Problem 195 - A Complete Resource Book in Chemistry for JEE Main

Question

HBr reacts with \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{OCH}_{3}\) under anhydrous conditions at room temperature to give \(\quad\) [2006] (a) \(\mathrm{CH}_{3} \mathrm{CHO}\) and \(\mathrm{CH}_{3} \mathrm{Br}\) (b) \(\mathrm{BrCH}_{2} \mathrm{CHO}\) and \(\mathrm{CH}_{3} \mathrm{OH}\) (c) \(\mathrm{BrCH}_{2}-\mathrm{CH}_{2}-\mathrm{OCH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CHBr}-\mathrm{OCH}_{3}\)

Step-by-Step Solution

Answer

The reaction yields \\(\text{H}_3\text{C}-\text{CHBr}-\text{OCH}_3\\) according to Markovnikov's rule, so the correct option is (d).

Step 1: Identify the Reaction Type

The reaction of HBr with an alkene is typically an addition reaction. Here, HBr will add across the double bond of the alkene \(CH_2=CH-OCH_3\).

Step 2: Markovnikov's Rule Application

According to Markovnikov's rule, in the addition of HBr to an unsymmetrical alkene, the hydrogen (H) will attach to the carbon with the most hydrogen atoms, leading the bromine (Br) to attach to the more substituted carbon atom. Thus, in \(CH_2=CH-OCH_3\), Br will attach to the \(CH\) attached to the \(OCH_3\) group.

Step 3: Draw the Product Structure

Following the application of Markovnikov's rule, the major product will have Br attached to the second carbon and H to the first carbon. This creates the product \(H_3C-CHBr-OCH_3\).

Step 4: Identify the Correct Option

The correct structure from the options provided is \(\text{H}_3\text{C}-\text{CHBr}-\text{OCH}_3\), which matches option (d).

Key Concepts

Addition Reaction

An addition reaction is a fundamental concept in organic chemistry that involves two or more molecules combining to form a larger molecule. This type of reaction is especially significant when it comes to double bonds, like those present in alkenes. During these reactions, the double bond breaks open, allowing new atoms or groups to attach to the carbon atoms.
These reactions are crucial for transforming unsaturated compounds, such as alkenes, into saturated compounds. Since the original unsaturated compound has a double bond, the breaking of this bond makes room for additional atoms.
Addition reactions can occur in one of two ways:

• Electrophilic Addition: Typically occurs with alkenes where a pair of electrons from the double bond is used to form a new bond with an electrophile, such as HBr.
• Nucleophilic Addition: Often involves carbonyl compounds reacting with a nucleophile.

Understanding addition reactions is vital for grasping more complex organic processes, such as polymerization, where small monomer units add together to form large polymer chains.

Alkenes

Alkenes are hydrocarbons characterized by the presence of at least one carbon-carbon double bond. This double bond not only defines alkenes but also dictates much of their chemical behavior. Due to their unsaturated nature, alkenes are highly reactive and readily participate in addition reactions.
Alkenes can be categorized based on the number of carbon atoms connected to the double-bonded carbons:

• Mono-Substituted: One carbon atom is attached to the double-bonded carbons.
• Di-Substituted: Two carbon atoms are attached.
• Tri-Substituted: Three carbon atoms are attached.
• Tetra-Substituted: All four substituents around the double bond are carbon atoms.

Their reactivity allows alkenes to serve as building blocks in the synthesis of more complex compounds. In the context of HBr reactions, alkenes undergo electrophilic addition where the pi bond electrons attack an electrophile, facilitating the reaction.

HBr Reaction

The HBr reaction with alkenes is a common example of electrophilic addition. In this reaction, hydrogen bromide (HBr) adds across the double bond present in an alkene. The reaction occurs in two main steps:

• Step 1: Formation of a Carbocation Intermediate: The electrophilic hydrogen from HBr attaches to one of the carbon atoms in the double bond, resulting in the formation of a carbocation intermediate and a bromide ion (Br^-).
• Step 2: Bromide Ion Attack: The bromide ion, being a nucleophile, quickly attacks the positively charged carbocation, forming the final product.

Markovnikov's rule plays a crucial role in determining the regiochemistry of the reaction, predicting where the hydrogen and bromine will add on an unsymmetrical alkene. This rule states that the hydrogen atom will add to the carbon that already has more hydrogen atoms, directing the bromine to the more substituted carbon.

Organic Chemistry

Organic chemistry is the branch of chemistry that studies the structure, properties, and reactions of organic compounds, which contain carbon atoms. It is a vast and intricate field that serves as the foundation for studying various chemical processes and synthesizing countless compounds.
Alkenes, being a central component of organic chemistry, showcase the complexity and versatility of carbon. Understanding their behavior in addition reactions, such as with HBr, is essential for grasping larger concepts like synthesis and polymerization.
Key areas of focus within organic chemistry include:

• Functional Groups: Specific groupings of atoms within molecules that determine the chemical reactions of those molecules. The double bond in alkenes is a prime example of a functional group.
• Reaction Mechanisms: Detailed step-by-step descriptions of how reactions occur, such as the formation of intermediates during HBr addition.
• Synthesis: The design and creation of complex compounds from simpler ones, heavily relying on precise reactions and mechanisms.

Organic chemistry shapes our understanding of molecular interactions and transformations, providing the tools needed to manipulate chemical structures at the molecular level.