Question

The correct order of nucleophile in decreasing order of of \(\mathrm{S}_{\mathrm{N}} 2\) reactivity is \(\mathrm{Me}_{3} \mathrm{CO}^{\ominus}, \mathrm{MeO}^{\ominus}, \mathrm{MeCH}_{2} \mathrm{O}^{\ominus}, \mathrm{Me}_{2} \mathrm{CHO}^{\Theta}\) (A) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}>\mathrm{V}\) (B) II \(>\mathrm{III}>\mathrm{IV}>\mathrm{V}>\mathrm{I}\) (C) \(\mathrm{II}>\mathrm{III}>\mathrm{V}>\mathrm{IV}>\mathrm{I}\) (D) \(\mathrm{I}>\mathrm{IV}>\mathrm{V}>\mathrm{III}>\mathrm{II}\)

Short answer

The correct order of nucleophiles in terms of decreasing SN2 reactivity is \(II > III > IV > I\), which corresponds to option (C).

Step-by-step solution

Identify the given nucleophiles

The given nucleophiles are: 1. Me3CO⁻ (I) 2. MeO⁻ (II) 3. MeCH2O⁻ (III) 4. Me2CHO⁻ (IV)

Analyze the charge of nucleophiles

All of the given nucleophiles carry a negative charge on the oxygen atom, indicating the oxygen will act as the nucleophile.

Analyze electronegativity of central atoms

The nucleophilicity of negatively charged species is inversely proportional to the electronegativity of the central atom. All the oxygen atoms are negatively charged and are of about the same electronegativity, so this factor does not affect the order in this case.

Analyze steric hindrance of nucleophiles

Steric hindrance is the effect of bulky groups near the negatively charged atom on the nucleophilicity of that atom. Greater the steric hindrance, lesser the reactivity of the nucleophile in SN2 reactions due to difficulties in reaching the reaction center. Analyzing the given nucleophiles based on steric hindrance: 1. Me3CO⁻ (I) - Most sterically hindered due to three methyl (Me) groups 2. MeO⁻ (II) - Least sterically hindered due to only one Me group 3. MeCH2O⁻ (III) - Intermediate steric hindrance with one Me group and a hydrogen atom 4. Me2CHO⁻ (IV) - Intermediate steric hindrance with two Me groups

Determine the order of nucleophiles

Based on the steric hindrance, the order of nucleophiles in terms of decreasing SN2 reactivity should be: II > III > IV > I This matches the order provided in option (C). Therefore, the correct order of nucleophiles is: (C) II > III > IV > I

Key concepts

Nucleophile Strength

Understanding the strength of a nucleophile is essential for predicting the outcome of chemical reactions, especially the S_N2 mechanism.

Nucleophiles are species that donate an electron pair to electrophiles during chemical reactions. The strength of a nucleophile is influenced by several factors, with the most crucial being the nucleophile's charge, the solvent, and the polarizability of the nucleophile.

A negative charge usually enhances nucleophilicity, making anions stronger nucleophiles than their neutral counterparts. Solvents play a significant role as well. For instance, polar aprotic solvents tend to increase nucleophile strength because they do not solvate anions as effectively, allowing the nucleophile to remain 'free' and reactive. In contrast, protic solvents can reduce nucleophile strength by solvating anions through hydrogen bonding.

Steric Hindrance in Organic Chemistry

Steric hindrance is a key concept when evaluating reaction mechanisms such as S_N2. It refers to the increased difficulty in the approach of a nucleophile to a reactive center due to the presence of bulky groups around the site.

In the context of the S_N2 reactivity order, the presence of larger groups around the negatively charged oxygen atom will impede the reaction. Nucleophiles that are less hindered, typically smaller in size or with fewer bulky groups, are able to more easily approach and react with the electrophile.

When considering steric hindrance, envision the reactive site as a crowded doorway. A larger nucleophile is akin to a person trying to enter this doorway while carrying wide luggage, obviously facing more difficulty than a slim person carrying nothing. This metaphor allows us to appreciate how steric factors influence reactivity in organic molecules.

Nucleophilicity vs. Electronegativity

While nucleophilicity and electronegativity might appear similar, as they both deal with the distribution of electrons, they are in fact distinct concepts with inverse relationships.

Nucleophilicity refers to an atom's or molecule's tendency to donate an electron pair, whereas electronegativity pertains to an atom's ability to attract and hold onto electron pairs. Typically, an increase in electronegativity coincides with a decrease in nucleophilicity. This is because atoms with high electronegativity are less willing to donate their electron pairs. On the other hand, atoms with low electronegativity, such as those found down the periodic table, are often better nucleophiles as they are less tightly holding onto their electron pairs and thus are more inclined to share.

For example, consider the halogen group. Fluorine, being highly electronegative, is a poor nucleophile. Conversely, iodine, with much lower electronegativity, is a much more effective nucleophile.