Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions will yield 2 , 2-dibromopropane? (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHBr}+\mathrm{HBr} \longrightarrow\) (b) \(\mathrm{HC}=\mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HBr} \longrightarrow\) (d) \(\mathrm{CH}_{3} \mathrm{C}=\mathrm{CH}+2 \mathrm{HBr} \longrightarrow\)

Step-by-Step Solution

Answer

Reaction (d) yields 2,2-dibromopropane.

Step 1: Analyze the Target Compound

The target compound is 2,2-dibromopropane, which has the formula \( \text{CH}_3\text{CBr}_2\text{CH}_3 \). This structure contains a propane backbone with two bromine atoms attached to the second carbon.

Step 2: Identify Key Features of Reactions

We are dealing with hydrohalogenation reactions, where an alkene reacts with HBr in such a way that the addition follows Markovnikov's rule. This means the bromine attaches to the more substituted carbon atom of the double-bonded carbon.

Step 3: Examine Reaction (a)

In reaction (a), the substrate is \( \text{CH}_3\text{CH}=\text{CHBr} \) which reacts with HBr. The double bond is between two carbons with a bromine already attached to one carbon, making it possible to form 1,2-dibromopropane, not the 2,2-dibromopropane target.

Step 4: Examine Reaction (b)

Reaction (b) starts with \( \text{HC}=\text{CH} \), an alkyne, and adds 2 equivalents of HBr. The product will be 1,1-dibromoethane, which has only two carbons, not the required three carbon chain.

Step 5: Examine Reaction (c)

The substrate \( \text{CH}_3\text{CH}=\text{CH}_2 \) can react with HBr following Markovnikov's rule to form \( \text{CH}_3\text{CHBr} \) (bromine on the second carbon), another addition is required for dibromination which is not considered here.

Step 6: Examine Reaction (d)

For reaction (d), an alkyne \( \text{CH}_3\text{C}=\text{CH} \) reacts with 2 HBr. The reaction proceeds by adding one HBr to form the vinyl bromide \( \text{CH}_3\text{CBr}=\text{CH}_2 \), and then another HBr converts it to \( \text{CH}_3\text{CBr}_2\text{CH}_3 \), which is 2,2-dibromopropane. Hence, this reaction results in the desired product.

Key Concepts

Alkenes

Alkenes are a class of hydrocarbons known for having at least one carbon-carbon double bond. This double bond is crucial as it influences the chemical reactions in which the alkene can participate. The presence of the double bond makes alkenes more reactive than alkanes, which only have single bonds.
This reactivity allows for various transformations such as addition reactions, where atoms or groups are added to the carbons involved in the double bond. Alkenes are often the starting point in organic synthesis because of their ability to participate in such diverse chemical reactions. Understanding the behavior of alkenes in reactions like hydrohalogenation is essential for predicting the outcome of organic chemical processes.

Markovnikov's Rule

Markovnikov's Rule is a guiding principle in chemistry that predicts the outcomes of hydrochlorination and hydrohalogenation reactions involving alkenes. According to this rule, when an alkene undergoes an addition reaction with a hydrogen halide (like HBr), the hydrogen atom from the halide will attach to the carbon atom with more hydrogen atoms—often referred to as the less substituted carbon. Conversely, the halogen from the halide attaches to the carbon with fewer hydrogen atoms—known as the more substituted carbon.
This rule is critical because it helps chemists anticipate which product will be most favored during a reaction, ensuring that the correct isomer is formed. Markovnikov's Rule can be a game changer in synthetic chemistry, particularly in designing molecules with specific properties.

Reaction Mechanisms

A reaction mechanism is a detailed step-by-step description of how a chemical reaction occurs at the molecular level. It details the sequence in which bonds are formed and broken, the intermediates formed, and often the transition states along the reaction pathway.
For the hydrohalogenation of alkenes, the mechanism begins with the protonation step, where the hydrogen from HBr attaches to one of the double-bonded carbons, leading to the formation of a carbocation intermediate. Following this, the bromide ion attacks the carbocation, resulting in the addition of bromine to the more substituted carbon, as predicted by Markovnikov's Rule.
Understanding reaction mechanisms allows chemists to predict the products of chemical reactions and to design pathways for synthesizing complex molecules.

Organic Chemistry Reactions

Organic chemistry reactions encompass a broad category of chemical reactions that involve organic compounds. These reactions are the backbone of organic synthesis and include processes like addition, substitution, elimination, and rearrangement reactions.
Hydrohalogenation is one type of addition reaction, where halogens are added across the double bonds of alkenes. This type of reaction is fundamental in organic chemistry because it transforms a simple molecule into a more complex one, often setting the stage for further chemical reactions.

• Substitution reactions involve replacing one atom or group in a molecule with another.
• Elimination reactions involve removing elements from a molecule to form a double or triple bond.
• Rearrangement reactions involve the reorganization of the molecular structure.

Understanding these types of reactions provides a foundation for predicting and deriving novel organic compounds in chemistry.