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 order is (b) \(\mathrm{HS}^- > \mathrm{RCOO}^- > \mathrm{RCOOH} > \mathrm{ROH}\).

Step 1: Understand Nucleophilicity

Nucleophilicity refers to the ability of a chemical species to donate a pair of electrons. It is affected by charge, electronegativity, steric hindrance, and the solvent.

Step 2: Analyze the Chemical Species

The given species are: 1. \(\mathrm{HS}^-\), which is a negatively charged ion (hydrosulfide ion).2. \(\mathrm{RCOO}^-\), which is the carboxylate ion.3. \(\mathrm{RCOOH}\), a neutral carboxylic acid.4. \(\mathrm{ROH}\), a neutral alcohol.

Step 3: Evaluate Effects that Influence Nucleophilicity

The negative charge generally enhances nucleophilicity, making anions more nucleophilic than neutral molecules. Thus, \(\mathrm{HS}^-\) and \(\mathrm{RCOO}^-\) are more nucleophilic compared to \(\mathrm{RCOOH}\) and \(\mathrm{ROH}\).\(\mathrm{HS}^-\), being less electronegative, is expected to be a stronger nucleophile than \(\mathrm{RCOO}^-\).

Step 4: Compare and Rank Nucleophiles

Since negatively charged ions are stronger nucleophiles than their neutral counterparts:- \(\mathrm{HS}^-\) is the strongest due to its charge and lesser electronegativity.- \(\mathrm{RCOO}^-\) follows next as it is also charged.- Neutral \(\mathrm{RCOOH}\) and \(\mathrm{ROH}\) are weaker nucleophiles in comparison.- \(\mathrm{ROH}\) is less acidic than \(\mathrm{RCOOH}\), thus slightly more nucleophilic.

Step 5: Determine the Correct Order

Based on the evaluations, the order of decreasing nucleophilicity is: \(\mathrm{HS}^- > \mathrm{RCOO}^- > \mathrm{ROH} > \mathrm{RCOOH}\). This corresponds to option (b).

Key Concepts

Nucleophile

A nucleophile is a chemical species that has an affinity for positive centers, or nuclei, in chemical reactions. It donates a pair of electrons to form a chemical bond. Nucleophilicity is a measure of how strong a nucleophile is and is influenced by several factors:

• Charge: Negatively charged ions are generally stronger nucleophiles because the extra electrons increase the ability to donate an electron pair.
• Electronegativity: Atoms with lower electronegativity are more willing to share their electrons, thus acting as better nucleophiles.
• Steric Hindrance: Bulkier molecules are less effective as nucleophiles because their size makes it difficult to approach and attack the target molecule.
• Solvents: Protic solvents can hinder nucleophilicity by forming hydrogen bonds with the nucleophile, reducing its ability to participate in reactions.

Each of these factors plays a role in the relative nucleophilicity of different chemical species, such as carboxylate ion, hydrosulfide ion, and others.

Carboxylate Ion

The carboxylate ion (\(\mathrm{RCOO}^-\)) is a negatively charged species derived from carboxylic acids. It consists of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxide ion, which loses a proton, resulting in a negative charge.

• As a result of its negative charge, carboxylate ions are potent nucleophiles.
• They are more nucleophilic than their neutral counterpart, carboxylic acid (\(\mathrm{RCOOH}\)), due to the presence of an extra electron pair ready for donation.
• Despite the lone pairs on oxygen, carboxylate ions can sometimes be less nucleophilic than similar species like hydrosulfide ions (\(\mathrm{HS}^-\)) due to differences in electronegativity and resonance stabilization.

Hydrosulfide Ion

The hydrosulfide ion (\(\mathrm{HS}^-\)) is a negatively charged ion consisting of a sulfur atom bonded to a hydrogen atom. It’s notable for being an excellent nucleophile because:

• Sulfur is less electronegative than oxygen, making the hydrosulfide ion more willing to share its electrons compared to ions like the carboxylate.
• This lower electronegativity results in a better ability to donate its electron pair, making\(\mathrm{HS}^-\)more nucleophilic than the carboxylate ion (\(\mathrm{RCOO}^-\)).
• Despite sulfur’s larger atomic size, which could introduce steric hindrance, the \(\mathrm{HS}^-\)ion remains highly nucleophilic, often used in reactions that require strong nucleophiles.

Understanding these properties helps in predicting the outcome of reactions involving the hydrosulfide ion, as it tends to be a preferred choice when a strong nucleophile is needed.