Question

The hydrocarbon which can react with sodium in liquid ammonia is: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{3}\)

Short answer

Option (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) is the hydrocarbon that can react with sodium in liquid ammonia.

Step-by-step solution

Identify the Type of Reaction

The question involves the reaction of hydrocarbons with sodium in liquid ammonia. This scenario typically suggests a dissolving metal reduction, where sodium in liquid ammonia is used to create radicals that can lead to a transformation or reaction with hydrocarbons. The reaction with sodium usually indicates that the hydrocarbon must be an alkyne with at least one terminal hydrogen because only terminal alkynes (acetylide ions) are acidic enough to release a proton and react with sodium.

Evaluate Each Hydrocarbon

Examine the structural formulas provided:(a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) is an internal alkyne without a terminal hydrogen.(b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) is a terminal alkyne as it has a hydrogen atom connected at the end of the carbon chain, facilitating proton release.(c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) is an alkene, not characterized by significant acidity to convert to a carbanion.(d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CCH}_{2} \mathrm{CH}_{3}\) is another internal alkyne without a terminal hydrogen.

Determine the Reactivity with Sodium in Liquid Ammonia

Identify if these hydrocarbons can react with sodium in liquid ammonia. Only option (b), the terminal alkyne, can efficiently react in this environment because it can donate its terminal hydrogen atom as a proton, forming a negatively charged acetylide ion, which is stabilized. No other candidates (a, c, d) have this capability.

Conclude Which Hydrocarbon Can React

Based on the analysis, the only hydrocarbon which reacts with sodium in liquid ammonia is (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\), as it is the only one with a terminal alkyne structure that can lose a hydrogen to form an acetylide ion.

Key concepts

Sodium in Liquid Ammonia

Sodium in liquid ammonia is a powerful reagent used in organic chemistry, commonly for reactions involving the reduction of certain hydrocarbons. Liquid ammonia acts as a solvent that not only dissolves sodium but also allows it to release electrons into the solution. These electrons act as reducing agents.

When sodium is dissolved in liquid ammonia, it forms a deep blue solution due to the presence of solvated electrons. These electrons are highly reactive and can easily transfer to other molecules, facilitating various chemical transformations. This environment is particularly effective for reducing functionalities like alkynes to alkenes. The procedure is often very selective and gentle, making it a valuable tool in synthetic chemistry. For instance, it's widely used to perform dissolving metal reductions and transform triple bonds in alkynes into more reactive intermediates.

Dissolving Metal Reduction

The term "dissolving metal reduction" refers to a reaction where metals such as sodium, in liquid ammonia, are used to reduce organic compounds. This reduction process is different because it doesn't require traditional oxidizing agents or heat.

There are several key advantages to this approach:

• It allows for the rapid addition of electrons to a system, quickly generating radicals or anions.
• It is mild compared to other reduction methods, reducing the chance of damaging sensitive functional groups.
• It often proceeds with high selectivity, giving control over how complex molecules are modified.

Understanding this method is essential in organic synthesis, as it enables chemists to achieve transformations that might be difficult using other methods. This technique is especially useful for chemoselective reductions, where certain bonds are reduced while others remain intact.

Acetylide Ions

Acetylide ions are negatively charged ions formed when a terminal alkyne loses a hydrogen atom. Terminal alkynes have a hydrogen atom attached directly to a carbon-carbon triple bond. This hydrogen is slightly acidic, meaning it can be removed by a strong base.

When a terminal alkyne is exposed to a strong base, such as sodium in liquid ammonia, it can lose its acidic hydrogen to form an acetylide ion. Here's why this process is significant:

• Acetylide ions are highly nucleophilic, meaning they can attack electrophilic centers in other molecules, making them useful in forming carbon-carbon bonds.
• They are crucial intermediates in many organic synthesis reactions, allowing for the construction of complex carbon skeletons.
• Acetylide anions also play a vital role in coupling reactions, where two hydrocarbon chains can be joined together.

Remember, only terminal alkynes have enough acidity to form acetylide ions, making them unique in this context.

Hydrocarbon Reactivity

Hydrocarbons are the foundation of organic chemistry, consisting of only hydrogen and carbon. Their reactivity largely depends on their structure, with different types behaving distinctively under chemical conditions.

Alkanes are generally unreactive unless strong activation energy is applied. Alkenes, with their double bonds, are more reactive due to electron density that can interact with electrophiles or participate in addition reactions. However, alkynes, especially terminal ones, are even more reactive for several reasons:

• The presence of a triple bond creates electron-rich regions, attracting certain reagents for addition or replacement reactions.
• Terminal alkynes can form acetylide ions, enabling them to participate actively in substitution and coupling reactions.
• The acidity of terminal hydrogens is significant in reaction settings like sodium in liquid ammonia, allowing them to transform into potent intermediates.

This unique reactivity profile allows alkynes, particularly terminal ones, to serve as versatile building blocks in many synthetic pathways.