Question

Which compound can react with \(\mathrm{Cl}_{2}\) in light as well as in dark? (A) (B) \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) (C) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{3}\) (D) None of these

Short answer

The compound that can react with \(\mathrm{Cl}_{2}\) in both light and dark conditions is (C) \(\mathrm{CH}_{3}\mathrm{-CH}=\mathrm{CH}_{3}\).

Step-by-step solution

Identify the reactions

First, we will identify the types of reactions that can take place between the given compounds and \(\mathrm{Cl}_{2}\) in light and dark conditions. In light conditions, free radical halogenation occurs, and in dark conditions, electrophilic addition takes place. Now, we will examine each option.

Option (A)

Here, we don't have any information about the compound, so we cannot determine if it reacts with \(\mathrm{Cl}_{2}\) in both conditions or not.

Option (B) - \(\mathrm{CH}_{3}\mathrm{-CH}_{3}\)

In the light condition, free radical halogenation takes place, and \(\mathrm{CH}_{3}\mathrm{-CH}_{3}\) can react with \(\mathrm{Cl}_{2}\). However, in the dark condition, electrophilic addition is not possible because there is no double bond or any other reactive site.

Option (C) - \(\mathrm{CH}_{3}\mathrm{-CH}=\mathrm{CH}_{3}\)

In the light condition, free radical halogenation takes place, and \(\mathrm{CH}_{3}\mathrm{-CH}=\mathrm{CH}_{3}\) can react with \(\mathrm{Cl}_{2}\). In the dark condition, electrophilic addition is possible due to the presence of the double bond, and \(\mathrm{CH}_{3}\mathrm{-CH}=\mathrm{CH}_{3}\) can also react with \(\mathrm{Cl}_{2}\) in this case.

Option (D) - None of these

Since we found that option (C) \(\mathrm{CH}_{3}\mathrm{-CH}=\mathrm{CH}_{3}\) can react with \(\mathrm{Cl}_{2}\) in both light and dark conditions, this option is incorrect. In conclusion:

Final answer

The compound that can react with \(\mathrm{Cl}_{2}\) in both light and dark conditions is (C) \(\mathrm{CH}_{3}\mathrm{-CH}=\mathrm{CH}_{3}\).

Key concepts

Free Radical Halogenation

Free radical halogenation is a chemical reaction involving the substitution of hydrogen atoms in an organic compound with halogen atoms, like chlorine, under the influence of light or heat. This process is initiated by the breaking of the chlorine molecule into two chloride radicals, \( ext{Cl}\cdot \), when exposed to light or heat. These highly reactive radicals can then react with the hydrogen atoms in organic compounds.

• The process begins with the initiation step, where \( ext{Cl}_2 \) molecules split into two chlorine radicals.
• This is followed by propagation steps, where these radicals react with the compound, creating new radicals and perpetuating the reaction.
• The reaction finally ends in a termination step, where radicals join to form non-radical products.

In summary, free radical halogenation primarily occurs when ultraviolet light or heat is present, and it can effectively replace hydrogen atoms in alkanes with chlorine atoms, transforming compounds like \( ext{CH}_3 ext{-CH}_3 \) into chlorinated products.

Electrophilic Addition

Electrophilic addition is a chemical reaction that takes place primarily in alkenes and alkynes, where molecules with carbon-carbon double or triple bonds add atoms or groups of atoms across these bonds. Unlike free radical halogenation, electrophilic addition typically happens in the absence of light and involves the interaction with electrophiles, like \( ext{Cl}_2 \).
During this reaction, the electron-rich double bond attracts an electrophilic chlorine molecule, resulting in the addition of chlorine across the bond. Here’s a simplified breakdown of the process:

• In the initiation phase, the double bond in the alkene attacks a chlorine molecule, leading to the formation of a chloronium ion.
• Next, the trapped chlorine atom bonds to the carbon atoms, breaking the double bond and resulting in a saturated product.

This reaction is key for compounds like \( ext{CH}_3 ext{-CH}= ext{CH}_3 \), where the presence of a double bond allows for reactivity with \( ext{Cl}_2 \), effectively adding chlorine across the double bond and forming an alkane.

Alkenes Reactivity

Alkenes possess significant reactivity due to the presence of their carbon-carbon double bonds. These bonds create sites of electron richness which can be targeted by various reagents. This characteristic makes alkenes highly versatile in chemical reactions both in light and dark conditions.
In processes like free radical halogenation, alkenes can engage with radicals to swap hydrogen atoms with halogens under light conditions. However, it’s their behavior in dark conditions that defines their unique reactivity, especially through electrophilic addition reactions:

• The electron-rich double bond is primed to interact with and accept electrophiles.
• This reactivity allows alkenes to undergo transformations, adding different atoms or groups across the double bond without requiring light to initiate the process.
• This dual capability makes alkenes like \( ext{CH}_3 ext{-CH}= ext{CH}_3 \) participate in reactions efficiently in varied conditions, explaining their ability to react with \( ext{Cl}_2 \) both in light and darkness.

Thus, the reactivity of alkenes provides ample opportunity for chemical synthesis, utilizing their double bonds as fundamental reactive sites in organic chemistry.