Problem 204 - A Complete Resource Book in Chemistry for JEE Main

Question

Acid catalysed hydration of alkenes, except ethene, leads to the formation of (a) primary alcohol (b) secondary or tertiary alcohol (c) mixture of primary and secondary alcohols (d) mixture of secondary and tertiary alcohols

Step-by-Step Solution

Answer

The acid-catalyzed hydration of alkenes (except ethene) forms secondary or tertiary alcohols.

Step 1: Understanding the Reaction

In acid-catalyzed hydration of alkenes, water ( H_2O) is added across the double bond of the alkene in the presence of an acid, usually sulfuric acid (H_2SO_4). This reaction follows Markovnikov's rule, which states that the hydrogen atom from water will attach to the less substituted carbon, and the hydroxyl group (OH) will attach to the more substituted carbon.

Step 2: Application of Markovnikov's Rule

According to Markovnikov's rule, when dealing with alkenes other than ethene, after the double bond receives the proton (H⁺) from the acid to form a more stable carbocation, the hydroxyl group (OH⁻) from water will attach to the more substituted carbon. This typically results in the formation of secondary or tertiary alcohols depending on the alkene's structure.

Step 3: Exception of Ethene

Ethene is an exception because it forms ethanol, which is a primary alcohol. However, the problem specifically mentions 'except ethene,' so we apply the discussion from previous steps, focusing on other alkenes.

Step 4: Conclusion from Reaction Mechanism

Since we are considering alkenes other than ethene and given the rule of Markovnikov addition, the resulting alcohol will always attach the hydroxyl group to the more substituted carbon atom, leading to the formation of either secondary or tertiary alcohols.

Key Concepts

Markovnikov's Rule

Markovnikov's rule is a fundamental principle in organic chemistry guiding the addition reactions of alkenes. Named after the Russian chemist Vladimir Markovnikov, the rule essentially helps predict the outcome of certain chemical reactions involving alkenes. When dealing with acid-catalyzed hydration, Markovnikov's rule specifies how water (H₂O) adds across the double bond of an alkene in the presence of an acid.

According to this rule:

• The hydrogen atom from the acid will attach to the less substituted carbon of the double bond.
• The hydroxyl group (OH) will bond with the more substituted carbon.

The term "substituted" refers to the number of carbon atoms directly attached to the carbon in question. A more substituted carbon generally has more carbon atoms attached, making it a favorable site for addition. This preference is due to the formation of a more stable carbocation intermediate, which is crucial in the reaction pathway. Through this rule, chemists can predict that the reaction will mainly produce secondary or tertiary alcohols, as the hydroxyl group attaches to the more substituted carbon.

Carbocation Formation

Carbocation formation is a critical step in the acid-catalyzed hydration of alkenes. A carbocation is a positively charged ion with the positive charge residing on a carbon atom. This occurs when the proton from the acid adds to an alkene, breaking its double bond.

The formation of a carbocation is a key reason why Markovnikov's rule applies to these reactions. Here's a closer look at the process:

• Initially, the alkene reacts with a proton (H⁺) from the acid catalyst.
• This addition results in the double bond opening and forming a carbocation at the more substituted carbon, because it holds the positive charge more stably.
• The stability of a carbocation increases with its degree of substitution: tertiary > secondary > primary.

Therefore, the more substituted carbon, being in the position to form a more stable carbocation, typically receives the hydroxyl group during hydration. This realization underscores the production of mainly secondary or tertiary alcohols in these reactions.

Alkene Hydration

Alkene hydration is a chemical reaction that involves the conversion of alkenes into alcohols, employing water and often an acid catalyst. The process is termed 'hydration' because it involves the addition of water across the carbon-carbon double bond of an alkene.

In acid-catalyzed alkene hydration, the steps are typically as follows:

• The alkene reacts with a proton from the acid, forming a carbocation.
• Water then acts as a nucleophile, attacking the carbocation.
• Finally, a deprotonation step releases a proton to regenerate the acid catalyst, completing the reaction cycle.

This reaction transforms the alkene into an alcohol, with the water molecule specifically splitting into H and OH. The H atom adds to the less substituted carbon, while the OH group adds to the more substituted carbon, as per Markovnikov's rule. Alkenes excluding ethene typically lead to secondary or tertiary alcohols, reinforcing the importance of understanding how alkene structure influences product formation.