Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

The product formed by acid catalysed hydration of 2-phenyl propene is (a) 3-phenyl-2-propanol (b) 1-phenyl-2-propanol (c) 2-phenyl-2-propanol (d) 2-phenyl-1-propanol

Step-by-Step Solution

Answer

The product is (c) 2-phenyl-2-propanol.

Step 1: Understand the Reaction Type

The exercise involves an acid-catalyzed hydration of an alkene. This means that water is added to an alkene, facilitated by an acid, following Markovnikov's rule. It leads to the formation of alcohols.

Step 2: Analyze the Reactant Structure

2-Phenyl propene is an alkene with a phenyl ring attached to the first carbon and a propene group (C=C) starting from the second carbon. We need to identify where the -OH group will be added in the molecule.

Step 3: Apply Markovnikov's Rule

According to Markovnikov's rule, the hydrogen atom from water will attach to the less-substituted carbon atom in the double bond, while the -OH group will attach to the more substituted carbon atom. Here, the double bond is between the second and third carbon in 2-phenyl propene.

Step 4: Identify the Product

By applying Markovnikov's rule, the -OH group attaches to the third carbon (
7CI(CH3)CH-Ph
7CI). This forms 2-phenyl-2-propanol as the major product.

Key Concepts

Markovnikov's Rule

Markovnikov's rule is a guiding principle in organic chemistry used to predict the outcome of certain chemical reactions, specifically the addition of compounds to alkenes. When a molecule like

• water (H₂O)
• hydrogen halides (HCl, HBr, etc.)

adds across the double bond of an alkene, Markovnikov's rule helps determine where each component will attach. The rule states that in the addition of HX to an alkene, the hydrogen atom (H) bonds to the less-substituted carbon atom of the double bond (the one with fewer carbon chains attached), and the "other" group (e.g., -OH from water) bonds to the more-substituted carbon atom (the one with more carbon chains attached). This happens because more-substituted carbons form more stable carbocations during the reaction. In the acid-catalyzed hydration of 2-phenyl propene, we see this rule in action. The -OH group adds to the more substituted carbon (the third carbon in this case), while the H adds to the less-substituted carbon, resulting in alcohol formation with regiochemistry influenced by Markovnikov's rule.

Alkene Reaction

An alkene reaction involves the participation of carbon-carbon double bonds (C=C) present in an organic molecule. These bonds are characterized by having one sigma bond and one pi bond, with the latter being more reactive. This reactivity stems from the electron-rich nature of the pi bond, which readily participates in additional reactions. The transformation of alkenes through a reaction like acid-catalyzed hydration involves

• breaking the pi bond
• forming two new bonds with the substituent groups added (in this case, hydrogen and hydroxyl groups)

The process begins when the alkene reacts with a catalyst, such as a strong acid (commonly sulfuric acid, H₂SO₄), which protonates the pi bond. Subsequently, the water molecule attacks the carbocation intermediate, adding the -OH part to yield an alcohol. This kind of transformation is common in many organic syntheses, as it transforms alkenes into useful alcohol derivatives, expanding the utility and application range of the original alkene.

Alcohol Formation

Alcohol formation through acid-catalyzed hydration of alkenes is a crucial synthetic process in organic chemistry. The goal of this reaction is to convert the alkene into an alcohol, which involves adding water across the double bond. The acid catalyst, such as sulfuric acid, enhances the reaction by providing protons, facilitating the initial formation of a carbocation. Once the alkene reacts with the acid, it forms a carbocation at the relatively more stable carbon atom. The stability of carbocations follows the order:

• tertiary > secondary > primary

After carbocation formation, the hydroxide ion (OH^-) from water attaches to the carbocation, resulting in the formation of an alcohol. This process

is governed by Markovnikov's rule, ensuring that the -OH group attaches to the most substituted carbon, thus forming the most stable product. In our example with 2-phenyl propene, the main product is 2-phenyl-2-propanol, showcasing how the principles of carbocation stability and regioselective addition work hand in hand to direct the formation of the alcohol.