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

Question

Which of the following reactions will yield 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) \(\mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}+2 \mathrm{HBr} \longrightarrow\)

Step-by-Step Solution

Answer

Option (d) yields 2-dibromopropane.

Step 1: Understanding the Goal

We need to determine which chemical reaction results in the formation of 2-dibromopropane. This requires identifying the correct reactants and reaction conditions that lead to the addition of bromine at the appropriate positions in the propane structure.

Step 2: Analyzing Option (a)

Option (a) is the reaction of an alkene, \(\mathrm{CH}_{3}\mathrm{CH}=\mathrm{CHBr},\)with \(\mathrm{HBr}\). The reaction adds bromine to the carbon atoms involved in the double bond. The resulting product would be 1,2-dibromo-2-propene, not 2-dibromopropane, because the presence of Br on C-1 already means another Br usually binds to C-2 but not necessarily forming 2-dibromopropane.

Step 3: Analyzing Option (b)

Option (b) involves the addition of 2 moles of \( \mathrm{HBr}\) to acetylene, \( \mathrm{HC} \equiv \mathrm{CH}\). The addition of \( \mathrm{HBr}\) across the triple bond leads to the formation of a geminal dibromoalkane. However, the molecule is not long enough or structured correctly to form 2-dibromopropane.

Step 4: Analyzing Option (c)

Option (c) involves an alkene, \(\mathrm{CH}_{3}\mathrm{CH}=\mathrm{CH}_{2}\), reacting with \( \mathrm{HBr}\). This reaction adds two bromine atoms, via the formation of a Markovnikov product, resulting in 2-bromopropane initially, and with a further reaction, it would form 1,2-dibromopropane not 2-dibromopropane.

Step 5: Analyzing Option (d)

Option (d) involves propyne, \(\mathrm{CH}_{2}\mathrm{C} \equiv \mathrm{CH}\), reacting with 2 moles of \( \mathrm{HBr}\). This reaction adds bromines across the triple bond following Markovnikov's rule. The product formed is 2,2-dibromopropane, which matches the description of adding both bromines to the central carbon.

Step 6: Conclusion

After analyzing all options, option (d) yields 2-dibromopropane. The reaction of \(\mathrm{CH}_{2}\mathrm{C} \equiv \mathrm{CH}\) with 2 moles of \( \mathrm{HBr}\) adds two bromine atoms to the second carbon, resulting in 2,2-dibromopropane.

Key Concepts

Addition Reactions

In chemistry, addition reactions are fundamental, especially for organic molecules like alkenes and alkynes. These reactions occur when two or more molecules combine to form a larger molecule. An addition reaction typically involves the breaking of a double or triple bond and the addition of new atoms or groups.

• In the context of alkenes and alkynes, the double or triple bonds act as reactive sites for additions.
• These reactions are essential for forming new compounds, which can be useful in various applications.

Addition of hydrogen bromide (HBr) is a common example. Here, the hydrogen and bromine atoms add across the carbon-carbon double or triple bonds, expanding the carbon chain. Understanding the specifics of these reactions helps predict the structure of the resulting compound.

Markovnikov's Rule

Markovnikov's Rule is a pivotal guideline in organic chemistry, helping predict outcomes of addition reactions. It explains how a reagent adds across a carbon-carbon double or triple bond. According to the rule, in the addition of a protic acid like HBr to an alkene or an alkyne, the hydrogen atom will attach to the carbon with the greatest number of hydrogen atoms already attached.

• This rule ensures the halide ion adds to the more substituted carbon atom—generally, the carbon with fewer hydrogen atoms and more alkyl groups.
• The principle allows chemists to predict major products of a reaction, ensuring the production of branched or secondary structures.

For example, when HBr adds to an alkene like propene, the bromine attaches to the second carbon rather than the first. This leads to a more stable carbocation intermediate, favoring the reaction's progression.

Alkenes and Alkynes

Alkenes and alkynes are hydrocarbons characterized by carbon-carbon multiple bonds, making them highly reactive. Alkenes contain a carbon-carbon double bond, while alkynes possess a carbon-carbon triple bond.

• Both these compounds are unsaturated, meaning they have fewer hydrogen atoms than saturated hydrocarbons like alkanes.
• The presence of these multiple bonds provides unique reactivity patterns, especially for addition reactions.

Reacting alkenes and alkynes with various reagents leads to diverse products. For instance, the addition of hydrogen bromide can yield different organobromides based on the starting compound and reaction conditions. These reactions form the basis for many synthetic processes in organic chemistry.

Organic Chemistry

Organic chemistry is a vast and fascinating field centered around studying carbon-based compounds. It covers a broad range of substances, from simple molecules such as methane to complex ones like enzymes and DNA. Addition reactions, according to Markovnikov's Rule, play a key role in this domain, helping to build complex organic molecules from simpler ones.

• Organic chemistry is fundamental to the development of pharmaceuticals, polymers, and many essential products.
• Understanding the behavior of carbon compounds allows scientists to manipulate molecular structures for desired functions.

Knowledge of chemical reaction mechanisms, like those for alkenes and alkynes, is crucial for advancements in fields such as materials science and biochemistry. The study of organic chemistry enables the efficient synthesis and transformation of molecules, making it indispensable in various scientific and industrial applications.