Problem 204 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of increasing basicity of the given conjugate bases \(\left(\mathrm{R}=\mathrm{CH}_{3}\right)\) is \([2010]\) (a) \(\mathrm{RCO} \overline{\mathrm{O}}<\mathrm{HC} \equiv \overline{\mathrm{C}}<\overline{\mathrm{R}}<\overline{\mathrm{N}} \mathrm{H}_{2}\) (b) \(\overline{\mathrm{R}}<\mathrm{HC} \equiv \overline{\mathrm{C}}<\mathrm{R} \mathrm{CO} \overline{\mathrm{O}}<\overline{\mathrm{N}} \mathrm{H}_{2}\) (c) \(\mathrm{RCO} \overline{\mathrm{O}}<\overline{\mathrm{N}} \mathrm{H}_{2}<\mathrm{HC} \equiv \overline{\mathrm{C}}<\overline{\mathrm{R}}\) (d) \(\mathrm{RCO} \overline{\mathrm{O}}<\mathrm{HC} \equiv \overline{\mathrm{C}}<\overline{\mathrm{N}} \mathrm{H}_{2}<\overline{\mathrm{R}}\)

Step-by-Step Solution

Answer

The correct order of increasing basicity is option (a).

Step 1: Identify Conjugate Bases

The conjugate bases given are: \(\text{RCO} \overline{\text{O}}\), \(\text{HC} \equiv \overline{\text{C}}\), \(\overline{\text{R}}\), and \(\overline{\text{N}} \text{H}_2\). These are derived from their respective acids.

Step 2: Determine Conjugate Acid Strength

Conjugate acids of these bases are as follows: \(\text{RCOOH}\) for \(\text{RCO} \overline{\text{O}}\), \(\text{HC} \equiv \text{CH}\) for \(\text{HC} \equiv \overline{\text{C}}\), \(\text{RH}\) for \(\overline{\text{R}}\), and \(\text{NH}_3\) for \(\overline{\text{N}} \text{H}_2\). The stronger the acid, the weaker its conjugate base.

Step 3: Assess Acid Strength and Inverse Basicity

\(\text{RCOOH}\) is a moderately strong carboxylic acid, \(\text{HC} \equiv \text{CH}\) is a very weak acid (alkyne), \(\text{RH}\) is a very weak acid (alkane), and \(\text{NH}_3\) is weakly acidic (amine). Therefore, the order of increasing strength of the conjugate bases is the reverse: \(\text{RCO} \overline{\text{O}} < \text{HC} \equiv \overline{\text{C}} < \overline{\text{R}} < \overline{\text{N}} \text{H}_2\).

Step 4: Match with Given Options

Compare the determined order of basicity \(\text{RCO} \overline{\text{O}} < \text{HC} \equiv \overline{\text{C}} < \overline{\text{R}} < \overline{\text{N}} \text{H}_2\) with the options provided. Option (a) matches this order as it lists these conjugate bases in increasing order of basicity.

Key Concepts

Conjugate Bases

Conjugate bases are what you get when an acid donates a proton ( H^{+} ). They carry one less proton and often have additional electron density available, making them potential proton acceptors. This additional electron density is what gives a conjugate base its basicity. For example, when acetic acid ( CH_3COOH ) loses a proton, it forms its conjugate base ( CH_3COO^- ). Understanding the nature of conjugate bases can help predict the reactivity and behavior of similar compounds in solutions.

• **Formed by acid losing a proton:** Gain electron density.
• **Act as bases:** Can accept protons.
• **Predict behavior:** Help understand reactions and stability.

In the context of the exercise, predicting the basicity of different conjugate bases helps understand their reactivity order, from the weakest to the strongest base.

Acid-Base Strength

The relationship between acids and their conjugate bases is pivotal in organic chemistry. A simple principle guides this: the stronger the acid, the weaker the conjugate base. The reverse is also true. This is known as the "inverse relationship."

• **Strong acids have weak conjugate bases:** They easily donate protons, leaving behind a less reactive base.
• **Weak acids have strong conjugate bases:** They hold onto their protons more tightly, resulting in a more reactive base when they do lose the proton.

Consider carboxylic acids like RCOOH , which are stronger acids relative to alcohols or alkanes. Thus, their conjugate base ( RCOO^- ) is weaker compared to the conjugate bases derived from weaker acids, such as amines and hydrocarbons.

Organic Chemistry Concepts

Organic chemistry often involves understanding how molecules interact, react, and evolve, especially under acidic or basic conditions. Basicity and acidity aren't just about numbers; they involve molecular structure, stability, and charge distribution.

• **Stability of ions:** The more stable a conjugate base, generally the weaker it is.
• **Electronegativity:** Highly electronegative atoms in a molecule tend to pull electron density away, stabilizing the conjugate base.
• **Resonance effects:** If the negative charge on a conjugate base is delocalized, it's often more stable (thus weaker as a base).

Applying these concepts, the basicity of the conjugate bases listed in the exercise can be explained by considering the electronegativity and charge distribution in each molecule. The weaker the conjugate base's interaction with H^+ , the stronger the acid it came from, and vice versa. This exercise not only tests basicity but encourages integrating different aspects of organic molecular behavior.