Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions will yield propan-2-ol? \(1 \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O} \quad \stackrel{\mathrm{H}^{+}}{\longrightarrow}\) \(4 \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{3} \stackrel{\text { Neutral } \mathrm{KMnO}_{4}}{\longrightarrow}\) (a) 1 and 2 (b) 2 and 3 (c) 3 and 1 (d) 2 and 4

Step-by-Step Solution

Answer

Reaction 1 yields propan-2-ol, so option (a) is correct.

Step 1: Analyze the first reaction

The first reaction is an addition of water (hydration) to an alkene, in the presence of an acid catalyst (H+). This type of reaction is likely to result in the formation of an alcohol. The starting material is propene (CH2=CH-CH3). Adding H2O in the presence of H+ will yield propan-2-ol through the Markovnikov's rule where the OH group attaches to the more substituted carbon atom.

Step 2: Analyze the second reaction

The second reaction is the reaction of propene with neutral potassium permanganate (KMnO4). This is a dihydroxylation reaction, which adds an OH group to each carbon of the double bond, forming a diol. However, this will not give propan-2-ol but instead 1,2-propanediol.

Step 3: Evaluate the options

Considering the analysis from steps 1 and 2: reaction 1 yields propan-2-ol, but reaction 2 does not. So, options that include reaction 1 are possible candidates. Option (c) mentions reaction 3, which is not provided in the choices, indicating an error. Therefore, the only correct option is the one which only involves reaction 1.

Step 4: Conclusion

After analyzing all possibilities, the correct reaction that yields propan-2-ol is only the first reaction. Therefore, the correct answer must include option 1.

Key Concepts

Hydration of Alkenes

Hydration of alkenes is a fundamental reaction in organic chemistry, transforming an alkene into an alcohol. The reaction involves the addition of water (H₂O) across the alkene's double bond. This process typically requires the presence of an acid catalyst, such as sulfuric acid or phosphoric acid, to proceed.

In the hydration of an alkene, the electrophilic addition mechanism is involved. Here’s a simplified breakdown of the process:

• Initially, the acid catalyst donates a proton (H⁺) to the alkene. This forms a carbocation by breaking the double bond and attaching a hydrogen atom to one of the carbon atoms.
• Next, the water molecule attacks the positively charged carbocation, resulting in the formation of a protonated alcohol.
• Finally, a deprotonation step occurs, where a hydrogen ion is removed to form the stable alcohol product.

Overall, this reaction is a great way to synthesize alcohols from alkenes, and it serves as a classic example of interconverting functional groups in organic synthesis.

Markovnikov's Rule

Markovnikov's rule is a key principle in understanding the outcome of hydrohalogenation and hydration reactions involving alkenes. According to this rule, in the addition of protic acids (like HBr, HCl, or H₂O in the presence of acid) to alkenes, the hydrogen atom adds to the carbon with the most hydrogen atoms already attached (the least substituted carbon), while the addendum (like OH or Br) attaches to the most substituted carbon.

This tendency arises because the formation of the more stable carbocation during the reaction's intermediate steps is favored. More substituted carbocations are more stable due to hyperconjugation and induction effects. For example, in the hydration of propene, the OH group attaches to the central carbon atom, resulting in the production of propan-2-ol.

• Ensures the major product of the reaction is typically the one formed by adherence to these stability principles.
• Helps predict the outcomes in organic synthesis, aiding chemists in designing synthetic pathways efficiently.

Understanding and applying Markovnikov's rule is crucial in explaining the regioselectivity of these addition reactions.

Dihydroxylation

Dihydroxylation is a type of chemical reaction that introduces two hydroxyl groups (OH) into a molecule. Specifically, for alkenes, this transformation involves adding an OH group to each of the carbon atoms involved in the double bond.

A common reagents used in dihydroxylation includes neutral potassium permanganate (KMnO₄). This results in syn-dihydroxylation, meaning both hydroxyl groups add to the same side of the former double bond.

• Creates vicinal diols, compounds with OH groups on adjacent carbons.
• This process is important in synthetic organic chemistry for building more complex molecular structures.

Dihydroxylation enriches the chemical functionality of compounds by converting simple alkenes into molecules with multiple hydroxyl groups, enhancing their reactivity and applications in further reactions.

Organic Reaction Mechanisms

Organic reaction mechanisms describe the step-by-step sequence of events at the molecular level that lead to the conversion of reactants to products. Understanding these mechanisms is crucial because they explain how and why reactions occur.

Each mechanism involves several key concepts:

• **Nucleophiles and Electrophiles:** Understanding these reactive species is fundamental. Nucleophiles donate electrons, while electrophiles are electron-deficient and seek additional electrons.
• **Transition States and Intermediates:** These are high-energy structures formed during the reaction. Transition states represent the momentary position the atoms may pass through, and intermediates are temporary structures with a definite lifetime.
• **Energy Profiles:** Depicted using reaction coordinate diagrams, these profiles help visualize activation energy peaks and relative stability of intermediates.

By learning organic reaction mechanisms, chemists can predict reaction products and design pathways to synthesize desired compounds efficiently. This understanding also aids in optimizing conditions and solutions for challenging transformations.