Problem 80 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following statements best explains why 1-propyne can be deprotonated by the ethyl anion \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\right) ?\) (a) The acetylide anion is a stronger base than the ethyl anion. (b) Ethane has a lower \(\mathrm{pK}\), than acetylene. (c) The lone-pair orbitals in acetylide anions have more s character than those in alkyl anions. (d) The acetylide anion is a weaker base than an alkyl anion.

Step-by-Step Solution

Answer

Option (c) is correct: Acetylide anions have more s character in lone-pair orbitals.

Step 1: Analyze the Base Strength and Acidity Relationship

To deprotonate 1-propyne, we need to consider the strength of the base (ethyl anion) and the acidity of 1-propyne. A stronger base can deprotonate a weaker acid, forming a stronger conjugate acid.

Step 2: Understand the Acidity of 1-Propyne

1-propyne is a terminal alkyne, and terminal alkynes have acidic hydrogen due to the sp-hybridized carbon, making them more acidic than typical alkanes or alkenes.

Step 3: Consider the Characteristics of Acetylide and Alkyl Anions

Acetylide anions, derived from terminal alkynes like 1-propyne, have more s character in their lone-pair orbitals compared to alkyl anions. This higher s character results in greater electron density closer to the nucleus, stabilizing the negative charge better.

Step 4: Compare Strength of Bases and Resulting Anions

The ethyl anion is a strong enough base to abstract the acidic hydrogen from 1-propyne, forming an acetylide anion, which is the conjugate base of 1-propyne. The acetylide anion is resonance-stabilized due to its sp-hybridized carbon.

Step 5: Evaluate the Explanation in Context of the Options

Given the description of the sp character and resonance stabilization of the acetylide anion, option (c) 'The lone-pair orbitals in acetylide anions have more s character than those in alkyl anions.' best explains why ethyl anion can deprotonate 1-propyne.

Key Concepts

Deprotonation

Deprotonation in organic chemistry is a fundamental reaction where a proton (+) is removed from a molecule, typically by a base, resulting in the formation of an anion. This process is crucial for reactions involving acids and bases in organic chemistry. In the context of 1-propyne, deprotonation occurs when the ethyl anion (\( \text{CH}_3\text{CH}_2^- \)) removes the acidic hydrogen from the terminal alkyne. It's important to note:

• A deprotonation reaction requires a base strong enough to remove the proton.
• The product of deprotonation is an anion, such as the acetylide anion in this case.
• A stronger base can deprotonate a weaker acid, whereas a weaker base cannot remove the proton from a stronger acid.

This reaction highlights how the basicity of the bases and the acidity of the molecules are compared to facilitate the deprotonation step. Understanding these concepts is key to predicting the direction of acid-base reactions in organic chemistry.

Terminal Alkynes

Terminal alkynes, such as 1-propyne, are hydrocarbons where the triple bond terminates at the end of the carbon chain. They possess a unique property: the presence of an acidic hydrogen. This acidity is significantly higher than in alkanes or alkenes due to the distinct hybridization of the carbon involved in the triple bond. Key features of terminal alkynes include:

• The sp-hybridized carbon, which is more electronegative compared to sp^2 or sp^3 hybrids.
• The increase in s-character influences the acidity, making it easier for bases to remove the hydrogen.
• This s-character draws more electron density towards the nucleus, increasing the acidity.

The acidic nature of terminal alkynes is exploited in synthesis and transformations in organic chemistry, where their deprotonated form, the acetylide anion, can be utilized in further reactions such as forming carbon-carbon bonds.

s Character

s Character refers to the contribution of the s orbital in the hybridization of atomic orbitals. In the context of acetylides, sp-hybridized orbitals have 50% s character, as opposed to sp^2 (33%) or sp^3 (25%). Here's why this matters:

• The greater s character of sp orbitals brings the lone pair electrons closer to the nucleus, stabilizing the negative charge on the carbon.
• This increased stabilization makes acetylide anions less reactive and more stabilized compared to alkyl anions.
• Consequently, the sp-hybridized anion can effectively stabilize the negative charge due to increased electron density towards the nucleus.

As a result, this concept is crucial in understanding why acetylide anions, which form upon deprotonation of terminal alkynes, exhibit relatively high stability and impact their basic and nucleophilic properties in organic reactions.