Question

By using which of the following can the conversion of \(\mathrm{PhC} \equiv \mathrm{CH} \longrightarrow \mathrm{PhC}=\mathrm{CHCH}_{3}\) be achieved? (1) \(\mathrm{Br}_{2} \mathrm{CCl}_{4}\), then \(\mathrm{KOH}\) (2) \(\mathrm{Na}\), then \(\mathrm{CH}_{3} \mathrm{CH}_{3} \mathrm{I}\) (3) \(\mathrm{Na}\), then \(\mathrm{CH}_{3} \mathrm{I}\) (4) \(\mathrm{CH}_{2} \mathrm{~N}_{2}\)

Short answer

Option 3 (Na, then CH₃I)

Step-by-step solution

Identify the target functional groups

Examine the initial and final compounds. The initial compound is PhC≡CH (a terminal alkyne) and the target compound is PhC=CHCH₃ (an alkene). The goal is to convert a triple bond to a double bond with the addition of a CH₃ group.

Analyze the given options

Consider each of the provided reagents and their typical reactions with an alkyne. Determine which set of reagents can achieve the conversion from a triple bond to a double bond while adding a CH₃ group.

Evaluate Option 1

Option 1 suggests using Br₂/CCl₄ followed by KOH. Br₂/CCl₄ would add bromine to the alkyne (Br₂ addition), forming a dibromoalkane, and KOH would be used for a dehydrohalogenation to form an alkyne or alkene. This path does not add a CH₃ group, so it doesn't match our goal.

Evaluate Option 2

Option 2 involves using Na first, followed by CH₃CH₃I. Sodium can deprotonate the terminal alkyne forming a sodium acetylide, but CH₃CH₃I does not introduce a new bonding site directly related to the desired change, pointing to the wrong path.

Evaluate Option 3

Option 3 uses Na followed by CH₃I. Sodium acetylide reacts with methyl iodide (CH₃I) to form a new C-C bond, resulting in PhC≡CCH₃. A partial hydrogenation then may achieve the desired alkene (PhC=CHCH₃) formation. This seems promising.

Evaluate Option 4

Option 4 involves CH₂N₂, which is typically used for cyclopropanation reactions, not for converting triple bonds to an alkene with the addition of a methyl group.

Conclude the correct reagent

Based on the evaluations, Option 3 (Na, then CH₃I) is the correct reagent combination to achieve the conversion of PhC≡CH to PhC=CHCH₃.

Key concepts

reagents in organic chemistry

Reagents are substances or compounds added to a system to cause a chemical reaction or used to see if a reaction occurs. In organic chemistry, they are essential to performing transformations and synthesizing new compounds. For the conversion of an alkyne (PhC≡CH) to an alkene (PhC=CHCH₃), specific reagents must be used. One important reagent is sodium (Na), which can deprotonate the terminal alkyne to form a highly reactive acetylide ion. This acetylide ion can then react with an alkyl halide like methyl iodide (CH₃I) to form a new carbon-carbon bond. Ultimately, choosing the right combination of reagents is crucial to achieving the desired chemical transformation.

In the example provided:

• Na forms a sodium acetylide.
• CH₃I reacts with the sodium acetylide to add a CH₃ group, forming PhC≡CCH₃.

functional group transformations

Functional group transformations are central to organic synthesis. These transformations involve modifying the functional groups of molecules to achieve a specific target compound. For instance, converting an alkyne to an alkene involves:

• Understanding the functional groups involved (e.g., an alkyne with a triple bond and an alkene with a double bond).
• Knowing the steps to convert these groups (often through reagents and specific reaction conditions).

In our exercise, the transformation of PhC≡CH (an alkyne) to PhC=CHCH₃ (an alkene) requires the addition of a methyl group (CH₃) and the reduction of the triple bond to a double bond. This step-by-step progression is essential in achieving the desired functional group change.

For the conversion:

• Add a CH₃ group to form PhC≡CCH₃ using Na and CH₃I.
• Hydrogenate the triple bond partially to convert it into an alkene, leading to PhC=CHCH₃.

reaction mechanisms

A reaction mechanism describes the step-by-step process by which reactants are converted into products. Understanding these mechanisms allows chemists to predict the outcome of reactions and to design pathways for synthesis.
For the conversion from an alkyne to an alkene, the mechanism can be outlined as:

• Deprotonation: Sodium (Na) removes a hydrogen atom from the terminal alkyne (PhC≡CH), creating a sodium acetylide (PhC≡C⁻Na⁺).
• Nucleophilic substitution: The acetylide ion formed reacts with methyl iodide (CH₃I) through a nucleophilic attack, resulting in the formation of a new C-C bond, giving PhC≡CCH₃.
• Partial hydrogenation: The triple bond in PhC≡CCH₃ is partially hydrogenated, reducing it to a double bond, and resulting in the target compound PhC=CHCH₃.

By understanding each step in the mechanism, we can see how the reactions are carried out and what intermediates are formed, which is crucial for mastering organic synthesis.