Question

An alkene on mild oxidation with dil. aqueous \(\mathrm{KMnO}_{4}\) gave a product which on further treatment with \(\mathrm{HIO}_{4}\) gave hexane- 2,5 -dione. The alkene is (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}_{2}\) (c) Cyclohexane (d) 1, 2 dimethyl cyclobutene

Short answer

Based on the given information and step-by-step analysis, the initial alkene was: \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\)

Step-by-step solution

Understand the reactions involved

In this problem, there are two key reactions: 1. Mild oxidation of alkene with dilute aqueous \(\mathrm{KMnO}_{4}\) (Baeyer's test): This reaction results in the formation of vicinal diol (glycol) from the alkene. 2. Reaction with \(\mathrm{HIO}_{4}\) (Periodic acid cleavage): In this reaction, the vicinal diol formed earlier is cleaved to produce two carbonyl compounds.

Identify the functional groups present in the intermediate product

Based on the given information, the intermediate product that forms upon treatment with \(\mathrm{HIO}_{4}\) is hexane-2,5-dione. Hexane-2,5-dione has two carbonyl groups at positions 2 and 5. From this information, we can reason that the intermediate product (formed after the oxidation of alkene with \(\mathrm{KMnO}_{4}\)) must have been a vicinal diol that could be cleaved by \(\mathrm{HIO}_{4}\) to form hexane-2,5-dione.

Identify the starting alkene

We have to backtrack from hexane-2,5-dione to the intermediate vicinal diol and then to the initial alkene. Based on the intermediate product (hexane-2,5-dione), the possible vicinal diol would be: 3-hydroxypentane-2,4-diol. This vicinal diol would have been formed from the initial alkene by the mild oxidation with dilute aqueous \(\mathrm{KMnO}_{4}\). Therefore, the starting alkene must have a double bond between carbon atoms 2 and 3 (numbering from left to right). Thus, the alkene is: \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) Comparing with the given options, it corresponds to option (a).

Key concepts

Baeyer's Test

Baeyer's Test is an important qualitative reaction that helps identify the presence of unsaturation, such as double bonds, in organic compounds. It uses a dilute solution of potassium permanganate ($KMnO\_4$), which is a strong oxidizing agent that reacts with alkenes to form vicinal diols. In this reaction, the color of the purple KMnO4 solution changes to brown due to the formation of manganese dioxide ($MnO\_2$), indicating a positive test.

The mild conditions of Baeyer's test make it suitable for detecting alkenes without breaking the carbon skeleton, which can be crucial for subsequent reactions. For students understanding this concept, the color change and the conditions under which the reaction occurs are highly relevant, as they signal the presence of a double bond in the tested molecule.

Vicinal Diol Formation

The term 'vicinal' refers to two functional groups being on adjacent carbon atoms. During an alkene oxidation, such as the one in Baeyer's test, the double bond of the alkene reacts with the oxidizing agent to form a vicinal diol, where two hydroxyl (-OH) groups are attached to the carbon atoms that were previously involved in the double bond.

Understanding vicinal diol formation is crucial for students because it illustrates how a specific type of functional group transformation occurs. Recognizing the formation of these diols is also a stepping stone for further chemical transformations, like cleavage with periodate, which can be used to deduce the structure of the original alkene, as seen in the textbook exercise.

Periodic Acid Cleavage

Periodic acid cleavage involves the splitting of vicinal diols into carbonyl compounds using periodic acid ($HIO\_4$) as an oxidizing agent. The vicinal diol's carbon-carbon bond is broken, and one oxygen from the diol combines with each carbon to form the respective carbonyl groups. This reaction is a useful tool in organic chemistry for fragmenting molecules and deducing structures.

For the textbook exercise, the application of this reaction suggests that the alkene's oxidation with potassium permanganate must have produced a diol that, when cleaved with periodic acid, gives hexane-2,5-dione. It is an essential step for students to visualize how complex molecules can be dissected into simpler components for identification purposes.

Functional Group Identification

Identifying functional groups in organic compounds is fundamental to understanding their chemical behavior and reactions. In the given exercise, we focused on recognizing the presence of vicinal diols and carbonyl groups. Each functional group imparts different properties and reactivity, which is pivotal when predicting the outcome of chemical reactions.

Students can improve their grasp by practicing how to infer the presence of certain functional groups based on the products formed in reactions like oxidation or cleavage. This skill is integral in organic chemistry, as it assists in mapping reaction pathways and understanding mechanisms at a molecular level.

Organic Reaction Mechanisms

Organic reaction mechanisms provide a detailed step-by-step description of how a chemical reaction proceeds at an atomic level. They illustrate the breaking and formation of bonds, the rearrangement of atoms, and the changes in electron density throughout the process. For students to solve exercises like the one given, it is crucial to understand the underlying mechanisms of the Baeyer's test and periodic acid cleavage.

By breaking down reactions into individual steps, students can better predict the products and understand the role of each reactant. This enhances their ability to logically deduce the structure of unknown compounds and to design syntheses for target molecules. In educational contexts, clearly and succinctly explaining these mechanisms is vital for fostering students' comprehension of complex organic chemistry concepts.