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Organic Chemistry – Some Basic Principles and Techniques - Isomerism

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Isomerism is the phenomenon where two or more organic compounds possess the same molecular formula but differ in their physical or chemical properties due to differences in the arrangement of atoms.

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Structural Isomerism occurs when compounds have the same molecular formula but different structures (connectivity). It is further classified into Chain, Position, Functional, and Metamerism.

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Chain Isomerism arises when two or more compounds have the same molecular formula but differ in the nature of the carbon skeleton, such as nn-butane and isobutane (C4H10C_4H_{10}).

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Position Isomerism occurs when the functional group or substituent occupies different positions on the same carbon chain, e.g., CH3CH2CH2OHCH_3CH_2CH_2OH (Propan-1-ol) and CH3CH(OH)CH3CH_3CH(OH)CH_3 (Propan-2-ol).

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Functional Isomerism happens when compounds have the same molecular formula but different functional groups, such as alcohols and ethers (e.g., C2H6OC_2H_6O can be CH3CH2OHCH_3CH_2OH or CH3OCH3CH_3OCH_3).

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Metamerism arises due to unequal distribution of alkyl groups around the same polyvalent functional group, such as in ethers like C2H5OC2H5C_2H_5OC_2H_5 (Ethoxyethane) and CH3OC3H7CH_3OC_3H_7 (1-Methoxypropane).

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Stereoisomerism involves compounds with the same connectivity but different spatial arrangements. It includes Geometrical isomerism (ciscis and transtrans isomers) and Optical isomerism.

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Geometrical Isomerism is common in alkenes due to restricted rotation around the C=CC=C double bond. ciscis-isomers have similar groups on the same side, while transtrans-isomers have them on opposite sides.

📐Formulae

CnH2n+2 (General formula for Alkanes)C_nH_{2n+2} \text{ (General formula for Alkanes)}

CnH2n (General formula for Alkenes)C_nH_{2n} \text{ (General formula for Alkenes)}

CnH2n−2 (General formula for Alkynes)C_nH_{2n-2} \text{ (General formula for Alkynes)}

Degree of Unsaturation (DU)=C+1−H+X−N2Degree\ of\ Unsaturation\ (DU) = C + 1 - \frac{H + X - N}{2}

Number of structural isomers for C4H10=2\text{Number of structural isomers for } C_4H_{10} = 2

Number of structural isomers for C5H12=3\text{Number of structural isomers for } C_5H_{12} = 3

💡Examples

Problem 1:

Identify the type of isomerism between CH3CH2CH2CH2OHCH_3CH_2CH_2CH_2OH and CH3CH2CH(OH)CH3CH_3CH_2CH(OH)CH_3.

Solution:

Position Isomerism.

Explanation:

Both compounds have the same molecular formula C4H10OC_4H_{10}O and the same carbon skeleton (butane chain). They differ only in the position of the hydroxyl (−OH-OH) group, which is at C−1C-1 in Butan-1-ol and C−2C-2 in Butan-2-ol.

Problem 2:

Draw the ciscis and transtrans isomers for But−2−eneBut-2-ene (CH3CH=CHCH3CH_3CH=CHCH_3).

Solution:

ciscis-But-2-ene: High priority groups on same side. transtrans-But-2-ene: High priority groups on opposite sides.

Explanation:

In ciscis-But-2-ene, both −CH3-CH_3 groups are on the same side of the C=CC=C double bond. In transtrans-But-2-ene, the −CH3-CH_3 groups are diagonally opposite.

Problem 3:

Determine if CH3OCH2CH2CH3CH_3OCH_2CH_2CH_3 and CH3CH2OCH2CH3CH_3CH_2OCH_2CH_3 are metamers.

Solution:

Yes, they are metamers.

Explanation:

Both share the molecular formula C4H10OC_4H_{10}O and the same functional group (ether −O−-O-). They differ in the distribution of alkyl groups: one has methyl and propyl groups, while the other has two ethyl groups.