Basic Principles of Organic Chemistry

Understanding organic chemistry principles is essential for mastering reaction mechanisms and solving JEE problems on organic compounds.

Overview

graph TD
    A[Organic Principles] --> B[Nomenclature]
    A --> C[Isomerism]
    A --> D[Electronic Effects]
    A --> E[Reaction Mechanisms]
    B --> B1[IUPAC Rules]
    C --> C1[Structural]
    C --> C2[Stereoisomerism]
    D --> D1[Inductive]
    D --> D2[Resonance]
    E --> E1[Bond Fission]
    E --> E2[Reaction Types]

Tetravalency of Carbon

Carbon has 4 valence electrons and can form 4 covalent bonds.

Hybridization States

Hybridization	Geometry	Bond Angle	Example
sp³	Tetrahedral	109.5°	CH₄, C₂H₆
sp²	Trigonal planar	120°	C₂H₄, Benzene
sp	Linear	180°	C₂H₂, HCN

IUPAC Nomenclature

Steps for Naming

1. Select parent chain (longest continuous carbon chain with highest priority functional group)
2. Number the chain (lowest locant to principal functional group)
3. Identify substituents
4. Name: Prefix + Root word + Primary suffix + Secondary suffix

Root Words

Carbons	Root	Carbons	Root
1	Meth-	6	Hex-
2	Eth-	7	Hept-
3	Prop-	8	Oct-
4	But-	9	Non-
5	Pent-	10	Dec-

Functional Group Priority (Decreasing)

$$-COOH > -SO_3H > -COOR > -COCl > -CONH_2 > -CHO > C=O > -OH > -NH_2 > C=C > C≡C$$

Common Substituent Names

Group	Prefix
-CH₃	Methyl
-C₂H₅	Ethyl
-F, -Cl, -Br, -I	Fluoro, Chloro, Bromo, Iodo
-NO₂	Nitro
-NH₂	Amino
-OH	Hydroxy

Isomerism

Structural Isomerism

graph TD
    A[Structural Isomerism] --> B[Chain]
    A --> C[Position]
    A --> D[Functional Group]
    A --> E[Metamerism]
    A --> F[Tautomerism]

Type	Definition	Example
Chain	Different carbon skeleton	Butane, Isobutane
Position	Different position of substituent	1-Propanol, 2-Propanol
Functional Group	Different functional group	C₂H₅OH (alcohol), CH₃OCH₃ (ether)
Metamerism	Different alkyl groups on either side	CH₃OC₂H₅, C₂H₅OC₂H₅
Tautomerism	Keto-enol interconversion	Acetone ⇌ Propen-2-ol

Stereoisomerism

Geometrical (cis-trans) Isomerism:

• Requires restricted rotation (C=C or ring)
• Different groups on each carbon

Optical Isomerism:

• Requires chiral center (asymmetric carbon)
• Non-superimposable mirror images (enantiomers)

Chirality

A molecule is chiral if it is not superimposable on its mirror image.

Conditions for optical activity:

1. Presence of chiral center(s)
2. Absence of plane of symmetry
3. Absence of center of symmetry

Number of stereoisomers = $2^n$ (where n = number of chiral centers)

Electronic Effects

Inductive Effect (I Effect)

Permanent displacement of σ-electrons along a chain.

-I Effect (Electron withdrawing):

$$-NO_2 > -CN > -COOH > -F > -Cl > -Br > -I > -OCH_3 > -C_6H_5$$

+I Effect (Electron donating):

$$-(CH_3)_3C > -(CH_3)_2CH > -C_2H_5 > -CH_3 > -H$$

Resonance Effect (R/M Effect)

Delocalization of π-electrons through conjugated systems.

+R Effect (Electron donating):

$$-NH_2 > -NHR > -OH > -OR > -NHCOR > -OCOR > -F > -Cl > -Br > -I$$

-R Effect (Electron withdrawing):

$$-NO_2 > -CN > -CHO > -COR > -COOH > -COOR > -CONH_2$$

Hyperconjugation

Interaction between σ-bond and empty/partially filled p-orbital.

Stability of carbocations:

$$3° > 2° > 1° > CH_3^+$$

Electromeric Effect

Temporary electron displacement in presence of attacking reagent.

Bond Fission

Homolytic Fission

$$A:B \rightarrow A\cdot + B\cdot$$

Forms free radicals (odd electrons)

Heterolytic Fission

$$A:B \rightarrow A^+ + B^-$$

or

$$A:B \rightarrow A^- + B^+$$

Forms ions (carbocations or carbanions)

Reactive Intermediates

graph TD
    A[Reactive Intermediates] --> B[Carbocation]
    A --> C[Carbanion]
    A --> D[Free Radical]
    A --> E[Carbene]
    B --> B1[sp², planar, empty p-orbital]
    C --> C1[sp³, pyramidal, lone pair]
    D --> D1[sp², planar, one unpaired e⁻]

Stability Order:

Carbocations: $3° > 2° > 1° > CH_3^+$

Carbanions: $CH_3^- > 1° > 2° > 3°$

Free radicals: $3° > 2° > 1° > CH_3\cdot$

Types of Organic Reactions

Substitution

One group replaces another.

Nucleophilic (Sₙ):

• Sₙ1: First order, via carbocation
• Sₙ2: Second order, concerted

Electrophilic:

• Common in aromatic compounds

Addition

Two groups add across a multiple bond.

Electrophilic Addition:

• To C=C (alkenes)
• Markovnikov’s rule applies

Elimination

Two groups leave to form multiple bond.

• E1: First order, via carbocation
• E2: Second order, concerted

Rearrangement

Atoms rearrange within the molecule.

• Carbocation rearrangements (hydride shift, methyl shift)

Markovnikov’s Rule

In addition of HX to unsymmetrical alkene:

• H adds to carbon with more H atoms
• X adds to carbon with fewer H atoms (more substituted)

$$CH_3-CH=CH_2 + HBr \rightarrow CH_3-CHBr-CH_3$$

Anti-Markovnikov Addition

In presence of peroxides, reversal occurs (for HBr only).

Practice Problems

1. Give IUPAC name: $CH_3-CH(OH)-CH_2-CHO$

2. How many stereoisomers are possible for 2,3-dichlorobutane?

3. Arrange in order of decreasing acidity: Phenol, p-nitrophenol, p-methoxyphenol

4. Predict the major product: 2-Butene + HBr

Further Reading

• Hydrocarbons
• Halogen Compounds