Isomerism MCQ Quiz in বাংলা - Objective Question with Answer for Isomerism - বিনামূল্যে ডাউনলোড করুন [PDF]

CONCEPT:

Specific Rotation and Observed Rotation

• The specific rotation [α] is a property of a chiral compound and represents the rotation of plane-polarized light per unit concentration (g/mL) and path length (dm).
• The observed rotation (α) can be calculated using the formula:
  \(\alpha = [\alpha]_{\text{specific}} \times \frac{l \cdot c}{100} \), where:
  • [\(\alpha_{\text{specific}} \)] = Specific rotation of the substance (in degrees).
  • l = Path length of the polarimeter tube (in dm).
  • c = Concentration of the solution (in g/100 mL).

CALCULATION:

• Given data:
  • Specific rotation [\(\alpha_{\text{specific}}\)] = +15°
  • Concentration (c) = 5 g/100 mL
  • Path length (l) = 10 cm = 1 dm
• Using the formula:
  \(\alpha = [\alpha]_{\text{specific}} \times \frac{l \cdot c}{100} \)
• Substitute the values:
  \(\alpha = 15^\circ \times \frac{1 \cdot 5}{100} = 15^\circ \times 0.05 = 0.75^\circ\)

CONCLUSION:

The observed rotation is +0.75°.

CONCEPT:

Structural Isomerism of C₃H₆O

• Isomers are compounds that have the same molecular formula but different structural arrangements or functional groups.
• Structural isomerism can occur as chain isomers (different carbon chain arrangements) and functional isomers (different functional groups).
• The molecular formula C₃H₆O allows for both chain and functional isomers, including aldehydes, ketones, and alcohols, as well as compounds with different bonding patterns.

CONCLUSION:

The molecular formula C₃H₆O gives rise to a total of nine structural isomers, including aldehydes, ketones, alcohols, ethers, and cyclic compounds.

The correct answer is 1,4-dimethylcyclohexane with both methyl groups in equatorial positions) Means trans 1,4-dimethylcyclohexane

Concept:-

1,3-Diaxial interactions are steric interactions between an axial substituent located on carbon atom 1 of a cyclohexane ring and the hydrogen atoms (or other substituents) located on carbon atoms 3 and 5.

• All structures are dimethylcyclohexane isomers in chair conformation
• Analyzing each option based on:
  • Position of methyl groups (1,4 vs 1,3 vs geminal)
  • Orientation of methyl groups (axial vs equatorial)

Option 1 (1,4-dimethyl):

• Both methyl groups in equatorial positions
• Maximum spacing between methyl groups (1,4 position)
• No 1,3-diaxial interactions
• Minimal steric strain

Option 2 (1,4-dimethyl):

• One axial and one equatorial methyl
• 1,3-diaxial interaction present
• Higher energy than option 1

Option 3 (1,3-dimethyl):

• One axial and one equatorial methyl
• 1,3-diaxial interaction present
• Closer methyl groups cause more steric strain

Option 4 (geminal dimethyl):

• Both methyls on same carbon
• One forced to be axial
• Maximum steric strain
• Gem-dimethyl effect

In chair conformation of cyclohexane we have two position in the conformer.
1. Axial position
2. Equatorial position

• When a substitutent is present at axial position, the conformer will be less stable because it has 1, 3 diaxial interaction which is a steric interaction of axial group. This will increase the energy of the conformer and make it less stable.
• When a substituent is present at equatorial position, the conformer is stable because the substituent has more room and fewer steric interactions when it is in an equatorial position.

For given trans-1, 4 dimethyl cyclohexane compound, the stable conformer will be the one which has both the methyl substituents at equatorial position.

Key Points:

• Factors affecting chair conformer stability:
  • Equatorial positions are more stable than axial by ~1.7 kcal/mol (per methyl)
  • 1,3-diaxial interactions increase strain energy
  • Substitutents prefer equatorial positions to minimize steric interactions
  • Spacing between substituents affects overall stability (further apart = more stable)
• The general stability order is: diequatorial > equatorial-axial > diaxial 1,4-position > 1,3-position > geminal position

Concept:

Structural isomers are compounds with the same molecular formula but different connectivity of atoms. Below are some key points about structural isomers:

• Different Connectivity: Isomers have the same molecular formula but different arrangements of their atoms.

• Variety in Structure: They can include chain isomers, position isomers, and functional group isomers.

• Physical and Chemical Properties: Isomers often exhibit significantly different physical and chemical properties.

• Examples: Benzene derivatives (e.g., xylenes) and other forms like alkenes and alkanes with the same molecular formula.

Explanation:

The molecular formula C₈H₁₀ allows for several types of structural isomers, particularly involving aromatic compounds like benzene derivatives. 

Aromatic Isomers

• o-Xylene (1,2-Dimethylbenzene): Two methyl groups attached to adjacent carbon atoms on the benzene ring.

  • 

• m-Xylene (1,3-Dimethylbenzene): Two methyl groups attached to carbon atoms separated by one carbon on the benzene ring.

  • 

• p-Xylene (1,4-Dimethylbenzene): Two methyl groups attached to carbon atoms directly opposite each other on the benzene ring.

  • 

• Ethylbenzene: A single ethyl group (C₂H₅) attached to the benzene ring.

  • 

Conclusion:

Possible aromatic structural isomer for the compound with molecular formula C₈H₁₀ is: 4.

CONCEPT:

Geometrical Isomerism:

• Geometrical isomerism arises due to restricted rotation around a double bond or in cyclic compounds.
• Each double bond can exist in two possible configurations: cis (Z) and trans (E).
• The number of geometrical isomers depends on the number of stereocenters and symmetry in the molecule.

EXPLANATION:

• The given molecule has 3 stereocenters around double bonds.
• Each stereocenter contributes to two configurations: E (trans) and Z (cis).
• However, due to the symmetrical nature of the molecule, the total number of geometrical isomers is reduced.
• In this case, the total possible geometrical isomers are:
  • EE: Both double bonds are in the E configuration.
  • ZZ: Both double bonds are in the Z configuration.
  • EZ: One double bond is E, and the other is Z (two distinct isomers due to asymmetry).

CONCLUSION:

Number of Geometrical Isomers: 4 (EE, ZZ, and two EZ isomers)

Concept:

Isomers are compounds with the same molecular formula but different structural arrangements of atoms.

When a compound undergoes a reaction that forms a mixture of products with mirror images (non-superimposable), it forms a racemic mixture. Racemic mixtures contain equal amounts of left- and right-handed enantiomers of a chiral molecule.

In the process of monochlorination, only one hydrogen atom in a molecule is replaced by a chlorine atom. The number of different positions where chlorine can replace a hydrogen atom determines the number of isomeric products formed. Each unique position leads to a different structural isomer.

Explanation:

Given:

• Starting compound: 2-methylbutane (C₅H₁₂)

The structural formula of 2-methylbutane is:

CH₃-CH₂-CH(CH₃)-CH₃

Monochlorination takes place in the presence of sunlight, which induces a radical chain reaction. Here, a chlorine atom replaces one hydrogen atom in the molecule, leading to different isomeric products. We need to consider all possible hydrogen atoms that can be replaced by chlorine:

• Primary hydrogens at the terminal carbons (positions 1 and 4):
• Secondary hydrogens on the second carbon (position 2):
• Tertiary hydrogens on the central carbon (position 3):

Reaction: 

There are total 2 racemic mixture product and 2 other product making a total of 6 isomer possible.

Conclusion:

The number of isomeric products formed by monochlorination of 2-methylbutane is: 6

Concept:

Stepwise Approach to Solving Monochloro Derivatives Problems:

1. Draw the structure of the molecule: Clearly lay out the carbon chain and any substituents.

2. Identify the unique carbon positions: Determine which carbon atoms are distinct based on their environment and symmetry.

3. Count the unique positions: Consider equivalent positions due to symmetry to avoid overcounting.

4. Consider substituent positions: For any substituents (like methyl groups), determine if substitution can occur there and if those positions are unique.

5. Summarize the possible derivatives: Combine your counts of unique substitution positions to determine the total number of monochloro derivatives.

Explanation:

To determine the number of distinct monochloro derivatives,

• C-1 and C-7: These positions are equivalent due to molecular symmetry.

• C-2 and C-6: These positions are equivalent, each having a distinct environment.

• C-3 and C-5: Due to symmetry, these positions are equivalent.

• C-4: This is a unique position placed centrally in the chain.

• Methyl groups on C-2 and C-6: They are in identical environment as C-1 and C-7.

This totals 4 uniquely identifiable positions for monochlorination:

• C-1 or C-7

• C-2 or C-6

• C-3 or C-5

• C-4

Conclusion:

The maximum number of monochloro derivatives possible for 2,6-dimethylheptane is: 4.

Concept:

Optical activity: Optical activity is a property of a compound to rotate the plane of polarized light. For a molecule to be optically active, it must meet the following conditions:

• The molecule must contain at least one chiral center (an atom, usually carbon, with four different groups attached).

• Chiral centers give rise to non-superimposable mirror images (enantiomers), which are optically active.

• An optically active compound does not possess a plane of symmetry or center of symmetry.

Explanation:

• 1) CH₃COCH₂CH₃ (2-Butanone): This structure is a ketone without any chiral center. It does not have four different groups attached to any carbon atom and cannot be optically active.

• 2) CH₃CH₂CH₂CHO (Butanal): This structure is an aldehyde without any chiral center. It does not have four different groups attached to any carbon atom and cannot be optically active.

• 3) (CH₃)₂CHCHO (2-Methylpropanal): This structure is an aldehyde as well. The central carbon (carbonyl carbon) does not have four different groups attached, so it cannot be optically active.

• 4) CH₂=CH-CH(OH)CH₃ (3-Buten-2-ol): This structure contains a carbon atom (the one with the -OH group) that has four different groups attached (OH, H, CH₃, and CH₂=CH). This carbon is a chiral center, making the compound optically active.

Conclusion:

The correct structure of the given compound C₄H₈O that is optically active is: CH₂=CH-CH(OH)CH₃

CONCEPT:

Decarboxylation Reaction

• The reaction of a sodium salt of a carboxylic acid (such as CH₃COONa) with sodium hydroxide (NaOH) in the presence of calcium oxide (CaO) and heat is known as a decarboxylation reaction.
• During decarboxylation, the carboxyl group (-COOH) of the acid is removed as carbon dioxide (CO₂), producing an alkane.
• General reaction:

  RCOONa + NaOH → RH + Na₂CO₃ (in the presence of CaO and heat)

• Here, R is the alkyl group, and RH is the resulting alkane.

EXPLANATION:

• In the given reaction:

  CH₃COONa + NaOH → A + B (in the presence of CaO and heat)

  • The sodium acetate (CH₃COONa) undergoes decarboxylation.
  • The carboxylic group (-COO^-) is removed as carbon dioxide (CO₂), leaving behind methane (CH₄).
  • Along with methane, sodium carbonate (Na₂CO₃) is formed as a byproduct.
• Thus:

  A = CH₄ (methane), B = Na₂CO₃ (sodium carbonate)

Therefore, the correct answer is Option 4: CH₄ ; Na₂CO₃.

Concept:

Isomers:

• These are the compounds that have the same molecular formula but different structures or stereochemistries.
• There is a wide range of classification of organic molecules based on their structures.

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• Metamerism: When isomers have the same molecular formula but differ in nature of alkyl groups attached to it.

• A class of compounds that have the same molecular formula but can have different functional groups exhibit functional isomerism.

• Positional isomers: Isomers having different positions of their functional groups.

(iii) C₃H₇Cl represents two position isomers

• Optical isomerism:

  • When two isomers rotate plane polarised light in different directions, then they are known as optical isomers.
  • The phenomenon is known as optical isomerism.

Explanation:

• The molecular formula of propyl alcohol is C₃H₈O.
• In the positional isomers, these things might differ:
  • The position of the substituent.
  • The position of the functional group.
  • Arrangement of carbon atoms in the chain.
• The structure of  n- propyl alcohol and isopropyl alcohol are:

• It is clearly seen that in n-propyl alcohol, the functional group -OH is in position 1 whereas in isopropyl alcohol it is in position two.

Hence, they are positional isomers.