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Overview

Trace the origin of organic chemistry and the fall of the vital-force theory through Wöhler’s experiment.
Compare organic with inorganic compounds in structure, solubility, melting and boiling points, combustion and bonding.
Distinguish molecular from structural formulae and explain isomerism using C₂H₆O.
Detect carbon and hydrogen in an organic compound, name alkanes by IUPAC rules, and state how methane is prepared and how alkanes react.

🌳 Concept tree

Organic Chemistry and the Alkanes

├── Origins

│ ├── Ancient use of fats, oils, sugar, vinegar, alcohol, dyes
│ ├── Berzelius (1806) organic = plant/animal origin

│ ├── Vital-force theory

│ └── Wöhler (1828)→urea from ammonium cyanate

├── Why organic compounds are so numerous

│ ├── Catenation (C–C, C=C, C≡C)

│ └── Straight, branched chains and rings

├── Molecular vs structural formula · isomerism (C₂H₆O)
├── Detecting C and H (CuO, then anhydrous CuSO₄, then limewater)

├── Classification of hydrocarbons

│ ├── Aliphatic open chain (alkanes
alkenes
alkynes)

│ └── Aromatic · cyclic aliphatic

└── Alkanes (paraffins) CₙH₂ₙ₊₂

├── Homologous series

├── Alkyl radical R– and IUPAC naming

├── Methane→preparation and uses

├── Physical properties

└── Chemical properties→combustion, halogenation, cracking, carbon black, water gas

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 1 / 11

The birth of organic chemistry and the vital-force theory

In ancient times

Since antiquity humans used substances from animals and plants: fats, oils, sugar, vinegar, alcohols and perfumes.
The ancient Egyptians used drugs in mummification and fast dyes still visible on the walls of temples.

Organic compoundsextracted from a plant or animal origin.

Inorganic compoundsobtained from mineral sources in the earth.

The vital-force theory

Berzelius held that an organic compound is formed inside the cells of a living organism by a vital force, and therefore cannot be prepared in the laboratory.

Wöhler’s experiment (1828)

The German chemist Friedrich Wöhler prepared urea — an organic compound found in the urine of mammals — from two inorganic compounds.
He mixed ammonium chloride with silver cyanate to form ammonium cyanate, then heated it and obtained urea:

NH₄Cl + AgCNO → AgCl ↓ + NH₄CNO

NH₄CNO —(heat)→ H₂N–CO–NH₂ (urea)

This was the death blow to the vital-force theory; scientists then went on to prepare drugs, detergents, dyes, plastics, fertilisers and pesticides.

The modern definition

Comparison aspect	Main idea	Keywords
•	An organic substance is defined by its composition not its source because most laboratory-prepared compounds do not exist in living organisms	organicsubstancedefinedcomposition
Organic chemistry	is the study of the compounds of carbon excluding oxides of carbon carbonates and cyanides	studycompoundscarbonexcluding
Inorganic chemistry	is the study of the remaining elements and their compounds	studyremainingelementscompounds

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 2 / 11

Why organic compounds are so abundant

Organic compounds exceed ten million and the number grows daily, while inorganic compounds do not exceed half a million — a ratio of about 20 : 1.

The carbon atom can bond to itself and to other atoms in many ways.
Atoms join by single, double or triple bonds C–C, C=C, C≡C.
Carbon atoms form continuous chains, branched chains, homogeneous rings (all carbon) and heterogeneous rings (another element inside the ring).
Because of this enormous number, compounds had to be classified into groups and rules of naming established.

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 3 / 11

Organic versus inorganic compounds

Comparison aspect	Main idea	Keywords
Chemical composition	organic: carbon must be present · inorganic: carbon may be present	organiccarbonmustpresent
Solubility	organic: mostly insoluble in water soluble in organic solvents · inorganic: mostly soluble in water	organicmostlyinsolublewater
Melting and boiling points	organic: low · inorganic: high	organicinorganichigh
Odour	organic: usually characteristic · inorganic: usually odourless	organicusuallycharacteristicinorganic
Combustion	organic: burn to give always CO₂ and H₂O · inorganic: mostly non-combustible	organicburngivealways
Type of bonds	organic: covalent · inorganic: ionic and covalent	organiccovalentinorganicionic
Conduction	organic: non-electrolytic do not conduct · inorganic: mostly electrolytic and conduct	organicelectrolyticinorganicmostly
Reaction rate	organic: slow (between molecules) · inorganic: fast (between ions)	organicslowbetweenmolecules
Polymerisation	organic: many form polymers · inorganic: rarely	organicmanyformpolymers
Isomerism	organic: common · inorganic: rare	organiccommoninorganicrare

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 4 / 11

Molecular formula, structural formula and isomers

The molecular formula shows only the kind and number of atoms in the molecule, not how they are linked. Examples: CH₄, C₂H₆, C₂H₆O.
The structural (displayed) formula shows the kind and number of atoms and how they are joined by covalent bonds. Each covalent bond represents one valency, and valency is fixed in organic compounds: carbon = 4, hydrogen = 1, oxygen = 2, nitrogen = 3, halogen = 1.
Condensed structural formula of ethanol CH₃–CH₂–OH.

Isomerism

Isomerism→is the existence of several compounds that share one molecular formula but differ in their structural formula and therefore in their physical and chemical properties; such compounds are called isomers.

The example C₂H₆O

Comparison aspect	Main idea	Keywords
Structural formula	ethyl alcohol: CH₃–CH₂–OH · dimethyl ether: CH₃–O–CH₃	ethylalcoholdimethylether
Melting point	−114 °C · −138 °C
Boiling point	78 °C · −24 °C
Reaction with sodium	the alcohol reacts and sodium replaces the hydroxyl hydrogen the ether does not react	alcoholreactsreplaceshydroxyl
•	A structural formula drawn on paper makes the molecule look flat while in reality it is three-dimensional molecular models (coloured balls one colour and size per element) are used to show the true shape	structuralformuladrawnpaper
•	Methane for instance is a tetrahedral molecule with bond angles of 109.5° the four bonds arranged symmetrically to minimise electron-pair repulsion	Methaneinstancetetrahedralmolecule

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 5 / 11

Detecting carbon and hydrogen in an organic compound

Steps

Mix a little of the organic substance (cloth, leather, paper or plastic) with dry copper(II) oxide powder in a heat-resistant test tube.
Heat the tube and pass the vapours first over white anhydrous copper sulphate, then into clear limewater.

White CuSO₄ turns blue because it absorbs water vapour ⟹ the compound contains hydrogen.
Limewater turns milky because CO₂ is liberated ⟹ the compound contains carbon.
CuO oxidises the elements of the compound C + 2CuO → CO₂ + 2Cu, and 2H + CuO → H₂O + Cu.

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 6 / 11

Hydrocarbons and the alkane series

Hydrocarbons→are compounds containing only carbon and hydrogen. They are divided into:

Aliphatic open-chainsaturated (alkanes, CₙH₂ₙ₊₂), unsaturated with a double bond (alkenes, CₙH₂ₙ) or a triple bond (alkynes, CₙH₂ₙ₋₂).

Cyclic aliphaticsaturated cycloalkanes (CₙH₂ₙ) such as cyclohexane.

Aromaticcontaining a benzene ring, such as benzene and toluene.

The alkanes (paraffins)

Alkanes form a homologous series of general formula CₙH₂ₙ₊₂:

Comparison aspect	Main idea	Keywords
•	Methane CH₄ · ethane C₂H₆ · propane C₃H₈ · butane C₄H₁₀ · pentane C₅H₁₂ · hexane C₆H₁₄	MethaneethaneC₂H₆propane
What is observed in the series	each member differs from the next by a –CH₂– unit (14 in molecular mass) they share one general formula and similar chemical properties while physical properties change gradually with mass	memberdiffersnextunit
Importance and sources	alkanes are the main constituents of natural gas and of the light fraction of petroleum used as fuels and as raw material for many chemicals	alkanesmainconstituentsnatural

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 7 / 11

The alkyl radical and IUPAC naming

Comparison aspect	Main idea	Keywords
•	Removing one hydrogen atom from an alkane gives the alkyl radical R– written CₙH₂ₙ₊₁–: CH₃– methyl C₂H₅– ethyl C₃H₇– propyl	Removinghydrogenatomalkane
Steps of IUPAC naming	Choose the longest continuous carbon chain — it gives the parent name (meth- eth- prop- but- pent- hex-… + -ane). Number the chain from the end nearest the first branch so that substituents get the lowest possible numbers. Name the substituents (alkyl groups halogens as fluoro/chloro/bromo/iodo) with their numbers use di- tri- tetra- for repeated groups and separate numbers by commas and numbers from words by hyphens. Arrange substituents alphabetically then give the parent name	Chooselongestcontinuouscarbon
Examples	CH₃–CH₂–CH₂–CH₃ → butane. (CH₃)₂CH–CH₃ → 2-methylpropane (common name: isobutane). CH₃–CHCl–CH₂–CH₃ → 2-chlorobutane. CH₃–CH(CH₃)–CH₂–CH₃ → 2-methylbutane never “3-methylbutane” — the lower number wins	butanemethylpropanecommonname

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 8 / 11

Methane, physical and chemical properties of alkanes

Preparation of methane in the laboratory

Heating sodium acetate with soda lime (NaOH + CaO) CH₃COONa + NaOH —CaO/heat→ CH₄ ↑ + Na₂CO₃. CaO makes the mixture porous and prevents the tube from fusing.
It is collected over water because it is insoluble in water and lighter than air.

The first four members (C₁–C₄) are gases, C₅–C₁₇ are liquids and the higher ones are solids; boiling point rises with molecular mass.
Non-polar , so they are insoluble in water and soluble in organic solvents; they are lighter than water and burn with a luminous flame.

Comparison aspect	Main idea	Keywords
Combustion	CH₄ + 2O₂ → CO₂ + H₂O + heat. Complete combustion gives CO₂ and H₂O incomplete combustion gives CO (poisonous) or free carbon (soot)	heatCompletegivesincomplete
Reaction with halogens — substitution	in diffused sunlight hydrogen atoms are replaced one after another: CH₄ + Cl₂ → CH₃Cl + HCl → CH₂Cl₂ → CHCl₃ → CCl₄. Methane is unreactive in the dark hence the old name paraffin “little affinity”	diffusedsunlighthydrogenatoms
Uses of halogeno-derivatives	CHCl₃ (chloroform) as an anaesthetic and solvent CCl₄ as a solvent and in fire extinguishers Freon in refrigerators and iodoform as an antiseptic	CHCl₃chloroformanaestheticsolvent
Catalytic cracking	long-chain alkanes are broken at about 500 °C with a catalyst into shorter alkanes and alkenes — the industrial route to petrol and to unsaturated hydrocarbons: C₈H₁₈ → C₄H₁₀ + C₄H₈	longchainalkanesbroken
Obtaining fragmented carbon (carbon black)	incomplete combustion of methane gives carbon black used in rubber tyres inks and paints: CH₄ —(limited air)→ C + 2H₂	incompletecombustionmethanegives
Obtaining water gas	passing steam over red-hot coke gives a mixture of CO and H₂ (water gas) used as a fuel and as a source of hydrogen: C + H₂O → CO + H₂	passingsteamovercoke

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 9 / 11

Practice with answers

Which experiment destroyed the vital-force theory, and in what year? — Wöhler, 1828, preparing urea from ammonium cyanate.
Write the formula of the alkane with 10 carbon atoms. — C₁₀H₂₂ (CₙH₂ₙ₊₂).
CH₃–CH(CH₃)–CH₂–CH₂–CH₃. — 2-methylpentane.
CH₃–CCl₂–CH₃. — 2,2-dichloropropane.
C₂H₆ + O₂ → — 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Why is methane collected over water? — Because it is insoluble in water and does not react with it.
What is observed when a burning gas gives soot and carbon monoxide? — Incomplete combustion (limited air).
Distinguish isomers of C₃H₈O. — Propan-1-ol CH₃CH₂CH₂OH and propan-2-ol CH₃CHOHCH₃ (position isomers), plus methoxyethane CH₃–O–C₂H₅ (functional isomer).
Give the reason alkanes are called saturated hydrocarbons. — All carbon–carbon bonds are single covalent bonds, so each carbon carries the maximum number of hydrogens.

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 10 / 11

Ministry exam traps

Comparison aspect	Main idea	Keywords
Exclusions from organic chemistry.	CO₂ carbonates and cyanides are carbon compounds but are studied in inorganic chemistry	carbonatescyanidescarboncompounds
Low melting and boiling points are organic	high ones are inorganic reversing them is the most common objective-question trap	highonesinorganicreversing
Slow reactions are organic	(between molecules) fast ones inorganic (between ions)	betweenmoleculesfastones
Halogenation needs diffused sunlight	and gives substitution alkenes react in the dark by addition. Do not mix the conditions with the reaction type	givessubstitutionalkenesreact
The numbering rule	number from the end giving the substituent the lowest locant not the alphabetically-first group	givingsubstituentlowestlocant
Soda lime, not NaOH alone	is used to prepare methane CaO is there to make the mixture porous and protect the tube	usedpreparemethanemake
Cracking vs distillation	cracking is a chemical breakdown of large molecules fractional distillation is a physical separation	chemicalbreakdownlargemolecules

Introduction to Organic Chemistry, Wöhler’s Theory and Alkanes · 11 / 11