Problem 167 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following $$ \begin{array}{ll} \text { Column-I } & \text { Column-II } \\ \hline \begin{array}{ll} \text { (a) } \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}^{\oplus} & \text { (p) } \mathrm{sp}^{3} \text { (carbon carry- } \\ \text { ing the charge) } \end{array} \\ \begin{array}{ll} \text { (b) } \mathrm{CH}_{3}-\mathrm{CH}_{2}-\stackrel{\Theta}{\mathrm{C}} \mathrm{H}_{2} & \text { (q) } \mathrm{sp}^{2} \text { (carbon carry- } \\ \text { ing the charge) } \\ \text { (c) } \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{C} \mathrm{H}_{2} & \text { (r) Nucleophile } \\ \text { (d) } \mathrm{CH}_{3}-\mathrm{C}^{+}=\mathrm{O} & \text { (s) Electrophile } \\ &\text { (t) Planar } \end{array} \\ \end{array} $$

Step-by-Step Solution

Answer

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

Step 1: Analyzing Compound (a)

The compound given is \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}^{+} \). The carbon carrying the positive charge is a primary carbocation. In such a carbocation, the central carbon atom is \( \mathrm{sp}^{2} \) hybridized, resulting in a planar structure.

Step 2: Analyzing Compound (b)

The compound is \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\stackrel{\Theta}{\mathrm{C}}\mathrm{H}_{2} \). The negatively charged carbon carries a lone pair of electrons, making it \( \mathrm{sp}^{3} \) hybridized. This hybridization allows it to function as a nucleophile, which donates electron pairs to electrophiles.

Step 3: Analyzing Compound (c)

The structure \( \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{C} \mathrm{H}_{2} \) involves a benzyl cation, which is also \( \mathrm{sp}^{2} \) hybridized due to resonance with the phenyl group. Such an arrangement is planar.

Step 4: Analyzing Compound (d)

In \( \mathrm{CH}_{3}-\mathrm{C}^{+}=\mathrm{O} \), the carbon is part of a carbonyl group and bears a positive charge. This makes it an electrophile because it can accept electron pairs from nucleophiles.

Key Concepts

Carbocations

Carbocations are positively charged carbon atoms within organic molecules. They play a crucial role in many organic reactions, acting as intermediates during chemical transformations. Carbocations are formed when a carbon atom loses one of its covalent bonds acquiring a positive charge.

It is important to note that their stability is influenced by several factors:

• **Substitution Level**: Tertiary carbocations are more stable than secondary, which in turn are more stable than primary. This is because alkyl groups can donate electron density to the positively charged carbon, stabilizing the charge.
• **Hyperconjugation and Resonance**: Both effects help delocalize and stabilize the positive charge. A benzyl carbocation, which involves resonance with an aromatic ring, is quite stable.
• **Hybridization**: Most carbocations are sp² hybridized due to the loss of a typically tetrahedral environment, leading to a planar geometry.

Understanding carbocations is key, as they frequently appear in mechanisms like electrophilic addition or substitution reactions.

Hybridization

Hybridization describes the process of mixing atomic orbitals in an atom to create new hybrid orbitals. These are used to form sigma bonds and accommodate lone pairs, fitting within various molecular geometries. In organic chemistry, hybridization often involves carbon atoms.

Focusing on the types relevant to carbocations and related compounds:

• **sp³ Hybridization**: This occurs when a carbon atom forms four sigma bonds, or holds lone pairs, making it tetrahedral. Typical for saturated hydrocarbons and anionic carbon species (nucleophiles).
• **sp² Hybridization**: Involves three sigma bonds and one pi bond, resulting in a planar structure. This is common in carbocations due to the missing bond associated with the lost electron.
• **sp Hybridization**: Characterized by two sigma bonds and two pi bonds, giving a linear shape. Although less common in simple carbocations, it's important in alkyne structures.

Understanding hybridization helps in deciphering molecular shapes and reactivity traits of organic molecules.

Nucleophiles

Nucleophiles are species rich in electrons that actively seek positively charged regions to interact with. Their defining feature is the ability to donate an electron pair to an electrophile, creating a chemical bond.

Key characteristics of nucleophiles include:

• **Electron Pair Donors**: They usually have lone pairs or pi bonds that can be shared.
• **Charge**: Often, nucleophiles bear a negative charge, enhancing their electron donating capability. However, neutral molecules with lone pairs can also act as nucleophiles.
• **Strength**: Influenced by several factors such as charge density, solvent interactions, and bond strength.

Nucleophiles play a pivotal role in reactions like nucleophilic substitution, where they attack an electrophile and form a new bond. Recognizing nucleophiles is essential in predicting and controlling chemical reaction pathways.

Electrophiles

Electrophiles are chemical species that lack electron density and actively seek out electron-rich environments. Distinguished by their affinity for electrons, they often initiate reactions by accepting electron pairs from nucleophiles.

Electrophiles can be various species:

• **Positive Charge**: Many have a full or partial positive charge, like carbocations.
• **Electron Deficiency**: Molecules like carbonyl compounds or halogens in certain states can also act as electrophiles because they lack full octets at some atoms.
• **Polar Bonds**: Electrophilic nature can emerge from polar bonds where electronegative atoms withdraw electron density from adjacent atoms.

Their role in chemical reactions is crucial, as they often determine the direction and products of interactions, especially in foundational reactions like electrophilic addition where they bond with nucleophiles to form new compounds.