Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A biologically active compound, bombykol \(\left(\mathrm{C}_{16} \mathrm{H}_{30} \mathrm{O}\right)\) is obtained from a natural source. The structure of the compound is determined by the following reactions. (a) On hydrogenation, bombykol gives a compound A, \(\mathrm{C}_{16} \mathrm{H}_{34} \mathrm{O}\), which reacts with acetic anhydride to give an ester; (b) Bombykol also reacts with acetic anhydride to give another ester, which on oxidative ozonolysis \(\left(\mathrm{O}_{3} / \mathrm{H}_{2} \mathrm{O}_{2}\right)\) gives a mixture of butanoic acid, oxalic acid and 10 -acetoxydecanoic acid. Determine the number of double bonds in bombykol. Write the structures of compound \(\boldsymbol{A}\) and bombykol. How many geometrical isomers are possible for bombykol?

Step-by-Step Solution

Answer

Bombykol has 2 double bonds and 4 geometrical isomers. Compound \(A\) is a saturated alcohol.

Step 1: Chemical Formula Analysis

Let's start by analyzing the given molecular formulas. Bombykol has the formula \(\mathrm{C}_{16} \mathrm{H}_{30} \mathrm{O}\). After hydrogenation, it forms compound \(A\), which is \(\mathrm{C}_{16} \mathrm{H}_{34} \mathrm{O}\). The increase in hydrogen atoms suggests the addition of hydrogen across double bonds.

Step 2: Determine Number of Double Bonds

To determine the number of double bonds, count the added hydrogen atoms during hydrogenation: \(\mathrm{C}_{16} \mathrm{H}_{30} \mathrm{O}\rightarrow \mathrm{C}_{16} \mathrm{H}_{34} \mathrm{O}\). The hydrogen atom increase is 4 (from 30 to 34), which indicates there are 2 double bonds in bombykol, since each double bond adds 2 hydrogen atoms upon hydrogenation.

Step 3: Reaction with Acetic Anhydride to Form Ester

Bombykol forming an ester with acetic anhydride directly indicates that it contains a hydroxyl functional group (alcohol) in its structure, which reacts to form an ester linkage.

Step 4: Understanding Ozonolysis Products

Ozonolysis of bombykol's ester gives butanoic acid, oxalic acid, and 10-acetoxydecanoic acid. The fragmentation pattern suggests that the double bonds are interrupted by certain functional groups present in these products, indicating possible positions of C=C in the original structure.

Step 5: Determine Possible Geometrical Isomers

Two double bonds in bombykol can each exist as either \(\text{cis}\) or \(\text{trans}\) configurations. Therefore, the number of possible geometric isomers is \(2^2 = 4\), accounting for each double bond having two possible configurations.

Step 6: Draw Chemical Structures

Based on the information, compound \(A\) is \(\mathrm{C}_{16} \mathrm{H}_{34} \mathrm{O}\), likely a fully saturated alcohol (a 16-carbon straight-chain alcohol). Bombykol, therefore, should have a similar structure but with 2 double bonds and one hydroxyl group, considering the need to form an ester and the described ozonolysis products.

Key Concepts

Hydrogenation

Hydrogenation is a key reaction in organic chemistry, particularly when working with alkenes—compounds with carbon-carbon double bonds. In this process, hydrogen molecules add to the double bonds, effectively turning alkenes into alkanes, which are saturated hydrocarbons. For bombykol, hydrogenation results in the compound transforming from \(\mathrm{C}_{16} \mathrm{H}_{30} \mathrm{O}\) to a fully saturated \(\mathrm{C}_{16} \mathrm{H}_{34} \mathrm{O}\).

• This increase of hydrogen by 4 atoms signifies the presence of 2 double bonds originally in bombykol, since each double bond consumes two hydrogen molecules during the reaction.
• Hydrogenation is crucial for studying the structural aspects of organic compounds because it simplifies the understanding of molecule saturation levels.

Note that this process often requires a catalyst, like Palladium or Nickel, to facilitate the addition of hydrogen atoms. Understanding these reactions gives insights into how molecular structures can be modified in synthetic chemistry.

Ozonolysis

Ozonolysis is a powerful technique used to understand the positioning of double bonds within a molecule. In this reaction, ozone (\(\mathrm{O}_{3}\)) cleaves double bonds, leading to chain fragmentation and forming carbonyl-containing products.For bombykol, when subject to ozonolysis in the presence of hydrogen peroxide, it produces butanoic acid, oxalic acid, and 10-acetoxydecanoic acid.

• The appearance of these specific products reflects the positions where the double bonds in bombykol were originally located.
• This helps in mapping out the molecular structure before and after reaction, showing how substructures of the molecule are linked.

The method serves not only to confirm the presence of double bonds but also gives clues to the surrounding functional groups, as seen with the production of the acids mentioned above. Thus, ozonolysis is pivotal in organic synthesis and structural elucidation.

Molecular Formula Analysis

Molecular formula analysis is a foundational step in understanding organic compounds. The formula provides a count of each type of atom present in the molecule, allowing chemists to deduce possible structures and functionalities.In the case of bombykol, the formula \(\mathrm{C}_{16} \mathrm{H}_{30} \mathrm{O}\) indicates:

• Sixteen carbons, thirty hydrogens, and one oxygen atom.
• This can suggest the presence of multiple types of functional groups, such as double bonds (due to fewer hydrogens compared to a saturated chain) and possibly a hydroxyl group (alcohol) if it forms an ester.

Molecular formula analysis sets the stage for further confirmation via reactions like hydrogenation and ozonolysis, revealing specifics about the compound's structure and chemical properties.

Geometrical Isomers

Geometrical isomers arise from compounds with the same molecular formula but different spatial arrangements due to restricted bond rotation, typically around a double bond.In bombykol, two double bonds are present, allowing for possible geometrical isomers known as \(\text{cis}\) and \(\text{trans}\) configurations. Each double bond can independently be either \(\text{cis}\) (same side) or \(\text{trans}\) (opposite sides), leading to several isomer possibilities.

• For two double bonds, each offering two configurations, the total isomer count is \(2^2 = 4\).
• This highlights the significance of isomerism in determining the physical and chemical properties of molecules, as slight changes in geometry can lead to vastly different functionalities or interactions.

Understanding geometrical isomers is crucial, especially in biological contexts, as organisms may react differently to each isomer due to spatial considerations.