Question

The correct nucleophilicity order for \(\mathrm{S}_{\mathrm{N}} 2\) reaction in protic solvents is (a) \(\mathrm{CH}_{3} \mathrm{~S}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~S}^{-}>\mathrm{I}^{-}>\mathrm{CN}^{-}>\mathrm{CH}_{3} \mathrm{O}^{-}>\mathrm{OH}^{-}\) (b) \(\mathrm{CH}_{3} \mathrm{~S}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~S}^{-}>\mathrm{CN}^{-}>\mathrm{I}^{-}>\mathrm{CH}_{3} \mathrm{O}^{-}>\mathrm{OH}^{-}\) (c) \(\mathrm{CN}^{-}>\mathrm{CH}_{3} \mathrm{~S}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~S}^{-}>\mathrm{OH}^{-}>\mathrm{CH}_{3} \mathrm{O}^{-}>\mathrm{I}^{-}\) (d) \(\mathrm{OH}^{-}>\mathrm{CH}_{3} \mathrm{O}^{-}>\mathrm{CN}^{-}>\mathrm{CH}_{3} \mathrm{~S}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~S}^{-}>\mathrm{I}^{-}\)

Short answer

Answer: The correct nucleophilicity order is: \(\mathrm{CH}_{3} \mathrm{~S}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~S}^{-}>\mathrm{I}^{-}>\mathrm{CN}^{-}>\mathrm{CH}_{3} \mathrm{O}^{-}>\mathrm{OH}^{-}\).

Step-by-step solution

Understand the influence of Protic Solvents

In protic solvents, which have a hydrogen atom bound to an electronegative atom (such as water, alcohols, or carboxylic acids), nucleophile strength is influenced by their ability to form hydrogen bonds with the solvent. Stronger hydrogen bonding with the solvent makes the nucleophile less available for reaction, thus decreasing its nucleophilicity.

Analyze the given nucleophile species

We have the following nucleophiles: \(\mathrm{CH}_3 \mathrm{~S}^{-}\), \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{~S}^{-}\), \(\mathrm{I}^{-}\), \(\mathrm{CN}^{-}\), \(\mathrm{CH}_3 \mathrm{O}^{-}\), and \(\mathrm{OH}^{-}\). In protic solvents, larger and less basic anions are better nucleophiles due to their decreased ability to form hydrogen bonds, leading to increased availability for reaction.

Compare the given orders and look for the correct one

A useful trend to remember for protic solvents is: Polarizability and size increase nucleophilicity, whereas strong hydrogen bonding with the solvent decreases nucleophilicity. According to this, the correct order should look something like: - High nucleophilicity: Large, less basic anions (such as \(\mathrm{CH}_3 \mathrm{~S}^{-}\), \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{~S}^{-}\), \(\mathrm{I}^{-}\)) - Medium nucleophilicity: \(\mathrm{CN}^{-}\) - Low nucleophilicity: Small, more basic anions (such as \(\mathrm{CH}_3 \mathrm{O}^{-}\), \(\mathrm{OH}^{-}\)) Comparing the options with these trends, we can find that the correct nucleophilicity order for \(\mathrm{S}_{\mathrm{N}} 2\) reaction in protic solvents is: (a) $\mathrm{CH}_{3} \mathrm{~S}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~S}^{-}>\mathrm{I}^{-}>\mathrm{CN}^{-}>\mathrm{CH}_{3} \mathrm{O}^{-}>\mathrm{OH}^{-}$

Key concepts

Protic Solvents

When discussing SN2 reactions, the environment in which they occur is a critical aspect of understanding nucleophilicity. Protic solvents are characterized by their hydrogen atoms attached to electronegative elements, like oxygen or nitrogen. Examples include water, alcohols, and carboxylic acids. These solvents can form hydrogen bonds with nucleophiles, which can significantly affect reactivity.

In a protic solvent, a nucleophile's effectiveness is often diminished due to the formation of these hydrogen bonds, which shield the nucleophile and make it less accessible to the substrate it's supposed to react with. Therefore, nucleophiles that are less prone to hydrogen bonding are more reactive in these solvents. Understanding this can immensely help students to predict and rationalize the outcomes of SN2 reactions in protic environments.

Nucleophile Strength

The strength of a nucleophile is essentially how eager or effective it is at donating a pair of electrons. This is a central concept to grasp for SN2 reactions. Nucleophilic strength is influenced by a variety of factors like charge, basicity, solvent, and steric hindrance. In a typical SN2 process, a stronger nucleophile will accelerate the reaction by more readily attacking the electrophile.

Nucleophile strength is not to be confused with basicity, though they are related. A good nucleophile is often a poor base, particularly in protic solvents, because good bases tend to form stronger hydrogen bonds which make them less available to attack the electrophilic center. Therefore, teaching this distinction is crucial when explaining nucleophilicity orders.

Polarizability in Nucleophiles

The concept of polarizability pertains to a molecule's ability to distort its electron cloud in response to external electric fields, such as those from other molecules or ions. This distortion is especially important in nucleophiles.

A highly polarizable nucleophile, which is usually large with loosely held outer electrons, can more easily form partial bonds with the electrophile, increasing its reactivity in SN2 reactions. Larger atoms are typically more polarizable. This aspect of nucleophilic identity often correlates with increased nucleophilicity in protic solvents, as these nucleophiles are less hindered by hydrogen bonding and can interact more freely with the substrate.

Hydrogen Bonding and Nucleophilicity

Hydrogen bonding plays a pivotal role in nucleophilicity, especially within the context of protic solvents. Hydrogen bonds are a type of dipole-dipole interaction that occur when a hydrogen atom covalently bonded to a highly electronegative atom like oxygen or nitrogen is attracted to another electronegative atom with a lone pair of electrons.

These interactions affect nucleophilicity because they can tie up nucleophiles in solvent molecules, making them less available to attack an electrophile. Nucleophiles capable of strong hydrogen bonds are generally weaker in protic solvents. This is a nuanced point that clarifies why some small but strongly basic nucleophiles, such as hydroxide ions, are not as effective in protic solvents despite their high intrinsic activity.