Organic Chemistry: Some Basic Principles and Techniques

COMPREHENSIVE CHAPTER NOTES: ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES

GENERAL INTRODUCTION

**Organic chemistry** is the study of carbon-containing compounds (except carbonates, bicarbonates, and oxides of carbon). These compounds are vital for life and include DNA, proteins, carbohydrates, lipids, and synthetic polymers.

**Historical Context:**

Before 1828, scientists believed organic compounds could only be produced in living organisms due to a "vital force"

**Friedrich Wöhler's synthesis (1828)**: Synthesized urea (NH₂CONH₂) from inorganic ammonium cyanate (NH₄OCN), disproving the vital force theory

**Later syntheses**: Kolbe (1845) synthesized acetic acid; Berthelot (1856) synthesized methane

The development of electronic bonding theory modernized organic chemistry

**Importance**: Organic compounds are essential in medicines, dyes, plastics, fuels, textiles, and food products, making their study critical for understanding chemistry and material science.

---

TETRAVALENCE OF CARBON: SHAPES OF ORGANIC COMPOUNDS

The Shapes of Carbon Compounds

**Carbon's tetrahedral nature** results from **sp³ hybridization**, where four hybrid orbitals arrange tetrahedrally (109.5° bond angles). Understanding molecular geometry is fundamental to predicting organic compound properties.

**Hybridization Types and Bond Characteristics:**

| Hybridization | Geometry | Bond Angle | Bond Strength | Example |

|---|---|---|---|---|

| **sp³** | Tetrahedral | 109.5° | Weakest single bonds | CH₄ (methane) |

| **sp²** | Trigonal planar | 120° | Intermediate | CH₂=CH₂ (ethene) |

| **sp** | Linear | 180° | Strongest | HC≡CH (ethyne) |

**Key Concept**: sp orbitals contain more s-character (50%) than sp² (33%) or sp³ (25%), making them shorter, stronger, and closer to the nucleus. This affects:

**Bond length**: sp < sp² < sp³

**Bond strength**: sp > sp² > sp³

**Electronegativity**: Carbon with sp hybridization is most electronegative

**Examples:**

**Methane (CH₄)**: Carbon is sp³ hybridized; tetrahedral shape with H-C-H bonds of 109.5°

**Ethene (C₂H₄)**: Each carbon is sp² hybridized; trigonal planar, 120° bond angles

**Ethyne (C₂H₂)**: Each carbon is sp hybridized; linear molecule, 180° C-C-H angle

Characteristic Features of π Bonds

**π bonds** form between adjacent atoms with **parallel p-orbital orientation**, creating a "side-on" overlap above and below the bonding axis.

**Critical Properties:**

1. **Restricted Rotation**: In C=C double bonds, rotation about the C-C bond is **restricted or prevented** because rotation disrupts p-orbital overlap. This creates **geometric isomerism** (cis/trans isomers in alkenes)

2. **Electron Density Distribution**: The electron cloud of a π bond lies above and below the molecular plane, making these electrons:

Easily accessible to electrophilic reagents

More reactive than σ bonds

3. **Reactivity**: Molecules with π bonds (alkenes, alkynes, aromatics) are **more reactive** than saturated alkanes because π electrons are more polarizable

**Example**: In but-2-ene, the double bond prevents rotation:

**cis-but-2-ene**: CH₃ groups on same side (higher boiling point due to dipole)

**trans-but-2-ene**: CH₃ groups on opposite sides (lower boiling point)

**Exam Point**: σ bonds (from orbital head-on overlap) allow free rotation; π bonds (from parallel p-orbital overlap) restrict rotation and create double bond reactivity centers.

---

STRUCTURAL REPRESENTATIONS OF ORGANIC COMPOUNDS

Complete, Condensed, and Bond-line Structural Formulas

Organic structures are represented in multiple ways for clarity and simplicity:

**1. Complete Structural Formula (Lewis Structure)**

Shows every atom and every bond as a line (—) or pair of dots

Single bond: —; Double bond: ==; Triple bond: ≡

Lone pairs on heteroatoms may or may not be shown

Example: Ethane (C₂H₆)

```

H—C—C—H or H—C—C—H

| | H H

H H

```

**2. Condensed Structural Formula**

Atoms bonded to the same carbon are grouped together

Reduces lines but maintains structural information

Example: CH₃CH₂CH₃ (propane) instead of C₃H₈

**Examples:**

Ethane: CH₃CH₃

Ethene: CH₂=CH₂

2-methylbutane: CH₃CH(CH₃)CH₂CH₃

**3. Bond-line (Skeletal) Structural Formula**

**Most simplified** representation used by organic chemists

**Rules**:

Carbon and hydrogen atoms are **not shown**

Vertices (corners) and line junctions represent **carbon atoms**

Each carbon is bonded to enough hydrogens to satisfy tetravalency (4 bonds total)

**Only heteroatoms** (O, N, S, halogen, etc.) and their hydrogens are explicitly written

Double/triple bonds shown as ==, ≡

**Example transformations:**

CH₃CH₂CH(OH)CH₃ can be represented as:

```

OH

|

_____|

/

(skeletal form shows a bent line with OH branch)

```

**Cyclic compound examples:**

Cyclopropane: Triangle (3-membered ring)

Cyclopentane: Pentagon (5-membered ring)

Chlorocyclohexane: Hexagon with Cl labeled

**Exam Advantage**: Bond-line formulas appear frequently in questions; students must convert between all three formats.

Three-Dimensional Representation of Organic Molecules

Since organic molecules are **three-dimensional**, paper representations use specific conventions:

**Wedge-and-Dash Notation:**

**Solid wedge (▲)**: Bond projecting **toward observer** (out of page)

**Dashed wedge (▼)**: Bond projecting **away from observer** (into page)

**Normal line**: Bond lies **in the plane of paper**

Broad end of wedge always faces observer

**Example - Methane (CH₄)**:

```

H

|

H—C—H (with one H as wedge, one as dash)

|

H

```

**Molecular Models:**

Physical 3D models help visualize geometry:

**Framework model**: Shows only bonds (no atoms), emphasizes bonding pattern

**Ball-and-stick model**: Atoms as balls, bonds as sticks; good for showing angles

**Space-filling model**: Shows relative atomic sizes (van der Waals radii); emphasizes molecular volume

---

CLASSIFICATION OF ORGANIC COMPOUNDS

Organic compounds are classified by **structure** and **functional groups**:

Structural Classification

**I. Acyclic (Open-chain) Compounds**

Carbon atoms form straight or branched chains

Also called **aliphatic compounds**

Examples: butane (CH₃CH₂CH₂CH₃), 2-methylpropane

**II. Cyclic (Closed-chain/Ring) Compounds**

**A. Alicyclic Compounds**

Carbon atoms joined in a ring (no aromatic character)

Examples: cyclopropane (3-membered), cyclopentane (5-membered), cyclohexane (6-membered)

Exhibit mostly alkane-like properties

**B. Aromatic Compounds**

Contain benzene ring or related ring systems

Special stability and reactivity due to π-electron delocalization

**Benzenoid aromatics**: Benzene, naphthalene, aniline

**Non-benzenoid aromatics**: Tropone (7-membered ring with special properties)

**C. Heterocyclic Compounds**

Rings contain atoms other than carbon (N, O, S)

Examples: pyridine (C₅H₅N), furan (C₄H₄O), thiophene (C₄H₄S)

Can be alicyclic (tetrahydrofuran) or aromatic

Functional Group Classification

**Functional Group**: An atom or group of atoms bonded to carbon chain responsible for characteristic chemical properties

**Common Functional Groups:**

| Group | Name | Example | Parent Class |

|---|---|---|---|

| —OH | Hydroxyl | CH₃OH (methanol) | Alcohol |

| —CHO | Aldehyde | CH₃CHO (acetaldehyde) | Aldehyde |

| —CO— | Ketone | CH₃COCH₃ (acetone) | Ketone |

| —COOH | Carboxyl | CH₃COOH (acetic acid) | Carboxylic acid |

| —NH₂ | Amino | CH₃NH₂ (methylamine) | Amine |

| —X (Cl, Br, I) | Halogen | CH₃Cl (chloromethane) | Alkyl halide |

| —OR | Ether linkage | CH₃OCH₃ (dimethyl ether) | Ether |

**Polyvalent Compounds**: Contain two or more functional groups (e.g., amino acids contain —COOH and —NH₂)

Homologous Series

**Definition**: A series of organic compounds where:

1. Each member contains the same functional group

2. Successive members differ by exactly **one CH₂ unit** (molecular weight differs by 14)

3. All members follow the **same general formula**

4. They exhibit **gradual variation in physical properties** and **similar chemical properties**

**Examples:**

**Alkane series**: General formula CₙH₂ₙ₊₂

CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane)...

**Alkene series**: General formula CₙH₂ₙ

C₂H₄ (ethene), C₃H₆ (propene), C₄H₈ (butene)...

**Alcohol series**: General formula CₙH₂ₙ₊₂O

CH₃OH (methanol), C₂H₅OH (ethanol), C₃H₇OH (propanol)...

**Properties within homologous series**:

**Physical properties** change gradually (boiling point increases with chain length)

**Chemical properties** remain similar (all alcohols undergo similar reactions)

Difference in boiling points for successive members ≈ 20-30°C

---

NOMENCLATURE OF ORGANIC COMPOUNDS

IUPAC System of Nomenclature

The **International Union of Pure and Applied Chemistry (IUPAC)** system provides systematic names correlating structure to name—the reader can deduce structure from the name.

**Advantages over Trivial Names**:

Unambiguous identification

Predictable from structure

Allows naming of millions of unknown compounds

**Trivial/Common Names** (still used):

Acetic acid (CH₃COOH) instead of ethanoic acid

Formic acid instead of methanoic acid

Acetone instead of propanone

**General Strategy**:

**Systematic name = Prefix + Parent + Suffix**

Where:

**Prefix**: Indicates substituents/branches

**Parent**: Longest carbon chain (determines base name)

**Suffix**: Indicates functional group (-ane for alkane, -ene for alkene, -ol for alcohol, etc.)

IUPAC Nomenclature of Alkanes

**Straight-Chain Alkanes:**

Saturated hydrocarbons containing only C-C single bonds; names end in **-ane**

| Carbons | Name | Molecular Formula |

|---|---|---|

| 1 | Methane | CH₄ |

| 2 | Ethane | C₂H₆ |

| 3 | Propane | C₃H₈ |

| 4 | Butane | C₄H₁₀ |

| 5 | Pentane | C₅H₁₂ |

| 6 | Hexane | C₆H₁₄ |

| 7 | Heptane | C₇H₁₆ |

| 8 | Octane | C₈H₁₈ |

| 9 | Nonane | C₉H₂₀ |

| 10 | Decane | C₁₀H₂₂ |

**Alkyl Groups** (substituents derived by removing H from alkanes):

Prefix: "meth-", "eth-", "prop-", "but-" + suffix "-yl"

CH₃— = methyl; C₂H₅— = ethyl; C₃H₇— = propyl

**Branched Alkyl Groups**:

**n-Propyl**: CH₃CH₂CH₂—

**Isopropyl**: (CH₃)₂CH— (two CH₃ groups on same C)

**n-Butyl**: CH₃CH₂CH₂CH₂—

**sec-Butyl**: CH₃CH₂CH(CH₃)— (secondary position)

**Isobutyl**: (CH₃)₂CHCH₂—

**tert-Butyl**: C(CH₃)₃— (tertiary position)

**Neopentyl**: (CH₃)₃CCH₂—

Rules for Naming Branched Alkanes

**Step 1: Identify Longest Carbon Chain**

This becomes the **parent** alkane name

If multiple chains of same length, choose the one with most branches

Example: In a molecule with one 8-carbon chain and one 7-carbon chain, use octane as parent

**Step 2: Number the Chain**

Number from the end giving **lowest numbers** to substituents

If multiple choices give same total, lowest number goes to first point of difference

Example:

```

CH₃—CH—CH₂—CH—CH₃

| |

CH₃ CH₃

```

Numbering: C1—C2—C3—C4—C5 (gives 2,4)

Not: C5—C4—C3—C2—C1 (gives 2,4 also, but count from left)

Use 2,4-dimethylpentane

**Step 3: Name and Position Substituents**

Write position number, hyphen, then substituent name

If two identical substituents: use di-, tri-, tetra-, penta- (without considering these prefixes in alphabetical order)

**Step 4: Write Complete Name**

Alphabetical order of different substituents (but not counting di-, tri- etc.)

Separate numbers from letters by hyphens

Separate numbers from numbers by commas

No space between "methyl" and "pentane" in "2-methylpentane"

**Examples:**

**Example 1:**

```

CH₃

|

CH₃—CH—CH₂—CH₂—CH₃

1 2 3 4 5

```

Name: **2-methylpentane** (one methyl on carbon 2 of pentane chain)

**Example 2:**

```

CH₃ CH₃

| |

CH₃—CH—CH₂—CH—CH₃

1 2 3 4 5

```

Name: **2,4-dimethylpentane** (two methyl groups; numbering gives 2,4—not 2,4 from other end which would also be 2,4)

**Example 3:**

```

CH₂CH₃ CH₃

| |

CH₃—CH—CH₂—CH—CH₃

1 2 3 4 5

```

Name: **4-ethyl-2-methylpentane** (alphabetical: "ethyl" before "methyl"; no "di-" because different groups)

**Example 4:**

```

CH₃—CH—CH₂—CH₂—CH—CH₂—CH₃

| |

CH₃ CH₂CH₃

1 2 6 7

```

Longest chain = 7 carbons (heptane)

Numbering from correct end: C1—C2—C3—C4—C5—C6—C7 (gives positions 2 and 6)

Name: **6-ethyl-2-methylheptane**

Nomenclature of Alkenes and Alkynes

**Alkenes** (C=C double bond):

General formula: CₙH₂ₙ

Suffix: **-ene**

Position of double bond indicated by **lowest number** of carbons at either end

Examples:

CH₂=CH—CH₂—CH₃ → **but-1-ene** (double bond at C1)

CH₃—CH=CH—CH₃ → **but-2-ene** (double bond at C2)

**Alkynes** (C≡C triple bond):

General formula: CₙH₂ₙ₋₂

Suffix: **-yne**

Position indicated by lowest number

Examples:

HC≡C—CH₂—CH₃ → **but-1-yne**

CH₃—C≡C—CH₃ → **but-2-yne**

Nomenclature of Compounds with Functional Groups

When functional groups are present, priority is given in this order:

1. Carboxylic acid (—COOH) → suffix **-oic acid**

2. Aldehyde (—CHO) → suffix **-al**

3. Ketone (C=O) → suffix **-one**

4. Alcohol (—OH) → suffix **-ol**

5. Amine (—NH₂) → suffix **-amine**

6. Alkene (C=C) → suffix **-ene** or **-enol** if OH also present

7. Alkyne (C≡C) → suffix **-yne**

**Examples:**

**1. Alcohol (—OH):**

CH₃CH₂OH → **ethanol** (2 carbons in chain + -ol)

HOCH₂CH₂CH₃ → **propan-1-ol** (number position of OH)

**2. Aldehyde (—CHO):**

CH₃CHO → **ethanal** (C in CHO counts as C1)

CH₃CH₂CHO → **propanal**

**3. Carboxylic Acid (—COOH):**

CH₃COOH → **ethanoic acid** (C in COOH = C1)

HOOC—CH₂—COOH → **propanedioic acid**

**4. With Multiple Functional Groups:**

HOCH₂CHO → **2-hydroxyethanal** (aldehyde has priority)

HOCH₂CH₂OH → **ethane-1,2-diol** (two OH groups)

---

ORGANIC REACTION MECHANISMS

Concept of Organic Reaction Mechanism

**Mechanism**: The **step-by-step sequence of elementary reactions** by which a reactant is converted to product, showing **movement of electrons** and **formation/breaking of bonds**.

**Why Important:**

Explains **reaction pathways** and **reaction rates**

Predicts **products and by-products**

Helps design synthetic routes

Explains effect of conditions on reactions

**Types of Bond Cleavage:**

**1. Homolytic Cleavage**

Bond breaks such that **each fragment gets one electron**

Produces **free radicals** (atoms with unpaired electrons, denoted by •)

Occurs with heat, light, or peroxides

Example: Cl—Cl → Cl• + Cl• (photochemical cleavage)

Results in **non-polar products** even if bond was polar

**2. Heterolytic Cleavage**

Bond breaks such that **both electrons go to one fragment**

Produces **cation and anion**

Occurs in polar solvents, with polar reagents

Example: C—Br → C⁺ + Br⁻ (in solution)

Results in **ionic intermediates**

Electronic Displacement Effects

**Definition**: Redistribution of electron density in a molecule due to substituents, affecting reactivity.

**Types:**

**1. Inductive Effect**

**Permanent shift** of electron density through σ bonds

Caused by **electronegativity difference** between atoms

Effect **decreases with distance** from source

**Electron-withdrawing groups** (EWG): F, Cl, Br, NO₂, CN (pull electrons toward themselves)

**Electron-donating groups** (EDG): alkyl, —OH, —OR (push electrons away)

Example: In chloromethane (CH₃Cl), chlorine's high electronegativity pulls electron density:

```

δ-Cl—CH₃(δ+)

```

The carbon becomes partially positive, affecting reactivity toward nucleophiles.

**2. Resonance Effect**

**Delocalization** of π electrons (or lone pairs) through π-bonded systems

Results in **stabilization** of carbocations and carbanions

**Electron-donating resonance**: —OR, —NR₂ groups donate electrons through resonance to conjugated system

**Electron-withdrawing resonance**: —C=O, —CN groups withdraw through resonance

Example: In aniline (C₆H₅NH₂), nitrogen's lone pair resonates into benzene ring, stabilizing it:

```

NH₂ ↔ [ring with N-C double bond] ↔ ...

```

Makes aniline **more reactive** toward electrophiles than benzene.

**3. Effect on Reactivity**

**π bonds** are major sites of electrophilic attack (high electron density above/below plane)

**EWG** (like —NO₂) **decreases electron density** at double bond, **deactivates** molecule

**EDG** (like —OH) **increases electron density**, **activates** molecule toward electrophiles

**Steric effects**: Bulky groups physically block access to reactive site

---

TYPES OF ORGANIC REACTIONS

Organic reactions are classified by **mechanism and outcome**:

Substitution Reactions

An atom or group is replaced by another atom or group.

**Types:**

**1. Nucleophilic Substitution (SN)**

Nucleophile (electron-rich species: OH⁻, CN⁻, etc.) attacks carbon bearing leaving group

Occurs at **sp³ carbon** (saturated)

Examples:

CH₃Br + OH⁻ → CH₃OH + Br⁻ (nucleophilic displacement)

RCl + NH₃ → RNH₂ + HCl (amine formation)

**2. Electrophilic Substitution (SE)**

Electrophile (electron-poor species: H⁺, Br⁺, NO₂⁺) attacks electron-rich π systems

Common in **aromatic rings**

Example: C₆H₆ + Br₂ → C₆H₅Br + HBr (bromination of benzene)

**3. Free Radical Substitution**

Mechanism involves **free radical intermediates**

Requires **heat or light**

Example: CH₄ + Cl₂ → CH₃Cl + HCl (chlorination under UV light)

Addition Reactions

Atoms are added across π bonds (C=C or C≡C), decreasing degree of unsaturation.

**Electrophilic Addition** (most common):

π electrons (nucleophilic) attacked by electrophile

Example: C₂H₄ + HBr → C₂H₅Br (addition to alkene)

Mechanism: Formation of carbocation intermediate

**Example mechanism:**

```

CH₂=CH₂ + HBr → [CH₃—CH₂⁺] → CH₃CH₂Br

(carbocation)

```

Elimination Reactions

Removal of atoms/groups from adjacent carbons forming π bonds.

**Example:**

```

CH₃—CH₂—Br + KOH → CH₂=CH₂ + KBr + H₂O

```

Removal of HBr from ethyl bromide yields ethene.

Oxidation-Reduction Reactions

Transfer of electrons between reactants.

**Examples:**

**Oxidation of alcohols**: CH₃CH₂OH + [O] → CH₃CHO (to aldehyde) or → CH₃COOH (to carboxylic acid)

**Hydrogenation of alkenes**: CH₂=CH₂ + H₂ → CH₃CH₃ (reduction using catalyst)

---

TECHNIQUES OF PURIFICATION OF ORGANIC COMPOUNDS

Physical Methods

**1. Crystallization**

**Principle**: Differential solubility in hot vs. cold solvent

**Procedure**:

Dissolve compound in hot solvent

Cool slowly; pure compound crystallizes

Filter crystals; wash with cold solvent

Dry crystals

**Best for**: Solid compounds with distinct solubility changes with temperature

**Example**: Recrystallization of naphthalene from hot ethanol

**2. Distillation**

**Principle**: Separation based on **boiling points**

**Procedure**:

Heat liquid mixture to vaporization

Vapor condenses at collection point

Liquid with lower boiling point distills first

**Variants**:

**Simple distillation**: For liquids differing by ≥30°C in boiling points

**Fractional distillation**: For liquids with similar boiling points (uses fractionating column)

**Steam distillation**: For heat-sensitive compounds; uses steam to lower boiling point

**Vacuum distillation**: For compounds that decompose at high temperature

**Example**: Separation of ethanol (b.p. 78°C) from water (b.p. 100°C)

**3. Sublimation**

**Principle**: Substance vaporizes directly from solid to gas without melting

**Requirements**: Compound must have **vapor pressure** at reasonable temperature

**Examples**: Dry ice (CO₂), iodine (I₂), camphor sublime under reduced pressure

**Advantage**: Very pure product (only sublime compound vaporizes)

**4. Chromatography**

**Principle**: **Differential distribution** of components between mobile and stationary phases

**Types**:

**Paper chromatography**: Mobile phase (solvent) travels up paper; compounds separate by affinity

**Thin-layer chromatography (TLC)**: Stationary phase = silica/alumina on plate; superior resolution

**Gas chromatography (GC)**: Gas is mobile phase; excellent for volatile compounds

**Column chromatography**: Stationary phase packed in column; solvent percolates; fractions collected

**Principle**: Different compounds have different **Rf values** (ratio of distance traveled by compound to distance traveled by solvent)

---

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS

**Objective**: Identify the **presence of elements** (C, H, N, S, halogens) and **functional groups** in organic compounds.

Detection of Elements

**Carbon and Hydrogen:**

**Test**: Heat compound with copper oxide (CuO)

**Reaction**:

C + 2CuO → 2Cu + CO₂↑ (black copper oxide turns red; CO₂ turns limewater white)

2H + CuO → Cu + H₂O (water condenses on glass rod)

**Conclusion**: Presence of CO₂ and H₂O confirms C and H

**Nitrogen:**

**Denitrification test**: Heat compound with soda lime (CaO + NaOH)

**Indicator**: Ammonia (NH₃) gas released; pungent smell; turns red litmus blue

**Equation**: Organic-N + NaOH/heat → NH₃↑ + Na-salt

**Sulfur:**

**Test**: Fuse compound with sodium metal; cool; add dilute HCl

**Reaction**: S + Na → Na₂S; Na₂S + HCl → NaHS + NaCl

**Detection**: Lead acetate paper turns black (Na₂S + Pb²⁺ → PbS↓, black)

**Halogens (Cl, Br, I):**

**Denitrification with sodium**: Heat with Na metal in alcohol

**Products**: NaCl, NaBr, NaI

**Test with AgNO₃**: Add dilute HNO₃ then AgNO₃

Cl⁻ → AgCl↓ (white precipitate)

Br⁻ → AgBr↓ (pale yellow)

I⁻ → AgI↓ (yellow)

Detection of Functional Groups

**Alkenes (C=C):**

**Baeyer's test**: Add dilute KMnO₄ (purple) in water

**Observation**: Purple color decolorizes; brown MnO₂ precipitate forms

**Equation**: C=C + KMnO₄ → diol + MnO₂↓

**Alkynes (C≡C):**

**Tollens' reagent test**: Add Tollens' reagent (ammoniacal silver solution)

**Observation**: Silver mirror forms (Ag⁺ → Ag metal)

**Only alkyne**: Reacts with Tollens' due to acidic terminal hydrogen in alkyne

**Alcohols (—OH):**

**Ferric chloride test**: Add neutral FeCl₃

**Observation**: No color change (unlike phenols which give colored complex)

**Alternative**: Oxidation test—oxidize with K₂Cr₂O₇₂ in acidic medium

Primary alcohols → aldehyde → carboxylic acid

Secondary alcohols → ketones

Color change: Orange Cr₂O₇²⁻ → green Cr³⁺

**Phenols (—OH on benzene):**

**FeCl₃ test**: Add ferric chloride

**Observation**: Characteristic colored complex (violet, purple, green depending on phenol type)

**Example**: Phenol + FeCl₃ → violet complex

**Aldehydes (—CHO):**

**Tollens' test**: Add Tollens' reagent (ammoniacal silver solution)

**Observation**: Silver mirror forms (RCHO oxidized to RCOOH; Ag⁺ → Ag)

**Fehling's test**: Add Fehling's reagent (Cu²⁺ complex in basic solution)

**Observation**: Blue color → red precipitate of Cu₂O↓

**Ketones (C=O):**

**2,4-DNP test**: Add 2,4-dinitrophenylhydrazine

**Observation**: Yellow/orange precipitate forms (2,4-DNP hydrazone)

**Note**: Aldehydes also give this test; use Tollens' to distinguish (aldehydes give silver mirror; ketones don't)

**Carboxylic Acids (—COOH):**

**Sodium carbonate test**: Add Na₂CO₃ solution

**Observation**: Effervescence (CO₂ gas); foam

**Equation**: RCOOH + Na₂CO₃ → RCOONa + H₂O + CO₂↑

**Amines (—NH₂):**

**Carbylamine test**: Add chloroform (CHCl₃) and alkali

**Reaction**: Amine + CHCl₃ + KOH → isocyanide (R—N=C)

**Observation**: Pungent odor (like decaying fish)

---

QUANTITATIVE ANALYSIS OF ORGANIC COMPOUNDS

**Objective**: Determine **percentage composition** of elements (C, H, N, S)