Question

Which of the following is the strongest nucleophile? (a) \(\mathrm{NH}_{2}^{-}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}\) (c) \(\mathrm{HC} \equiv \mathrm{C}^{-}\) (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}^{-}\)

Short answer

The strongest nucleophile is (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}\).

Step-by-step solution

Identify Nucleophile Properties

A nucleophile is a chemical species that donates a pair of electrons to form a new chemical bond. The strength of a nucleophile is influenced by several factors, including its charge, electronegativity, and steric hindrance. In general, more negatively charged species and those with lower electronegativity are stronger nucleophiles.

Analyze Each Option's Properties

Evaluate each species:- (a) \(\mathrm{NH}_{2}^{-}\): Negatively charged nitrogen; moderate electronegativity.- (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}\): Negatively charged carbon; lower electronegativity and less steric hindrance.- (c) \(\mathrm{HC} \equiv \mathrm{C}^{-}\): Extremely high electronegativity due to sp hybridization.- (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}^{-}\): Lower electronegativity but has resonance stabilization.Less electronegative elements with negative charges often act as stronger nucleophiles.

Compare Influence of Electronegativity and Resonance

Lower electronegativity typically enhances nucleophilicity because the atom does not hold its electrons as tightly and is more willing to donate them. Additionally, while resonance can stabilize a negative charge, it could also delocalize it, potentially reducing nucleophilicity.

Determine the Strongest Nucleophile

Comparing all options, \(\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}\) is the strongest nucleophile. It has lower electronegativity due to carbon and lacks resonance stabilization, meaning its negative charge is more localized and available for bonding.

Key concepts

Nucleophile Strength

Nucleophiles are an essential concept in chemistry, especially in relation to reactions and bonding. A nucleophile is a species that donates a pair of electrons to form a chemical bond. The strength of a nucleophile describes how readily it can donate its electrons to another atom, typically a positively charged or electron-deficient center.

• Negatively charged species are generally stronger nucleophiles compared to neutral ones.
• Atom or ion size can affect strength; smaller ions can often access electrophiles more easily.
• A strong charge localized on the species enhances its ability to donate electrons.

In the context of our exercise, for example, the \(CH_3-CH_2^-\) anion is a particularly strong nucleophile. It is negatively charged, and the charge is not stabilized by resonance or other effects, making its electron pair readily available for bonding.

Electronegativity Effects

Electronegativity refers to the ability of an atom to attract electrons towards itself. This property can significantly impact the strength of a nucleophile. The basic rule is: the lower the electronegativity of an atom with a negative charge, the stronger its nucleophilic properties.

• A highly electronegative atom holds onto its electrons tightly, reducing its nucleophilicity.
• Less electronegative elements with negative charges are often more willing to donate electrons, enhancing nucleophilic strength.

For instance, in our example, carbon in \(CH_3-CH_2^-\) has a lower electronegativity compared to other elements like nitrogen or oxygen, which makes it a stronger nucleophile compared to \(NH_2^-\) in this specific context.

Resonance Stabilization

Resonance stabilization refers to the distribution of electrons across multiple atoms in a molecule, which can delocalize a negative charge. While this can make the specie more stable, it can also reduce its ability to act as a strong nucleophile.

• Resonance can reduce nucleophilicity by distributing the electron density across several atoms.
• This delocalization often makes the electrons less available for forming bonds.

In the exercise, \(H_2C=CH^-\) has resonance stabilization. The negative charge is shared across the structure, thus reducing its nucleophilic abilities compared to non-resonance stabilized species.

Chemical Bonding Concepts

Understanding chemical bonding is crucial for grasping how nucleophiles work. Chemical bonds involve the sharing or transferring of electrons between atoms, and nucleophiles play a key role in the formation of these bonds.

• Nucleophiles donate electron pairs to form covalent bonds with electrophiles (electron-deficient species).
• The strength and directionality of these bonds are influenced by factors like hybridization and the atomic orbitals involved.

In our problem, nucleophiles like \(CH_3-CH_2^-\) are more effective at forming bonds because they have localized negative charges and are not hindered by effects like resonance or high electronegativity. This allows them to engage more readily in chemical reactions, making them strong bonding agents in the right context.