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) \(\mathrm{HC} \equiv \mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HBr} \longrightarrow\) (d) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}+2 \mathrm{HBr} \longrightarrow\)

Step-by-Step Solution

Answer

Reaction (d) will yield 2,2-dibromopropane.

Step 1: Understanding 2,2-dibromopropane

2,2-dibromopropane is an alkane with a propane chain, where two bromine atoms are attached to the second carbon. Its structure can be represented as \( \text{CH}_3\text{CBr}_2\text{CH}_3 \).

Step 2: Analyzing Option (a)

The reaction \( \text{CH}_3\text{CH} = \text{CHBr} + \text{HBr} \rightarrow \) involves an alkene with a halogen substituent and an additional HBr molecule. This reaction will lead to the formation of a dibrominated structure, but not 2,2-dibromopropane.

Step 3: Analyzing Option (b)

The reaction \( \text{HC} \equiv \text{CH} + 2 \text{HBr} \rightarrow \) involves acetylene and two HBr molecules. This reaction will form dibromoethane, not 2,2-dibromopropane, because acetylene has only two carbon atoms.

Step 4: Analyzing Option (c)

The reaction \( \text{CH}_3\text{CH} = \text{CH}_2 + \text{HBr} \rightarrow \) produces 2-bromopropane as an intermediate. This will not yield 2,2-dibromopropane as it would need a second addition of HBr to attach bromines on the second carbon.

Step 5: Analyzing Option (d)

The reaction \( \text{CH}_3\text{C} \equiv \text{CH} + 2 \text{HBr} \rightarrow \) involves propyne and two HBr molecules. With this reaction, the first HBr adds across the triple bond, yielding an alkene. The second HBr adds across the alkene's double bond, resulting in the attachment of one bromine to each carbon of what is now a three-carbon chain, thereby forming \( \text{CH}_3\text{CBr}_2\text{CH}_3 \) or 2,2-dibromopropane.

Key Concepts

Dibromination

Dibromination is an essential reaction in organic chemistry where two bromine atoms are added to a compound. This reaction is significant because it allows the transformation of unsaturated compounds, such as alkenes and alkynes, into saturated dibromides. In the context of the given exercise, dibromination involves the transformation of a compound like propyne into 2,2-dibromopropane.
An understanding of dibromination helps students predict the formation of such compounds based on the type and structure of reactants involved. In option (d), for example, the sequential addition of HBr to propyne results in a complete dibromination, leading to the desired product. It's a fascinating process, providing a glimpse into the strategic manipulation of molecular structures to yield specific outcomes.

• Primary mechanism: Addition of bromine atoms across double or triple bonds.
• Outcome: Formation of vicinal or geminal dibromides.
• Importance: Useful in synthesizing more complex molecules in organic synthesis.

Chemical Equation Balancing

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is followed. Each side of a chemical equation must have the same number of atoms for every element involved. This principle helps predict the amounts of reactants needed and products formed in chemical reactions. For example, in the given exercise, ensuring the equation is balanced means verifying that the reactants and products reflect the same number of each type of atom.
A correctly balanced chemical equation for the reaction forming 2,2-dibromopropane from propyne would show the precise transformation of carbon-carbon triple bonds into single bonds with bromine atom additions. This practice not only underpins theoretical chemistry but is also crucial for practical applications such as calculating reactant quantities and optimizing reaction conditions.

• Conservation of mass: Balanced equations ensure no atoms are lost or created.
• Stoichiometry: Enables calculation of reactant and product quantities.
• Reaction accuracy: Confirms that the predicted reaction pathway is chemically feasible.

Reaction Mechanism Analysis

Analyzing reaction mechanisms involves exploring the step-by-step changes that occur in chemical reactions. For organic reactions like dibromination, understanding the mechanism provides insight into how and why particular products form—like 2,2-dibromopropane in the exercise. Mechanism analysis often involves identifying intermediates, transition states, and describing electron movement through the reaction.
For instance, in option (d) involving propyne, the reaction proceeds via a two-step mechanism:

• Step 1: Addition of the first HBr across the triple bond forms an intermediate alkene.
• Step 2: Addition of the second HBr to the alkene, resulting in the final product, 2,2-dibromopropane.

Understanding these intermediates and transitions is invaluable in predicting the outcomes of organic reactions and is often employed to design new synthesis pathways in complex chemical research.