Problem 174 - A Complete Resource Book in Chemistry for JEE Main

Question

Column-I (a) Benzene (b) Neopentane (c) Buta \(-1,3\) - diene (d) \(2,2,3,3\) - Tetramethyl butaneColumn-II (p) Forms only one mono chloro derivative (q) Aromatic (r) Has only \(\mathrm{sp}^{2}\) hybridised carbon atoms (s) Undergoes 1,2 and 1,4 -addition (t) Does not show resonance

Step-by-Step Solution

Answer

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

Step 1: Match Benzene

First, consider benzene from Column-I. Benzene is a well-known aromatic compound, characterized by resonance. - Matching Column-II: (q) Aromatic is the correct match for benzene.

Step 2: Match Neopentane

Neopentane, also known as 2,2-dimethylpropane, is a compound where all carbon atoms are sp3 hybridized. Due to symmetry, it forms only one type of mono chloro derivative when one hydrogen is replaced by chlorine. - Matching Column-II: (p) Forms only one mono chloro derivative is the correct match for neopentane.

Step 3: Match Buta-1,3-diene

Buta-1,3-diene is a conjugated diene, consisting of alternating double and single bonds. It undergoes both 1,2 and 1,4 addition reactions. - Matching Column-II: (s) Undergoes 1,2 and 1,4 -addition is the correct match for buta-1,3-diene.

Step 4: Match 2,2,3,3-Tetramethylbutane

2,2,3,3-Tetramethylbutane is an alkane with no double bonds; hence it doesn't show resonance and all its carbon atoms are sp3 hybridized. - Matching Column-II: (t) Does not show resonance is the correct match for 2,2,3,3-tetramethylbutane.

Key Concepts

Aromatic Compounds

Aromatic compounds are a fascinating category in organic chemistry marked by their unique stability and distinct electronic structure. The most famous example is benzene, a compound consisting of six carbon atoms arranged in a ring, with alternating double and single bonds. This structure is not fixed, but a resonance hybrid, meaning the electrons are delocalized across the ring, increasing its stability. The delocalization leads to an equal sharing of electrons, which is often represented as a circle within the hexagon structure.
Aromatic compounds must satisfy Huckel's rule, which states that a molecule must have a planar ring of \(4n + 2\) \(\pi\) electrons to be aromatic, where \(n\) is a non-negative integer. This rule is a key characteristic that enables these compounds to maintain a stable structure.

• Benzene, being aromatic, is part of many essential chemical reactions and compounds found in nature and synthesized in labs.
• Aromaticity imparts special properties, such as increased chemical stability and unique spectral characteristics.

Understanding aromatic compounds allows chemists to predict their reactions and utilize their stability in designing new molecules.

Hybridization

Hybridization is a fundamental concept in organic chemistry, describing the mixing of atomic orbitals to form new hybrid orbitals for the bonding process. This mixing helps explain the geometry and bonding properties of carbon-containing compounds, among others.
In organic compounds, carbon most commonly exhibits three types of hybridization: \(sp^3\), \(sp^2\), and \(sp\). Each of these determines the shape and angle of the bonds formed.

• \(sp^3\) hybridization occurs when one s orbital mixes with three p orbitals, forming four equivalent hybrid orbitals, and typically results in tetrahedral geometry with bond angles of approximately 109.5°.
• \(sp^2\) hybridization involves an s orbital mixing with two p orbitals, creating three planar hybrid orbitals and resulting in a trigonal planar geometry with angles of 120°.
• \(sp\) hybridization, where one s orbital mixes with one p orbital, leads to two linear hybrid orbitals with a bond angle of 180°.

Each type of hybridization explains the specific bonding and geometric configurations that determine the properties and reactivity of organic molecules.

Addition Reactions

Addition reactions are a crucial class of reactions in organic chemistry where atoms are added to a molecule, typically involving double or triple bonds in alkenes and alkynes. These reactions are key in converting unsaturated compounds into saturated ones.
In the case of conjugated dienes like Buta-1,3-diene, these reactions can occur in two main ways: 1,2-addition and 1,4-addition.

• 1,2-addition happens when the adding groups attach themselves to the first and second carbon atoms of the diene.
• 1,4-addition involves the addition across the first and fourth carbon atoms, effectively skipping over double bonds.

Both forms of addition reactions depend on the conditions and the nature of the reactants. The reactivity and orientation in addition reactions are critical because they determine the structure and properties of the final product.

Resonance in Chemistry

Resonance in chemistry refers to a way of describing delocalized electrons within molecules where the bonding cannot be expressed by a single Lewis structure. Instead, resonance structures show different possible configurations where electrons can exist, providing a more comprehensive understanding of a molecule's structure.
This concept is especially important in explaining the stability of conjugated systems and aromatic compounds like benzene. In such systems, electrons are shared over multiple bonds, expanding beyond a single connection between atoms.

• Resonance structures imply that actual electron distribution is a blend of different Lewis structures, known as resonance hybrids.
• This leads to greater molecular stability and is a pivotal factor in the unique properties of many organic compounds.

Understanding resonance is crucial in predicting chemical behavior, stability, and reactivity, ultimately influencing how molecules interact and react in various chemical environments.