Organic chemistry doesn’t have to be confusing. This guide breaks it down into simple, memorable patterns.
What Is Organic Chemistry?
Organic chemistry is the study of carbon compounds. Carbon can form four bonds, allowing for countless different molecules.
Basic Hydrocarbons
Hydrocarbons contain only carbon and hydrogen.
Alkanes:
- Single bonds only
- – General formula: CnH2n+2
- – Saturated hydrocarbons
First four alkanes:
- Methane: CH4
- – Ethane: C2H6
- – Propane: C3H8
- – Butane: C4H10
Alkenes:
- Contain a double bond
- – General formula: CnH2n
- – Unsaturated hydrocarbons
First four alkenes:
- Ethene: C2H4
- – Propene: C3H6
- – Butene: C4H8
- – Pentene: C5H10
Note: No “methene” – need at least 2 carbons for a double bond.
Homologous Series
A homologous series is a family of compounds with:
- Same functional group
- – Same general formula
- – Similar chemical properties
- – Gradual change in physical properties
Each successive member differs by CH2.
Functional Groups
Alkanes: C-C single bonds
Alkenes: C=C double bonds
Alcohols: -OH group
Carboxylic acids: -COOH group
Naming Compounds
Prefixes indicate carbon count:
- Meth- : 1 carbon
- – Eth- : 2 carbons
- – Prop- : 3 carbons
- – But- : 4 carbons
- – Pent- : 5 carbons
Suffixes indicate functional group:
- -ane : alkane
- – -ene : alkene
- – -ol : alcohol
- – -oic acid : carboxylic acid
Alkanes
Properties:
- Unreactive
- – Colourless gases/liquids
- – Burn in air
Reactions:
- Combustion: alkane + oxygen → CO2 + H2O
- – Substitution with halogens (UV light needed)
Example: CH4 + Cl2 → CH3Cl + HCl
Alkenes
Properties:
- More reactive than alkanes
- – Colourless gases/liquids
Reactions:
- Combustion: alkene + oxygen → CO2 + H2O (sootier flame)
- – Addition reactions
Addition with hydrogen (hydrogenation):
C2H4 + H2 → C2H6
Addition with water (hydration):
C2H4 + H2O → C2H5OH
Addition with halogens:
C2H4 + Br2 → C2H4Br2 (decolorises bromine water)
Test for alkenes: Bromine water turns from orange to colourless.
Alcohols
General formula: CnH2n+1OH
First three alcohols:
- Methanol: CH3OH
- – Ethanol: C2H5OH
- – Propanol: C3H7OH
Properties of ethanol:
- Colourless liquid
- – Volatile
- – Miscible with water
- – Good solvent
Uses of ethanol:
- Alcoholic drinks
- – Fuel
- – Solvent
Production of ethanol:
- Fermentation: glucose → ethanol + CO2 (yeast, 37°C)
- 2. Hydration of ethene: C2H4 + H2O → C2H5OH
Carboxylic Acids
General formula: CnH2n+1COOH
First three:
- Methanoic acid: HCOOH
- – Ethanoic acid: CH3COOH
- – Propanoic acid: C2H5COOH
Properties:
- Weak acids
- – Sharp smell (vinegar)
- – React with metals, bases, carbonates
Reactions:
- With metals: acid + metal → salt + hydrogen
- – With bases: acid + base → salt + water
- – With carbonates: acid + carbonate → salt + water + CO2
Esters
Formation: alcohol + carboxylic acid → ester + water
Esters have fruity smells and are used in flavourings and perfumes.
Polymers
Polymers are large molecules made from repeating units (monomers).
Addition Polymerisation:
- Alkenes join together
- – No by-products
- – Double bonds break and form single bonds
Example: n(C2H4) → (-C2H4-)n
Ethene → poly(ethene) or polythene
Condensation Polymerisation:
- Two different monomers join
- – Small molecule (usually water) eliminated
Natural polymers:
- Proteins (from amino acids)
- – Starch (from glucose)
- – Cellulose (from glucose)
Synthetic polymers:
- Polythene (from ethene)
- – PVC (from chloroethene)
- – Nylon (condensation polymer)
Key Patterns to Remember
- Functional groups determine reactions
- 2. Homologous series have similar properties
- 3. Names follow a pattern (meth-, eth-, prop-, but-)
- 4. Formulas follow general formulae
- 5. Reactions are predictable by functional group
Common Mistakes
- Mixing up -ane, -ene, -ol
- 2. Wrong molecular formulae
- 3. Forgetting conditions for reactions
- 4. Not balancing equations
- 5. Confusing polymer and monomer
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