Problem 151 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following order/statement (s) is/are correct? (a) In \(\mathrm{S}_{\mathrm{N}} 1\) reactions, \(\mathrm{AgNO}_{3}\) increases the rates of solvolysis (b) Soft bases have enhanced nucleophilicities towards \(\mathrm{S}_{\mathrm{N}} 2\) reactions. (c) \(\mathrm{PhSO}_{3}^{-}>\mathrm{Cl}_{3} \mathrm{CCOO}^{-}>\mathrm{CH}_{3} \mathrm{COO}^{-}>\mathrm{PhO}^{-}\)(leav- ing group ability) (d) HCOOH may lead a tertiary halide towards \(\mathrm{S}_{\mathrm{N}} 2\).

Step-by-Step Solution

Answer

Statements (a), (b), and (c) are correct.

Step 1: Understanding SN1 Reaction and AgNO3 Influence

In an SN1 reaction, the rate-determining step involves the formation of a carbocation. AgNO3 is often used to promote the ionization of alkyl halides by forming a precipitate with the halide ion, thereby increasing the rate of solvolysis. Thus, statement (a) is correct.

Step 2: Analyzing Nucleophilicity in SN2 Reactions

SN2 reactions depend on the nucleophile's strength. In general, softer bases (which have more polarizable electrons) show higher nucleophilicity in SN2 reactions because they are better at donating electrons. Therefore, statement (b) is correct.

Step 3: Evaluating Leaving Group Ability

Leaving group ability is determined by how easily the leaving group can stabilize a negative charge. Generally, the order given in statement (c) is reasonable as sulfonates like PhSO3- are better leaving groups than Cl3CCOO- (due to resonance stabilization), which is better than CH3COO-, which in turn is better than PhO-. Thus, statement (c) is correct.

Step 4: Understanding SN2 Reaction and Solvent Influence

SN2 reactions proceed with primary and secondary substrates more favorably due to steric effects. A tertiary halide undergoing SN2 is highly unlikely due to steric hindrance, even in a polar aprotic solvent like HCOOH. Hence, statement (d) is incorrect.

Key Concepts

SN1 Reaction

An SN1 reaction is a type of nucleophilic substitution reaction that is known for its two-step process. The "SN" stands for substitution nucleophilic, and the "1" signifies that the rate-determining step is unimolecular. This means that the reaction rate only depends on the concentration of the substrate, not the nucleophile. The reaction begins with the formation of a carbocation intermediate, which involves the loss of a leaving group. This step is crucial because the stability of the carbocation heavily influences the reaction's feasibility. Tertiary carbocations are more stable and typically form more readily than primary or secondary ones. In the presence of \({\mathrm{AgNO}_{3}}\), the rate of SN1 reactions often increases. This is because silver nitrate can precipitate the departing halide ion, like chloride, which helps drive the reaction forward by facilitating ionization of the alkyl halide. By increasing the likelihood of carbocation formation, \({\mathrm{AgNO}_{3}}\) essentially speeds up the process of solvolysis.

SN2 Reaction

The SN2 reaction is another type of nucleophilic substitution but differs significantly from the SN1 mechanism. For SN2, the number "2" indicates a bimolecular rate-determining step, meaning both the substrate and the nucleophile affect the reaction rate. During an SN2 reaction, the nucleophile attacks at the same time the leaving group departs. This simultaneous action forms a transition state where bonds are partially formed and broken. As a result, the stereochemistry of the substrate is inversed. Nucleophilicity plays a crucial role here. Generally, soft bases—nucleophiles with highly polarizable electrons—are more effective in SN2 reactions. They can better donate an electron pair to the electrophile, resulting in a quicker reaction. Furthermore, steric hindrance can affect SN2 reactions significantly. Bulkier substrates tend to slow down or prevent the reaction, making primary substrates more favorable than tertiary ones.

Nucleophilicity

Nucleophilicity is a measure of a species' ability to donate an electron pair and form a bond with an electrophile. It's often discussed in the context of substitution reactions like SN1 and SN2. Several factors, including charge, electronegativity, and solvent, can affect nucleophilicity. Typically, a negatively charged species is a stronger nucleophile compared to its neutral counterpart due to the increased electron density. However, comparing nucleophilicity across different atoms, lower electronegativity indicates higher nucleophilicity because the atom doesn’t hold onto its electrons as tightly and is more willing to share them. In SN2 reactions, where nucleophilicity is particularly important, softer nucleophiles are preferred. A soft nucleophile has more diffused electrons and can easily polarize, enhancing its reaction speed. This concept becomes even more relevant when considering the demands of the specific solvent environment, often favoring polar aprotic solvents which do not interfere with the nucleophile via hydrogen bonding.

Leaving Group Ability

Leaving group ability is another important factor in understanding nucleophilic substitution reactions like SN1 and SN2. It refers to how easily a group can depart from a molecule, taking with it the electrons of the bond it leaves behind. A good leaving group often stabilizes the extra electrons it carries away through mechanisms like resonance or inductive effects. Generally, stronger leaving groups are weak bases since they are less reactive and better at accommodating an extra charge. Examples of good leaving groups include halides like \({\mathrm{Cl^-}}\) and sulfonate ions like \({\mathrm{PhSO_3^-}}\). These stabilize the negative charge very efficiently, allowing the reaction to proceed more smoothly. In contrast, groups that are poor at stabilizing an extra charge are poor leaving groups, such as hydroxide ions.