Question

Alkynes are less reactive than alkenes towards electrophilic reagents. But the alkyne chemistry is useful for organic synthesis, in the form of hydration of the triple bond, formation of metalacetylides, selective reduction of alkynes and few other related reactions. Acetylene and HCHO react together in the presence of \(\mathrm{NaOCH}_{3}\) to give (a) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{OH}\) (b) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{HOH}_{2} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CHOH}-\mathrm{CH}_{3}\)

Short answer

Answer: (b) H2C=CH-CH2OH

Step-by-step solution

Write the reaction equation

Write down the reaction between acetylene and HCHO in the presence of NaOCH3. Acetylene has the formula C2H2, and HCHO is formaldehyde (CH2O). C2H2 + CH2O --[NaOCH3]--> Product

Recognize the nature of the reactants

The given reactants are acetylene, which is an alkyne, and formaldehyde, which is an aldehyde. Both of these are electrophilic in nature; however, the methoxide ion (OCH3-) from NaOCH3 acts as a nucleophile.

Analyze the possible reaction mechanism

The methoxide ion, being a nucleophile, attacks the electrophilic carbon atom of the formaldehyde and pushes the electrons towards the oxygen atom. This creates an intermediate where the methoxide ion is now bonded to the carbon atom, forming a methoxy alcohol group.

Attach the new functional group to the alkyne

The intermediate then reacts with acetylene, where the nucleophilic methoxy alcohol group attacks the electrophilic alkyne carbon atom, resulting in the addition of the new functional group across the triple bond. This forms a vinyl alcohol (enol) structure.

Identify the final product

Based on the above reaction mechanism, we find that the enol formed in the reaction is: H2C=CH-CH2OH Comparing this with the given options, we see that this product matches option (b). Thus, the correct answer is (b) H2C=CH-CH2OH.

Key concepts

Alkyne Reactivity

Alkynes contain a carbon-carbon triple bond, which consists of one sigma bond and two pi bonds. This structure makes them a unique kind of unsaturated hydrocarbon. Although they have a high degree of unsaturation, alkynes are generally less reactive towards electrophiles compared to alkenes. This is due to the higher energy and more stable nature of triple bonds.
Despite this, alkynes have important roles in organic synthesis. They can participate in a variety of reactions, such as:

• Hydration of the triple bond, which can lead to ketone and aldehyde formations
• Formation of metal acetylides, useful in C-C bond formation
• Selective reduction, enabling control over the degree of unsaturation in a molecule

Learning to manipulate the reactivity of alkynes is crucial for synthesizing complex organic molecules.

Nucleophilic Addition

Nucleophilic addition is a fundamental organic reaction mechanism where a nucleophile interacts with an electrophilic site, resulting in the formation of a new bond. This type of reaction is keenly observed in the interaction between nucleophiles and carbon-oxygen double bonds, such as those found in aldehydes and ketones.
In the exercise provided, the methoxide ion o ( O C H _3 -) from sodium methoxide acts as the nucleophile. It attacks the electron-deficient carbon atom in formaldehyde (HCHO), breaking the pi bond between the carbon and oxygen, and forming a new intermediate.
This mechanism is vital because it showcases how nucleophiles can help form complex structures by linking simple molecules together. Understanding nucleophilic addition opens doors to synthesizing a wide range of alcohols and other important organic compounds.

Vinyl Alcohol Formation

Vinyl alcohols contain a hydroxyl group (-OH) adjacent to a carbon-carbon double bond. They are often formed via nucleophilic addition reactions involving alkynes, where the addition results in an enol structure.
For example, in the reaction from the exercise, acetylene (an alkyne) reacts with formaldehyde in the presence of a methoxide ion. The addition of the nucleophilic methoxy group leads to the formation of a vinyl alcohol. This is because the hydroxyl group attaches to one of the carbons in the former triple bond, transforming it into a double bond and thus creating an enol.
Vinyl alcohols are interesting both for their reactivity and their fleeting nature, as they often rapidly tautomerize to more stable keto forms. This aspect emphasizes the importance of understanding reaction pathways and potential product transformations.

Electrophilic Reaction

Electrophilic reactions involve an electrophile, which is an atom or molecule with a positive charge or partially positive charge that looks to pair with an electron-rich site.
In the exercise, the alkyne (acetylene) acts less as an individual electrophile and more as a reactive site capable of accepting nucleophiles. However, alkynes can exhibit electrophilic character in reactions under different conditions, such as halogenation or other reactions leading to the formation of dihaloalkenes.
Understanding electrophilic reactions is crucial because it helps in predicting the site of attack and directing group transformations. These reactions can effectively transform larger substrates, which is key for building more complex structures from simpler beginnings in organic synthesis.