Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following nucleophiles: \(\mathrm{H}_{2} \mathrm{O}, \quad \mathrm{CH}_{3} \mathrm{COO}^{-}, \quad \overline{\mathrm{O} H}, \quad \mathrm{CH}_{3} \mathrm{O}^{-}\) \(\begin{array}{llll}\text { I II II } & \text { II } & \text { III } & \text { IV }\end{array}\) The correct order of decreasing nucleophilicity is, (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) IV \(>I I I>I I>I\) (c) IV > I > II > III (d) IV > II > III >I

Step-by-Step Solution

Answer

The correct order of decreasing nucleophilicity is (b): IV > III > II > I.

Step 1: Understand Nucleophilicity

Nucleophilicity refers to the ability of a substance to donate a pair of electrons to form a chemical bond. It usually increases with increasing negative charge and decreases with increasing electronegativity or steric hindrance.

Step 2: Analyze the Ionization and Electronegativity

The nucleophiles mentioned are in order: 1. \(\mathrm{H}_{2} \mathrm{O}\), a neutral molecule. 2. \(\mathrm{CH}_{3} \mathrm{COO}^{-}\), a negatively charged ion. 3. \(\overline{\mathrm{O} H}\), also a negatively charged ion. 4. \(\mathrm{CH}_{3} \mathrm{O}^{-}\), another negatively charged ion.Negatively charged species are generally more nucleophilic than neutral molecules like \(\mathrm{H}_{2} \mathrm{O}\).

Step 3: Compare the Nucleophiles

\(\mathrm{CH}_{3} \mathrm{O}^{-}\) is the most nucleophilic because it is a strong base and has no resonance stabilization.\(\overline{\mathrm{O} H}\) is also a strong base but slightly less than \(\mathrm{CH}_{3} \mathrm{O}^{-}\) due to less polarizability.\(\mathrm{CH}_{3} \mathrm{COO}^{-}\) is less nucleophilic due to resonance stabilization which delocalizes the negative charge.\(\mathrm{H}_{2} \mathrm{O}\) is the least nucleophilic because it's neutral.

Step 4: Arrange in Order of Decreasing Nucleophilicity

Based on the analysis: 1. \(\mathrm{CH}_{3} \mathrm{O}^{-}\) (IV)2. \(\overline{\mathrm{O} H}\) (III)3. \(\mathrm{CH}_{3} \mathrm{COO}^{-}\) (II)4. \(\mathrm{H}_{2} \mathrm{O}\) (I)This corresponds to option (b): IV > III > II > I.

Key Concepts

Nucleophiles

Nucleophiles are molecules or ions that are attracted to positive charges, making them great at forming chemical bonds. They achieve this by donating a pair of electrons to another atom that is deficient in electrons, typically referred to as an electrophile.
Many factors can influence how great a substance is as a nucleophile:

• **Negative Charge:** Generally, species with a negative charge are more nucleophilic. They have more electrons to donate, thus readily forming chemical bonds. For example, consider the negatively charged species like \( \mathrm{CH}_{3} \mathrm{O}^{-} \) and \( \overline{\mathrm{O} H} \), both are excellent nucleophiles compared to neutral molecules like water.
• **Size of the Atom:** Smaller atoms tend to be better nucleophiles because they can approach electrophiles more closely and donate their electrons directly. However, larger atoms are more polarizable, which can also be advantageous in certain cases.
• **Solvent Interactions:** Nucleophiles can be affected by the solvent they're in. Polar protic solvents can hinder nucleophilicity by surrounding and stabilizing nucleophiles, decreasing their activity. Meanwhile, polar aprotic solvents tend to enhance nucleophilicity by not stabilizing nucleophiles.

Electronegativity

Electronegativity is the measure of an atom's ability to attract and hold electrons. Understanding how electronegativity relates to nucleophilicity can demystify why certain substances behave the way they do in reactions.
Here's how electronegativity impacts nucleophilicity:

• **Inversely Related:** Generally, as electronegativity increases, nucleophilicity decreases. This is because more electronegative atoms hold onto their electrons more tightly, making them less available for donation to form new chemical bonds.
• **Periodic Trend:** As you move across a period on the periodic table, electronegativity increases and, thus, nucleophilicity tends to decrease. Conversely, moving down a group, electronegativity decreases, which often enhances nucleophilicity.
• **Role in Comparison:** In comparing \( \mathrm{OH}^{-} \) and \( \mathrm{CH}_{3} \mathrm{O}^{-} \), for example, both are similar in negative charge, but the presence of the methyl group in \( \mathrm{CH}_{3} \mathrm{O}^{-} \) means it has slightly lower overall electronegativity, aiding its nucleophilicity.

Resonance Stabilization

Resonance stabilization is a form of stability achieved when the negative charge of a molecule is spread over multiple atoms. This clever distribution can diminish the reactive nature of a nucleophile.
Here's what happens in resonance stabilization:

• **Delocalization of Electrons:** Resonance involves the delocalization of electrons over two or more atoms. This spreads out the charge, allowing the molecule to be more stable, but often less reactive. For instance, in \( \mathrm{CH}_{3} \mathrm{COO}^{-} \), resonance stabilization occurs, making it a weaker nucleophile compared to others like \( \mathrm{CH}_{3} \mathrm{O}^{-} \).
• **Stability vs Reactivity:** While resonance stabilization offers stability, it decreases reactivity. In the realm of nucleophiles, stability is not a priority since a nucleophile's job is to react and form bonds.
• **Comparison:** If we compare different nucleophiles, those like \( \mathrm{CH}_{3} \mathrm{COO}^{-} \) with resonance will often take a backseat in reactivity to those without, such as \( \mathrm{CH}_{3} \mathrm{O}^{-} \), despite identical charges.