Problem 177 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{Na}+\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br} \longrightarrow\) Number of hydrogen atoms attached to sp \(^{2}\) carbon in the product is

Step-by-Step Solution

Answer

0 hydrogen atoms are attached to sp² carbon in the product.

Step 1: Understanding the Reaction

We are given a reaction involving an ethyl sodium compound, \(\mathrm{CH}_{3}-\mathrm{CH}_{2}\mathrm{Na}\), and an ethyl propyl bromide, \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br}\). This reaction is a typical nucleophilic substitution where the sodium ethyl acts as a nucleophile.

Step 2: Identifying the Reaction Type

This reaction is an SN2 reaction where the bromine atom on \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br}\) is replaced by the ethyl group from \(\mathrm{CH}_{3}-\mathrm{CH}_{2}\mathrm{Na}\), creating butane, \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}\mathrm{CH}_{3}\).

Step 3: Analyzing the Product Structure

The product formed is butane, \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\), which only contains sp3-hybridized carbons, not sp2.

Step 4: Identifying Hybridization

In butane, all carbon atoms are sp3 hybridized. The structure is \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\), indicating that there are no sp2 carbon atoms present.

Step 5: Conclusion

Since there are no sp2 carbon atoms in the product butane, the number of hydrogen atoms attached to sp2 carbon is zero.

Key Concepts

Nucleophilic Substitution

In organic chemistry, nucleophilic substitution is a fundamental type of reaction that involves the exchange of one atom or group of atoms in a molecule, known as the leaving group, with a nucleophile. A nucleophile itself is a chemical species that is electron-rich, allowing it to donate a pair of electrons to bond with an electrophile, an electron-deficient species. In the context of the given exercise, the role of the nucleophile is played by the sodium ethyl group (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}\mathrm{Na}\)), which targets and displaces the bromide ion in ethyl propyl bromide (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Br}\)).
This type of reaction, in particular, follows the SN2 mechanism.

• The step is a one-step process.
• It involves a concerted reaction, meaning bond-breaking and bond-making occur simultaneously.
• Since the nucleophile directly attacks the carbon with the leaving group, typically from the opposite side, this reaction results in an inversion of configuration at that carbon.

Such substitution reactions are critical for synthesizing a wide range of compounds in organic chemistry.

Sp3 Hybridization

Hybridization in chemistry refers to the mixing of atomic orbitals to form new hybrid orbitals. In a molecule like butane, every carbon atom undergoes sp3 hybridization. An sp3 hybridized carbon atom is one where an s orbital combines with three p orbitals to form four equivalent sp3 hybrid orbitals. Each sp3 orbital can form one sigma bond, providing the carbon atom with a tetrahedral geometry and bond angles of about 109.5 degrees.
This hybridization is characteristic of single-bonded carbon structures and is essential for constructing stable organic molecules like alkanes. For example, in butane (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\)), all carbons are bonded to hydrogens or other carbons with single covalent bonds, enhancing stability and contributing to the molecule's overall low reactivity compared to unsaturated hydrocarbons.

Organic Chemistry

The field of organic chemistry is the study of carbon-containing compounds and their properties, structures, and reactions. This branch of chemistry is foundational for understanding how various organic molecules interact, behave in different conditions, and transform during chemical reactions. The reaction in the exercise is a classic example of an organic reaction—a nucleophilic substitution.

• Organic chemistry explores diverse reactions like additions, eliminations, and variations of substitutions depending on the reactants involved.
• Reactivity, reaction mechanisms, and molecular stability are core themes.
• Organic reactions are crucial in industries, ranging from pharmaceuticals to petrochemicals.

Understanding concepts like nucleophilic substitution helps elucidate how complex molecules can be synthesized from simpler ones, forming the backbone of molecular design and synthesis in organic chemistry.