Question

The basic strength order of following in \(\mathrm{H}_{2} \mathrm{O}\) is (I) \(\mathrm{MeNH}_{2}\) (II) \(\mathrm{Me}_{2} \mathrm{NH}\) (III) \(\mathrm{Me}_{3} \mathrm{~N}\) (IV) \(\mathrm{Me}_{4} \mathrm{NOH}\) (A) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (B) \(\mathrm{IV}>\mathrm{III}>\mathrm{II}>\mathrm{I}\) (C) II > IV > I \(>\) III (D) IV > II > I > III

Short answer

The short answer based on the step-by-step solution is: In water, the basic strength order of the given compounds is (C) II > IV > I > III, taking into account the electron-donating nature and steric hindrance of the substituents on nitrogen.

Step-by-step solution

Know the Lewis acid and Lewis base concept

To solve this problem, we should be familiar with Lewis acid and Lewis base theory. A Lewis acid is a compound that can accept a lone pair of electrons from another compound. In contrast, a Lewis base is a compound that can donate a lone pair of electrons to a Lewis acid. In this case, we're looking at the basic strength of different compounds. This means we will be considering them as Lewis bases (electron-pair donors) in water.

Determine factors affecting basicity

One of the primary factors affecting the basicity of a compound is the presence of electron pair-donating elements or groups. In the given compounds, all have nitrogen atoms that can donate their lone pair of electrons. Another factor that may affect the basic strength is the sterical hindrance and electron-donating or withdrawing nature of the substituents.

Comparing the basic strength of all compounds

Now, let's break down each compound and determine its basic strength to rank them in order. (I) \(\mathrm{MeNH}_{2}\) has one methyl \((CH_3)\) group attached to the nitrogen. Methyl groups are slightly electron-donating in nature, meaning it will increase the electron density on the nitrogen atom, making it more basic. (II) \(\mathrm{Me}_{2} \mathrm{NH}\) has two methyl groups attached to the nitrogen. The increased number of electron-donating methyl groups will increase the basic strength of this compound compared to (I). (III) \(\mathrm{Me}_{3} \mathrm{~N}\) has three methyl groups attached to the nitrogen. Having more electron-donating groups should, in theory, increase the basic strength. However, the steric hindrance of the bulky groups will make it more challenging for the lone pair of nitrogen to donate to a Lewis acid. (IV) \(\mathrm{Me}_{4} \mathrm{NOH}\) has a quaternary ammonium counterion with a hydroxyl anion. The hydroxyl is the basic part of the compound as it can donate its lone pair, but the presence of four methyl groups on nitrogen will generate strong electron density on the nitrogen, reducing the basic character of the molecule.

Rank the given compounds in basic strength order

From the analysis above, the order of basic strength in \(H_2O\) is as follows: II > I > III > IV Thus, the correct answer is (C) II > IV > I > III.

Key concepts

Lewis Acid and Base Theory

When exploring the basic strength of organic compounds in water, it's essential to understand the Lewis acid and base theory. This concept revolves around the transfer of electron pairs - with Lewis bases being the donors and Lewis acids the acceptors. Lewis bases contain a lone pair of electrons that they can provide to form a bond, thus initiating a reaction.

For example, nitrogen-containing organic compounds, such as amines, are typical Lewis bases because nitrogen has a lone pair of electrons. When dissolved in water, these amines can donate their lone pair to the hydrogen ion (H⁺), which is a common Lewis acid. The ability of an organic compound to donate an electron pair to a Lewis acid determines its basic strength in water.

Factors Affecting Basicity

The basicity of a compound is influenced by various factors, which determine how readily it can donate an electron pair. One critical attribute is the electronegativity of the atom holding the lone pair; more electronegative atoms will hold on tightly to their electrons, making them less available to donate.

Electron-Delocalization

In addition to the electronegativity, electron delocalization that occurs through resonance or inductive effects can either increase or decrease the electron density on the atom with the lone pair, thus affecting its basicity. For instance, the presence of electron-donating groups will typically enhance basicity by increasing electron density, while electron-withdrawing groups will do the opposite.

Solvent Effects

Moreover, the solvent can play a vital role. In water, solvation effects can either stabilize or destabilize the lone pair, modifying how available it is to act as a base.

Understanding these factors helps clarify why some compounds are stronger bases than others, as it's the interplay between these characteristics that determines the overall basicity.

Steric Hindrance in Organic Bases

Steric hindrance is a physical obstruction caused by the size and shape of molecules, which can affect how compounds react. In the context of basicity, steric hindrance can impede the ability of a base to donate its electron pair to an acid.

For example, when a nitrogen atom in an amine is bonded to several bulky groups, such as methyl groups, the crowded space around the nitrogen atom makes it less accessible to acids like H⁺. Even though additional electron-donating groups may increase the electron density on the nitrogen (and theoretically its basicity), if the Lewis acid cannot efficiently come into contact with the lone pair due to steric constraints, the basic strength of the compound decreases.

The impact of steric hindrance is well observed in tetrasubstituted ammonium compounds, where the availability of the lone pair for bonding is significantly reduced. Thus, this spatial consideration is key to accurately predicting the basic strength of organic compounds.