Question

4-chloropyridine reacts with sodium methoxide about 230 million times faster than chlorobenzene at \(50^{\circ} \mathrm{C}\), itis because of: (A) \(\mathrm{C}-\mathrm{Cl}\) bond in 4-chloropyridine is weaker (B) Ionic intermediate of 4-chloropyridine has its negative charge on nitrogen (C) 4-chloropyridine is more nucleophilic (D) None of these

Short answer

The correct reason for 4-chloropyridine reacting with sodium methoxide about 230 million times faster than chlorobenzene at \(50^{\circ} \mathrm{C}\) is (B) the ionic intermediate of 4-chloropyridine has its negative charge on nitrogen, which lowers the activation energy needed for the reaction, speeding up the reaction.

Step-by-step solution

Compare C-Cl bond strengths in 4-chloropyridine and chlorobenzene

If the C-Cl bond in 4-chloropyridine is weaker, then it would more easily break and react with sodium methoxide. So, first, we need to analyze the bond strengths in both 4-chloropyridine and chlorobenzene. 4-chloropyridine has a nitrogen atom in the aromatic ring that attracts electrons toward itself, slightly weakening the C-Cl bond. Nonetheless, chlorobenzene also has a resonance structure that weakens the C-Cl bond. Therefore, it is unlikely that the bond strength difference alone can account for the significant difference in reactivity.

Examine the ionic intermediate

Consider the reaction between 4-chloropyridine and sodium methoxide. Sodium methoxide acts as a nucleophile and attacks the carbon linked to the chlorine, which then kicks off the chlorine as a leaving group. In the process, a negative charge could potentially be distributed among different atoms in the intermediate. In 4-chloropyridine, this negative charge is more likely to be on the nitrogen, due to its higher electronegativity. This ionic intermediate lowers the activation energy needed for the reaction, speeding up the reaction. Therefore, this fact might play a role in the higher reactivity of 4-chloropyridine.

Assess nucleophilicity

Nucleophilicity refers to the capability of a molecule to donate electrons and form a bond with an electrophile. However, in this case, both 4-chloropyridine and chlorobenzene are electrophiles, not nucleophiles. Sodium methoxide is the nucleophile here, which means that the reaction rate should depend on the electrophilicity of the reactants, not their nucleophilicity. Based on the analysis, the correct reason is: (B) Ionic intermediate of 4-chloropyridine has its negative charge on nitrogen.

Key concepts

C-Cl bond strengths

The strength of the carbon-chlorine bond ( C-Cl bond) is an important factor in chemical reactions. A weaker C-Cl bond is easier to break, making the molecule more reactive. In the case of 4-chloropyridine, the presence of nitrogen in the aromatic ring contributes to a slight weakening of the C-Cl bond. This occurs because nitrogen is more electronegative and tends to pull electron density towards itself.
This electron-withdrawing effect can marginally destabilize the C-Cl bond. However, chlorobenzene also has resonance structures that help to weaken the C-Cl bond through electron delocalization. Despite these influences, the impact on bond strength is not substantial enough to explain the drastic difference in reaction rates between 4-chloropyridine and chlorobenzene.
Therefore, while the C-Cl bond may seem to have a weaker interaction in 4-chloropyridine due to nitrogen's influence, it is not the sole factor contributing to its heightened reactivity with sodium methoxide.

ionic intermediate

An ionic intermediate is a crucial step in many reactions. It involves a temporary structure where ions are formed, allowing the reaction to proceed. When 4-chloropyridine reacts with sodium methoxide, an ionic intermediate is formed. Sodium methoxide, as a nucleophile, attacks the carbon bonded to chlorine, prompting the chloride ion to leave.
In this intermediate, the negative charge has to be distributed across the remaining molecule. In 4-chloropyridine, the nitrogen atom, being highly electronegative, is the most likely candidate to bear this negative charge. This allocation of charge stabilizes the intermediate, making it easier for the reaction to occur.
Stabilization of this ionic intermediate through the nitrogen atom effectively reduces the energy barrier, or activation energy, for the reaction. Consequently, 4-chloropyridine reacts much faster than chlorobenzene due to this additional stabilization.

nucleophilicity

Nucleophilicity describes a molecule’s ability to donate electrons to form a bond with an electrophile. Although it might seem relevant in the discussion of reaction rates, nucleophilicity actually refers to the behavior of the attacking species in this context.
In the reaction of interest, sodium methoxide acts as the nucleophile, attacking the 4-chloropyridine. Both 4-chloropyridine and chlorobenzene function as electrophiles, meaning they accept electrons rather than donate them. Therefore, the focus is on their electrophilicity rather than nucleophilicity.
Considering electrophilicity here, the presence of nitrogen in the pyridine ring makes 4-chloropyridine a more accommodating target for the nucleophile, because of the ionic stabilization discussed earlier. Thus, nucleophilicity of 4-chloropyridine is irrelevant to its reaction rate against sodium methoxide.