Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct reactions are (a) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HBr} \rightarrow\) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HCl} \rightarrow\) \(\mathrm{CH}_{3}-\mathrm{CH}(\mathrm{Cl})-\mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HI} \stackrel{\text { peracide }}{\mathrm{HI}}\) \(\mathrm{CH}_{3}-\mathrm{CHI}^{2}-\mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HBr} \stackrel{\text { peratide }}{\longrightarrow}\) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br}\)

Step-by-Step Solution

Answer

Correct reactions: (b) HCl addition, (d) HBr with peroxide.

Step 1: Understand addition reactions

Alkenes like \( \text{CH}_3-\text{CH}=\text{CH}_2 \) undergo addition reactions with halogen acids (HX) such as HBr, HCl, and HI. The double bond opens up, allowing the hydrogen and the halogen to attach to the carbon atoms.

Step 2: Analyze reaction (a)

In reaction (a) \( \text{CH}_3-\text{CH}=\text{CH}_2 \) reacts with HBr. According to Markovnikov’s rule, the hydrogen (H) from HBr will bond with the carbon that has more hydrogen atoms, resulting in the product \( \text{CH}_3-\text{CH}Br-\text{CH}_3 \), not \( \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{Br} \). Thus, reaction (a) is incorrect.

Step 3: Analyze reaction (b)

For reaction (b) \( \text{CH}_3-\text{CH}=\text{CH}_2 \) with HCl, Markovnikov’s rule also applies. Hydrogen bonds with \( \text{CH}_2 \) leading to \( \text{CH}_3-\text{CHCl}-\text{CH}_3 \) as stated. This reaction is correct.

Step 4: Analyze reaction (c)

Reaction (c) involves \( \text{CH}_3-\text{CH}=\text{CH}_2 \) reacting with HI and a peracid. Typically, without a rearrangement or a peracid, the HI addition would go through Markovnikov’s rule, but the presence of a peracid might suggest a rearrangement to form \( \text{CH}_3-\text{CHI}_2-\text{CH}_3 \) (although this compound is unusual and incorrect due to valency rules). Thus, this reaction is likely incorrect.

Step 5: Analyze reaction (d)

In reaction (d), \( \text{CH}_3-\text{CH}=\text{CH}_2 \) reacts with HBr in the presence of a peroxide, which suggests a free radical mechanism (anti-Markovnikov addition). This would ideally form \( \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{Br} \) as the less substituted carbon gets the bromine by this mechanism, which matches the given product, making this reaction correct.

Key Concepts

Addition Reactions

When we talk about addition reactions in organic chemistry, we are referring to a process where atoms are added to a molecule without taking any away. This often happens with alkenes, such as in our example molecule, \( \text{CH}_3-\text{CH}=\text{CH}_2 \). An alkene has a characteristic carbon-carbon double bond. In an addition reaction, this double bond "opens up," allowing other atoms to attach to the carbon atoms.

Commonly, these reactions involve alkenes reacting with halogen acids like hydrogen bromide (HBr), hydrogen chloride (HCl), or hydrogen iodide (HI). During these reactions, the hydrogen atom typically bonds with one carbon atom, and the halogen atom typically bonds with the other. The reaction proceeds via two different mechanisms, which we will discuss later, depending on the conditions of the reaction.

Anti-Markovnikov Addition

The term Anti-Markovnikov Addition might sound a bit daunting, but it's quite fascinating once understood. This concept is all about the unusual way that hydrogen atoms add to molecules during specific reactions, unlike what Markovnikov's Rule would predict.

Originally, Markovnikov’s Rule predicted that in addition reactions, the hydrogen atom attaches to the carbon with the most hydrogen atoms already attached (this is known as the more substituted carbon). However, under certain conditions, such as when a peroxide is present, the reaction can follow an Anti-Markovnikov pathway. This means that the hydrogen atom will bond to the less substituted carbon atom instead.

• This mechanism is particularly observed with HBr and when peroxides, which initiate free radical mechanisms, are involved.
• The result of this reaction is often different compared to those predicted by Markovnikov's regular rules, influencing the type of molecule that's formed.

Understanding these variations helps chemists manipulate reactions to get specific products by choosing the correct conditions.

Free Radical Mechanism

Free radical mechanisms are the key to unlocking the mysteries of Anti-Markovnikov additions. A free radical is a molecule or atom that has an unpaired electron, making it highly reactive. In the presence of peroxides, addition reactions can proceed through a free radical mechanism.

Here’s how it typically works:

• Peroxides decompose to form free radicals, which initiate a chain reaction.
• The free radical reacts with hydrogen bromide (HBr), breaking its bond to form a bromine radical.
• This bromine radical then adds to the alkene, where the bromine prefers the less substituted carbon, resulting in an Anti-Markovnikov product.

This mechanism is notably different from the ionic additions seen in Markovnikov's Rule, because the presence of free radicals changes how the components interact (highlighting the reactive nature of these compounds). Understanding the free radical mechanism not only helps in predicting the products but is crucial in the synthesis of complex organic compounds.