Introduction to Organic Chemistry, Wöhler's Theory and Alkanes

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.

Berzelius's division (1806)

• Organic compounds: extracted from a plant or animal origin.
• Inorganic compounds: obtained 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.

• 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 reason

• 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.

• 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.

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. Methane, for instance, is a tetrahedral molecule with bond angles of 109.5°, the four bonds arranged symmetrically to minimise electron-pair repulsion.

Steps

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

Observations, explanation and conclusion

• 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.

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

• Aliphatic open-chain: saturated (alkanes, CₙH₂ₙ₊₂), unsaturated with a double bond (alkenes, CₙH₂ₙ) or a triple bond (alkynes, CₙH₂ₙ₋₂).
• Cyclic aliphatic: saturated cycloalkanes (CₙH₂ₙ) such as cyclohexane.
• Aromatic: containing a benzene ring, such as benzene and toluene.

The alkanes (paraffins)

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

• Methane CH₄ · ethane C₂H₆ · propane C₃H₈ · butane C₄H₁₀ · pentane C₅H₁₂ · hexane C₆H₁₄.
• 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.
• 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.

• Removing one hydrogen atom from an alkane gives the alkyl radical R–, written CₙH₂ₙ₊₁–: CH₃– methyl, C₂H₅– ethyl, C₃H₇– propyl.
• Steps of IUPAC naming:
  1. Choose the longest continuous carbon chain — it gives the parent name (meth-, eth-, prop-, but-, pent-, hex-… + -ane).
  2. Number the chain from the end nearest the first branch so that substituents get the lowest possible numbers.
  3. 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.
  4. Arrange substituents alphabetically, then give the parent name.
• 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.

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.

Physical properties

• 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.

Chemical properties

1. Which experiment destroyed the vital-force theory, and in what year? — Wöhler, 1828, preparing urea from ammonium cyanate.
2. Write the formula of the alkane with 10 carbon atoms. — C₁₀H₂₂ (CₙH₂ₙ₊₂).
3. Name: CH₃–CH(CH₃)–CH₂–CH₂–CH₃. — 2-methylpentane.
4. Name: CH₃–CCl₂–CH₃. — 2,2-dichloropropane.
5. Complete and balance: C₂H₆ + O₂ → — 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
6. Why is methane collected over water? — Because it is insoluble in water and does not react with it.
7. What is observed when a burning gas gives soot and carbon monoxide? — Incomplete combustion (limited air).
8. Distinguish isomers of C₃H₈O. — Propan-1-ol CH₃CH₂CH₂OH and propan-2-ol CH₃CHOHCH₃ (position isomers), plus methoxyethane CH₃–O–C₂H₅ (functional isomer).
9. 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.