Organic Chemistry – Some Basic Principles and Techniques — CBSE Class 11 Chemistry

Organic chemistry is the study of carbon compounds. Carbon's unique ability to form four covalent bonds and to bond with itself in long chains (catenation) gives rise to millions of organic compounds. These range from simple methane to complex biomolecules.

Tetravalency of Carbon and Catenation

Carbon (atomic number 6) has the electronic configuration 2,4. It forms four bonds — single, double, or triple — with other carbon atoms and with elements like H, O, N, S, and halogens. This leads to open chains (aliphatic), closed rings (cyclic), and aromatic structures.

Structural Representation

Complete structural formula: All atoms and bonds drawn.
Condensed formula: Groups shown without drawing all bonds (e.g., CH_3CH_2OH).
Bond-line (skeletal) formula: Carbon chain shown as a zig-zag; carbon and hydrogen atoms implied at each vertex and end.

Classification of Organic Compounds

Acyclic (open chain): Alkanes, alkenes, alkynes.
Cyclic: Alicyclic (non-aromatic rings) and Aromatic (benzene and related).
By functional group: alcohols (-OH), aldehydes (-CHO), ketones (C=O), carboxylic acids (-COOH), amines (-NH₂), etc.

Functional Groups and Homologous Series

A functional group is an atom or group of atoms that determines the characteristic properties of an organic compound. A homologous series is a family of compounds with the same functional group and general formula, each member differing by -CH₂-.

IUPAC Nomenclature (Systematic Naming)

1.Find the longest carbon chain (parent chain) — gives the root word (meth-, eth-, prop-, but-, pent-, hex-...).
2.Identify the principal functional group and add the appropriate suffix (-ol, -al, -one, -oic acid, -amine).
3.Number the chain from the end nearest the principal functional group.
4.Name and number all substituents as prefixes (methyl-, ethyl-, chloro-, bromo-, etc.).
5.Use di-, tri-, tetra- for multiple identical substituents.
6.Alphabetical order for substituents in the name.

Types of Organic Reactions

Substitution: One atom/group replaces another (e.g., halogenation of alkanes).
Addition: Two reactants combine to give one product (e.g., H₂ adding across a double bond).
Elimination: Removal of atoms/groups to form a double/triple bond.
Rearrangement: Structural rearrangement of atoms within a molecule.

Fission of Covalent Bonds

Homolytic fission: Each atom in the bond gets one electron → forms free radicals (neutral, highly reactive).
Heterolytic fission: Both electrons go to one atom → forms carbocations (C+) or carbanions (C-).

Electronic Effects

Inductive effect (I): Shifting of electrons through sigma bonds due to electronegativity differences. Electron-withdrawing groups (-I): -F, -Cl, -NO₂. Electron-donating groups (+I): alkyl groups.
Resonance: Delocalisation of pi electrons across adjacent p-orbitals; described by resonance structures.
Hyperconjugation: Delocalization of sigma bonding electrons of C-H into adjacent empty or pi orbitals.
Electromeric effect: Complete transfer of pi electrons to one atom under influence of an attacking reagent.

Purification Techniques

Crystallisation: For purifying solids.
Distillation / Fractional distillation: For liquids with different boiling points.
Chromatography (TLC, column): Separates components by differential migration on a stationary phase.
Sublimation: For solids that sublime (e.g., camphor, naphthalene).
Extraction with solvents: Using immiscibility of solvents.

---

Worked Examples

Example 1

Give the IUPAC name of CH_3CH_2CH_2OH.
Longest chain = 3C (propane root). OH at C1 → propan-1-ol.

Example 2

Name CH_3CHOHCH₃.
Three carbons, OH on C2 → propan-2-ol.

Example 3

Identify the functional group in CH_3CHO.
-CHO is the aldehyde functional group.

Example 4

Classify the fission in CH₄ → CH₃• + H•.
Each fragment gets one electron → homolytic fission, producing free radicals.

Example 5

Arrange in decreasing +I effect: CH₃, C_2H₅, (CH₃)_3C.
More alkyl branches → stronger +I effect. Order: (CH₃)_3C > C_2H₅ > CH₃.

Example 6

Draw the resonance structures of the formate ion (HCOO^-).
The negative charge and double bond delocalize between the two oxygen atoms:
H-C(=O)-O^- ↔ H-C(-O^-)=O. Both C-O bonds are equivalent.

Example 7

Write the homologous series formula for alcohols and give the first three members.
General formula: C_nH_2n+2O or C_nH_2n+1OH.
n=1: methanol (CH_3OH), n=2: ethanol (C_2H_5OH), n=3: propan-1-ol (C_3H_7OH).

---

Key Formulas / Rules

General formula: Alkanes C_nH_2n+2, Alkenes C_nH_2n, Alkynes C_nH_2n-2
Degree of unsaturation = (2C + 2 + N - H - X) / 2 (for CcHhNnOoXx)
IUPAC suffix: -ane (single bond), -ene (double bond), -yne (triple bond)

Common mistakes

Numbering the parent chain from the wrong end — always number from the end closer to the principal functional group.
Confusing inductive effect (permanent, through sigma bonds) with electromeric effect (temporary, through pi bonds, only when reagent approaches).
Writing resonance structures with different numbers of electrons — all resonance structures must have the same number of electrons.

Summary

Organic chemistry is built on carbon's tetravalency and catenation. Systematic IUPAC naming, understanding of functional groups, electronic effects (inductive, resonance, hyperconjugation), and bond fission types form the backbone of this chapter. Purification techniques are also essential practical knowledge.

Organic Chemistry – Some Basic Principles and Techniques