Problem 121 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The structure of alkyne is linear. Terminal alkyne is acidic in character. It reacts with base to give acid-base reaction. Alkyne is nucleophile and gives electrophilic as well as nucleophilic addition reaction. Which one of the following will react with \(\mathrm{NaNH}_{2}\) ? (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}\) (d) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Option (c) \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH} \) reacts with \( \mathrm{NaNH}_{2} \).

Step 1: Identify Terminal Alkyne

A terminal alkyne has the structure \( \mathrm{R}-\mathrm{C} \equiv \mathrm{CH} \) where \( \mathrm{R} \) is a carbon group, and it has an acidic hydrogen at the end. From the options provided, option (c) \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH} \) is a terminal alkyne.

Step 2: Analyze the Reaction with \( \mathrm{NaNH}_{2} \)

\( \mathrm{NaNH}_{2} \) is a strong base that will react with acidic hydrogens. Terminal alkynes have relatively acidic hydrogens, especially at the terminal \( \equiv \mathrm{CH} \) group. Therefore, a terminal alkyne like option (c) \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH} \) would react with \( \mathrm{NaNH}_{2} \).

Step 3: Rule Out Other Compounds

Other hydrocarbons provided in the options, such as alkanes (\( \mathrm{CH}_{3}-\mathrm{CH}_{3} \)), alkenes (\( \mathrm{CH}_{2} = \mathrm{CH}_{2} \)), or non-terminal alkynes (\( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3} \)), do not have acidic hydrogens that would react with \( \mathrm{NaNH}_{2} \).

Key Concepts

Acid-Base Reaction

Terminal alkynes, such as \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH} \), have a unique characteristic due to their acidic hydrogen at the terminal carbon. This acidic hydrogen is essential for acid-base reactions, especially when reacting with strong bases like sodium amide \( \mathrm{NaNH}_{2} \).
In an acid-base reaction, a proton donor (acid) reacts with a proton acceptor (base). Here, the terminal alkyne acts as the acid. The \( \mathrm{NaNH}_{2} \), being a strong base, abstracts the acidic hydrogen atom from the terminal alkyne. This forms an acetylide ion \( \mathrm{R} - \mathrm{C} \equiv \mathrm{C}^{-} \) and ammonia \( \mathrm{NH}_{3} \). The produced acetylide ion is an excellent nucleophile. This enables it to participate in further reactions.
The acidity of terminal alkynes is particularly noteworthy. Compared to alkanes and alkenes, terminal alkynes have more acidic hydrogens. This is due to the sp-hybridized carbon, which holds the electrons closer, stabilizing the negative charge from the loss of hydrogen. As a result, they are able to effectively participate in acid-base reactions, especially with strong bases.

Nucleophilic Addition Reaction

Alkynes, including terminal ones, can act as nucleophiles. This behavior is due to the presence of a carbon-carbon triple bond, which is electron-rich. Integrating their role as nucleophiles explains how they can add to electrophilic centers in various reactions.
In nucleophilic addition reactions, the electron-rich alkyne attacks an electron-deficient region—or electrophile—forming a new bond. For example, when an alkyne reacts with an electrophile like hydrogen halides, it initially forms a vinylic halide. Further reaction can eventually lead to a geminal dihalide.
The ability of terminal alkynes to participate in nucleophilic addition stems from their polarizable triple bond. Though they are not as strong as double bonds, their triple bonds offer sufficient electron density to initiate an attack. Besides, when converted into acetylide ions through the acid-base reaction, their nucleophilic nature is significantly enhanced, making them formidable in a variety of reactions.

Strong Base Reaction

Reactions involving strong bases are central to understanding how terminal alkynes can be activated. Strong bases like \( \mathrm{NaNH}_{2} \) are capable of deprotonating terminal alkynes, rendering them more reactive.
A strong base is characterized by its ability to remove a proton from even the less acidic hydrogen atoms. When \( \mathrm{NaNH}_{2} \) interacts with a terminal alkyne such as \( \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH} \), it abstracts the terminal hydrogen. This deprotonation reaction transforms the alkyne into an acetylide ion, which is an excellent nucleophile because of the extra electron lone pair on carbon.
The effectiveness of a strong base like \( \mathrm{NaNH}_{2} \) in these reactions lies in its conjugate acid, \( \mathrm{NH}_{3} \), being a very weak acid. This means it strongly favors the formation of the acetylide ion. Thus, understanding how strong bases function with terminal alkynes sheds light on various synthetic strategies in organic chemistry.