Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following behaves both as a nucleophile and as an electrophile? (a) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\) (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{OH}\) (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{ONa}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}\) behaves as both.

Step 1: Understand Nucleophiles and Electrophiles

Nucleophiles are chemical species that donate an electron pair to an electrophile in order to form a chemical bond. They are typically rich in electrons and often negatively charged or neutral with lone pairs. Electrophiles, on the other hand, are species that accept electron pairs. They are usually electron-deficient, either positively charged or neutral atoms with vacant orbitals.

Step 2: Analyze Each Option

Examine each molecule: (a) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\) has a lone pair on nitrogen, making it nucleophilic. (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{OH} \) has a lone pair on oxygen, also nucleophilic. (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}\) has a triple bond with a nitrogen atom possessing a lone pair, can act as a nucleophile. (d) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{ONa}\) can act as a nucleophile due to negatively charged oxygen.

Step 3: Identify Electrophilic Characteristics

Next, determine if any of the options can also act as electrophiles. A molecule capable of acting as both will have an electron-deficient center. In \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}\), the presence of the positive charge on nitrogen and the polarization of the triple bond can allow it to accept an electron pair, thus behaving as an electrophile.

Step 4: Conclusion

Option (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}\) behaves both as a nucleophile due to the lone pair on nitrogen and as an electrophile due to the polarization across the triple bond, which can accept electrons at its carbon end.

Key Concepts

Chemical Bonding

Chemical bonding is all about how atoms connect to form molecules. At the core of it, atoms bond by sharing or transferring electrons. These bonds hold the atoms together in a molecule and determine the molecule's structure and properties. There are different types of chemical bonds, including:

• Covalent Bonds: These result from the sharing of electron pairs between atoms. They occur mainly between non-metal atoms.
• Ionic Bonds: These occur when one atom donates an electron to another, resulting in attraction between charged ions.
• Metallic Bonds: These involve a 'sea of electrons' shared over many metal atoms, characteristic of metals.

In the context of nucleophiles and electrophiles, chemical bonds are formed when nucleophiles donate their electrons to electron-deficient electrophiles, creating a stable molecule. This interaction is critical in many chemical reactions.

Electron Pairs

In chemistry, electron pairs are two electrons occupying the same orbital in an atom or molecule. They can be bonded or lone pairs. Understanding electron pairs is essential as they affect how molecules react and interact.

• Lone Pairs: These are electron pairs that are not involved in bonding. They sit on an atom and can be donated to form bonds. Nucleophiles usually have lone pairs ready to donate.
• Bonding Pairs: These are electron pairs shared between atoms, forming a chemical bond. They hold the molecule together.

For example, in a molecule like \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\)\, nitrogen has a lone pair, which can be used to form bonds with electrophiles in reactions. Electron pairs play a crucial role in determining a molecule's reactivity and stability.

Polarization in Molecules

Polarization in molecules refers to the distribution of electrical charge over the atoms joined by the bond. It occurs due to differences in electronegativity between atoms.

• Electronegativity: It measures an atom's ability to attract and hold onto electrons in a bond. Differences in electronegativity lead to polarization.
• Polar Bonds: In a polar bond, electrons are shared unequally, causing one atom to be slightly negative and the other slightly positive. This creates a dipole moment.
• Examples: In \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{N}\)\, the polarization of the triple bond makes the carbon potentially electron-deficient, allowing it to act as an electrophile while the nitrogen can act as a nucleophile.

Polarization plays a fundamental role in determining the behavior of molecules in chemical reactions, influencing how nucleophiles and electrophiles interact.