Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

................ is most acidic. (Ethane, Ethene, Ethyne)

Step-by-Step Solution

Answer

Ethyne is the most acidic.

Step 1: Understand Molecular Structure

First, let's examine the molecular structures of ethane, ethene, and ethyne. Ethane ( C_2H_6 ) consists of single bonds between carbon atoms. Ethene ( C_2H_4 ) has a double bond between its carbon atoms. Ethyne ( C_2H_2 ) contains a triple bond between the carbon atoms.

Step 2: Consider Hydrogen Atom Bonding

Next, notice the different bonds affect the acidity. Hydrogen atoms bonded to carbon with more s-character tend to be more acidic. In ethyne, the carbon atoms each share one s and two p orbitals in their sp hybridization (50% s-character), while in ethene, there's sp^2 hybridization (33% s-character), and ethane has sp^3 hybridization (25% s-character).

Step 3: Relate Hybridization to Acidity

Realize that the greater the s-character, the more acidic the hydrogen. This is because electrons are held closer to the nucleus in orbitals with more s-character, stabilizing the conjugate base.

Step 4: Conclusion based on Acidity

From step 3, we know ethyne, with the highest s-character ( sp hybridization), is the most acidic, followed by ethene, and then ethane.

Key Concepts

Hybridization and Acidity

Hybridization is a concept that helps us understand the types of bonds formed between atoms in a molecule. It involves the mixing of atomic orbitals to create new hybrid orbitals. The impact of hybridization on acidity becomes evident when examining the molecules in question: ethane, ethene, and ethyne.

In ethane (\(C_2H_6\)), the carbon atoms are \(sp^3\) hybridized, which means that one s orbital combines with three p orbitals to form four identical orbitals. Ethene (\(C_2H_4\)), on the other hand, has carbon atoms that are \(sp^2\) hybridized, resulting in one s orbital and two p orbitals forming three identical hybrid orbitals. Lastly, in ethyne (\(C_2H_2\)), the carbon atoms are \(sp\) hybridized, meaning one s orbital and one p orbital merge to create two equal hybrid orbitals.

The degree of s-character affects the acidity of a molecule. As the s-character increases, the acidity also increases. In simple terms, the closer the bonding electrons are to the nucleus, the more stable the conjugate base will be after the hydrogen is removed. This results in greater acidity. Ethyne, with its \(sp\) hybridization and 50% s-character, is the most acidic of the three. Ethene follows with \(sp^2\) hybridization and 33% s-character, and lastly, ethane with \(sp^3\) hybridization and just 25% s-character.

s-character in Chemical Bonds

The s-character in a chemical bond refers to the proportion of an s orbital used in hybridization. It is crucial in determining the shape, bond angles, and acidity of that particular bond.

When analyzing s-character, think of it like a tug-of-war between the s and p orbitals. The more s-character a bond has, the more the electrons are closer to the nucleus. This proximity leads to a more compact and stable electron cloud.

• In \(sp^3\) hybridization, such as in ethane, the s-character is 25% since one s orbital out of four hybridized orbitals is used.
• In \(sp^2\) hybridization, like in ethene, the s-character increases to 33%, using one s orbital in three total.
• In \(sp\) hybridization, as observed in ethyne, the s-character peaks at 50% because one s orbital is involved in a total of two hybridized orbitals.

As s-character increases, the acidity generally increases. This is because electrons closer to the nucleus mean that the conjugate base, being the ion remaining after an acid molecule loses a hydrogen ion, is more stable.

Molecular Structure of Hydrocarbons

The molecular structure of hydrocarbons plays a crucial role in determining the properties such as acidity, boiling points, and overall reactivity. Understanding the basic structure of molecules like ethane, ethene, and ethyne provides a solid foundation for grasping more complex chemical principles.

• Ethane (\(C_2H_6\)) involves single carbon-carbon bonds, indicating a saturated hydrocarbon structure. It is fully hydrogenated and generally has low acidity due to minimal s-character.
• Ethene (\(C_2H_4\)) contains a double bond between carbon atoms, showing a degree of unsaturation. The presence of this double bond leads to added s-character and increased acidity compared to ethane.
• Ethyne (\(C_2H_2\)), with a triple carbon-carbon bond, reflects the highest degree of unsaturation. This structure results in a linear arrangement with significant s-character, making it highly acidic relative to ethane and ethene.

Recognizing these structural differences helps in predicting the behavior of hydrocarbons in various chemical reactions. The nature of the bonds and hybridization in these molecules directly impacts their reactivity and other chemical properties, including acidity.