Question

Which of the following statements is correct? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a stronger base, but weaker nucleophile than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\). (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base, but more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a stronger base and more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\). (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base and less nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\).

Short answer

Answer: (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base, but more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\).

Step-by-step solution

Compare the basicity of oxygen and sulfur

Notice that oxygen is more electronegative than sulfur. Due to their respective positions on the periodic table, oxygen has a higher tendency to attract electrons, while sulfur is more polarizable. Since basicity involves accepting protons, a more electronegative atom (oxygen) will have a weaker ability to accept protons, making the corresponding anion a weaker base.

Compare the nucleophilicity of oxygen and sulfur

Nucleophilicity depends on the ability of an atom to donate electrons to a suitable electrophile. Sulfur is more polarizable and can stabilize the negative charge over a larger volume, resulting in a higher nucleophilicity compared to oxygen. In polar aprotic solvents (like ethers or nitriles), the trend in nucleophilicity is mainly determined by the size and polarizability of the atoms. In these solvents, sulfur has a higher nucleophilicity than oxygen.

Relate the basicity and nucleophilicity to the given statements

From our previous analysis, we can conclude that ethylthiolate (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\)) is a weaker base but more nucleophilic than ethoxide (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)).

Choose the correct statement

Based on our previous conclusions, we can now identify the correct statement. Statement (b) states that "\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\) is a weaker base, but more nucleophilic than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)", which is consistent with our analysis. Therefore, statement (b) is the correct answer.

Key concepts

Nucleophilicity

In organic chemistry, nucleophilicity refers to the ability of a molecule or ion to donate a pair of electrons to an electrophile, forming a new chemical bond. It is an essential concept for understanding reaction mechanisms, especially in substitution reactions. Various factors influence nucleophilicity, including the atom's size and the solvent in which the reaction occurs.

Size and polarizability play a significant role; larger atoms can better stabilize their negative charge, enhancing their electron-donating ability. For example, sulfur is larger than oxygen and thus more nucleophilic because it can more effectively share its electron pair.

• Nucleophiles are often negatively charged or neutral species with high electron density.
• Solvents also impact nucleophilicity; in polar aprotic solvents, larger, more polarizable atoms like sulfur are stronger nucleophiles compared to smaller atoms like oxygen.

Understanding the distinction between nucleophilicity and basicity is important, as they are related but not the same.

Basicity Comparison

Basicity is the measure of how readily a molecule or atom accepts protons (H⁺ ions). It relates closely to the concept of electronegativity, which is the tendency of an atom to attract electrons. In general, the higher the electronegativity, the weaker the base. This is because strong electronegative atoms will more likely hold onto their electrons rather than donate them back in an interaction with a proton.

A classic example we have is comparing sulfur and oxygen. Oxygen, being more electronegative, is less inclined to accept an additional electron (a proton) compared to sulfur. Therefore,

• Oxygen-based anions (like ethoxide) tend to be stronger bases than sulfur-based anions (like ethylthiolate) because they are less prone to bear an additional proton.
• The stronger the base, the higher its ability to neutralize acids.

Understanding this helps predict how these molecules will behave in various chemical reactions.

Polarizability

Polarizability is the ability of an atom or molecule’s electron cloud to be distorted by an electric field, which effectively enhances nucleophilicity. It is an important factor when comparing nucleophiles. The bigger the atom, the more polarizable it is, primarily because larger atoms have more loosely held outer electrons that can be more easily pulled away.

When we compare sulfur and oxygen, sulfur is more polarizable due to its larger size and the more diffuse nature of its valence electrons.

• This makes sulfur an excellent nucleophile, as its electrons are more readily available for bonding.
• Polarizability also contributes to developing attractions between atoms in different molecules, influencing reaction rates and stabilities.

Additionally, polarizability helps guide the choice of reagents and conditions in synthetic chemistry to control reaction pathways.

Periodic Table Trends

The periodic table is a fantastic tool for predicting the properties and behaviors of elements, including nucleophilicity, basicity, and polarizability. Understanding periodic trends is crucial when analyzing chemical reactions and comparing atoms.

Moving across a period from left to right, electronegativity generally increases, while atom size decreases. Therefore:

• Nucleophilicity often decreases across a period due to increasing electronegativity.
• Basicity also decreases as electronegativity increases, given that atoms are less likely to share electrons when they are highly electronegative.
• Down a group, atom size and polarizability increase, making these atoms excellent nucleophiles, despite often being weaker bases.

Overall, these trends help chemists predict how different elements might behave under certain conditions, assisting in the planning of reactions and synthesis pathways.