Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following reactions gives a secondary alcohol? (a) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2} \frac{\text { peroxide }}{\mathrm{HBr}} \stackrel{\mathrm{H}_{2} \mathrm{O}, \mathrm{OH}^{-}}{\longrightarrow}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2} \frac{\text { (i) } \mathrm{BH}_{3}(\mathrm{THF})}{\text { (ii) } \mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{OH}^{-}}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2} \stackrel{\mathrm{H}_{3} \mathrm{SO}_{4}}{\mathrm{H}_{2} \mathrm{O}}\) (d) \(\underbrace{\mathrm{CH}_{2}-\mathrm{CH}_{2}}_{\mathrm{O}} \frac{\text { (i) } \mathrm{CH}_{3} \mathrm{MgBr}}{\text { (ii) } \mathrm{H}_{3} \mathrm{O}^{+}}\)

Step-by-Step Solution

Answer

Reaction (c) gives a secondary alcohol.

Step 1: Understanding the Reaction Type for Each Option

For option (a), we have a hydrohalogenation to an alkene, but since it involves peroxide, it follows the anti-Markovnikov rule, meaning the Br atom attaches to the least substituted carbon. For option (b) with hydroboration-oxidation, the alcohol forms at the less substituted carbon following the anti-Markovnikov rule. Option (c) involves a hydration reaction, likely to follow Markovnikov's rule where OH attaches to the more substituted carbon. For option (d), a Grignard reagent reacts with an epoxide, which typically leads to ring-opening at the less hindered carbon.

Step 2: Identify the Products of Each Reaction

For reaction (a), the product will be 1-bromo-2-propanol, a combination scenario unlikely since the question prioritizes the occurrence of secondary alcohols directly. For option (b), the product will be propan-2-ol, a secondary alcohol. In option (c), hydration forms propan-2-ol, another secondary alcohol. In option (d), the reaction forms prop-2-en-1-ol through a ring opening on the linear molecule side then a subsequent processing.

Step 3: Determine Which Reaction Results in a Secondary Alcohol

Our task is to find the reaction that directly leads to the formation of a secondary alcohol. Option (a) does not directly lead to an alcohol formation reaction. Although option (b) produces propan-2-ol, option (c) also does produce propan-2-ol through an acid-catalyzed hydration process. Option (d) will lead to an alcohol but not primarily as a secondary alcohol.

Step 4: Conclude with the Correct Reaction

Upon analyzing each option, reaction (c) results in a secondary alcohol through direct acid-catalyzed hydration, which commonly targets the more substituted carbon for OH attachment.

Key Concepts

Markovnikov's Rule

In organic chemistry, Markovnikov's Rule helps predict how molecules react when adding a reagent to an unsymmetrical alkene. According to Markovnikov's Rule, the hydrogen atom of the reagent will attach to the carbon in the alkene that has the most hydrogen atoms already. Meanwhile, the other part of the reagent will attach to the carbon that has fewer hydrogen atoms. This results in the more substituted carbon becoming the new attachment site for components such as halogens or hydroxyl groups.

In the context of alcohol formation, this rule often plays a crucial part in determining where the OH group will add. For example, in hydration reactions where water adds to an alkene, Markovnikov's Rule predicts the OH will attach to the more substituted carbon, resulting in a secondary or tertiary alcohol, depending on the accompanying structure.

This rule is especially useful when dealing with reactions like acid-catalyzed hydration, as seen in option (c) of the exercise. It helps to explain why propan-2-ol, a secondary alcohol, is typically formed because the OH prefers the more substituted center as guided by Markovnikov's principles.

Anti-Markovnikov Rule

The Anti-Markovnikov Rule suggests a contrasting approach where the addition of atoms across the bond occurs at the least substituted carbon. This usually involves radical processes rather than the typical ionic processes suggested by Markovnikov's Rule. When peroxides are involved, this rule takes precedence as it circumvents the usual ionic pathway.

Such reactions are characteristic of hydrohalogenation processes in the presence of a peroxide. This rule is evident in the case of option (a) of the exercise, where using HBr with a peroxide causes the bromine to attach to the less substituted carbon. This produces 1-bromo-2-propanol, emphasizing the role of peroxides in directing the reaction path towards the Anti-Markovnikov addition.

Radical intermediates formed under these conditions ensure that the reagents will attach themselves following the Anti-Markovnikov orientation, opposite to what traditional ionic paths might predict.

Hydroboration-Oxidation

Hydroboration-oxidation is a two-step process that adds across a double bond with a specific preference confirmed by the Anti-Markovnikov Rule.
The first step involves the addition of borane ( BH_3 ) in a tetrahydrofuran (THF) solution, which adds hydrogen and boron across the double bond. This reaction is concerted, meaning it occurs simultaneously, and leads to syn addition where both components add to the same side of the molecule.

The second step completes with the oxidation of the boron group by hydrogen peroxide ( H_2O_2 ) in an alkaline medium, replacing the boron with a hydroxyl group ( OH ). This typically results in the formation of an alcohol on the less substituted carbon, as seen in option (b) of the exercise, where the resultant product is propan-2-ol, a secondary alcohol.

This method is invaluable for providing anti-Markovnikov alcohol additions in a simple and direct manner.

Grignard Reaction

Grignard Reagents are powerful tools used to form carbon-carbon bonds. These are organometallic compounds generally of the form R-MgX , with R being an organic group, and X a halogen. In Grignard reactions, these reagents can effectively add to carbon-oxygen bonds like those found in epoxides.

When a Grignard reagent reacts with an epoxide, the reaction typically opens the epoxide ring at the least hindered carbon due to sterics, resulting in the addition of the Grignard reagent to compensate. After acid work-up with H_3O^+ , an alcohol is formed with the newly introduced carbon chain extending from the original epoxide. In option (d), the addition of CH_3MgBr leads to an epoxide ring opening to form prop-2-en-1-ol.

This process is a practical application of Grignard reagents and demonstrates their versatility in forming alcohols from simple epoxides.

Epoxide Ring-Opening

Epoxide Ring-Opening occurs when an epoxide, a cyclic ether with a three-membered ring, is reacted with nucleophiles. The strain in the small three-membered ring makes epoxides highly reactive and capable of undergoing ring-opening reactions.

The reaction can proceed under either acidic or basic conditions, with each condition favoring different sites for the attack.

• Under acidic conditions, the nucleophile typically attacks the more substituted carbon due to its higher stability.
• Under basic conditions, it preferentially attacks the less hindered carbon.

In option (d) from the exercise, a Grignard reagent acts as the nucleophile, favoring the less hindered site, showing how conditions affect the ring opening and the structure of the final alcohol product.

This versatile reaction, particularly useful in synthesizing complex alcohols, is widely applied in organic synthesis due to the functional flexibility of the epoxide group.