. For ethyne ( C_2H_2 ), the carbon is sp hybridized, meaning there is 50% s-character in its bonding. Benzene is sp^2 hybridized with 33% s-character, and -hexane is sp^3 hybridized with 25% s-character.

Relating s-character to Acidity

Higher s-character results in a more stable conjugate base upon deprotonation, thus increasing acidity. Having 50% s-character, ethyne is more acidic than benzene which has 33% s-character, and much more than -hexane with 25% s-character.

Arranging the Compounds

Based on their acidity, the order from most acidic to least acidic is: Ethyne > Benzene > -hexane.

Explaining the Reasoning

The increased s-character in ethyne allows it to hold onto its lone pairs more effectively and stabilize the conjugate base better than benzene and -hexane, which have lower s-character and thus are less acidic.

Key concepts

s-character

The concept of s-character is crucial in understanding why different molecules exhibit varied acidity levels. The term "s-character" refers to the proportion of the s orbital in a hybridized orbital. For hybridized orbitals, such as sp, sp², and sp³, the s-character varies. The sp hybridization, seen in ethyne, has a high s-character of 50%, meaning half of the hybrid orbital is sourced from the s orbital.
This has significant implications for acidity behavior. A higher s-character means that the electrons are held more closely to the nucleus. This proximity increases the electronegativity, allowing the molecule to attract and hold a proton (H⁺) less tightly. As a result, it's easier for the compound to lose a proton and produce a stable conjugate base.
Therefore, molecules with higher s-character tend to be more acidic. Ethyne, with its high s-character, is more acidic than benzene and n-hexane, demonstrating how s-character impacts acidity behavior.

hybridization

The hybridization of a molecule's bonding orbitals plays a key role in determining its acidity. During hybridization, atomic orbitals mix to form new hybrid orbitals. The types of hybridization include sp, sp², and sp³, with sp having the highest s-character.

• In ethyne ( C_2H_2 ), the carbon atoms are sp hybridized, resulting in an s-character of 50%.
• Benzene ( C_6H_6 ) features sp² hybridization, with an s-character of 33%.
• n-Hexane, on the other hand, has sp³ hybridized carbons showing an s-character of 25%.

Understanding the relationship between hybridization and acidity requires recognizing that compounds with higher s-character (such as sp hybridization) are more acidic. This is due to the closer proximity of the electrons to the nucleus, which increases electronegativity and stabilizes the conjugate base. Consequently, ethyne is more acidic than benzene, which in turn is more acidic than n-hexane.

conjugate base stability

Conjugate base stability is a fundamental factor in the acidity of compounds. When a compound loses a proton (H⁺), it forms a conjugate base. The more stable this conjugate base, the more acidic the original compound.
The stability of a conjugate base is influenced significantly by the s-character of the molecule's hybridization. A higher s-character provides greater overlap between orbitals, allowing the lone pair of electrons in the conjugate base to be held more tightly and closer to the nucleus.
This increased stability is seen in ethyne due to its sp hybridization. When ethyne loses a proton, it forms a very stable conjugate base because the remaining electrons can better stabilize themselves by being closer to the nucleus. On the other hand, benzene and n-hexane have lower s-character, resulting in less stable conjugate bases when they lose a proton. Hence, increased s-character enhances conjugate base stability and leads to greater acidity, helping to explain why ethyne is more acidic than benzene and n-hexane.