Question

Which of the following has least mucleophilicity? (1) \(\left(\mathrm{CII}_{3}\right)_{3} \mathrm{C}^{\Theta}=\) (2) \(\left(\mathrm{CII}_{3}\right)_{2} \mathrm{C}_{\mathrm{II}}^{\Theta}\) (3) \(\mathbf{C H}_{3}-\ddot{C} \mathrm{H}_{2}\) (4) : \(\stackrel{\ominus}{\mathrm{C}} \mathrm{H}_{3}\)

Short answer

\(\left(\mathrm{CII}_{3}\right)_{3} \mathrm{C}^{\Theta}\) has the least nucleophilicity due to significant steric hindrance.

Step-by-step solution

Understand Nucleophilicity

Nucleophilicity refers to the ability of a nucleophile to donate a pair of electrons to an electrophile. A good nucleophile is generally less electronegative, less sterically hindered, and has a negative or lone pair of electrons available for donation.

Analyze Each Compound

Examine each given nucleophile for factors that decrease nucleophilicity, namely steric hindrance (bulkiness) and electron-donating ability.

Examine (1) \(\left(\mathrm{CII}_{3}\right)_{3} \mathrm{C}^{\Theta}\)

The nucleophile \(\left(\mathrm{CII}_{3}\right)_{3} \mathrm{C}^{\Theta}\) has three bulky triiodomethyl groups attached to the central carbon, causing significant steric hindrance.

Examine (2) \(\left(\mathrm{CII}_{3}\right)_{2} \mathrm{C}_{\mathrm{II}}^{\Theta}\)

This nucleophile \(\left(\mathrm{CII}_{3}\right)_{2} \mathrm{C}_{\mathrm{II}}^{\Theta}\) is also hindered by two bulky triiodomethyl groups, though somewhat less hindered than (1).

Examine (3) \(\mathbf{CH}_{3}-\ddot{C} \mathrm{H}_{2}\)

The nucleophile \(\mathbf{CH}_{3}-\ddot{C} \mathrm{H}_{2}\) has only small hydrogen atoms around it, making it less sterically hindered and relatively good at donating electrons.

Examine (4) \(:\stackrel{\ominus}{\mathrm{C}} \mathrm{H}_{3}\)

This nucleophile \(:\stackrel{\ominus}{\mathrm{C}} \mathrm{H}_{3}\) is a methyl anion, which is very small and has a high electron density, making it highly nucleophilic.

Identify the Least Nucleophilic

Based on the analysis, \(\left(\mathrm{CII}_{3}\right)_{3} \mathrm{C}^{\Theta}\) has the most significant steric hindrance due to the three triiodomethyl groups, making it the least nucleophilic.

Key concepts

nucleophile

Nucleophiles are species that donate an electron pair to form a chemical bond in a reaction. They are typically characterized by having negative charges or lone pairs of electrons. The strength of a nucleophile is termed nucleophilicity, which is an important factor in determining the reactivity of various compounds in organic synthesis.

A strong nucleophile is one that can easily donate its electron pair to an electrophile, which is a positively charged or electron-deficient species. The ability to donate electrons makes nucleophiles key players in many chemical reactions, such as substitution and addition reactions.

In our exercise, the nucleophiles provided are compared based on factors that affect their electron-donating abilities and their effectiveness in attacking an electrophile. These factors are further analyzed in the next sections.

steric hindrance

Steric hindrance refers to the restriction of reactivity due to the spatial arrangement of atoms within a molecule. Larger groups attached to a reactive site can block other reactants from easily approaching and reacting, thereby reducing the nucleophile’s ability to donate electrons to an electrophile.

Let's consider the example of (1) \((\mathrm{CII}_{3})_{3}\mathrm{C}^{\Theta}\). The presence of three bulky triiodomethyl groups creates significant steric hindrance around the central carbon, making it difficult for this nucleophile to interact with an electrophile. This makes it the least nucleophilic among the examples given.

Comparatively, (2) \((\mathrm{CII}_{3})_{2}\mathrm{C}_{\mathrm{II}}^{\Theta}\) is slightly less hindered but still has substantial bulkiness, affecting its nucleophilicity. In contrast, smaller groups like hydrogens around \(\mathbf{CH_{3}-CH_{2}}\) create much less steric hindrance, enhancing its nucleophilic character.

electron-donating ability

The electron-donating ability of a nucleophile is a critical factor in its reactivity. This ability depends on the availability of lone pairs or negative charges that can be donated to an electrophile. A nucleophile with a high electron density can donate electrons more easily and is therefore more reactive.

For instance, consider (4) \(:\stackrel{\ominus}{\mathrm{C}}\mathrm{H}_{3}\). This compound, a methyl anion, is very small and possesses a high electron density due to its extra electron compared to a neutral carbon atom. This makes it a very strong nucleophile.

On the other hand, nucleophiles that are less effective at donating electrons, either because of steric hindrance or other electronic effects, will exhibit lower nucleophilicity. Therefore, evaluating both the steric bulk and the electron-donating capacity provides a full picture of a nucleophile’s potential reactivity.