Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

The strongest Bronsted base among the following ions is (a) \(\mathrm{CH}_{3} \mathrm{O}^{-}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHO}^{-}\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CO}^{-}\)

Step-by-Step Solution

Answer

The strongest Bronsted base is \\(\mathrm{CH}_{3} \mathrm{O}^{-}\\).

Step 1: Understand the Concept of Bronsted Base

Bronsted bases are species that can accept a proton (H⁺). The strength of a Bronsted base generally increases with the stability of its conjugate acid. Also, bases that are less hindered (smaller in size) usually accept protons more readily.

Step 2: Identify the Conjugate Acids

List the conjugate acids of the given bases: - For \(\mathrm{CH}_{3} \mathrm{O}^{-}\), the conjugate acid is \(\mathrm{CH}_{3} \mathrm{OH}\).- For \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\), the conjugate acid is \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\).- For \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHO}^{-}\), the conjugate acid is \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}\).- For \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CO}^{-}\), the conjugate acid is \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\).

Step 3: Analyze Steric Effects

Bases with more alkyl groups experience steric hindrance, making it difficult for them to accept protons. The more substituted alkoxide ions such as \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CO}^{-}\) are therefore weakest.

Step 4: Compare the Basicity of Ions

Comparing the ions based on size and steric effects, the conjugate acid \(\mathrm{CH}_{3} \mathrm{OH}\) is a simple, small molecule, indicating that \(\mathrm{CH}_{3} \mathrm{O}^{-}\) is less hindered and therefore the strongest base among the options.

Key Concepts

Conjugate Acid

In chemistry, whenever you have an acid-base reaction, acids and bases transform into their conjugates. A conjugate acid is what's formed when a base gains a proton ( H^+ ). Understanding conjugate acids helps in determining the strength of a base. For example, CH_3O^- gains a proton to form CH_3OH , which is its conjugate acid.
The strength of a base is often linked to the stability of its conjugate acid. If the conjugate acid is stable, the original base is strongly willing to accept a proton. This is because the resulting conjugate acid does not destabilize the system.
For alkoxide ions like C_2H_5O^- , (CH_3)_2CHO^- , and (CH_3)_3CO^- , analyzing their conjugate acids ( C_2H_5OH , (CH_3)_2CHOH , and (CH_3)_3COH respectively) reveals their tendency to either donate or accept protons, affecting their base strength.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the size of substituents attached to reacting molecules. It plays a significant role in the reactivity of ionic species. In the context of Bronsted bases, larger substituents near the reactive center can block or hinder reactions by physically denying access to other molecules or ions.
For example, in the case of (CH_3)_3CO^- , it has three methyl groups surrounding the central oxygen. This makes it bulky and hard for the species to accept a proton easily, hence, weaker as a base.
Meanwhile, CH_3O^- , with less steric hindrance due to its smaller size, can more readily accept a proton, making it a stronger Bronsted base. Understanding steric hindrance helps predict how reactive a base might be, based on its molecular size and arrangement.

Alkoxide Ions

Alkoxide ions are negatively charged species ( RO^- ) that derive from alcohols where the OH group loses a proton. An alkoxide ion like CH_3O^- is formed from methanol ( CH_3OH ). These ions are known for being strong bases.
The ability of an alkoxide ion to act as a base is influenced by its structure. Specifically, the number and type of alkyl groups attached influence both electronic effects and steric hindrance.
In this context:

• CH_3O^- , which has a single methyl group, is less sterically hindered and a stronger Bronsted base.
• Conversely, (CH_3)_3CO^- has three bulky methyl groups, making it less effective in accepting protons due to increased steric hindrance.

Understanding the structure of alkoxide ions is key to predicting their behavior in acid-base chemistry.