Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following compounds: 1\. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHCl}-\mathrm{CH}_{3}\) 2\. \(\mathrm{CH}_{2}=\mathrm{CH}^{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Cl}\) 3\. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}^{2} \mathrm{Cl}\) These compounds are dehydrohalogenated by treatment with a strong base under identical conditions. The correct sequence of the increasing order of reactivity of these compounds in the given reaction is (a) \(3,1,2\) (b) \(3,2,1\) (c) \(1,2,3\) (d) \(2,1,3\)

Step-by-Step Solution

Answer

The correct sequence is (a) 3,1,2.

Step 1: Understand Dehydrohalogenation

Dehydrohalogenation is a chemical reaction that involves the removal of a halogen (such as chlorine) and a hydrogen atom from adjacent carbon atoms, resulting in the formation of a double bond. This reaction typically requires a strong base.

Step 2: Analyze Compound 1

The structure of Compound 1, \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHCl}-\mathrm{CH}_{3}\), shows that the chlorine atom is attached to a secondary carbon. This setup can lead to stable carbocation intermediate forms during reactions, hence moderate reactivity in dehydrohalogenation reactions.

Step 3: Analyze Compound 2

Compound 2, \(\mathrm{CH}_{2}=\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}\mathrm{Cl}\), contains an allylic chloride. The allylic position enhances the reactivity due to resonance stabilization when the chloride is removed, making this compound the most reactive in dehydrohalogenation.

Step 4: Analyze Compound 3

Compound 3, \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}\mathrm{Cl}\), involves a primary chloride, which does not benefit from any resonance stabilization. This primary carbon will be the least reactive because it forms a less stable intermediate during the reaction.

Step 5: Determine the Order of Reactivity

Summarizing the analyses: Compound 2 is the most reactive due to the allylic position, Compound 1 has moderate reactivity due to the secondary carbon, and Compound 3 is the least reactive with its primary chloride. The increasing order of reactivity is: Compound 3, Compound 1, Compound 2.

Step 6: Select the Correct Sequence

The order from least to most reactive is Compound 3, then Compound 1, and finally Compound 2. Therefore, the correct sequence is option (a) \(3,1,2\).

Key Concepts

Halogen Removal

In many organic reactions, a key step is the removal of halogens from a compound, known as halogen removal. This process is crucial in dehydrohalogenation reactions where a halogen and a hydrogen atom are removed from adjacent carbon atoms.

When a halogen atom, like chlorine, is attached to a carbon chain, it can be removed by treating the compound with a strong base. This removal forms a double bond between the former carbon sites. This reaction is fundamental in creating unsaturated compounds from saturated ones.

Understanding this allows us to predict the behavior of various compounds when exposed to such reactions. The removal is more effective when the leaving group (halogen) is on a carbon that can stabilize any charges that form during the reaction, which leads us to the types of carbons involved.

Strong Base Reaction

Strong bases play a crucial role in dehydrohalogenation reactions by facilitating the removal of halides and hydrogens from compounds. Commonly used strong bases include potassium hydroxide (KOH) and sodium ethoxide (NaOEt). These bases abstract a hydrogen atom next to the halide in the carbon chain.

The role of the base is to initiate a reaction that leads to the removal of the halogen. This process often involves forming a carbanion intermediate where the base takes a hydrogen, leading to removal of the halogen via an elimination reaction.

• High-energy bases are required to remove the electrons involved in these reactions.
• These interactions often depend on the stability of involved intermediates.

This type of reaction helps convert alkanes to alkenes by creating additional pi bonds and is a common method for transforming saturated hydrocarbons into more reactive unsaturated ones.

Carbocation Stability

When discussing the mechanism of dehydrohalogenation, understanding carbocation stability is key. Carbocations are positively charged carbon intermediates that can form during these reactions. Their stability significantly influences the reactivity of the compound.

Several factors stabilize carbocations:

• Hyperconjugation: The presence of alkyl groups stabilizes the positive charge.
• Resonance: Delocalization of electrons over multiple atoms stabilizes the charge.

Carbocations can be primary, secondary, or tertiary, with tertiary being the most stable due to greater hyperconjugation and resonance opportunities. This stability directly affects how likely a compound is to undergo dehydrohalogenation, making the stabilizing factors crucial in understanding reactivity patterns.

Allylic Chloride

Allylic chlorides, such as in Compound 2, significantly impact reactivity in dehydrohalogenation due to their position next to a double bond. The "allylic" position refers to a carbon atom that is adjacent to a carbon-carbon double bond, which introduces special resonance stability.

The presence of a double bond allows electrons to be delocalized, creating an additional stability that makes removing the halogen more favorable. This contributes to a higher reactivity compared to simple alkyl halides.

• This type of compound can often stabilize charged intermediates during the reaction.
• The delocalized electrons enable easier formation of pi bonds.

The special position of allylic chlorides creates unique reaction conditions, enhancing the effectiveness of generating new alkenes from the halogenated starting materials.

Resonance Stabilization

Resonance stabilization is a fascinating aspect of organic chemistry that plays a crucial role in many reactions, including dehydrohalogenation. It arises when electrons can be shared across multiple atoms, effectively spreading out and stabilizing charges.

In the context of carbocations and dehydrohalogenation, resonance helps stabilize intermediates by allowing electron pairs to delocalize. Compounds that can offer resonance stabilization, such as those with double bonds or lone pairs, tend to be more reactive.

• This effect lowers the energy of the transition state.
• It facilitates the removal of the halogen leaving group.

Resonance is particularly significant in compounds like allylic chlorides, where the positive charge can be spread over multiple carbon atoms, enhancing their reactivity in elimination reactions.