Problem 6 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The synthesis of 3-octyne is achieved by adding a bromoalkane into a mixture of sodium amide and an alkyne. The bromoalkane and alkyne respectively are [2010] (a) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) (b) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\) (c) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\) (d) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}\)

Step-by-Step Solution

Answer

(c) Bromoalkane: \( \mathrm{BrCH_2CH_2CH_2CH_2CH_3} \), Alkyne: \( \mathrm{CH_3C \equiv CH} \).

Step 1: Understand the Target Molecule

The target molecule is 3-octyne, which has the structure \\( \mathrm{CH_3(CH_2)_2C \equiv C(CH_2)_2CH_3} \). This implies a carbon chain with a total of eight carbon atoms and a triple bond between the third and fourth carbon atoms.

Step 2: Determine the Required Fragments

To synthesize 3-octyne, we need to identify the fragments that could be combined in the reaction. The bromoalkane will contribute carbon atoms to one part of the chain, and the alkyne will contribute the alkynyl group and possibly part of the chain as well.

Step 3: Analyze Option (a)

In option (a), the bromoalkane is \( \mathrm{BrCH_2CH_2CH_2CH_2CH_3} \) (5 carbons) and the alkyne is \( \mathrm{CH_2CH_2C \equiv CH} \) (4 carbons). Combining these does not provide the correct placement of the triple bond for 3-octyne.

Step 4: Analyze Option (b)

In option (b), the bromoalkane is \( \mathrm{BrCH_2CH_2CH_3} \) (3 carbons) and the alkyne is \( \mathrm{CH_3CH_2CH_2C \equiv CH} \) (4 carbons). This would make a product with a 3-carbon chain plus a 4-carbon chain forming hept-3-yne, not octyne.

Step 5: Analyze Option (c)

In option (c), the bromoalkane is \( \mathrm{BrCH_2CH_2CH_2CH_2CH_3} \) (5 carbons) and the alkyne is \( \mathrm{CH_3C \equiv CH} \) (2 carbons). When combined, these can form 3-octyne: the bromoalkane forms a chain with the alkyne to provide a six-carbon entity which adds another carbon through coupling to satisfy the structure of 3-octyne.

Step 6: Analyze Option (d)

In option (d), the bromoalkane is \( \mathrm{BrCH_2CH_2CH_2CH_3} \) (4 carbons) and the alkyne is \( \mathrm{CH_3CH_2C \equiv CH} \) (3 carbons). This also does not place the triple bond correctly for 3-octyne, as the combined structure would have the wrong placement for the alkyne.

Step 7: Identify the Correct Option

Among the options, option (c) provides the correct approach. Using the 5-carbon bromoalkane and 2-carbon alkyne allows the formation of 3-octyne, aligning with the structure of the desired product.

Key Concepts

Bromoalkane

Bromoalkanes are a type of organic molecule where a bromine atom is bonded to an alkane (a chain of carbon and hydrogen atoms). In this context, the bromoalkane functions as a carbon chain donor in the synthesis reaction.
In the synthesis of compounds like 3-octyne, bromoalkanes offer a convenient way to add specific lengths of carbon chains to the molecule being synthesized. For example, using a bromoalkane like \( \mathrm{BrCH_2CH_2CH_2CH_2CH_3} \) provides a five-carbon segment that eventually helps form a part of the desired 3-octyne structure.
The reaction involving bromoalkanes frequently uses them in coupling processes, where they react with other components to extend or modify the carbon backbone. This makes them vital in forming complex structures, especially in organic synthetic chemistry.

Sodium Amide

Sodium amide (\( \mathrm{NaNH_2} \)) is a potent base commonly utilized in organic chemistry for various reactions, including the synthesis of alkynes. Its strong basic nature enables it to deprotonate molecules, which is a key step in many synthesis pathways.
In the case of alkyne synthesis, sodium amide is used to remove a hydrogen atom from a terminal alkyne (an alkyne with a hydrogen atom at the end). This creates an alkynide ion, which is highly reactive and can attack an electrophile, like a bromoalkane, to form more complex organic molecules.
When sodium amide is added to a mixture with a terminal alkyne and a bromoalkane, it initiates substitution reactions that lead to the creation of new bonds, essential in building the kind of structures seen in molecules like 3-octyne. This ability to facilitate bond formation by altering the electronic structure of molecules is why sodium amide is so valuable in this type of synthetic chemistry.

Carbon Chain

The carbon chain is the backbone of organic molecules, defining their length and shape, which are crucial for determining the chemical properties and functionalities of these compounds. In the synthesis of 3-octyne, the carbon chain must be carefully constructed to match the desired structure.
For example, when synthesizing 3-octyne, you must ensure that there are exactly eight carbon atoms with a triple bond between the third and fourth carbons. This involves not only selecting the correct bromoalkane and alkyne but also correctly coupling these components through reactions facilitated by reagents such as sodium amide.
Manipulating carbon chains allows chemists to create complex molecules that are used in various applications, from pharmaceuticals to materials science. Thus, understanding how to control and modify these chains is a fundamental skill in organic chemistry. In scenarios like the given exercise, selecting the right components and understanding how they come together to form specific carbon chain structures is key to successful synthesis.