Problem 150 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct statements : (a) \(\mathrm{H}_{2} \mathrm{O}<\mathrm{CH}_{3} \mathrm{COO}^{-}<\mathrm{CH}_{3} \mathrm{O}^{-}\)[basic strength] (b) \(\mathrm{H}_{2} \mathrm{O}<\mathrm{CH}_{3} \mathrm{COO}^{-}<\mathrm{CH}_{3} \mathrm{O}^{-}\)[nucleophilicity] (c) \(\mathrm{F}<\mathrm{Cl}^{-}<\mathrm{Br}^{-}<\mathrm{I}^{-} \quad\) [basic strength \(]\) (d) \(\mathrm{F}^{-}<\mathrm{Cl}^{-}<\mathrm{Br}^{-}<\mathrm{I}^{-} \quad\) [nucleophilicity]

Step-by-Step Solution

Answer

Only (a) is correct. (b), (c) and (d) are incorrect.

Step 1: Analyzing Basic Strength of (a)

Basic strength refers to the ability of a species to accept a proton. Order of basic strength often follows the inverse order of acidity, meaning that weaker acids have stronger conjugate bases. In the case of (a), the acetate ion \(\text{CH}_3\text{COO}^-\) is the conjugate base of acetic acid, which is weaker than the methoxide ion \(\text{CH}_3\text{O}^-\), the conjugate base of methanol, and water is the weakest base. Therefore, the correct order of basic strength here is \(\text{CH}_3\text{O}^- > \text{CH}_3\text{COO}^- > \text{H}_2\text{O}\), which matches statement (a). The statement is correct.

Step 2: Evaluating Nucleophilicity of (b)

Nucleophilicity refers to the ability of a species to donate an electron pair. For compounds with similar structures, nucleophilicity often parallels basicity. Comparing \(\text{H}_2\text{O}\), \(\text{CH}_3\text{COO}^-\), and \(\text{CH}_3\text{O}^-\), methoxide (strong base and great electron pair donor) is a better nucleophile than acetate, and acetate is a better nucleophile than water. Therefore, the sequence \(\text{H}_2\text{O} < \text{CH}_3\text{COO}^- < \text{CH}_3\text{O}^-\) is incorrect for nucleophilicity, as the correct order should actually be \(\text{CH}_3\text{O}^- > \text{CH}_3\text{COO}^- > \text{H}_2\text{O}\). (b) is incorrect.

Step 3: Analyzing Basic Strength of Halogens in (c)

For halogens, the smaller the atomic radius, the stronger the base due to higher density of negative charge on a smaller area. Fluoride is the smallest ion and thus the strongest base of the group. Hence, the order \(\text{F}^- > \text{Cl}^- > \text{Br}^- > \text{I}^-\) is based on basic strength. Therefore, the given order in (c) is incorrect because it should be reversed to match this concept. (c) is incorrect.

Step 4: Evaluating Nucleophilicity of Halogens in (d)

In a polar aprotic solvent, larger ions which are less solvated are better nucleophiles due to increased electron pair availability. Thus, iodide, being the largest ion, is the best nucleophile in polar aprotic solvents, followed by bromide, chloride, and fluoride. Therefore, the correct order for nucleophilicity is \(\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-\). Given order in (d) is incorrect. (d) is incorrect.

Key Concepts

Basic Strength

Basic strength in chemistry refers to a molecule's ability to accept protons. This property is closely linked to acid-base chemistry. Let's dive into the interplay between acids and bases to understand this concept better.

In Lewis acid-base theory, a base is defined as a species that donates a lone pair of electrons. However, in the more common Brønsted-Lowry concept, a base is a proton acceptor. The basic strength of a compound can often be visualized by looking at its conjugate acid. A strong base typically has a weak conjugate acid and vice versa.

For example:

• The methoxide ion ( ext{CH}_3 ext{O}^-) is a conjugate base of methanol. Methoxide is a strong base due to methanol being a weak acid.
• The acetate ion ( ext{CH}_3 ext{COO}^-) is a weaker base compared to methoxide because its conjugate acid, acetic acid, is stronger than methanol.
• Finally, water is the weakest base in this comparison since its conjugate acid, hydronium ion ( ext{H}_3 ext{O}^+), is much stronger than methanol or acetic acid.

This basic strength communicates molecular behavior and predictions we can make about reaction tendencies.

Conjugate Base

Understanding the concept of conjugate bases is vital in grasping acid-base reactions. A conjugate base is formed when an acid donates a proton ( ext{H}^+).

Consider the following points about conjugate bases:

• A strong acid will yield a weak conjugate base, as the acid readily donates protons and the conjugate is less inclined to accept protons.
• Conversely, a weak acid will have a strong conjugate base, indicating the acid's reluctance to relinquish its proton, leaving the conjugate with enhanced proton-accepting capabilities.
• For example, methanol (a weak acid) gives rise to the methoxide ion ( ext{CH}_3 ext{O}^-), a strong base, while acetic acid (stronger than methanol) forms the acetate ion ( ext{CH}_3 ext{COO}^-), a relatively weaker base.

A robust understanding of conjugate bases aids in predicting the direction of acid-base reactions and the outcome of equilibrium in chemical systems.

Acids and Bases

The world of acids and bases is expansive, encompassing important reactions and concepts used in various fields of chemistry. An understanding of this fundamental category is pivotal when examining nucleophilicity and basicity.

There are few core principles to keep in mind:

• Acids are proton donors or electron pair acceptors, according to Brønsted-Lowry and Lewis theories respectively.
• Bases are proton acceptors or electron pair donors. This dual nature provides a strong base for forming strong ionic or coordinate bonds in reactions.

Delving further into acidity or basicity:

• Strong acids include hydrochloric acid ( ext{HCl}) and sulfuric acid ( ext{H}_2 ext{SO}_4) while weak acids include acetic acid ( ext{CH}_3 ext{COOH}).
• Some bases like sodium hydroxide ( ext{NaOH}) are considered strong due to their high tendency to accept protons.

Whether in industrial applications or biological systems, this partnership of acids and bases shapes reactivity and chemical interactions.

Halogens

Halogens are fascinating elements that reside in Group 17 of the periodic table. They include fluorine ( ext{F}), chlorine ( ext{Cl}), bromine ( ext{Br}), and iodine ( ext{I}). Each has unique chemical properties, and their interactions showcase important trends in chemistry.

Let's uncover key insights:

• Halogens are known for their roles as strong oxidizing agents due to their high electronegativities and tendencies to gain electrons to achieve a full valence shell.
• When behaving as anions, these elements are also noteworthy in the context of nucleophilicity and basicity.
• Their nucleophilic behavior interestingly diverges in different solvents. In polar aprotic solvents, larger halide ions like iodide ( ext{I}^-) become more efficient nucleophiles than smaller ones like fluoride ( ext{F}^-), due to less extensive solvation shielding their electrons.

The progression from fluoride to iodide shows decreasing electronegativity and increasing atomic size, inversely impacting nucleophilicity and basic strength. Trends among halogens allow predictions about their reactivity in various chemical settings.