Problem 168 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

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

Step 1: Analyze Reaction (a)

In reaction (a), the reactants are acetylene (\( \mathrm{CH} \equiv \mathrm{CH} \)) and 2 moles of hydrobromic acid (\(2 \mathrm{HBr} \)). The addition of HBr to acetylene leads to the formation of vicinal dibromoalkenes due to anti-addition, resulting in the formation of 1,2-dibromoethene as the product. This will not form 2,2-dibromopropane.

Step 2: Analyze Reaction (b)

In reaction (b), the reactants are propyne (\( \mathrm{H}_{2}\mathrm{CC} \equiv \mathrm{CH} \)) and 2 moles of HBr. The reaction proceeds with a Markovnikov addition of HBr across the triple bond, forming first 3-bromo-1-propene and then 2,2-dibromopropane. This reaction will yield the desired product, 2,2-dibromopropane.

Step 3: Analyze Reaction (c)

In reaction (c), the reactants are propene (\( \mathrm{CH}_{3}\mathrm{CH} = \mathrm{CH}_{2} \)) and HBr. Propene undergoes Markovnikov addition, where the bromine atom adds to the more substituted carbon due to carbocation rearrangement, leading to the formation of 2-bromopropane. This will not yield 2,2-dibromopropane.

Step 4: Analyze Reaction (d)

In reaction (d), the reactants are allyl bromide (\( \mathrm{CH}_{3}\mathrm{CH}=\mathrm{CHBr} \)) and HBr. The reaction proceeds by the addition of another bromine across the remaining double bond, producing 2,3-dibromo-1-propane. This does not yield 2,2-dibromopropane.

Key Concepts

Markovnikov's Rule

Markovnikov's Rule is a guiding principle in organic chemistry that comes into play during the addition of hydrogen halides, such as HBr, to unsaturated hydrocarbons like alkenes and alkynes. This rule helps predict the outcome of reactions by stating that when a hydrogen halide is added to an unsymmetrical alkene or alkyne, the hydrogen atom will attach to the carbon with the greatest number of hydrogen atoms already bonded to it. In simpler terms, the rich get richer for hydrogen!

The underlying mechanism involves the formation of a temporary carbocation, a positively charged species that is more stable when the positive charge is on a more substituted carbon. Thus, in reactions, the major product is formed when the more electronegative halogen is added to the carbon atom with fewer hydrogen atoms already attached.

• This is what occurs in reaction (b) with propyne ( \( \mathrm{H}_{2}\mathrm{CC} \equiv \mathrm{CH} \) ), where HBr follows Markovnikov's Rule to eventually produce 2,2-dibromopropane.

Understanding Markovnikov's Rule is crucial for predicting the major products of many organic reactions involving the addition of acids to unsaturated carbons.

Anti-addition

Anti-addition is another important concept in organic chemistry, especially when dealing with reactions involving unsaturated compounds like alkenes and alkynes. This refers to the process where two substituents are added to opposite faces or sides of a double or triple bond, resulting in a trans-configuration of the molecules.

This addition typically occurs in reactions involving reagents that provide stereochemical outcomes, such as halogens (e.g., \( \mathrm{Br}_{2} \) ) in solution, creating a cyclic intermediate that opens up to give the products. In the context of the original exercise, anti-addition can be observed in reaction (a) where acetylene ( \( \mathrm{CH} \equiv \mathrm{CH} \) ) interacts with 2 moles of HBr.

• Each bromine atom adds to a different carbon, leading to vicinal dibromoalkenes.

Anti-addition helps in understanding how different stereochemical outcomes can be produced in organic reactions, making it a key concept for controlling the 3D structure of resulting compounds.

Alkyne Reactions

Alkynes, with their carbon-carbon triple bonds, exhibit unique chemical behaviors that distinguish them from alkenes and more saturated hydrocarbons. When reacting with hydrogen halides like HBr, alkynes can undergo both Markovnikov and anti-Markovnikov additions, depending on the reagents and conditions used.

In our context, the reaction of an alkyne with two equivalents of HBr can lead to different products based on whether it follows Markovnikov's Rule or displays anti-addition. Whereas the first addition often follows Markovnikov's preference, a second equivalent can add in an anti-fashion if conditions are right.

• For example, reaction (b) with propyne illustrates how alkynes react with HBr, first forming a vinyl halide which then undergoes further Markovnikov addition to produce 2,2-dibromopropane.
• Stereochemistry and regioselectivity are the two main considerations in alkyne reactions.

Understanding alkyne reactions requires a keen insight into how these triple-bonded carbons behave, offering a canvas for organic chemists to manipulate molecules into desired configurations and functionalities.