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

Question

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

Step-by-Step Solution

Answer

(a) 1 and 2

Step 1: Analyze Reaction 1

Reaction 1 is the hydration of propene, \(\text{CH}_2=\text{CH}-\text{CH}_3\), in the presence of an acid catalyst (\(\text{H}^+\)). This addition reaction will lead to propan-2-ol by Markovnikov's rule, where water adds across the double bond, placing the -OH group on the secondary carbon to form propan-2-ol (\(\text{CH}_3-\text{CH(OH)}-\text{CH}_3\)).

Step 2: Analyze Reaction 2

In Reaction 2, ethanal (\(\text{CH}_3-\text{CHO}\)) is reacted with methylmagnesium iodide (\(\text{CH}_3\text{MgI}\)), a Grignard reagent, followed by hydrolysis (\(\text{H}_2\text{O}\)). This Grignard reaction adds a -CH3 group to the carbonyl carbon, forming a secondary alcohol, propan-2-ol (\(\text{CH}_3-\text{CH(OH)}-\text{CH}_3\)).

Step 3: Analyze Reaction 3

Reaction 3 involves formaldehyde (\(\text{CH}_2\text{O}\)) reacting with ethylmagnesium bromide (\(\text{C}_2\text{H}_5\text{MgBr}\)), followed by hydrolysis. This sequence results in the production of ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)), not propan-2-ol.

Step 4: Analyze Reaction 4

In Reaction 4, propene (\(\text{CH}_2=\text{CH}-\text{CH}_3\)) is reacted with neutral potassium permanganate (\(\text{KMnO}_4\)). This reaction is known as syn-dihydroxylation, which adds two hydroxyl groups to the molecule, converting the alkene to a glycol, not propan-2-ol.

Step 5: Determine the Correct Answer

From the analyses, Reaction 1 and Reaction 2 both yield propan-2-ol, while Reactions 3 and 4 do not. Therefore, the correct choices are 1 and 2, corresponding to option (a).

Key Concepts

Markovnikov's Rule

Markovnikov's rule is a guideline used in organic chemistry to predict the outcome of reactions involving alkenes. Specifically, it helps determine where atoms or groups, like -OH in hydration reactions, will attach to a carbon chain. When an unsymmetrical molecule like

• Water (H-OH) adds to an unsymmetrical alkene (like propene),
• The hydrogen (H) will attach to the carbon with more hydrogens (less substituted),
• The added group (like OH) will add to the more substituted carbon with fewer hydrogens.

This rule is crucial in predicting and understanding the formation of major products in reactions. In the context of this exercise, Markovnikov's rule explains that in the hydration of propene, the -OH group adds to the middle carbon, forming propan-2-ol. The process follows an electrophilic addition mechanism:

• The alkene's pi bond attacks the hydrogen of the H-OH, forming a carbocation at the more substituted carbon.
• The water molecule then adds as an -OH to the carbocation, stabilizing it and forming the alcohol.

Grignard Reagent

The Grignard reagent is a key player in organic synthesis, especially in forming carbon-carbon bonds. Named after French chemist Victor Grignard, this reagent is a complex of an organomagnesium compound, typically represented as RMgX, where R is an organic group and X is a halogen. They have the incredible ability to react with various electrophilic centers to form

• New carbon-carbon bonds,
• Alcohols when reacted with carbonyl compounds.

In the exercise's step-by-step solution, the Grignard reaction involves an ethanal molecule reacting with methylmagnesium iodide. This reaction completes in two key stages:

• First, the Grignard reagent attacks the carbonyl carbon of ethanal, passing its electrons to form a new carbon-magnesium bond, and leading to the formation of an alkoxide ion.
• Upon hydrolysis, the alkoxide ion transforms into the desired secondary alcohol, propan-2-ol.

Grignard reagents are indispensable in synthetic chemistry due to their versatility and ability to facilitate complex carbon frameworks.

Hydration Reaction

Hydration reactions are a class of chemical reactions where a water molecule is added to a compound. In organic chemistry, these reactions usually involve alkenes like propene or ethene treated with water in the presence of an acid catalyst. A classic example of this is the acid-catalyzed hydration where propene undergoes:

• Water and an acid react with the alkene,
• The pi bond breaks, and the molecule adopts a more stable carbocationic state.

The acid catalyst, often a mineral acid like sulfuric acid, protonates the alkene, helping form a carbocation. The

• Water then acts as a nucleophile and adds to the carbocation.

After rearrangements and deprotonations, an alcohol is formed. In the exercise's case, the hydration of propene forms propan-2-ol. This is due to the intermediary carbocation structure that allows the hydroxyl group from water to attach at the most stable carbon, leading to product formations as suggested by Markovnikov's rule.