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 (c) \(\mathrm{HS}^{-} > \mathrm{RCOO}^{-} > \mathrm{ROH} > \mathrm{RCOOH}\).

Step 1: Understanding Nucleophilicity

Nucleophilicity refers to a species' ability to donate an electron pair to an electrophile, generally forming a bond. Stronger bases tend to be stronger nucleophiles. While comparing nucleophilicity, we need to consider both charge and electronegativity. Anions are typically more nucleophilic than neutral species.

Step 2: Analyzing the Anions

Among the given species, \(\mathrm{HS}^{-}\) and \(\mathrm{RCOO}^{-}\) are anions. Generally, sulfur-containing anions like \(\mathrm{HS}^{-}\) are stronger nucleophiles compared to oxygen-containing anions like \(\mathrm{RCOO}^{-}\), primarily due to sulfur's larger size and lower electronegativity allowing it to donate electron pairs more effectively.

Step 3: Evaluating Neutral Species

The neutral species \(\mathrm{RCOOH}\) (carboxylic acid) and \(\mathrm{ROH}\) (alcohol) are poor nucleophiles compared to the anions. Among these, \(\mathrm{ROH}\) can potentially act as a nucleophile through the lone pairs on oxygen, while \(\mathrm{RCOOH}\) is even less nucleophilic due to the resonance stabilization of its lone pairs.

Step 4: Arranging in Decreasing Order of Nucleophilicity

Based on the analysis, \(\mathrm{HS}^{-}\) is the strongest nucleophile, followed by \(\mathrm{RCOO}^{-}\), \(\mathrm{ROH}\), and finally \(\mathrm{RCOOH}\). Thus, the correct order is \(\mathrm{HS}^{-} > \mathrm{RCOO}^{-} > \mathrm{ROH} > \mathrm{RCOOH}\).

Key Concepts

Anions vs Neutral Species

Anions and neutral species have different roles in chemical reactions due to their charge properties. Anions, such as \(\mathrm{HS}^{-}\) and \(\mathrm{RCOO}^{-}\), are negatively charged ions. This negative charge makes them more nucleophilic compared to neutral species like \(\mathrm{RCOOH}\) and \(\mathrm{ROH}\) that do not carry a charge.

Anions are capable of donating electron pairs more readily because their negative charge creates a strong attraction to positively charged or electron-deficient centers in other molecules, also known as electrophiles.

Neutral species, on the other hand, lack this additional electron density, and their ability to act as nucleophiles is hampered by the absence of a charge. This means that in general, anions are more aggressive and effective in providing electrons compared to their neutral counterparts.

In the given problem,

• the anions \(\mathrm{HS}^{-}\) and \(\mathrm{RCOO}^{-}\) distinctly outperform the neutral \(\mathrm{RCOOH}\) and \(\mathrm{ROH}\) in nucleophilicity.
• The presence of a charge significantly affects their reactivity in forming new bonds.

Nucleophiles and Electrophiles

In the realm of organic chemistry, nucleophiles and electrophiles are key players. A nucleophile is an 'electron-rich' species that loves to donate electrons, while an electrophile is an 'electron-deficient' species that loves to accept electrons.

Nucleophiles are often negatively charged ions or molecules with lone pairs of electrons. Their tendency to donate electron pairs makes them crucial in forming covalent bonds with electrophiles. Common nucleophiles include anions like \(\mathrm{HS}^{-}\) and \(\mathrm{RCOO}^{-}\). Neutral molecules such as \(\mathrm{ROH}\) and \(\mathrm{RCOOH}\) can also act as nucleophiles when the condition facilitates electron donation.

Electrophiles, by contrast, are usually positively charged ions or molecules that have an electron deficit. They attract electrons from nucleophiles to complete their valence shells, leading to the formation of stable products.

Understanding the dynamics between nucleophiles and electrophiles is crucial in predicting chemical reactions and their outcomes. It helps explain how bonds are formed and why certain reactions occur the way they do.

Electronegativity and Nucleophilicity

Electronegativity is the ability of an atom to attract electrons towards itself. It plays a crucial role in determining the nucleophilicity of a species. Generally, elements with - lower electronegativity- have a higher propensity to donate electron pairs, making them better nucleophiles.

In comparing \(\mathrm{HS}^{-}\) and \(\mathrm{RCOO}^{-}\), sulfur has a lower electronegativity compared to oxygen. This means that sulfur atoms, like those found in \(\mathrm{HS}^{-}\), can donate electrons more effectively than oxygen-containing anions like \(\mathrm{RCOO}^{-}\).

Another factor affecting nucleophilicity in anions is the size of the atom. Larger atoms like sulfur have their valence electrons further from the nucleus, making these electrons easier to donate. Hence, despite sulfur's larger size, its lower electronegativity enhances \(\mathrm{HS}^{-}\)'s nucleophilicity over \(\mathrm{RCOO}^{-}\).

The trend in nucleophilicity often follows the opposite of electronegativity values, explaining why \(\mathrm{HS}^{-}\) is a stronger nucleophile compared to other species in the problem statement.