Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following pairs is correctly matched? (a) \(\mathrm{R}-\mathrm{Cl} \frac{\text { Nal }}{\text { Acetone }}-\) Finkelstain reaction (b) \(\mathrm{R}-\mathrm{X} \stackrel{\mathrm{Na}}{\longrightarrow}\) Frankland reaction (c) \(\mathrm{R}-\mathrm{X} \stackrel{\mathrm{Zn} / \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}}{\longrightarrow}\) Wurtz reaction (d) \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{3} \stackrel{\text { Alc. KOH }}{\mathrm{X}}\) Grignard reaction

Step-by-Step Solution

Answer

Option (a) is correctly matched with the Finkelstein reaction.

Step 1: Identify each reaction

We need to identify what each reaction in the options corresponds to: - **Option (a):** Finkelstein reaction involves the exchange of halogens, typically using alkyl halides in acetone. - **Option (b):** Frankland reaction is an earlier name for an older coupling reaction involving alkyl radicals. - **Option (c):** Wurtz reaction involves coupling of alkyl halides with sodium to form higher alkanes. - **Option (d):** Grignard reactions involve the formation of Grignard reagents using alkyl or aryl halides in the presence of magnesium.

Step 2: Match correctly with descriptions

Now, compare the definitions of each reaction to the given pairs: - **Option (a):** Correctly describes Finkelstein reaction as it involves the exchange of halogens (e.g., R-Cl becomes R-I with NaI). - **Option (b):** Incorrect because it describes a Frankland-type metal-organic reaction, which involves alkyl radicals but not with sodium as written. - **Option (c):** Incorrect because the Wurtz reaction uses sodium, not zinc, and typically occurs in dry ether. - **Option (d):** Incorrectly described as Grignard, because the conditions and reactions cited do not form Grignard reagents.

Step 3: Conclusion

From the analysis, option (a) is the correctly matched because the conditions describe the Finkelstein reaction accurately, which is specifically about halogen exchange.

Key Concepts

Finkelstein Reaction

The Finkelstein reaction is a nifty organic chemistry process that focuses on swapping halogens in alkyl halides. Named after Hans Finkelstein, this transformation primarily involves the replacement of a chlorine or bromine atom with an iodine atom. It’s like a magic trick in the chemistry world, where one halogen poof—turns into another! Here's how it typically works:

• You start with an alkyl halide, say, \(\mathrm{R}-\mathrm{Cl}\).
• Mix it with sodium iodide (NaI) in acetone, a polar solvent where sodium iodide easily dissolves.
• The reaction favors the formation of alkyl iodide because the product, sodium chloride (NaCl), or sodium bromide (NaBr), is not soluble in acetone and precipitates out of the solution.

This insolubility drives the reaction forward. It's an elegant example of how solubility principles in organic chemistry guide reactions to completion.

Wurtz Reaction

The Wurtz reaction is another fascinating reaction in organic chemistry specifically designed for coupling reactions. It serves as an effective method for synthesizing alkanes from alkyl halides. Imagine linking molecular blocks together to form a bigger structure. In the classic Wurtz reaction:

• Get alkyl halides like \(\mathrm{R}-\mathrm{X}\) as starting materials.
• Mingle them with freshly sliced pieces of sodium metal.
• The mixture needs to be under anhydrous (water-free) conditions, often using dry ether as a solvent.
• The result? Higher alkanes formed by the \(\mathrm{R}-\mathrm{R'}\) bond!

However, note that the Wurtz reaction can lead to side reactions and is generally limited to the formation of symmetrical alkanes. It's a perfect example of why reaction conditions are key in organic reactions.

Frankland Reaction

Now, onto the Frankland reaction—a historical gem in the field of organic chemistry. This reaction dates back to Edward Frankland in the mid-1800s and serves as one of the earliest examples of organometallic chemistry. Here's what it involves:

• It is essentially an organometallic coupling reaction.
• Typically, alkyl halides react with metals like zinc, or earlier with other metals, to form metal-organic compounds.
• Imagine constructing new molecules by forming metal–carbon bonds as building blocks!

Though not as widely practiced today, the Frankland reaction paved the way for modern metal-catalyzed coupling reactions that are vital in synthetic organic chemistry.

Grignard Reagent

Grignard reagents are the superstars in organic synthesis, first developed by Victor Grignard at the start of the 20th century. These compounds are prized for their ability to forge new carbon-carbon bonds, transforming simple starting materials into complex molecules. So, what exactly is a Grignard reagent?

• They are organomagnesium compounds with the general formula \(\mathrm{R}-\mathrm{MgX}\), where \(\mathrm{R}\) is an organic group and \(\mathrm{X}\) a halogen.
• Formed by reacting an alkyl or aryl halide with magnesium metal in dry ether.
• They are very reactive, which makes them incredibly useful for forming new bonds, but also means they should be handled carefully.

One of the main uses of Grignard reagents is in adding carbon chains to various compounds, helping build up more complex molecular structures. These reagents are a staple in many synthetic strategies in laboratories worldwide.