Problem 158 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions will yield 2-propanol? (1) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{H}^{\prime}}{\longrightarrow}\) (2) \(\mathrm{CH}_{3} \mathrm{CHO} \stackrel{\text { (i) } \mathrm{CH}_{3} \mathrm{MgI}, \text { (ii) } \mathrm{H}_{2} \mathrm{O}}{\longrightarrow}\) (3) \(\mathrm{CH}_{2} \mathrm{O} \quad\) (i) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{Mgl}\), (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 3 (b) 2 and 4 (c) 1 and 2 (d) 3 and 4

Step-by-Step Solution

Answer

(c) 1 and 2

Step 1: Identify the Desired Reaction

We want to produce 2-propanol, which is an alcohol with the formula \( ext{CH}_3 ext{CHOH} ext{CH}_3 \). This molecule has a three-carbon chain with a hydroxyl group (-OH) on the second carbon.

Step 2: Analyze Reaction 1

Reaction 1 is an acid-catalyzed hydration of propene (\( ext{CH}_2= ext{CH}- ext{CH}_3 \)) which can produce 2-propanol by adding water across the double bond in Markovnikov orientation. This means the hydroxyl group adds to the more substituted carbon, which is the second carbon, forming 2-propanol.

Step 3: Analyze Reaction 2

Reaction 2 involves the reaction of ethanal (\( ext{CH}_3 ext{CHO} \)) with methylmagnesium iodide (a Grignard reagent), followed by hydrolysis. The Grignard reagent adds a methyl group to the carbonyl carbon, converting it into a secondary alcohol. This yields 2-propanol.

Step 4: Analyze Reaction 3

Reaction 3, starting from formaldehyde (\( ext{CH}_2 ext{O} \)), involves a Grignard reagent which will lead to a primary alcohol after hydrolysis, but not 2-propanol.

Step 5: Analyze Reaction 4

Reaction 4 uses neutral \( ext{KMnO}_4 \) which will result in syn-dihydroxylation, adding hydroxyl groups across the double bond. This gives glycols like 1,2-propanediol, not 2-propanol.

Step 6: Determine the Correct Answer

Since reactions 1 and 2 yield 2-propanol, we identify 1 and 2 as the correct reactions. Therefore, the correct answer based on the options is (c) 1 and 2.

Key Concepts

Acid-catalyzed hydration

Acid-catalyzed hydration is an important reaction used to convert alkenes (molecules with carbon-carbon double bonds) into alcohols. In this reaction, water is added to the alkene in the presence of an acid catalyst like sulfuric acid or hydrochloric acid. This process allows the components of water 8 the hydrogen and hydroxyl group 8 to add across the double bond. The reaction follows a specific rule known as Markovnikov's rule, which dictates where the hydrogen and hydroxyl group will add on the alkene. The reason acid is used is to protonate the double bond, making it more susceptible to nucleophilic attack by water. Initially, the double bond attacks the proton, forming a more stable carbocation at the position that can best accommodate the positive charge. Then, water acts as a nucleophile and attacks the carbocation, leading to the formation of an alcohol. This transformation is particularly useful for turning simple alkenes like propene into alcohols such as 2-propanol.

Grignard reaction

The Grignard reaction is a versatile and powerful tool in organic chemistry for forming carbon-carbon bonds. It involves the use of a Grignard reagent, which is typically an organomagnesium compound formed by reacting an alkyl halide with magnesium metal in anhydrous ether. These reagents are highly nucleophilic and can react with a variety of electrophiles, including carbonyl compounds such as aldehydes and ketones, to produce alcohols. When a Grignard reagent reacts with a carbonyl group, it adds to the electrophilic carbon, forming a new carbon-carbon bond. After the addition, the intermediate is treated with water or another suitable acid in a process known as hydrolysis to neutralize the alkoxide ion, yielding the desired alcohol. For example, when methylmagnesium iodide (a Grignard reagent) reacts with ethanal, the product after hydrolysis is 2-propanol. This is because the Grignard reagent has added a methyl group to the aldehyde, transforming it into a secondary alcohol.

Markovnikov orientation

Markovnikov's rule is a guideline in organic chemistry that predicts the outcome of addition reactions involving alkenes. According to Markovnikov's rule, when a protic acid (an acid that can donate a proton) adds to an unsymmetrical alkene, the proton will attach to the carbon with more hydrogen atoms already attached, while the other component of the acid (such as a hydroxyl group from water in hydration reactions) attaches to the carbon with fewer hydrogen atoms. This orientation is often determined by the stability of the carbocation intermediates formed during the reaction. Alkenes that form more stable carbocations are favored, leading to the Markovnikov product. In simple terms, the rich get richer 8 the carbon that is already bonded to more hydrogens gets another 8 and this process yields the most stable product. For instance, during the acid-catalyzed hydration of propene, the hydroxyl group ends up on the more substituted second carbon, thus forming 2-propanol.