Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

The decreasing order of nucleophilicity of HS', \(\mathrm{RCOO}^{-}, \mathrm{RCOOH}\) and \(\mathrm{ROH}\) is (a) \(\mathrm{RCOO}^{-}>\mathrm{HS}^{->} \mathrm{RCOOH}>\mathrm{ROH}\) (b) \(\mathrm{HS}^{-}>\mathrm{RCOO}^{-}>\mathrm{RCOOH}>\mathrm{ROH}\) (c) \(\mathrm{HS}^{-}>\mathrm{RCOO}^{-}>\mathrm{ROH}>\mathrm{RCOOH}\) (d) \(\mathrm{RCOO}^{-}>\mathrm{HS}^{-}>\mathrm{ROH}>\mathrm{RCOOH}\)

Step-by-Step Solution

Answer

The correct answer is (c) HS^{-}>RCOO^{-}>ROH>RCOOH.

Step 1: Understanding Nucleophilicity

Nucleophilicity refers to the tendency of a species to donate an electron pair to a positively charged or electron-deficient site. It often correlates to the basicity of the species, where stronger bases tend to be stronger nucleophiles. However, steric factors and solvent can influence nucleophilicity as well.

Step 2: Analyzing Each Species

- HS⁻: A strong nucleophile due to its negative charge and less electronegative sulfur atom which can easily donate electrons. - RCOO⁻: Also negatively charged, but the resonance with the carbonyl group distributes the electron density, making it less nucleophilic than HS⁻. - RCOOH: A neutral molecule, less nucleophilic compared to those with a negative charge. - ROH: Neutral like RCOOH, but usually even less nucleophilic due to lack of additional oxidizable hydrogen like in RCOOH.

Step 3: Arranging in Decreasing Order

Considering charge and the ability to donate electron pairs, HS⁻ is the most nucleophilic. It is followed by RCOO⁻ due to its negative charge, despite resonance stabilization. ROH and RCOOH are less nucleophilic as they are neutral. However, RCOOH is even less nucleophilic than ROH due to its ability to donate hydrogen ions (it's more acidic).

Step 4: Conclusion

The correct order from most to least nucleophilic is: HS⁻ > RCOO⁻ > ROH > RCOOH.

Key Concepts

Chemical species

Chemical species refers to any set of atoms, molecules, ions, or radicals that share the same structure and properties. In the context of nucleophilicity, we look at species that have the ability to donate electrons. For instance, in our example, we deal with HS⁻, RCOO⁻, RCOOH, and ROH. Each of these chemical species behaves differently because of their molecular structure and charge:

• HS⁻ is an anion with a strong negative charge.
• RCOO⁻ is also an anion, but it has resonance that spreads the negative charge over the molecule.
• RCOOH and ROH are neutral molecules.

These differences are crucial because they greatly influence how these species interact in chemical reactions, particularly in their roles as nucleophiles.

Electron donation

Electron donation is a property where a chemical species gives away electrons to another species. This action is essential in many chemical reactions, particularly those involving nucleophiles and electrophiles. Nucleophiles, such as HS⁻, are known for their ability to donate electrons.
Their electron-rich nature enables them to attack electron-poor or electrophilic centers in other molecules.

• HS⁻: Due to its negative charge, it can easily donate electrons.
• RCOO⁻: While it can donate electrons, its efficiency decreases because of the resonance that spreads out its electron density.
• Neutral species like RCOOH and ROH have a lower tendency to donate electrons as they aren’t as electron-rich.

Understanding electron donation helps clarify why some species are better nucleophiles than others.

Resonance

Resonance is a concept in chemistry where electrons are delocalized in a molecule, spread over several structures rather than confined to a single location. In the case of RCOO⁻, resonance plays a significant role in its properties. The negative charge is shared between the oxygen atoms and the carbon.

• This delocalization of electrons gives stability to RCOO⁻ but decreases its nucleophilicity compared to HS⁻ because the electrons aren't as readily available for donation.

Resonance is a stabilization mechanism, but it often means a reduction in reactivity because the electron density that could participate in reactions is somewhat neutralized.

Acidity

Acidity is the ability of a molecule to donate a proton (H⁺) in a solution. It is inversely related to nucleophilicity.
For example, RCOOH is acidic as it can donate a hydrogen ion, becoming RCOO⁻ in the process. This ability to lose a proton and form a conjugate base is a defining feature of acidic compounds.

• The stronger the acidity of a molecule like RCOOH, the less likely it is to act as a nucleophile, since it prefers to donate protons rather than engage in electron donation.
• RCOO⁻, the conjugate base, shows how the acidity of RCOOH affects its nucleophilicity – once deprotonated, it becomes a moderate nucleophile due to its charge.

Understanding acidity helps predict the reactivity of different chemical species in a reaction.

Basicity

Basicity refers to the tendency of a molecule to accept a proton. It is closely related to nucleophilicity, as both involve the tendency to pair with a complementary species:

• HS⁻ is a strong base because it can easily accept protons, correlating with its role as a strong nucleophile.
• RCOO⁻, while a base, is less basic than HS⁻ due to its resonance stabilization, reflecting its moderate nucleophilicity.

High basicity often indicates a strong nucleophile because both rely on lone pair electrons to perform their roles in reactions.
However, the surrounding environment and structural features, like resonance, can influence these properties. Understanding basicity aids in predicting the effectiveness of a nucleophile.