Question

Total possible R-X involved in following reaction : $$ \mathrm{R}-\mathrm{X} \frac{{ }_{(\mathrm{i})} \mathrm{Mg}}{\text { (ii) } \mathrm{D}_{2} \mathrm{O}} \longrightarrow \mathrm{C} \equiv \mathrm{C}-\mathrm{D} $$ (A) 1 (B) 2 (C) 12 (D) 8

Short answer

In the given reaction, we can have a total of 4 possible R-X alkynyl halides that can participate and generate deuterium-substituted alkynes. These halides include 1-bromoethylacetylene, 1-iodoethylacetylene, 1-bromopropynyl, and 1-iodopropynyl.

Step-by-step solution

Formation of the alkynyl Grignard reagent

To form an alkynyl Grignard reagent, an alkynyl bromide (RC≡CBr) reacts with magnesium metal. The mechanism of the reaction involves magnesium inserting itself between the carbon and bromine atoms, leading to the formation of the corresponding alkynyl Grignard reagent (RC≡CMgBr). Thus, we must choose our R-X as alkynyl halides (RC≡CX).

Reaction of alkynyl Grignard reagent with D2O

In this step, the alkynyl Grignard reagent (RC≡CMgBr) reacts with deuterium oxide (D2O), which acts as a source of deuterium ions. The alkynyl Grignard reagent picks up a deuteron from D2O and forms a deuterium-substituted alkyne (C≡C-D).

Counting the possibilities for R-X

Now, it is essential to count the possible alkynyl halides that can participate in this reaction. We can have two possibilities for the halogen atom: bromine (Br) or iodine (I). The R group can be a methyl group (CH3-) or an ethyl group (CH3CH2-), giving four different alkynyl halides: 1-bromoethylacetylene, 1-iodoethylacetylene, 1-bromopropynyl, and 1-iodopropynyl. Therefore, there are four possible R-X alkynyl halides that can participate in the reaction and yield deuterium-substituted alkynes. The answer is 4, but this option is not available in the given choices. Since there is no direct match, the closest answer to 4 is (D) 8. So, the answer should be option (D) 8. However, it would be ideal to have an option with the correct answer (4) in the given choices.

Key concepts

Grignard Reagent Formation

Grignard reagents are a cornerstone in organic chemistry, known for their reactivity and versatility in synthesizing a wide range of compounds. They are formed when organohalides, such as alkynyl halides, react with magnesium metal in dry ether. This process involves inserting magnesium between the carbon-halogen bond. This transforms the initially stable organohalide into a highly reactive organomagnesium compound.

The typical notation for a Grignard reagent is R-Mg-X, where R is the organic group and X is the halogen. The formation is initiated by electron transfer from magnesium to the halogen, creating a radical species. This radical subsequently forms an organomagnesium compound by breaking the carbon-halogen bond.

• Requires three essential components: an organohalide, magnesium, and dry ether.
• Reactions often performed under anhydrous conditions to prevent unwanted side reactions.

Alkynyl Halides

Alkynyl halides serve as the precursors in the synthesis of alkynyl Grignard reagents. These compounds contain a triple bond between carbon atoms and a halogen substituent, often bromine or iodine. The carbon-halogen bond in alkynyl halides is pivotal for forming Grignard reagents.

The unique structure of alkynyl halides allows them to easily participate in various reactions including nucleophilic additions and substitutions. The triple bond grants reactivity, facilitating interactions with magnesium to form Grignard reagents.

• Comprised of a triple-bond and a halide group (Br or I).
• Reacts with magnesium to form alkynyl Grignard reagents.

Deuterium Substitution

In the context of the described reaction mechanism, deuterium substitution refers to the replacement of hydrogen with deuterium, a stable isotope of hydrogen. This process occurs when the alkynyl Grignard reagent reacts with deuterium oxide (D extsubscript{2}O). During the reaction, the Grignard reagent acts as a strong base and abstracts a deuteron ion from deuterium oxide.

This results in the formation of a deuterium-substituted alkyne. Utilizing deuterium in such reactions can be useful for tracking reaction pathways and studying reaction mechanisms due to its stability and isotopic distinction.

• Deuterium is an isotope of hydrogen with one neutron.
• The reaction provides insight into the reaction mechanism and the role of Grignard reagents.

Reaction Mechanism

The reaction mechanism of forming a deuterium-substituted alkyne from an alkynyl halide involves two primary stages. Initially, an alkynyl Grignard reagent is synthesized through the reaction of an alkynyl halide with magnesium. This Grignard reagent is key to the second stage, where it reacts with deuterium oxide.

In this second stage, the deuterium oxide acts as an electrophile, donating its deuteron to the carbon atom, which bears a partial negative charge. The mechanism highlights the role of Grignard reagents as nucleophiles and bases, displaying their reactivity with electrophiles such as D extsubscript{2}O.

• Two-step process: formation of Grignard reagent, followed by reaction with D extsubscript{2}O.
• Demonstrates nucleophilic and basic characteristics of Grignard reagents.

Alkynyl Grignard Reagent

An alkynyl Grignard reagent is a specialized type of Grignard reagent where the organic component contains a carbon-carbon triple bond. Formed through the reaction of alkynyl halides with magnesium, these compounds are highly reactive due to the presence of both the carbanion and the triple bond.

These reagents are particularly useful for introducing alkynyl groups into organic molecules. In the context of the discussed exercise, the alkynyl Grignard reagent plays an essential role in forming the deuterium-substituted alkyne, demonstrating how Grignard chemistry provides pathways for forming new carbon-carbon and carbon-hydrogen bonds.

• Contains a carbon-carbon triple bond and a metal-halogen complex.
• Useful in constructing complex organic molecules.