Question

Cold and dil. \(\mathrm{KMnO}_{4}\) combines with but-2-yne to form: (a) Butane-1, 3-diol (b) Butane-2, 3-diol (c) Butane-2, 3-dione (d) Butan-2-one

Short answer

The product is (b) butane-2,3-diol.

Step-by-step solution

Identify the Reagents and Reaction Type

The exercise gives cold and diluted potassium permanganate (\(\text{KMnO}_4\)) reacting with but-2-yne. Cold, dilute \(\text{KMnO}_4\) is known for oxidizing alkenes and alkynes to glycols through syn-dihydroxylation. But-2-yne is an alkyne, which can undergo this dihydroxylation under these conditions.

Analyze the Structure of But-2-yne

But-2-yne has a carbon-carbon triple bond between the 2nd and 3rd carbon. Its structure can be represented as\(\text{CH}_3-\text{C} \equiv \text{C}-\text{CH}_3\).

Apply Syn-Dihydroxylation Mechanism

Cold, dilute \(\text{KMnO}_4\) adds two hydroxyl groups (OH) across the triple bond in a syn-addition manner. This converts the triple bond into a double bond with hydroxyl groups on the 2nd and 3rd carbons.

Identify the Final Product

After the syn-dihydroxylation reaction, the alkyne is converted into a vicinal diol, specifically butane-2,3-diol because the hydroxyl groups are on the 2nd and 3rd carbon atoms.

Select the Correct Option

Based on the described reaction and product, the correct answer is the option that corresponds to butane-2,3-diol. This corresponds to option (b).

Key concepts

Syn-Dihydroxylation

Syn-dihydroxylation is a key reaction mechanism that involves the addition of two hydroxyl groups ( 6 0H) to an unsaturated carbon-carbon bond, specifically in an alkene or alkyne. In this process, both 6 0H groups are added to the same side of the bond, which is why it is referred to as 'syn' addition.
This reaction is vital for converting unsaturated hydrocarbons into their alcohol derivatives, known as diols, due to the presence of two 6 0H groups.
Syn-dihydroxylation is widely used in synthetic chemistry for its ability to precisely modify unsaturated bonds, and plays a crucial role in the conversion of alkynes into vicinal diols. Understanding this reaction sets a foundation for mastering more complex oxidation reactions in organic chemistry.

Cold Dilute KMnO4

Potassium permanganate ( 6 0K 0M 0n 0O_4) is a powerful oxidizing agent used extensively in organic chemistry. When used cold and in a dilute state, it facilitates gentler reactions, such as the dihydroxylation of alkenes and alkynes to form glycols.
Cold dilute 6 0K 0M 0n 0O_4 can transform triple bonds into diols without over-oxidizing the molecule. This condition is optimal for achieving syn-dihydroxylation, avoiding the production of excessive by-products. The use of cold dilute 6 0K 0M 0n 0O_4 is preferred for reactions where careful control of oxidation level is important, particularly in transforming potent triple bonds into more stable and reactive forms.

But-2-yne Oxidation

But-2-yne is an alkyne consisting of four carbon atoms connected by a carbon-carbon triple bond between the 2nd and 3rd carbons. Its structural formula can be depicted as 6 0H_3- 6 0C 6 6 6 0C- 6 0H_3.
During oxidation, especially with cold dilute 6 0K 0M 0n 0O_4, the triple bond undergoes syn-dihydroxylation, adding hydroxyl groups to these carbons. This oxidation is highly selective, ensuring that both hydroxyls are appended to the same side in a controlled syn addition, forming a more stable compound known as a vicinal diol without proceeding to further oxidation stages.

Vicinal Diol Formation

A vicinal diol is an organic compound with two hydroxyl groups bonded to adjacent carbon atoms. During the syn-dihydroxylation of but-2-yne, the reaction results in the formation of butane-2,3-diol.
This process involves the conversion of the carbon-carbon triple bond into a single bond, while simultaneously adding hydroxyls to the resultant carbon atoms. This switch not only saturates the molecule but also introduces new functionality, making the molecule significantly more reactive and versatile for further chemical transformations.

• Vicinal diols are important intermediates in organic synthesis.
• They open pathways for synthesizing other compounds by further oxidation or reduction reactions.

Mastering the formation and handling of vicinal diols is crucial for students aiming to understand complex synthetic routes in organic chemistry.