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

Question

Which one of the following reactions gives a ary 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) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2} \frac{\mathrm{H}_{2} \mathrm{SO}_{4}}{\mathrm{H}_{2} \mathrm{O}}\) (d) \(\underbrace{\mathrm{CH}_{2}-\mathrm{CH}_{2}}_{\mathrm{O}} \frac{\text { (i) } \mathrm{CH}_{3} \mathrm{MgBr}}{\text { (ii) } \mathrm{H}_{1} \mathrm{O}^{+}}\)

Step-by-Step Solution

Answer

None of the options form an aryl alcohol directly.

Step 1: Analyze Option (a)

The reaction process involves the addition of HBr with peroxide, followed by water in a basic medium. This is known as the anti-Markovnikov addition of HBr. The OH group eventually adds to the secondary carbon. Thus, it forms a secondary (2°) alcohol.

Step 2: Analyze Option (b)

In the presence of boron trifluoride in tetrahydrofuran (THF) and hydrogen peroxide and base, hydroboration-oxidation occurs. This process attaches the hydroxyl group (OH) to the less substituted carbon of the alkene, which would result in a primary alcohol. However, the structure provided aligns with a secondary alcohol, not ary.

Step 3: Analyze Option (c)

The process suggested by this option is the acid-catalyzed hydration of an alkene. This moves via the Markovnikov addition, where the OH adds to the more substituted carbon, resulting in a secondary alcohol.

Step 4: Analyze Option (d)

The epoxide is opened by the Grignard reagent (CH₃MgBr), leading to the formation of an alkoxide, which after hydrolysis, forms a primary alcohol. Although the ring structure from this reaction might suggest a type of aromatic behavior, it does not directly result in an aryl alcohol.

Step 5: Determine the Presence of Aryl Alcohol

For an alcohol to be classified as aryl, it must have the hydroxyl group directly attached to an aromatic ring. Thus, none of the given options produce a true aryl alcohol directly from the selected reagents.

Key Concepts

Anti-Markovnikov Addition

In organic chemistry, understanding how different addition reactions work is key. Anti-Markovnikov addition is a reaction rule that helps predict the outcome of certain addition reactions involving hydrogen halides, especially HBr, across alkenes in the presence of peroxides, like hydrogen peroxide. The unique feature of this addition is that the halogen, unlike in Markovnikov addition, attaches itself to the less substituted carbon of the double bond.
This happens because of the radical mechanism initiated by peroxides. Here’s how:

• Firstly, peroxides promote the formation of free radicals.
• These radicals induce the anti-Markovnikov addition, leading to the halogen joining at the less hindered side of the alkene.
• In the end, you’ll often end up creating secondary or even primary alcohols following the addition and subsequent steps.

In the context of our exercise, option (a) represents this process, showing an eventual secondary alcohol formation due to this mechanism.

Hydroboration-Oxidation

Hydroboration-oxidation is a two-step process highly valued in organic synthesis for its ability to add water across a double bond in an anti-Markovnikov fashion. Here, borane (BH₃) or its complex with THF acts as the principal player in bringing the OH group to the least substituted carbon, which is crucial for forming specific alcohol structures.
Here’s a clear step-by-step:

• The first step is the hydroboration phase, where the boron atom forms a bond with the less substituted carbon of the alkene employing syn addition.
• This is followed by oxidation using hydrogen peroxide and a base, often NaOH, which replaces the boron with a hydroxyl group.
• The result? A primary alcohol, in cases where the alkene hasn't been shifted or rearranged.

In the exercise, option (b) reflects this pathway, but suggests a secondary alcohol due to the specific structure involved, rather than forming an aryl alcohol.

Acid-Catalyzed Hydration

In acid-catalyzed hydration, a classic example of a Markovnikov addition, water is added to alkenes using an acid catalyst, commonly sulfuric acid (H₂SO₄). This reaction usually places the hydroxyl group on the more substituted carbon, making it Markovnikov in nature.
Here's what typically happens:

• The alkene first protonates, forming a carbocation on the more stable carbon, which is often the more substituted one.
• Next, water attacks this positively charged site, attaching an OH group where the carbocation previously was.
• Finally, the structure rearranges, if necessary, to stabilize, yielding a more stable alcohol, often a secondary or tertiary one.

In the context of the exercise, option (c) indicates application of this concept, producing a secondary alcohol, but not an aryl one, since it lacks the aromatic ring structure inherent to aryl alcohols.

Grignard Reaction

The Grignard reaction is a highly versatile method in synthetic organic chemistry for forming carbon-carbon bonds, generating alcohols using organomagnesium halides, called Grignard reagents. These reagents are prepared by reacting alkyl halides with magnesium turnings in anhydrous ether.
This method involves:

• The nucleophilic attack of the Grignard reagent on an electrophilic carbon, typically a carbonyl carbon or an epoxide, opening the ring in the latter case.
• Water or another proton source then facilitates conversion of this intermediate into an alcohol.
• The resulting product type depends largely on the original setup: with carbonyls, a range from primary to tertiary alcohols, while epoxides usually yield primary alcohols when opened by simple Grignard reagents.

As noted in option (d) of the exercise, despite the theoretical pathway potentially suggesting an aromatic character, this does not result in a genuine aryl alcohol, as the hydroxyl group is not directly bonded to an aromatic ring.