Topic 8

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Topic 8 — Key Facts for DU Admission (Bangladesh) Core concept: Organic Chemistry — structure, naming, and reactions of organic compounds High-yield point: IUPAC nomenclature, functional groups, and named reactions ⚡ Exam tip: Questions on organic reactions and nomenclature appear every year — focus on homologues and isomers

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Topic 8 — DU Admission (Bangladesh) Study Guide Overview: Organic Chemistry covers carbon compounds, functional groups, and their reactions Core principles: Covalent bonding, hybridization, IUPAC naming Key points: Homologous series, isomerism, reaction mechanisms Study strategy: Draw structures daily, memorize functional group properties and reactions

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Organic Chemistry — Complete Study Notes

Introduction to Organic Chemistry

Organic chemistry is the study of carbon compounds. Carbon’s unique property of catenation (forming long chains) gives rise to millions of organic compounds. The DU admission test focuses on aliphatic hydrocarbons, functional groups, and their characteristic reactions.

Why Carbon Is Special

• Tetravalency: Forms 4 covalent bonds
• Catenation: Bonds with itself to form chains and rings
• Multiple bonding: Forms double and triple bonds
• Small size: Forms strong C-C and C-H bonds

Hydrocarbons

Compounds containing only carbon and hydrogen.

Saturated Hydrocarbons (Alkanes)

• General formula: CₙH₂ₙ₊₂
• Single bonds only (sp³ hybridization)
• Methane CH₄ — simplest alkane
• Homologous series: Methane, Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀)…
• Chemical nature: Unreactive (saturated — no double/triple bonds)
• Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O

Unsaturated Hydrocarbons

Alkenes (CₙH₂ₙ):

• One double bond (sp² hybridization)
• Ethene (C₂H₄) — simplest alkene
• More reactive than alkanes due to π-bond

Alkynes (CₙH₂ₙ₋₂):

• One triple bond (sp hybridization)
• Ethyne (C₂H₂) — simplest alkyne
• Most reactive due to two π-bonds

IUPAC Nomenclature

Rules for naming organic compounds:

1. Longest chain of carbon atoms is selected as parent chain
2. Number the chain to give lowest locants for functional group
3. Name substituents with appropriate prefixes
4. Suffix indicates main functional group or bond type

Prefixes for number of carbons: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6)

Suffixes:

• -ane → Alkane (single bond)
• -ene → Alkene (double bond)
• -yne → Alkyne (triple bond)
• -ol → Alcohol (-OH)
• -al → Aldehyde (-CHO)
• -one → Ketone (>C=O)
• -oic acid → Carboxylic acid (-COOH)

Example: CH₃-CH₂-CH₂-OH → Propan-1-ol (or Propanol)

Isomerism

Two or more compounds with same molecular formula but different structures.

Structural Isomerism

Same molecular formula, different bonding sequence:

• Chain isomerism: Different carbon skeleton (e.g., Butane vs Isobutane)
• Position isomerism: Different position of functional group (e.g., Propan-1-ol vs Propan-2-ol)
• Functional group isomerism: Different functional groups (e.g., C₂H₆O = Ethanol or Dimethyl ether)

Geometric Isomerism (Cis-Trans)

Occurs due to restricted rotation around double bond.

• Cis: Similar groups on same side
• Trans: Similar groups on opposite sides
• Requires: (1) Double bond, (2) Two different groups on each carbon

Optical Isomerism (Enantiomerism)

Non-superimposable mirror images. Requires:

• Chiral center (usually carbon with 4 different groups)
• Plane of symmetry absence

Functional Groups

1. Haloalkanes (R-X)

• X = F, Cl, Br, I
• Primary, secondary, tertiary based on carbon bearing halogen
• Undergoes nucleophilic substitution (SN1, SN2)

2. Alcohols (R-OH)

• -OH group attached to sp³ carbon
• Methanol (wood spirit): CH₃OH — highly toxic, causes blindness
• Ethanol: CH₃CH₂OH — drinking alcohol, produced by fermentation
• Oxidation: 1° alcohol → Aldehyde → Carboxylic acid
• 2° alcohol → Ketone
• 3° alcohol → Cannot be easily oxidized

3. Aldehydes and Ketones (R-CHO, R-CO-R)

• Carbonyl group (C=O)
• Aldehyde: -CHO at end of chain
• Ketone: -CO- in middle of chain
• Formaldehyde (HCHO): preservative, simplest aldehyde
• Acetone (CH₃COCH₃): simplest ketone, nail polish remover
• Fehling’s test: Aldehydes reduce Cu²⁺ to Cu⁺ (brick red precipitate) — Ketones do not

4. Carboxylic Acids (R-COOH)

• -COOH functional group
• Acetic acid (CH₃COOH): Vinegar
• Benzzoic acid: Simplest aromatic carboxylic acid
• Reactions: Esterification, neutralization, decarboxylation

5. Esters (R-COOR’)

• Pleasant/fruity odor
• Saponification (alkaline hydrolysis) → Soap + Alcohol
• Natural occurrence: Fats and oils are glyceryl esters

Important Organic Reactions

Addition Reactions

• Occur in unsaturated compounds (alkenes, alkynes)
• H₂ addition: Hydrogenation (Ni catalyst, heat)
• HX addition: Markovnikov’s rule (H to carbon with more H)
• Halogen addition: Br₂ in CCl₄ — decolorizes (test for unsaturation)

Substitution Reactions

• One atom/group replaced by another
• Alkanes + Cl₂ (UV light) → Chloroalkane
• Benzene + Br₂/FeBr₃ → Bromobenzene

Elimination Reactions

• Removal of atoms/group from adjacent carbons
• Dehydration of alcohols (conc. H₂SO₄, 170°C) → Alkene
• Dehydrohalogenation of haloalkanes (alc. KOH) → Alkene

Oxidation Reactions

• KMnO₄ (cold, dilute): Alkenes → Dihydroxy alcohols (glycols)
• KMnO₄ (hot, conc.): Alkenes → Ketones/Carboxylic acids
• Alkaline KMnO₄ + I₂: Used in iodometry

Polymerization Reactions

• Addition polymerization: Monomers with double bonds (Polythene, PVC)
• Condensation polymerization: With elimination of small molecule (Nylon, Terylene)

Benzene (Aromatic Chemistry)

• Formula: C₆H₆ — highly unsaturation but behaves as saturated
• Aromaticity: 6 π-electrons (Hückel’s rule: 4n+2 = 6, n=1)
• Delocalization: π-electrons spread over ring (resonance)
• Reactions: Electrophilic substitution (not addition)
  • Nitration: HNO₃/H₂SO₄ → Nitrobenzene
  • Halogenation: Br₂/FeBr₃ → Bromobenzene
  • Friedel-Crafts alkylation: RCl/AlCl₃ → Alkylbenzene

Biomolecules

Carbohydrates

• Monosaccharides: Glucose (C₆H₁₂O₆) — aldehyde sugar, open chain ↔ cyclic form
• Disaccharides: Sucrose = Glucose + Fructose; Maltose = Glucose + Glucose
• Polysaccharides: Starch, Cellulose, Glycogen

Proteins

• Amino acids joined by peptide bonds
• 20 standard amino acids
• Primary → Secondary → Tertiary → Quaternary structure

Lipids

• Fats: Esters of glycerol and fatty acids
• Saturated fats: No double bonds (solid — butter)
• Unsaturated fats: Double bonds (liquid — vegetable oils)

Must-Remember Facts

Compound	Formula	Important Property
Methane	CH₄	Simplest alkane, natural gas
Ethanol	C₂H₅OH	Drinking alcohol
Acetic acid	CH₃COOH	Vinegar (6% in solution)
Formaldehyde	HCHO	preservative, carcinogenic
Acetone	CH₃COCH₃	Ketone, nail polish remover
Benzene	C₆H₆	Carcinogenic, aromatic
Glucose	C₆H₁₂O₆	Blood sugar, energy source

Common DU Admission Questions

1. IUPAC naming of organic compounds
2. Isomerism — structural vs geometric
3. Addition vs substitution vs elimination reactions
4. Tests for aldehydes vs ketones (Fehling’s, Tollens’)
5. Homologous series — properties trend
6. Markownikoff’s rule application
7. Petrochemical fractions and uses

Exam Tips

• Count carbons correctly in complex molecules
• For addition to alkene with HX: “H goes to the carbon with more hydrogens already” (Markovnikov’s rule)
• Aromatic compounds undergo substitution, NOT addition
• Alcohols with conc. H₂SO₄ at 140°C give ethers; at 170°C give alkenes
• Aldehydes give positive Tollens’ test (silver mirror); ketones generally do not

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