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Overview

🌳 Concept tree

Lesson 1→Introduction to Organic Chemistry, Wöhler's Synthesis and Alkanes

├── Historical background

├── Modern definition

├── Why are there so many organic compounds?
organic and inorganic compounds

├── Molecular formula

├── Structural formula

├── Isomerism

├── Detecting carbon and hydrogen in an organic compound

├── Classification of hydrocarbons

├── Structure and the homologous series

├── Alkyl radicals (R–)

├── IUPAC nomenclature of alkanes

├── Methane — occurrence and preparation

├── Physical properties

└── Chemical properties of alkanes

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 1 / 17

Historical background

People have used substances from plants and animals since ancient times.
The ancient Egyptians, for example, used natural dyes, drugs and resins in mummification and in colouring; some of the colours on their temples still shine today.
In 1806 Berzelius divided compounds into two groups:
Organic compounds those obtained from living things (plants and animals);
Inorganic compounds those from mineral sources in the earth.
He believed organic compounds were made by a "vital force" present only in living cells, so they could never be made in a laboratory.
In 1828 the German chemist Wöhler destroyed this idea.
He heated a solution of two inorganic substances, ammonium chloride and silver cyanate.
The reaction first gives ammonium cyanate, which rearranges on heating to urea — a typical organic compound found in the urine of mammals:

AgCNO + NH₄Cl → AgCl↓ + NH₄CNO

NH₄CNO (heat) → H₂N–CO–NH₂ (urea)

This experiment is regarded as the birth of modern organic chemistry.
Chemists soon prepared huge numbers of organic compounds in every field (drugs, dyes, plastics, fibres, perfumes), and most of the organic compounds made today have never existed in any living organism.
So the old definition based on "source" had to be abandoned.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 2 / 17

Modern definition

Organic chemistry is the chemistry of carbon compounds

(hydrocarbons and their derivatives), with the exception of the carbon oxides (CO, CO₂), carbonates, bicarbonates and metal cyanides, which are studied with inorganic chemistry.
Inorganic chemistry deals with all the other elements.

Comparison aspect	Main idea	Keywords
•	One might guess that inorganic compounds are more numerous since they involve all the other elements	mightguessinorganiccompounds
The truth is the opposite	there are more than 10 million known organic compounds (and the number grows daily) whereas inorganic compounds do not exceed about half a million	millionknownorganiccompounds
The ratio is roughly 20

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 3 / 17

Why are there so many organic compounds?

Because of the special properties of the carbon atom:

Carbon can join to other carbon atoms and to atoms of other elements (H, O, N, halogens, S …) by covalent bonds.
It can form single, double and triple bonds (C–C, C=C, C≡C; also C=O, C≡N).
Carbon atoms can link together in many different arrangements: straight chains, branched chains, and rings (homocyclic rings contain only carbon; heterocyclic rings contain carbon plus another atom such as N, O or S).
the same molecular formula can correspond to several different structures, multiplying the number of compounds.

Because there are so many compounds, chemists classify them into families (groups), each with its own functional group and naming rules; we study the important ones in this chapter.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 4 / 17

Comparison: organic and inorganic compounds

Property	Organic compounds	Inorganic compounds
Elements	built mainly on carbon (always C usually H often O halogens)	may contain carbon (e.g. carbonates) but mostly other elements
Bond type	mostly covalent	mostly ionic
Solubility	mostly insoluble in water soluble in organic solvents such as benzene	mostly soluble in water
Melting and boiling points	low	high
Odour	many have characteristic odours	mostly odourless
Combustibility	mostly burn, giving CO₂ and H₂O	mostly do not burn
Electrical conductivity	mostly non-conductors	solutions/melts conduct (ionic)
Reaction rate	slow because reactions occur between molecules	usually fast because reactions occur between ions
Polymerisation	many can polymerise	cannot
Isomerism	common	rare

(You can test these differences using paraffin wax, naphthalene, ethanol, acetone, glycerol as organic samples, and water, table salt and copper sulphate as inorganic ones, comparing solubility, melting point, boiling point, flammability, odour and conductivity.)

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 5 / 17

Molecular formula

The molecular formula shows the number and kind of atoms in one molecule (for example C₂H₆O), but it does not show how the atoms are joined.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 6 / 17

Structural formula

The structural formula shows the number and kind of atoms and how they are linked by covalent bonds.
In organic compounds the valency of each element is constant: carbon is tetravalent (4 bonds), hydrogen is monovalent (1), oxygen is divalent (2), nitrogen is trivalent (3), halogens are monovalent.
Each single bond = one valence unit.
So every carbon in a structural formula must have exactly four bonds around it.
Examples C₂H₆ is CH₃–CH₃; CH₄O is CH₃–OH; C₂H₄ is CH₂=CH₂; C₂H₂ is HC≡CH; C₂H₄Cl₂ can be CH₃–CHCl₂ or ClCH₂–CH₂Cl.
The structural formula is better than the molecular formula because one molecular formula can stand for several different compounds; only the structural formula tells which one is meant and explains its properties.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 7 / 17

Isomerism

Ethanol CH₃CH₂OH — b.p. 78.5 °C, m.p. −117 °C; contains an –OH group; reacts with sodium giving hydrogen.
Dimethyl ether CH₃–O–CH₃ — b.p. −24 °C (a gas at room temperature); no –OH, does not react with sodium.
(The much higher boiling point of ethanol is due to hydrogen bonds between its molecules.)

Further example, C₄H₁₀ has two isomersbutane CH₃CH₂CH₂CH₃ and methylpropane (isobutane) (CH₃)₂CHCH₃.

C₅H₁₂ has threepentane, 2-methylbutane and 2,2-dimethylpropane.

Practicedraw all isomers of C₅H₁₂ (or C₆H₁₄ – there are five).

Structural formulas look flat, but real molecules are three-dimensional (for example the four bonds of carbon in methane point toward the corners of a tetrahedron, H–C–H angle 109.5°).
Molecular models (coloured plastic balls and sticks) are used to picture the true shape.

🧊 3D Tetrahedral Molecular Geometry & sp³ Hybridization of Methane (CH₄) 3D Scientific Model Accurate 3D molecular visualization illustrating carbon sp³ hybridized orbital geometry with tetrahedral bond angle of 109.5°, demonstrating maximum electron-pair separation and minimum electrostatic repulsion.

Isomerism the phenomenon in which different compounds have the same molecular formula but different structural formulas, and therefore different physical and chemical properties.
Classic example, C₂H₆O:

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 8 / 17

Detecting carbon and hydrogen in an organic compound

Experiment mix a small sample of an organic substance (sugar, paper, wool, plastic) with dry copper(II) oxide in a hard-glass tube and heat strongly.
Pass the gases first over anhydrous white copper(II) sulphate and then through lime water.
Observations:

The white anhydrous CuSO₄ turns blue — water vapour was formed (hydrogen of the compound combined with oxygen of CuO).
The lime water turns milky (turbid) — carbon dioxide was formed (carbon of the compound combined with oxygen of CuO).

Conclusion→the compound contains carbon and hydrogen.

C + 2CuO → CO₂ + 2Cu

H + CuO → H₂O + Cu (more exactly→organic H → H₂O)

CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O

CuSO₄ (white) + 5H₂O → CuSO₄·5H₂O (blue)

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 9 / 17

Classification of hydrocarbons

Hydrocarbons

are organic compounds made of carbon and hydrogen only.
All other organic compounds are regarded as derivatives of them (one or more H atoms replaced by another atom or group).
Classification:

Aliphatic (open-chain) hydrocarbons

Saturated: alkanes (only single bonds)
Unsaturated: alkenes (≥ one C=C), alkynes (≥ one C≡C)

Cyclic hydrocarbons

Alicyclic: cycloalkanes (saturated rings)
Aromatic: benzene and compounds related to it

The word "aliphatic" comes from "fat" the earliest known members were obtained from fats (fatty acids), rich in hydrogen.
"Aromatic" compounds were first obtained from pleasant-smelling resins and natural products and contain relatively less hydrogen.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 10 / 17

Structure and the homologous series

Alkanes are open-chain hydrocarbons in which the carbon atoms are joined by single (sigma, σ) bonds only.
The σ bond is strong and hard to break, so alkanes are chemically rather unreactive (they were once called "paraffins", meaning "little affinity").
First ten members:

n	Name	Formula
1	methane	CH₄
2	ethane	C₂H₆
3	propane	C₃H₈
4	butane	C₄H₁₀
5	pentane	C₅H₁₂
6	hexane	C₆H₁₄
7	heptane	C₇H₁₆
8	octane	C₈H₁₈
9	nonane	C₉H₂₀
10	decane	C₁₀H₂₂

Comparison aspect	Main idea	Keywords
Observations	general formula CₙH₂ₙ₊₂ each member differs from the previous by –CH₂– (methylene group) all names end in -ane the prefix shows the number of carbons (meth-1 eth-2 prop-3 but-4 pent-5 hex-6 hept-7 oct-8 non-9 dec-10)	generalformulaCₙH₂ₙmember
Homologous series	a group of organic compounds with the same general formula the same functional group and similar chemical properties whose successive members differ by a CH₂ group and whose physical properties change gradually	grouporganiccompoundssame

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 11 / 17

Alkyl radicals (R–)

An alkyl radical is the group obtained by removing one hydrogen atom from an alkane.
It cannot exist alone.
General formula CₙH₂ₙ₊₁–, symbol R.
The name replaces -ane by -yl: methane → methyl (CH₃–), ethane → ethyl (C₂H₅–), propane → propyl (C₃H₇–), butane → butyl (C₄H₉–).
Examples of compounds: CH₃Cl methyl chloride, C₂H₅Br ethyl bromide, C₃H₇I propyl iodide, C₄H₉Cl butyl chloride.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 12 / 17

IUPAC nomenclature of alkanes

IUPAC = International Union of Pure and Applied Chemistry. Rules:

Choose the longest continuous chain of carbon atoms (straight or bent). Its alkane name is the parent name.
the carbons of that chain starting from the end nearer to the first branch, so the branches get the lowest possible numbers. (If the chain has no branch, number from either end.)
Name = position number(s) – branch name(s) + parent. Example: CH₃–CH(CH₃)–CH₂–CH₂–CH₃ = 2-methylpentane; CH₃CH₂CH(CH₃)CH₂CH₂CH₃ = 3-methylhexane.
If a branch is repeated, use di-, tri-, tetra- and give a number for each: 2,3-dimethylpentane; 3,3-dimethylpentane.
Substituents such as Cl, Br, NO₂ are named chloro, bromo, nitro (ending with o): CH₃–CHBr–CH₃ = 2-bromopropane; CH₃–CCl₂–CH₂–CH₃ = 2,2-dichlorobutane.
If different groups are present, list them alphabetically (ignoring di-, tri-), after numbering so that the sum of the numbers is lowest: e.g. 2-bromo-3-methylbutane; 3-chloro-4,4-dimethylhexane.
Numbers are separated by commas; numbers are separated from words by hyphens.

More examples→(CH₃)₃C–CH₂–CH₃ = 2,2-dimethylbutane; CH₃–CH(CH₃)–CH(CH₃)–CH₂CH₃ = 2,3-dimethylpentane; a nonane chain with ethyl at C4 and methyls at C2 and C7 = 4-ethyl-2,7-dimethyloctane (the longest chain must be octane in that case).

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 13 / 17

Methane — occurrence and preparation

Methane, CH₄, is the simplest organic compound.
It makes up about 50–90 % (often quoted ~93 %) of natural gas, is found with crude oil, in coal mines (where it forms explosive mixtures with air — "fire-damp"), and is released as bubbles from marshes where organic matter decays ("marsh gas").
Laboratory preparation heat anhydrous sodium acetate with soda lime (a mixture of NaOH and CaO) in a hard-glass tube; the gas is collected by downward displacement of water (methane is almost insoluble in water, lighter than air):
CH₃COONa + NaOH —(CaO, heat)→ CH₄↑ + Na₂CO₃

CaO is not a reactant; it keeps the NaOH dry (NaOH is hygroscopic and would attack the glass) and lowers the melting point of the mixture.
Sodium acetate must be anhydrous because water would prevent the reaction.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 14 / 17

Physical properties

The first four alkanes (C₁–C₄) are gases at room temperature; C₅–C₁₇ are liquids (gasoline, kerosene); above about C₁₇ they are waxy solids (paraffin wax). The boiling point rises with the number of carbon atoms (greater molecular mass → stronger intermolecular forces).
Propane (b.p. −42 °C) is more volatile than butane (b.p. −0.5 °C), so cooking-gas cylinders in cold countries contain a larger proportion of propane, while in warm countries they contain more butane.
Alkanes are non-polar, so they are insoluble in water but dissolve in organic solvents. For this reason heavy alkanes (grease, petroleum jelly) are used to coat metals to protect them against rust.
Methane is used as household fuel; propane–butane in cylinders; longer alkanes in petrol, kerosene, diesel, lubricating oils and wax. Alkanes are found in crude oil and are separated by fractional distillation.

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 15 / 17

Chemical properties of alkanes

The strong C–C and C–H σ bonds make alkanes relatively inert; they react only under special conditions.
1. Combustion. All alkanes burn in excess air giving CO₂ and H₂O and a lot of heat (highly exothermic) — hence their use as fuels.

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + heat

(With limited oxygen, carbon monoxide or soot forms.)
2. Substitution by halogens. In ultraviolet light or at about 400 °C, halogens replace hydrogen atoms one by one (substitution). With methane and chlorine, the products depend on the ratio:
CH₄ + Cl₂ —(UV)→ CH₃Cl + HCl (chloromethane, methyl chloride)

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane)

CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (trichloromethane, chloroform)
CHCl₃ + Cl₂ → CCl₄ + HCl (tetrachloromethane, carbon tetrachloride)
With ethane the first product is chloroethane CH₃CH₂Cl, then dichloroethanes (1,1- and 1,2-) etc.
Uses of halogenated alkanes:
Chloroform was used as an anaesthetic, but it is dangerous (dose is hard to control and it can cause death); halothane, CF₃–CHBrCl (2-bromo-2-chloro-1,1,1-trifluoroethane), is the safer modern anaesthetic.
1,1,1-Trichloroethane is used in dry cleaning.
Freons (chlorofluorocarbons, e.g. CF₂Cl₂ dichlorodifluoromethane) were used in refrigerators and air conditioners, as aerosol propellants and cleaning agents, because they are cheap, easily liquefied, non-poisonous and non-corrosive. But they destroy the ozone layer that protects us from harmful ultraviolet radiation, so an international agreement bans them.
3. Thermal (catalytic) cracking. Heating heavy, long-chain petroleum fractions at high temperature and pressure with a catalyst breaks them into smaller molecules, giving (a) short-chain alkanes used as petrol (gasoline) and (b) short-chain alkenes (ethene, propene) used for making polymers and other chemicals. For example:
C₁₂H₂₆ → C₈H₁₈ + C₄H₈ (one possible cracking; the products here: octane + butene)

C₈H₁₈ → C₄H₁₀ + C₄H₈ (octane → butane + butene)

Comparison aspect	Main idea	Keywords
•	4. Other reactions of methane	reactionsmethane
Heated to 1000 °C without air	CH₄ → C + 2H₂ gives finely divided carbon black used for tyres black paint polish and printing ink	givesfinelydividedcarbon
Reaction with steam (nickel catalyst, ~800 °C)	CH₄ + H₂O → CO + 3H₂ gives water gas (synthesis gas) a mixture of carbon monoxide and hydrogen used as a reducing agent and a fuel	giveswatersynthesismixture

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 16 / 17

Part 17

📖

⚖️ Governing Physical Laws & Mathematical Relations:

└── 3. Alkanes (General Formula: CnH2n+2)

Comparison aspect	Main idea	Keywords
├── Nomenclature	IUPAC rules (Longest carbon chain + lowest substituent numbering)	IUPACrulesLongestcarbon
├── Laboratory Preparation of Methane (CH4)	Dry distillation of anhydrous sodium acetate with soda lime	distillationanhydroussodiumacetate
•	│ └── CH₃COONa + NaOH (CaO catalyst) → CH₄(g) + Na₂CO₃	CH3COONaNaOHcatalystNa2CO3
•	├── Halogenation (Free Radical Substitution in UV light)	HalogenationFreeRadicalSubstitution
•	│ └── CH₄ + Cl₂ (UV) → CH₃Cl → CH₂Cl₂ → CHCl₃ (Chloroform) → CCl₄	CH3ClCH2Cl2CHCl3Chloroform
└── Economic Uses	Black carbon (printer ink & car tires) Water gas (CH4 + H2O at 725°C)	Blackcarbonprintertires
•	📋 Master Teacher's Blackboard Rules & Exam Secrets	MasterTeacherBlackboardRules
RULE 1	Soda lime is a mixture of NaOH and CaO. CaO does not enter the reaction but lowers the melting point of the mixture	SodalimemixtureNaOH
RULE 2	In IUPAC naming: Number from the end nearest to the branch. If equidistant number from the end that gives the lowest sum of locants. If still equal follow alphabetical order	IUPACnamingNumbernearest
RULE 3	Alkanes are chemically unreactive at room temperature because all C-C and C-H bonds are strong single sigma (σ) bonds that are difficult to break	Alkaneschemicallyunreactiveroom
EXAM TRAP	Halothane (2-bromo-2-chloro-1 1-trifluoroethane) is a safe modern anesthetic replacing chloroform which was discontinued due to inaccurate dosage fatalities	Halothanebromochlorotrifluoroethane
📊 Detailed Comparison Matrix	IUPAC Nomenclature and Physical Boiling Points of Straight-Chain Alkanes	IUPACNomenclaturePhysicalBoiling

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

Lesson 1 – Introduction to Organic Chemistry, Wöhler's Synthesis and Alkanes
Detailed Academic Lesson Overview:
Organic chemistry is the chemistry of carbon compounds.
Friedrich Wöhler famously demystified the Vital Force Theory in 1828 by synthesizing organic urea from inorganic ammonium cyanate.
Alkanes (paraffins, CnH2n+2) are saturated open-chain hydrocarbons characterized by strong single sigma (σ) bonds.

🌳 Hierarchy Classification & Conceptual Roadmap

Foundations of Organic Chemistry and Saturated Alkanes
├── 1. Demise of Vital Force Theory (Wöhler 1828)
│ └── NH₄Cl + AgCNO → AgCl(s) + NH₄CNO, then Δ → Urea (NH₂-CO-NH2)

├── 2. Structural Characteristics of Carbon

│ ├── Tetravalent carbon forming 4 covalent bonds (sp³
sp²
or sp hybridization)

Alkane Name	Molecular Formula	Physical State at Room Temp	Key Industrial / Household Use
Methane	CH₄	Gas (b.p. -161°C)	Natural gas fuel (>90%)
Propane & Butane	C₃H₈ and C₄H₁₀	Gases (liquefied under pressure)	LPG cylinder gas (Propane in winter Butane in summer)
Octane	C₈H₁₈	Liquid (b.p. 125°C)	Gasoline motor fuel rating component
Paraffin Wax	C20+ long chains	Solid	Candles and protective water-proofing

Lesson 1: Introduction to Organic Chemistry, Wöhler’s Synthesis and Alkanes · 17 / 17