Chapter 15 Organic Chemistry FBISE 1st year Keynotes with Solved Exercise

15.1 Catenation & Organic Chemistry Basics

🔗 Catenation Definition

From Greek “catena” meaning chain
Carbon’s ability to form long chains and rings
Unique property of carbon
Other elements show limited catenation (Si, S)

📝 Key Point: Catenation + Tetravalency + Small size = Millions of organic compounds

⚛️ Why So Many Organic Compounds?

C-C-C-C Chains & Rings

Single, Double, Triple Bonds

Attachment to H, O, N, Halogens

Catenation: Forms chains/rings of any length
Multiple bonding: Single, double, triple bonds
Isomerism: Same formula, different structures
Functional groups: Various reactive groups
Small atomic size: Strong C-C bonds (346 kJ/mol)

📊 Bond Energies Comparison

Bond	Energy (kJ/mol)	Bond	Energy (kJ/mol)
C-C	346	Si-Si	222
C-H	413	N-N	167
C=C	614	O-O	142
C≡C	839	S-S	226

15.2 Hydrocarbons & Functional Groups

⛽ Hydrocarbons Classification

Type	Bonds	General Formula	Example
Alkanes	Only single	CₙH₂ₙ₊₂	CH₄ (Methane)
Alkenes	Contains C=C	CₙH₂ₙ	C₂H₄ (Ethene)
Alkynes	Contains C≡C	CₙH₂ₙ₋₂	C₂H₂ (Ethyne)
Cycloalkanes	Ring, single	CₙH₂ₙ	C₆H₁₂ (Cyclohexane)

⚗️ Functional Groups

Atom/group determining chemical properties
Two parts: Reactive FG + Unreactive carbon network
Double/triple bonds = Functional groups
Reactions occur at functional groups

📈 Homologous Series

Same functional group, different chain length
Differ by -CH₂- (methylene group)
Similar chemical properties
Gradual change in physical properties
Same preparation methods

🎯 Examples:

Alkanes: CₙH₂ₙ₊₂ (CH₄, C₂H₆, C₃H₈…)

Alcohols: CₙH₂ₙ₊₁OH (CH₃OH, C₂H₅OH…)

Carboxylic acids: CₙH₂ₙ₊₁COOH (HCOOH, CH₃COOH…)

15.2.2 Empirical & Molecular Formulae

🔢 Empirical Formula

Simplest whole number ratio of atoms
May mislead identification
Examples: CH₂O for both glucose & acetic acid

Compound	Molecular Formula	Empirical Formula
Ethanol	C₂H₆O	C₂H₆O
Acetic acid	C₂H₄O₂	CH₂O
Glucose	C₆H₁₂O₆	CH₂O
Benzene	C₆H₆	CH
Ethyne	C₂H₂	CH

🧪 Example Calculations

Example 1: C=38.4%, H=4.8%, Cl=56.8%
Empirical Formula = C₂H₃Cl

Example 2: Empirical CH, Molar mass=78
Molecular Formula = C₆H₆ (Benzene)

📐 Structural Formulae Types

Condensed: CH₃CH₂CH₃ (shows connections)
Full/2D Displayed: Shows all atoms & bonds
Skeletal: Lines for bonds, vertices for C atoms
Stereochemical: 3D arrangement in space

15.5 Organic Reactions & Mechanisms

⚡ Types of Reagents

Reagent	Definition	Example
Free Radical	Atom/group with unpaired electron	Cl•, CH₃•
Electrophile	Electron-deficient species	CH₃⁺, H⁺
Nucleophile	Electron-rich species	OH⁻, NH₃

🔨 Bond Breakage Types

Homolytic Fission:
• Equal splitting
• Forms free radicals
• Cl₂ → 2Cl•
• Same/similar atoms Heterolytic Fission:
• Unequal splitting
• Forms ions (+ & -)
• CH₃Cl → CH₃⁺ + Cl⁻
• Different atoms

15.4 Nomenclature of Organic Compounds

🏷️ IUPAC Naming Rules

Step 1
Identify longest chain

→

Step 2
Number chain correctly

→

Step 3
Name substituents

Step 4
Assign locants

→

Step 5
Alphabetical order

→

Step 6
Write complete name

📝 Functional Group Suffixes

Class	Suffix/Prefix	Example
Alkanes	-ane	CH₃CH₂CH₃ (propane)
Alkenes	-ene	CH₂=CH₂ (ethene)
Alkynes	-yne	CH≡CH (ethyne)
Alcohols	-ol	CH₃CH₂OH (ethanol)
Aldehydes	-al	CH₃CHO (ethanal)
Ketones	-one	CH₃COCH₃ (propanone)
Carboxylic acids	-oic acid	CH₃COOH (ethanoic acid)
Amines	-amine	CH₃NH₂ (methanamine)
Nitriles	-nitrile	CH₃CN (ethanenitrile)

🎯 Special Naming Rules

Aldehydes: Carbon 1 always = CHO group
Ketones: Lowest number to carbonyl
Alcohols: Number closest to -OH
Multiple groups: Use di-, tri-, tetra-
Cyclic compounds: Prefix “cyclo”

15.6 Types of Organic Reactions

🔥 Free Radical Substitution

Methane Chlorination

CH₄ + Cl₂ → CH₃Cl + HCl (sunlight)

Mechanism: Initiation → Propagation → Termination

➕ Electrophilic Addition

Alkene + HBr

CH₂=CH₂ + HBr → CH₃CH₂Br

Electrophile (H⁺) attacks double bond

➖ Elimination Reactions

Dehydration of Alcohol

CH₃CH₂OH → CH₂=CH₂ + H₂O (H₂SO₄)

Removal of H₂O from adjacent carbons

🔄 Nucleophilic Substitution

Halogenoalkane + OH⁻

CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr

Nucleophile replaces halogen

💧 Hydrolysis & Condensation

Hydrolysis:
Ester + Water → Acid + Alcohol
CH₃COOCH₂CH₃ + H₂O →
CH₃COOH + CH₃CH₂OH Condensation:
Acid + Alcohol → Ester + Water
CH₃COOH + CH₃CH₂OH →
CH₃COOCH₂CH₃ + H₂O

15.7 Isomerism

🔄 Structural Isomerism Types

Chain Isomerism: Different carbon skeletons
C₄H₁₀: n-Butane & Isobutane
Positional Isomerism: Different functional group positions
C₃H₇OH: Propan-1-ol & Propan-2-ol
Functional Group Isomerism: Different functional groups
C₂H₆O: CH₃CH₂OH & CH₃OCH₃
Metamerism: Different alkyl groups on same functional group
C₄H₁₀O: CH₃OCH₂CH₂CH₃ & CH₃CH₂OCH₂CH₃
Tautomerism: Dynamic equilibrium (keto-enol)
CH₃COCH₂COOEt ⇌ CH₃C(OH)=CHCOOEt

📊 Isomer Count:

• Butane (C₄H₁₀): 2 isomers

• Pentane (C₅H₁₂): 3 isomers

• Hexane (C₆H₁₄): 5 isomers

• Decane (C₁₀H₂₂): 75 isomers!

MCQs & Short Questions – Complete Solutions

MCQ 1: Which molecule needs no number in its systematic name?

A. CH₃(CH₂)₄CHO (Hexanal)

B. CH₃CH=CHCH₂CH₃ (Pent-2-ene)

C. CH₃CH₂CH₂OH (Propan-1-ol)

D. CH₃CO(CH₂)₃CH₃ (Pentan-2-one)

Answer: A – CH₃(CH₂)₄CHO (Hexanal)

Explanation: Aldehydes always have the carbonyl carbon as carbon 1, so no number is needed for the aldehyde group. For hexanal, the -CHO group is automatically at position 1.

Other options:

B: Needs number for double bond position (2- or 3-)

C: Needs number for -OH position (1- or 2-)

D: Needs number for ketone position (2- or 3-)

MCQ 2: Which molecule has chiral carbon?

CH₃CHClCH₃

CH₃CHClCH₂CH₃

CH₂ClCH₂CH₃

CH₃CCl₂CH₃

Answer: B – CH₃CHClCH₂CH₃ (2-Chlorobutane)

Explanation: A chiral carbon has four different groups attached. In 2-chlorobutane (CH₃CHClCH₂CH₃):

Carbon 2 is attached to:

-CH₃ (methyl)
-Cl (chloro)
-H (hydrogen)
-CH₂CH₃ (ethyl)

All four groups are different → chiral center.

Other options: No chiral carbon due to symmetry or identical groups.

MCQ 3: Propene + H₂ (Ni catalyst) → Propane. What type of reaction?

A. Elimination

B. Addition

C. Hydrolysis

D. Oxidation

Answer: B – Addition reaction

Explanation: CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃

This is hydrogenation (addition of H₂ across double bond).

Characteristics:

Unsaturated → Saturated
Double bond → Single bond
Nickel catalyst at 200°C
Addition of H₂ molecule

Why not others:

A: Elimination removes atoms (opposite)

C: Hydrolysis involves water

D: Oxidation adds oxygen/removes hydrogen

Short Answer i: Define catenation and justify that this property of carbon is responsible for so large number of organic compounds.

Catenation: The ability of carbon atoms to form long chains and rings by bonding with other carbon atoms.

Justification:

Chain Formation: Carbon forms C-C-C chains of any length
Branching: Can form branched chains (isomerism)
Ring Formation: Forms cyclic compounds
Multiple Bonding: Single, double, triple bonds possible
Combination with Other Elements: Bonds with H, O, N, halogens
Stability: Strong C-C bonds (346 kJ/mol) due to small size

Result: These properties allow millions of unique structures from same atoms → vast number of organic compounds.

Short Answer ii: How was vital force theory rejected by Friedrich Wohler?

Vital Force Theory: Organic compounds could only be produced by living organisms (plants/animals) through a “vital force.”

Wohler’s Experiment (1828):

Ammonium cyanate (inorganic) → Urea (organic)

NH₄⁺ + OCN⁻ → (NH₂)₂CO (urea)

Significance:

First synthesis of organic compound from inorganic material
Proved organic compounds follow same chemical laws
No “vital force” needed
Beginning of synthetic organic chemistry

Short Answer iii: Differentiate between saturated and unsaturated hydrocarbons with examples.

Saturated Hydrocarbons

Only single C-C bonds
Maximum hydrogen
General formula: CₙH₂ₙ₊₂
Show substitution reactions
Less reactive
Examples:
CH₄ (Methane)

C₂H₆ (Ethane)

C₃H₈ (Propane)

Unsaturated Hydrocarbons

Contain C=C or C≡C bonds
Can add more atoms
General formulas: CₙH₂ₙ (alkenes), CₙH₂ₙ₋₂ (alkynes)
Show addition reactions
More reactive
Examples:
CH₂=CH₂ (Ethene)

CH≡CH (Ethyne)

CH₃CH=CH₂ (Propene)

Short Answer iv: Define empirical formula and give empirical formulae of ethanol, benzene and ethanoic acid.

Empirical Formula: The simplest whole number ratio of atoms of each element in a compound.

Compound	Molecular Formula	Empirical Formula
Ethanol	C₂H₆O	C₂H₆O
Benzene	C₆H₆	CH
Ethanoic acid	C₂H₄O₂	CH₂O

Note: Glucose also has empirical formula CH₂O, showing why empirical formula alone is insufficient for identification.

Short Answer v: Define electrophile and nucleophile with two examples each.

Electrophile (Electron-loving)

Electron-deficient species
Seeks electrons
Positively charged or neutral with empty orbital
Attacks electron-rich sites
Examples:
1. H⁺ (Proton)
2. CH₃⁺ (Carbocation)
3. NO₂⁺ (Nitronium ion)
4. AlCl₃ (Lewis acid)

Nucleophile (Nucleus-loving)

Electron-rich species
Donates electrons
Negatively charged or neutral with lone pairs
Attacks electron-deficient sites
Examples:
1. OH⁻ (Hydroxide ion)
2. NH₃ (Ammonia)
3. CN⁻ (Cyanide ion)
4. H₂O (Water with lone pairs)

Short Answer vi: Define chain isomerism. How differentiate between metamerism and positional isomerism?

Chain Isomerism: Isomers with same molecular formula but different carbon skeleton (chain branching).

Example: C₄H₁₀ → CH₃CH₂CH₂CH₃ (n-butane) & CH₃CH(CH₃)CH₃ (isobutane)

Differentiation:

Aspect	Positional Isomerism	Metamerism
Definition	Same carbon skeleton, different position of functional group	Different alkyl groups on same functional group
Functional Group	Same	Same
Carbon Skeleton	Same	Different around functional group
Example	C₃H₇OH: Propan-1-ol & Propan-2-ol	C₄H₁₀O: CH₃OCH₂CH₂CH₃ & CH₃CH₂OCH₂CH₃
Occurs in	All functional groups with multiple positions	Ethers, amines, ketones with -CO- group