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Hydrocarbons - Classification of Hydrocarbons

Grade 11CBSEChemistry

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

🔑Concepts

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Hydrocarbons are organic compounds composed entirely of carbon (CC) and hydrogen (HH) atoms.

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Classification based on structure: Hydrocarbons are primarily divided into Acyclic (Open-chain) and Cyclic (Closed-chain) compounds.

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Acyclic or Aliphatic Hydrocarbons: These are open-chain structures and are subdivided into saturated (Alkanes) and unsaturated (Alkenes and Alkynes) compounds.

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Saturated Hydrocarbons (Alkanes): Contain only single bonds between carbon atoms (C−CC-C). The carbon atoms are sp3sp^3 hybridized.

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Unsaturated Hydrocarbons: Contain at least one double bond (C=CC=C, Alkenes) or one triple bond (C≡CC \equiv C, Alkynes). Alkenes involve sp2sp^2 hybridization, while Alkynes involve spsp hybridization.

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Alicyclic Hydrocarbons: Cyclic compounds that exhibit properties similar to aliphatic hydrocarbons (e.g., Cyclopropane, Cyclohexane).

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Aromatic Hydrocarbons (Arenes): Special cyclic compounds that follow Hückel's rule (4n+2π4n+2 \pi electrons). They are classified into Benzenoid (containing a benzene ring, e.g., C6H6C_6H_6) and Non-benzenoid (aromatic without a benzene ring, e.g., Tropolone).

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General Formulae: Alkanes follow CnH2n+2C_nH_{2n+2}, Alkenes follow CnH2nC_nH_{2n}, and Alkynes follow CnH2n−2C_nH_{2n-2}.

📐Formulae

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

CnH2n (Alkenes or Cycloalkanes)C_nH_{2n} \text{ (Alkenes or Cycloalkanes)}

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

4n+2=π electrons (Hu¨ckel’s Rule for Aromaticity)4n + 2 = \pi \text{ electrons (Hückel's Rule for Aromaticity)}

💡Examples

Problem 1:

Classify the following hydrocarbons based on their molecular formulas: C3H8C_3H_8, C4H8C_4H_8, and C2H2C_2H_2.

Solution:

C3H8C_3H_8 is an Alkane; C4H8C_4H_8 is an Alkene (or Cycloalkane); C2H2C_2H_2 is an Alkyne.

Explanation:

Applying the general formulas: For C3H8C_3H_8, n=3n=3, 2n+2=82n+2=8 (fits CnH2n+2C_nH_{2n+2}). For C4H8C_4H_8, n=4n=4, 2n=82n=8 (fits CnH2nC_nH_{2n}). For C2H2C_2H_2, n=2n=2, 2n−2=22n-2=2 (fits CnH2n−2C_nH_{2n-2}).

Problem 2:

Is Benzene (C6H6C_6H_6) an aliphatic or aromatic hydrocarbon? Justify using Hückel's Rule.

Solution:

Benzene is an Aromatic hydrocarbon.

Explanation:

Benzene has a planar cyclic structure with 66 delocalized π\pi electrons. Applying Hückel's Rule: 4n+2=6  ⟹  4n=4  ⟹  n=14n+2 = 6 \implies 4n = 4 \implies n = 1. Since nn is an integer, it is aromatic.

Classification of Hydrocarbons - Revision Notes & Key Formulas | CBSE Class 11 Chemistry