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

(b) secondary or tertiary alcohol.

Step 1: Understanding the Reaction

Acid catalyzed hydration of alkenes is the process where water adds to an alkene in the presence of an acid catalyst (often sulfuric acid) to form an alcohol.

Step 2: Markovnikov's Rule

According to Markovnikov's Rule, during the addition of HX or water to an alkene, the hydrogen atom bonds to the carbon with more hydrogen atoms already present, while the other moiety, in this case, OH, bonds to the carbon with fewer hydrogen atoms.

Step 3: Analyzing the Product Type

In an acid-catalyzed hydration of alkenes (except ethene), the hydroxyl group typically adds to the more substituted carbon (as per Markovnikov's Rule), leading to the formation of secondary or tertiary alcohols, depending on the structure of the alkene.

Key Concepts

Markovnikov's Rule

When we talk about the acid-catalyzed hydration of alkenes, Markovnikov's Rule plays a central role. This rule helps predict which carbon atoms in an alkene will bond with a new group, such as in hydration, where water adds across the double bond. Simply put, in a reaction where water is introduced to the alkene, the hydrogen (H) atom from water prefers to bond with the carbon that already has more hydrogen atoms attached. Meanwhile, the hydroxyl group (OH) from water attaches to the carbon with fewer hydrogen atoms.

• This ensures that the more substituted carbon receives the OH group enhancing the formation of either a secondary or tertiary alcohol, instead of a primary alcohol.
• More substituted carbons generally lead to more stable carbocations, which means reaction pathways favor these intermediates.

In essence, Markovnikov's Rule provides a roadmap of how atoms are likely to arrange themselves in a hydration reaction, leading to specific types of alcohol as products.

Secondary Alcohol

Secondary alcohols are an essential product in the world of organic chemistry, especially during the hydration of alkenes. A secondary alcohol features the hydroxyl group (14 OH) attached to a carbon atom that is itself connected to two other carbon atoms. These alcohols often arise during acid-catalyzed hydration reactions when the hydroxyl group attacks a carbon in the middle of a carbon chain. The process can be highlighted with the following insights:

• When following Markovnikov’s Rule, if the more substituted carbon in an alkene ends up bonding with the hydroxyl group, a secondary alcohol results.
• Secondary alcohols are common products unless the alkene has a highly substituted carbon arrangement that leads to a tertiary alcohol instead.

Usually, the acid-catalyzed hydration of more straightforward alkenes results predominantly in secondary alcohols unless there's a possibility to form an even more stable tertiary carbocation.

Tertiary Alcohol

Tertiary alcohols emerge when the hydroxyl group attaches to a carbon atom bound to three other carbon atoms. In the context of acid-catalyzed hydration, tertiary alcohols often form when the carbon holding the double bond is highly substituted. Key Points:

• When Markovnikov's Rule applies, reactions favor forming stable carbocations, which, in highly branched alkenes, can be tertiary.
• The increased stability of tertiary carbocations makes the attachment of the hydroxyl group more favorable here, leading to tertiary alcohols.

Tertiary alcohols showcase the rule's ability to guide expected outcomes when specific structural configurations of alkenes are involved. The presence of more alkyl groups around the carbon bearing the hydroxyl group makes tertiary alcohols less likely to be deprotonated, thereby remaining stable in various conditions.