Problem 105 - A Complete Resource Book in Chemistry for JEE Main

Question

Isopropyl alcohol is obtained by reacting which of the following alkenes with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) followed by boiling with water? (a) 2 -methylpropene (b) ethylene (c) isoprene (d) propylene

Step-by-Step Solution

Answer

Isopropyl alcohol is obtained by reacting propylene with concentrated decomposed H₂SO₄ followed by boiling with water.

Step 1: Identify the Required Organic Reaction

To determine which alkene will produce isopropyl alcohol, we need to recognize the reaction of alkenes with concentrated sulfuric acid followed by hydrolysis. This sequence is typical for the production of alcohols from alkenes. The alkene undergoes an addition reaction where the double bond opens, and a hydrogen ion from the acid attaches to form a carbocation.

Step 2: Analyze Carbocations Formed

Next, analyze the type of carbocation each alkene in the options will form. We are looking for a stable secondary carbocation, which will lead to the formation of isopropyl alcohol. - (a) 2-methylpropene forms a tertiary carbocation. - (b) Ethylene forms a primary carbocation. - (c) Isoprene can form various unstable carbocations. - (d) Propylene forms a secondary carbocation.

Step 3: Determine the Final Product

The secondary carbocation formed from propylene reacts with water during boiling to form isopropyl alcohol. The reaction is: Propylene + H₂SO₄ → Secondary carbocation (+ ion) → Hyrolysis (adding H—OH) leads to Isopropyl alcohol (CH₃—CHOH—CH₃).

Step 4: Select the Correct Alkene

Based on the analysis, the alkene that produces isopropyl alcohol through this reaction process is propylene, as it forms the necessary secondary carbocation. The correct answer is therefore option (d).

Key Concepts

Alkene Reactions

Alkene reactions are key to understanding how different organic products are synthesized. Alkenes are hydrocarbons with at least one carbon-carbon double bond. This double bond is quite reactive, making alkenes suitable for various addition reactions. When alkenes undergo addition reaction with strong acids like sulfuric acid (\(\text{H}_2\text{SO}_4\)), the double bond breaks and new atoms are added.

• The double bond acts as a point of high electron density, attracting the positively charged acid components.
• First, a hydrogen ion (\(\text{H}^+\)) from the acid attaches to one of the carbon atoms, converting the double bond into a more stable, single-bonded structure.
• This process forms a carbocation, an intermediate with a positive charge.

As seen in the synthesis of isopropyl alcohol, specific types of alkenes and their configurations determine the stability of the resulting carbocation. This stability is crucial to the next stages of the reaction.

Carbocation Stability

Carbocations are intermediates formed when alkenes react with acids like \(\text{H}_2\text{SO}_4\). These species possess a positive charge on a carbon atom and are key determinants in the reaction pathway.

• Stability of a carbocation is influenced by its structure; primary, secondary, and tertiary describe the number of alkyl groups attached to the positively charged carbon.


• Secondary carbocations, like the one formed from propylene, are more stable than primary ones due to the dispersal of charge via electron-donating alkyl groups.
• In contrast, tertiary carbocations are the most stable with three alkyl groups aiding in neutralizing the charge.

The stability allows for a more efficient conversion into the desired product. For isopropyl alcohol, propylene gives rise to a secondary carbocation, making it the optimal precursor in such reactions.

H2SO4 Hydrolysis

The hydrolysis step in synthesizing alcohols, like isopropyl alcohol, is an important transformation. After the formation of a carbocation, the intermediate undergoes hydrolysis, typically by boiling with water.

• Water acts as a nucleophile, attacking the positively charged carbon in the carbocation and forming a new bond.
• This bond formation is usually followed by the rearrangement and release of the hydrogen ion (\(\text{H}^+\)) back into the solution, completing the transformation of alkene to alcohol.


• The sequence involves bonding water's oxygen to the carbocation carbon, resulting in the hydroxyl group (\(-\text{OH}\)) attachment, characteristic of alcohols.

This conversion not only provides the desired isopropyl alcohol (\(\text{CH}_3\text{CHOHCH}_3\)), but it also showcases the elegance of organic synthesis where simple mechanistic steps lead to complex products.