§ 8.1NCERT Class 11 · Chemistry · Chapter 8

Tetravalence of Carbon, Shapes & Classification of Organic Compounds

QCC Notes · Class 11 Chemistry NCERT-aligned Updated 26 Sep 2026 7 min read

Organic chemistry is the chemistry of carbon compounds. Carbon is uniquely suited to form the backbone of millions of biological and synthetic molecules because of two fundamental properties: its tetravalency and its unparalleled capacity for catenation (the ability to form stable, covalent C−C chains and rings of virtually limitless length).

1. Tetravalence of Carbon & Molecular Geometry

In its ground state, carbon has the electronic configuration 1s² 2s² 2p_x¹ 2p_y¹ 2p_z⁰ with only two unpaired electrons. Upon excitation, one 2s electron uncouples and promotes into the vacant 2p_z orbital, yielding the excited state configuration 1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z¹. By mixing these valence orbitals through hybridisation, carbon achieves directional tetravalence:

Hybridisation State	s-Character (%)	Geometry & Bond Angle	C−C Bond Length	Bond Energy (kJ/mol)	Representative Example
sp³	25%	Tetrahedral (109.5°)	154 pm	~348 kJ/mol	Ethane (CH₃−CH₃)
sp²	33.3%	Trigonal Planar (120°)	134 pm	~612 kJ/mol	Ethene (CH₂=CH₂)
sp	50%	Linear (180°)	120 pm	~837 kJ/mol	Ethyne (CH≡CH)

Electronegativity Correlation: Because s-electrons are spherically symmetric and penetrate closer to the nucleus than p-electrons, higher s-character draws shared electron pairs tighter to the carbon atom. Thus, electronegativity follows the sequence:

Electronegativity: sp-C (3.29) > sp²-C (2.75) > sp³-C (2.48)

This explains why terminal alkynes (containing sp-hybridized C−H bonds) exhibit measurable acidity (pK_a ≈ 25) and react with strong bases like NaNH₂.

2. Structural Representations of Organic Molecules

Organic chemists convey three-dimensional connectivity using three primary graphical conventions:

Complete Structural Formula (Lewis Dash Formula): Displays every individual atom and every covalent bond as a dash (− for single, = for double, ≡ for triple).
Condensed Structural Formula: Omission of single covalent dashes where bonds to multiple identical atoms are compressed (e.g., CH₃CH₂CH₂CH₃ or CH₃(CH₂)₂CH₃).
Bond-Line Structural Representation: The most universal, elegant notation. Carbon-carbon bonds are drawn as zig-zag lines. Carbon atoms are implied at every vertex and terminal line-end; hydrogen atoms attached to carbon are omitted and understood to satisfy four valencies. Heteroatoms (O, N, S, Halogens) and hydrogens bonded to heteroatoms are explicitly drawn.
Three-Dimensional (Wedge-Dash) Representation:
- Solid Line: Bond lying in the plane of the drawing paper.
- Solid Wedge (◂): Bond projecting forward out of the page towards the observer.
- Hashed/Dashed Wedge (|||||): Bond projecting backward behind the plane of the page away from the observer.

    Complete Formula:           Condensed:          Bond-Line:
         H   H   H
         |   |   |
     H - C - C - C - H         CH3-CH2-CH3            /         |   |   |                                   /           H   H   H                                  (Propane)

Fig 8.1A: Comparison of complete Lewis dash, condensed, and bond-line representations of propane.

3. Comprehensive Classification of Organic Compounds

Organic molecules are classified taxonomically based on their carbon skeletal architectures into broad structural categories:

   +--------------------------------------------------------------------------+
   |                TAXONOMY OF ORGANIC CHEMICAL COMPOUNDS                    |
   +--------------------------------------------------------------------------+
                              ORGANIC COMPOUNDS
                                      |
         +----------------------------+----------------------------+
         |                                                         |
         v                                                         v
   Acyclic (Open Chain / Aliphatic)                         Cyclic (Closed Ring)
   - Straight Chain (n-alkanes)                                    |
   - Branched Chain (isobutane)               +--------------------+--------------------+
                                              |                                         |
                                              v                                         v
                                    Homocyclic (Carbocyclic)                       Heterocyclic
                                              |                                    (Ring with O, N, S)
                   +--------------------------+--------------------------+
                   |                                                     |
                   v                                                     v
         Alicyclic Compounds                                   Aromatic Compounds
         (Cyclopropane, Cyclohexane)                                     |
                                              +--------------------------+--------------------------+
                                              |                                                     |
                                              v                                                     v
                                     Benzenoid Aromatics                               Non-Benzenoid Aromatics
                                     (Benzene, Naphthalene, Toluene)                   (Tropone, Azulene)

Fig 8.1B: Architectural taxonomy of organic compounds showing ring, heteroatom, and aromatic divisions.

Detailed Category Profiles:

Acyclic (Aliphatic / Open Chain) Compounds: Straight or branched chain molecules containing no rings. Examples: Methane, Isobutane, 2,2-Dimethylpropane.
Alicyclic (Carbocyclic) Compounds: Closed rings containing exclusively carbon atoms in the ring skeleton that behave chemically like open-chain aliphatic hydrocarbons. Examples: Cyclobutane, Cyclopentane, Cyclohexene.
Aromatic Benzenoid Compounds: Planar cyclic systems containing one or more six-membered benzene rings with (4n + 2) delocalized π-electrons. Examples: Benzene, Toluene, Anthracene.
Aromatic Non-Benzenoid Compounds: Planar conjugated aromatic systems that satisfy Huckel's rule but lack a classical benzene ring. Classic example: Tropone (cycloheptatrienone) and Azulene.
Heterocyclic Compounds: Cyclic systems containing at least one heteroatom (N, O, S) in the ring ring frame:
- Alicyclic Heterocycles: Oxirane (ethylene oxide), Tetrahydrofuran (THF).
- Aromatic Heterocycles: Furan (O), Thiophene (S), Pyrrole (N), Pyridine (N).

4. Functional Groups & Homologous Series

A functional group is an atom or bonded group of atoms within an organic molecule that confers characteristic chemical reactivity, regardless of the size or complexity of the attached hydrocarbon skeleton.

Homologous Series: A family of structurally related organic compounds whose successive members differ by a constant repeating methylene unit (−CH₂−), possessing a mass difference of 14 u. Homologs share:

The same general molecular formula (e.g. C_nH_2n+2 for alkanes).
Identical functional groups and similar chemical properties.
A smooth, continuous gradation in physical properties (boiling point, density, viscosity) due to progressive increase in molecular mass and van der Waals surface area.

JEE & NEET Solved Practice Problems

Problem 1: Determine the hybridization states of each carbon atom in the following molecule: CH₂=C=CH−CH₃. What is the total number of σ and π bonds?

Solution:
Let us number the carbon atoms from left to right: C1(=CH₂), C2(=C=), C3(−CH=), C4(−CH₃):
- C1: Forms two single C−H σ-bonds and one C=C double bond (one σ + one π). Steric number = 3 ⇒ sp² hybridised.
- C2: Forms two double bonds (one to C1 and one to C3), containing two σ-bonds and two π-bonds. Steric number = 2 ⇒ sp hybridised (allene central carbon, linear geometry).
- C3: Forms one double bond to C2, one single bond to C4, and one single bond to H. Steric number = 3 ⇒ sp² hybridised.
- C4: Forms four single σ-bonds (one C−C and three C−H). Steric number = 4 ⇒ sp³ hybridised.

Bond Count:
- C−H single bonds: 2 (at C1) + 1 (at C3) + 3 (at C4) = 6 σ-bonds.
- C−C single bonds: 1 (between C3 and C4) = 1 σ-bond.
- C=C double bonds: 2 (between C1=C2 and C2=C3) = 2 σ-bonds + 2 π-bonds.
Total σ-bonds = 6 + 1 + 2 = 9 σ-bonds; Total π-bonds = 2 π-bonds.

Problem 2: Arrange the following carbon-hydrogen bonds in decreasing order of bond dissociation energy: (I) H−C≡CH, (II) H−CH=CH₂, (III) H−CH₂−CH₃. Provide reasoning based on hybridisation.

Solution:
In (I), carbon is sp hybridised (50% s-character).
In (II), carbon is sp² hybridised (33.3% s-character).
In (III), carbon is sp³ hybridised (25% s-character).

Greater s-character pulls bonding electron density closer to the carbon nucleus, resulting in shorter, stronger, and more resilient covalent bonds. Consequently, bond dissociation energy decreases in the order:

H−C≡CH (sp, ~548 kJ/mol) > H−CH=CH₂ (sp², ~465 kJ/mol) > H−CH₂CH₃ (sp³, ~423 kJ/mol)

Order: (I) > (II) > (III).

Frequently Asked Questions

Q1. How does the hybridization of carbon influence bond length and electronegativity?

As the s-character in a hybridized orbital increases (sp3 = 25% s, sp2 = 33.3% s, sp = 50% s), electrons are held closer and more tightly to the positively charged carbon nucleus. Consequently, electronegativity increases in the order sp3 < sp2 < sp, and carbon-carbon bond length decreases in the order C-C single bond (154 pm) > C=C double bond (134 pm) > C#C triple bond (120 pm).

Q2. What is the difference between an alicyclic compound and an aromatic compound?

Alicyclic compounds are carbocyclic rings (like cyclopropane, cyclohexane) that exhibit chemical properties very similar to open-chain aliphatic hydrocarbons. In contrast, aromatic compounds contain cyclic conjugated planar pi-electron systems obeying Huckel's rule (4n + 2 pi electrons), conferring exceptional thermodynamic resonance stability and a marked preference for electrophilic aromatic substitution over addition.

Q3. What are heterocyclic aromatic compounds?

Heterocyclic aromatic compounds are cyclic conjugated planar rings where one or more ring carbon atoms are replaced by heteroatoms such as nitrogen, oxygen, or sulphur, while satisfying Huckel's (4n + 2) pi-electron rule. Classic examples include pyridine (C5H5N), furan (C4H4O), thiophene (C4H4S), and pyrrole (C4H5N).

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