Question

Which of the following has the highest nucleophi-licity? (a) \(\mathrm{F}^{-}\) (b) \(\mathrm{OH}^{-}\) (c) \(\mathrm{CH}_{3}^{-}\) (d) \(\mathrm{NH}_{2}^{-}\)

Short answer

\(\mathrm{CH}_{3}^{-}\) has the highest nucleophilicity.

Step-by-step solution

Understanding Nucleophilicity

Nucleophilicity refers to the ability of a nucleophile to donate an electron pair to an electrophile. It is influenced by factors such as charge, electronegativity, solvent effects, and steric hindrance. More negatively charged species are typically more nucleophilic, and elements with lower electronegativity tend to be better nucleophiles.

Analyze the Options

Consider the given anions: (a) \(\mathrm{F}^{-}\) is small and highly electronegative, thus less willing to share its electrons.(b) \(\mathrm{OH}^{-}\) is a strong nucleophile, but less than \(\mathrm{CH}_{3}^{-}\) and \(\mathrm{NH}_{2}^{-}\).(c) \(\mathrm{CH}_{3}^{-}\) is not electronegative and readily shares electrons because it is less hindered.(d) \(\mathrm{NH}_{2}^{-}\) is also a strong nucleophile as nitrogen is less electronegative than oxygen.

Comparing Electronegativity

The electronegativity of the atoms involved is important to consider. Fluorine and oxygen are more electronegative than nitrogen and carbon. Thus, \(\mathrm{F}^{-}\) and \(\mathrm{OH}^{-}\) are generally weaker nucleophiles compared to \(\mathrm{NH}_{2}^{-}\) and \(\mathrm{CH}_{3}^{-}\). Among the latter, carbon is the least electronegative, which typically provides it with higher nucleophilicity.

Examine Steric Effects

Steric hindrance can diminish nucleophilicity, but in this case \(\mathrm{CH}_{3}^{-}\) and \(\mathrm{NH}_{2}^{-}\) are both small and unhindered groups, allowing them to be very strong nucleophiles. \(\mathrm{CH}_{3}^{-}\) has the least steric hindrance overall.

Conclusion

Based on charge, electronegativity, and steric positioning, \(\mathrm{CH}_{3}^{-}\) is more nucleophilic than others because its low electronegativity and minimal steric hindrance allow it to donate electrons more readily.

Key concepts

Electron Pair Donation

Electron pair donation is a core feature of nucleophilicity, which refers to the ability of a species, known as a nucleophile, to donate an electron pair to another, known as an electrophile. This process can be compared to a friend generously sharing their resources. Nucleophiles usually have electron-rich environments, such as lone pairs or

• negative charges,
• that they can offer to form bonds.

When an electron pair donation occurs, it typically results in the formation of a new covalent bond. For instance, in nucleophilic substitutions, a nucleophile donates its electron pair to an electrophile, replacing a leaving group. The effectiveness of a nucleophile largely depends on its ability to freely donate its electrons without significant hindrance or reluctance. Here, species like

• methyl anion (\[\mathrm{CH_{3}^{-}}\])

which have highly available electron pairs, are exceptional at electron pair donation.

Electronegativity

The concept of electronegativity plays a vital role in understanding how different species behave as nucleophiles. Electronegativity is the tendency of an atom to attract electrons towards itself. If a nucleophile possesses high electronegativity, it tends to hold onto its electrons more tightly, making it less willing to donate electron pairs.

• Fluorine (\[\mathrm{F^{-}}\]), for instance, although negatively charged, is highly electronegative.
• This means it is less susceptible to donating its electron pair compared to other anions with lower electronegativity.

Conversely, atoms like

• carbon in a methyl anion (\[\mathrm{CH_{3}^{-}}\])
• or nitrogen in an amide ion (\[\mathrm{NH_{2}^{-}}\])

are less electronegative and thus more inclined to share their electrons. These atoms demonstrate how lower electronegativity often results in higher nucleophilicity.

Steric Hindrance

An important factor that affects nucleophilicity is steric hindrance, which refers to the physical obstruction to reaction processes due to the size of groups within a molecule. Essentially, the more room a nucleophile has around it, the more readily it can approach and interact with an electrophile. This is because a bulkier structure around the reactive site can impede the approach or bonding of the nucleophile.In the context of our examples:

• Methyl anion (\[\mathrm{CH_{3}^{-}}\]) experiences minimal steric hindrance. Its small size facilitates easier access to electrophiles, enhancing its nucleophilicity.
• Other potential nucleophiles that are more crowded or "sterically hindered," might struggle to approach and react efficiently.

Therefore, steric hindrance is directly related to the efficiency with which electron pairs can be donated. Smaller, less hindered nucleophiles often prove to be more rapidly reacting and effective.

Anions in Organic Chemistry

In organic chemistry, anions are significant as they often act as nucleophiles due to their negative charge. Anions are atoms or molecules that carry an extra electron, resulting in a negative charge, which can be advantageous for nucleophilic activity. This additional electron makes them more eager to participate in reactions by donating electron pairs.Examples of anions include:

• Hydroxide (\[\mathrm{OH^{-}}\]), which is quite a strong nucleophile,
• amide (\[\mathrm{NH_{2}^{-}}\]), and
• methyl anion (\[\mathrm{CH_{3}^{-}}\]), which showcases strong nucleophilic capabilities.

The presence of a negative charge makes these anions more reactive, as they are naturally inclined to share their electrons in search of stability by entering covalent interactions. Understanding the behavior of anions in organic chemistry is crucial for predicting reactivity and outcomes of chemical reactions.