Question

Identify the nucleophile, electrophile, and leaving group in each of the following substitution reactions. Predict whether equilibrium favors the reactants or products: (a) \(\mathrm{CH}_{3} \mathrm{OH}+\mathrm{I}^{-} \rightleftharpoons \mathrm{CH}_{3} \mathrm{OH}+\mathrm{NaOH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}+\mathrm{KCN} \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCN}+\mathrm{KCl}\)

Short answer

For reaction (a), \( I^- \) is the nucleophile, \( CH_3OH \) is the electrophile, and \( H^+ \) is the leaving group. The equilibrium favors the products. For reaction (b), \( KCN \) is the nucleophile, \( (CH_3)_2CHCl \) is the electrophile, and \( Cl^- \) ion is the leaving group. The equilibrium favors the products.

Step-by-step solution

Understanding the Key Terms

A nucleophile is a species that donates an electron pair to an electrophile to form a chemical bond in a reaction. An electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. The leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage.

Solution for Reaction (a)

In the reaction: \( CH_3OH + I^- \rightleftharpoons CH_3O^- + HI \), the \( I^- \) ion acts as the nucleophile since it donates an electron pair. The \( CH_3OH \) acts as the electrophile as it accepts an electron pair, and the \( H^+ \) from \( CH_3OH \) becomes the leaving group. The equilibrium favors the product side due to the formation of a stronger acid, \( HI \), compared to the reactant acid \( CH_3OH \).

Solution for Reaction (b)

In the reaction: \( (CH_3)_2CHCl + KCN \rightleftharpoons (CH_3)_2CHCN + KCl \), the \( KCN \) acts as a nucleophile, since it donates an electron pair. The \( (CH_3)_2CHCl \) acts as the electrophile as it accepts an electron pair, and \( Cl^- \) ion becomes the leaving group. Based on the principle of Le Chatelier's, when one of the products is removed, in this case, \( KCl \) which is an insoluble salt, the equilibrium shifts to the right side, thus, favors the product side.

Key concepts

Nucleophile

In chemistry, a nucleophile is a key player in substitution reactions. It is a chemical species that donates an electron pair to form a new chemical bond. By targeting an electron-deficient area, nucleophiles display their love for charge and seek to share their electrons.

• Nucleophiles tend to be negatively charged or neutral molecules with a lone pair of electrons.
• Common examples include ions like iodide (I^-) and neutral molecules like water (H₂O).
• They are essential in forming bonds and driving reactions forward.

Understanding nucleophiles is crucial because their characteristics determine their strength. Strong nucleophiles, which efficiently donate electron pairs, make reactions proceed more quickly. In the example given, the iodide ion ( uc{I}^- ) in the reaction (a) serves as the nucleophile.

Electrophile

Electrophiles are the opposite of nucleophiles; they seek to accept electron pairs rather than donate them. These species are attracted to electron-rich areas, as they typically bear either a positive charge or a partial positive charge.

• Electrophiles play a central role in chemical reactions as they form bonds by accepting electrons.
• They are typically attracted to nucleophiles, forming a stable bond through the interaction.
• In reactions, they are often the target of nucleophilic attacks.

In our reaction (a), methanol ( uc{CH}_3 ext{OH} ) acts as the electrophile because it accepts the electron pair from the iodide ion. Recognizing electrophiles is key to understanding reaction mechanisms and predicting the outcome of chemical processes.

Leaving Group

Leaving groups are critical in substitution reactions, as they are the portions of a molecule that depart with an electron pair during bond cleavage. A good leaving group stabilizes the departing negative charge, thereby facilitating the reaction.

• Leaving groups are usually weak bases or molecules able to accommodate the additional electron pair.
• They often participate in reactions like nucleophilic substitutions and eliminations.
• Typical leaving groups are halides like chloride (Cl^-) and iodide (I^-).

In reaction (b), the chloride ion ( uc{Cl}^- ) leaves the molecule, aiding the smooth progress of the reaction. Evaluating the leaving group's ability can help anticipate whether a reaction will proceed favorably.

Equilibrium in Chemical Reactions

Understanding equilibrium in chemical reactions is vital, as it determines whether the products or reactants are favored at the end of a reaction. Reactions strive to reach a state of balance between the forward and reverse directions.

• When reaction conditions change, equilibrium adjusts to minimize the effect, maintaining balance, as explained by Le Chatelier's principle.
• Factors like temperature, pressure, and concentration influence the position of equilibrium.
• Removing one of the products or adding additional reactants can shift the equilibrium toward the products and vice versa.

In the provided example (b), the equilibrium favors products because the formation of insoluble potassium chloride (KCl) drags the reaction toward product formation, as predicted by Le Chatelier's principle. A good grasp of equilibrium helps in predicting reaction outcomes and conditions needed to align with desired chemical processes.