ORGANIC CHEMISTRY BASICS

LEARNING OBJECTIVES

• Understand what organic chemistry is and why carbon is important

• Identify and describe alkanes and alkenes

• Write correct formulas for hydrocarbons

• Understand homologous series and their properties

• Identify isomers and draw simple structures

• Describe combustion of hydrocarbons

• Predict products of addition reactions

KEY DEFINITIONS

• Organic chemistry: Study of compounds containing carbon (usually bonded to hydrogen)

• Hydrocarbon: Compound containing only carbon and hydrogen atoms

• Saturated compound: Hydrocarbon containing only single bonds between carbon atoms

• Unsaturated compound: Hydrocarbon containing at least one double or triple bond between carbons

• Alkane: Saturated hydrocarbon with general formula CₙH₂ₙ₊₂

• Alkene: Unsaturated hydrocarbon with at least one C=C double bond, general formula CₙH₂ₙ

• Homologous series: Group of compounds with similar properties differing by CH₂ units

• Isomer: Compounds with same molecular formula but different structural arrangement

• Combustion: Burning of hydrocarbon in oxygen to produce CO₂ and H₂O

WHAT IS ORGANIC CHEMISTRY

• Study of carbon and its compounds

• Carbon is central to all living things

• Over 90% of known compounds are organic compounds

• Organic compounds contain C-C and C-H bonds

• Also contains many other elements (N, O, S, etc.) but always has carbon

Why is carbon special?

• Can form 4 covalent bonds

• Can bond with itself forming long chains

• Can form single, double, and triple bonds

• Forms stable compounds with hydrogen, nitrogen, oxygen, halogens

CARBON BONDING & STRUCTURE

Carbon valency :

• Carbon forms exactly 4 covalent bonds

• Can bond to other carbons or other elements

Single bond (C-C) :

• One pair of electrons shared between carbons

• Allows free rotation around the bond

• Found in alkanes

Double bond (C=C) :

• Two pairs of electrons shared between carbons

• Restricts rotation around the bond

• Found in alkenes

• Makes compound more reactive

Triple bond (C≡C) :

• Three pairs of electrons shared between carbons

• Very strong and stable

• Found in alkynes (not important in O/L)

  
  

Chain structures :

• Straight chain: carbons bonded in a line

• Branched chain: carbons bonded with side branches

• Ring/cyclic: carbons form a closed ring

ALKANES (SATURATED HYDROCARBONS)

General formula : CₙH₂ₙ₊₂

Definition:

• Hydrocarbons with only single C-C bonds

• Saturated - contain maximum number of hydrogen atoms

• Generally unreactive (inert)

Properties of alkanes :

• Mainly non-polar

• Insoluble in water (soluble in non-polar solvents)

• Low boiling points (increase with molecular size)

• Combustible - burn readily in oxygen

• Less reactive than alkenes

Common alkanes :

• Methane (CH₄) - gas, natural gas fuel

• Ethane (C₂H₆) - gas, in natural gas

• Propane (C₃H₈) - gas, cooking fuel

• Butane (C₄H₁₀) - gas/liquid, lighter fuel

• Pentane (C₅H₁₂) - liquid, solvent

• Hexane and above - liquids and waxy solids

Combustion of alkanes:

• Complete combustion (sufficient O₂) : Alkane + O₂ → CO₂ + H₂O

• Example : CH₄ + 2O₂ → CO₂ + 2H₂O

• Incomplete combustion (limited O₂) : produces CO and C (soot)

ALKENES (UNSATURATED HYDROCARBONS)

General formula : CₙH₂ₙ

Definition :

• Hydrocarbons with at least one C=C double bond

• Unsaturated - can add more atoms across the double bond

• More reactive than alkanes

Properties of alkenes :

• Contain C=C double bond

• More reactive than alkanes (due to double bond)

• Decolorize bromine water (turns from orange to colorless)

• Decolorize KMnO₄ solution (turns from purple to colorless)

• These colour changes are used to detect alkenes

Common alkenes :

• Ethene (C₂H₄) - gas, plant hormone, ripening of fruit

• Propene (C₃H₆) - gas, industrial chemical

• Butene (C₄H₈) - gas

Addition reactions of alkenes :

• Atoms add across the C=C double bond

• Double bond breaks, becomes single bond

• Product is saturated (no more double bond)

• Alkene + H₂ → Alkane (hydrogenation)

• Example: C₂H₄ + H₂ → C₂H₆

• Alkene + Br₂ → Dibromoalkane (orange color disappears)

• Example: C₂H₄ + Br₂ → C₂H₄Br₂ (colorless)

Combustion of alkenes :

• Complete combustion: Alkene + O₂ → CO₂ + H₂O

• Example: 2C₂H₄ + 5O₂ → 4CO₂ + 4H₂O

HOMOLOGOUS SERIES

Definition : Group of compounds with :

• Same general formula

• Similar chemical properties

• Successive members differ by CH₂

• Gradual change in physical properties

Alkane homologous series :

• CH₄ → C₂H₆ → C₃H₈ → C₄H₁₀ → ...

• Each member differs by CH₂

• General formula: CₙH₂ₙ₊₂

Alkene homologous series :

• C₂H₄ → C₃H₆ → C₄H₈ → C₅H₁₀ → ...

• Each member differs by CH₂

• General formula: CₙH₂ₙ

Properties within a homologous series :

• Same functional group (same type of bond)

• Similar chemical reactions

• Boiling point increases as chain length increases

• Density increases gradually

• State changes from gas to liquid to solid as chain length increases

ISOMERISM (STRUCTURAL ISOMERS)

Definition : Compounds with same molecular formula but different structural arrangement

Types of structural isomers (at O/L) :

Chain isomerism :

• Same molecular formula but different carbon chain arrangement

• Straight chain vs branched chain

• Example: C₄H₁₀ has two isomers:

• Butane (straight chain): CH₃-CH₂-CH₂-CH₃

• Isobutane (branched): CH₃-CH(CH₃)-CH₃ (2-methylpropane)

• Different properties: different boiling points, different densities

Position isomerism :

• Same molecular formula but functional group in different position

• Example: C₄H₈ has isomers:

• But-1-ene: CH₂=CH-CH₂-CH₃ (double bond at position 1)

• But-2-ene: CH₃-CH=CH-CH₃ (double bond at position 2)

ALKANES vs ALKENES COMPARISON