Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

and \(\mathrm{CH}_{3}=\mathrm{C}-\mathrm{CH}\), are C=C(C)C C=C(C)[O-] 0: (a) tautomers (b) optical isomers (c) geometrical isomers (d) resonating structures

Step-by-Step Solution

Answer

The molecules are resonating structures.

Step 1: Understanding Isomers

Isomers are compounds with the same molecular formula but different structural arrangements. The given SMILES represent different possible isomers.

Step 2: Analyzing the Provided SMILES

The first SMILES, 'C=C(C)C', denotes a propene molecule with a branching alkyl group. The second SMILES, 'C=C(C)[O-]', indicates a similar molecule with a deprotonated oxygen atom.

Step 3: Identifying Isomer Types

(a) Tautomers are isomers related by the repositioning of a proton. The SMILES do not suggest hydrogen shifting, so they are not tautomers. (b) Optical isomers (enantiomers) require chiral centers, which are absent in both SMILES. (c) Geometrical isomers involve different spatial arrangements around a double bond. The SMILES do not show different spatial configurations, hence are not geometrical isomers. (d) Resonating structures involve delocalized electrons within the same molecule with the same atom connectivity.

Step 4: Conclusion on Isomer Type

The two provided SMILES structures 'C=C(C)[O-]' show resonance between carbon and the charged oxygen, with the negative charge delocalizable across multiple structures.

Key Concepts

Structural Isomers

Structural isomers are fascinating because they share the same molecular formula but differ in how their atoms are connected or arranged. This means they are compounds with the same "recipe," but the structure or "shape" of the molecules differs. These differences can lead to variations in their physical and chemical properties. For instance:

• The number of carbon atoms forming a linear or branched chain can vary.
• The placement of functional groups such as -OH or -CH₃ can differ.

Consider a simple example: butane (C₄H₁₀) can exist as a straight chain known as n-butane or as a branched chain called isobutane. Despite having the same number of carbons and hydrogens, their distinct structures lead to different properties. In the exercise, the two SMILES strings represent molecules that could potentially share a molecular formula but have different arrangements. It's important to note that identifying structural isomers requires careful analysis of the atom-connectivity changes.

Resonating Structures

Resonating structures, also known as resonance structures, involve molecules with the same arrangement of atoms but a different distribution of electrons. In essence, these structures differ only in the position of electrons, particularly in the form of pi bonds and lone pairs. The concept of resonance helps us understand the delocalization of electrons across different parts of a molecule, which can impact its stability and reactivity. For example, in the case of C=C(C)[O-], we observe resonating structures as the negative charge can be shared between the oxygen and the surrounding carbon framework. This electron delocalization leads to a more stable molecule. Characteristics of resonating structures include:

• Atoms remain in the same position. Only electrons move.
• The real structure is a hybrid (average) of all possible resonating structures.
• The resonance hybrid is more stable than any individual structure.

Understanding these concepts boosts our insight into how molecules operate on an electronic level.

Geometrical Isomers

Geometrical isomers, also known as cis-trans isomers, are a fascinating type of stereoisomer where compounds have the same bonds but differ in the spatial arrangement of groups around a double bond or other rigid structure. These isomers are significant because the positioning of atoms can lead to drastic differences in chemical properties and biological activities. In a "cis" isomer, similar groups are on the same side of the bond, while in a "trans" isomer, they are on opposite sides. Consider the example of 2-butene:

• In the "cis" isomer, both methyl groups are on the same side of the double bond.
• In the "trans" isomer, they are on opposing sides, creating a molecule with distinct physical properties.

In the provided SMILES representation, the absence of multiple spatial arrangements due to double bonds implies a lack of geometrical isomers. Hence, understanding how these structures work ensures clarity when differentiating between isomer types.