Problem 165 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following $$ \begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \text { (a) } \mathrm{Hg}_{2}^{2+} & \text { (p) Nucleophile } \\ \text { (b) } \mathrm{AlCl}_{3} & \text { (q) Catalyst } \\ \text { (c) } \mathrm{Br} & \text { (r) Lewis acid } \\ \text { (d) } \mathrm{H}^{+} & \text {(s) Soft acid } \\ & \text { (t) Electrophile } \\ \hline \end{array} $$

Step-by-Step Solution

Answer

(a) - (s), (b) - (r), (c) - (p), (d) - (t)

Step 1: Identifying Characteristics

Analyze the properties of each entry in Column-I:- \( \mathrm{Hg}_{2}^{2+} \) is a soft acid as it is a metal ion that forms covalent bonds.- \( \mathrm{AlCl}_{3} \) is a well-known Lewis acid because it can accept electron pairs.- \( \mathrm{Br} \) (bromide ion) generally acts as a nucleophile due to its ability to donate an electron pair.- \( \mathrm{H}^{+} \) is a classic electrophile since it seeks electrons to fulfill its shell.

Step 2: Matching with Column-II

Now match the characteristics found in Step 1 with Column-II:- \( \mathrm{Hg}_{2}^{2+} \) (soft acid) matches with (s) Soft acid.- \( \mathrm{AlCl}_{3} \) (Lewis acid) matches with (r) Lewis acid.- \( \mathrm{Br} \) (nucleophile) matches with (p) Nucleophile.- \( \mathrm{H}^{+} \) (electrophile) matches with (t) Electrophile.

Step 3: Final Review

Review the matched pairs to ensure logical consistency with their properties:- \( \mathrm{Hg}_{2}^{2+} \) links to soft acid due to its soft, polarizable nature.- \( \mathrm{AlCl}_{3} \) is a typical Lewis acid.- \( \mathrm{Br} \) usually acts as a nucleophile, capable of donating an electron pair.- \( \mathrm{H}^{+} \) as an electrophile because it actively seeks electrons.

Key Concepts

Soft Acid

In the world of chemistry, a soft acid is a concept that describes certain types of metal ions and their ability to form bonds. This generally refers to cations like mercury(II) ion ( Hg_2^{2+} ). Soft acids possess characteristics that make them polarizable and usually have low charge density. This means they prefer to form covalent bonds rather than ionic bonds.
One distinctive feature of soft acids is that they tend to overlap their orbitals with soft bases. Such interactions are stronger due to their shared, evenly distributed electron cloud. This trait makes soft acids crucial in coordination chemistry, especially when forming complexes with soft bases such as phosphines or thiols.

• Soft acids have a flexible and diffuse electron cloud.
• They are often transition metal cations.
• They prefer covalent bond formations with similar soft bases.

Lewis Acid

Lewis acids are fundamental in chemical reactions where electron-pair acceptance is key. The concept, named after Gilbert N. Lewis, identifies substances that act as electron pair acceptors. A perfect example is aluminum chloride ( AlCl_3 ), a common Lewis acid.
The ability of a Lewis acid to form a bond by accepting donated electron pairs makes it a versatile participant in many industrial and biological reactions. These acids don't just include metal ions but also molecules like BF_3 and SiF_4 that have vacant orbitals to accept electron pairs. In essence, any species that can receive an electron pair can act as a Lewis acid.

• They can attract and accept electrons.
• Key participants in reactions like Friedel-Crafts acylation.
• Open vacant orbitals to form new bonds with electron donors.

Nucleophile

Nucleophiles are chemical species that donate electron pairs. Derived from the Greek words "nucleus" and "loving," they are attracted to positive or electron-deficient regions in molecules, making them essential players in chemical reactions. Bromide ions ( Br^- ), for instance, are classic nucleophiles.
Nucleophiles typically carry a negative charge or have lone pairs of electrons capable of being shared. This electron-rich nature enables nucleophiles to form bonds by attacking areas of molecules deficient in electrons, also known as electrophilic centers. Such behavior is crucial in substitution reactions where new chemical bonds are formed.

• They usually possess an extra electron pair to donate.
• Common in reactions like nucleophilic substitution and addition.
• Essential for forming stable chemicals and complex ions.

Electrophile

Electrophiles are the electron-deficient counterparts to nucleophiles. These species are attracted to regions where there is an abundance of electrons. The hydrogen ion ( H^+ ) is a prime example of a simple electrophile due to its lack of electrons.
In chemical reactions, electrophiles play the role of electron pair acceptors. They actively seek out electron-rich areas within molecules to form new chemical bonds. This makes them indispensable in various essential reactions, such as electrophilic substitution in aromatic chemistry. Without electrophiles, many crucial organic transformations could not take place.

• Typically, they lack electrons or have positively charged centers.
• Initiate electron pair bonding with electron donors.
• Key in processes that depend on charge attraction, like polymerization.