Question

Preparation and reactions of alcohols Alcohols can by prepared from alkyl halides, epoxides, alkenes, etc. When treated with acid, \(\mathrm{C}-\mathrm{OH}\) bond is cleaved heterolytically to form a carbocation which could then undergo substitution, elimination or rearrangement depending on the conditions and stability of the carbocation. When 1,2 -dimethyl cyclohexene is treated with cold dilute alkaline \(\mathrm{KMnO}_{4}\), the product obtained is

Short answer

Based on the given reaction, 1,2-dimethyl cyclohexene reacts with cold dilute alkaline KMnO4. This results in the oxidative cleavage of the alkene (double bond) via a dihydroxylation process. The final product formed is 1,2-dimethylcyclohexane-1,2-diol.

Step-by-step solution

1. Identify reactants and reaction conditions

In this reaction, we have 1,2-dimethyl cyclohexene as the reactant and cold dilute alkaline \(\mathrm{KMnO}_{4}\) as the reagent.

2. Find the mechanism of reaction

Cold dilute alkaline \(\mathrm{KMnO}_{4}\) is a strong oxidizing agent that is commonly used for the oxidative cleavage of alkenes (double bonds). In this case, the oxidizing agent reacts with the alkene and breaks the double bond. This process is called "dihydroxylation." The double bond will be cleaved to form two new \(\mathrm{C}-\mathrm{OH}\) bonds on the adjacent carbons where the double bond was initially present.

3. Determine the product

In the reaction of 1,2-dimethyl cyclohexene with cold dilute alkaline \(\mathrm{KMnO}_{4}\), dihydroxylation of the double bond occurs. This results in the formation of two new \(\mathrm{C}-\mathrm{OH}\) bonds where the double bond initially was, along with the retention of the two methyl groups present in the parent compound. The final product formed is 1,2-dimethylcyclohexane-1,2-diol.

Key concepts

Alcohols Preparation

Alcohols can be synthesized from various starting materials through different methods. One common method involves using alkyl halides. In this process, an alkyl halide is reacted with a nucleophile, such as \( ext{OH}^-\), to replace the halide group with a hydroxyl group (\( ext{OH}\)).
This leads to the formation of an alcohol. Another popular method involves the hydration of alkenes. Here, alkenes react with water in the presence of an acid, adding a hydroxyl group to one of the carbons of the double bond.

Another interesting method is the reduction of carbonyl compounds, where aldehydes or ketones are reduced by agents like \( ext{LiAlH}_4\) or \( ext{NaBH}_4\) to produce primary or secondary alcohols, respectively.
By choosing different starting materials and conditions, a wide array of alcohols can be synthesized efficiently, supporting diverse applications in industrial and laboratory settings.

Oxidative Cleavage

Oxidative cleavage is a reaction where a single molecule is split into two or more products using an oxidizing agent. This reaction is particularly useful in breaking double bonds, such as those found in alkenes. A common reagent for oxidative cleavage in organic chemistry is potassium permanganate (\( ext{KMnO}_4\)).
When cold, dilute, and alkaline, \( ext{KMnO}_4\) can mediate the oxidative cleavage of alkenes, transforming them into compounds with \-OH groups on what were previously double-bonded carbons.

This results in diols—molecules with two hydroxyl groups. Oxidative cleavage can be used strategically to modify or disrupt the compound's original structure, yielding new molecules that retain functionalities significant to different reactions.

Dihydroxylation

Dihydroxylation is a chemical process where a double bond in an alkene is converted into two hydroxyl groups across the former double bond. This process is an addition reaction that results in a "syn" orientation, meaning the groups add to the same side of the double bond.
Cold dilute alkaline \( ext{KMnO}_4\), as mentioned earlier, is a classic reagent for dihydroxylating alkenes.

Through dihydroxylation, significant transformations in organic chemistry are achieved. The resulting diols have added polarity and reactivity compared to the parent alkene, making them useful in further chemical transformations, such as protection of diols, or forming cyclic acetals. They also play a critical role in synthesizing complex natural products and pharmaceuticals.

Carbocation Reactions

Carbocations are positively charged intermediates that form during certain reactions, such as when an acid is used to cleave the \( ext{C-OH}\) bond in alcohols. Because carbocations are highly reactive, their formation opens the pathway to various transformations.

The stability of a carbocation is crucial. Tertiary carbocations are more stable than secondary or primary ones due to the electron-donating effect of surrounding alkyl groups. This stability influences the course of the reaction. Once formed, carbocations can undergo:

• Substitution - where another nucleophile can attack the carbocation to form a new compound.
• Elimination - which leads to the formation of alkenes.
• Rearrangement - a structural shift to a more stable carbocation.

Understanding carbocation behavior is essential in predicting the outcomes of many organic reactions and designing synthetic pathways.