Fundamental Principles of Organic Chemistry

1. Introduction

Organic chemistry is the chemistry of carbon compounds (mainly the hydrocarbons and their derivatives). Originally "organic" meant compounds obtained from living organisms, thought to need a "vital force"; this idea collapsed when Wohler synthesized urea (an organic compound) from an inorganic salt in 1828:

$\ce{NH4OCN ->[\Delta] (NH2)2CO}$

This chapter covers the basic concepts (why carbon is special, classification, nomenclature, functional groups, homologous series) and the fundamental principles (isomerism, and the electronic ideas behind reaction mechanisms).

2. Why Carbon Compounds are Studied Separately

Organic compounds are studied as a separate branch because:

• They are extraordinarily numerous (millions), far outnumbering all inorganic compounds.
• They are all based on carbon and share common features (covalent bonding, combustibility, similar reaction types).
• Their reactions are usually slow, molecular, and give side products, unlike the fast ionic reactions of inorganic chemistry.
• Many share functional groups that behave similarly, allowing systematic study.

2.1 Tetracovalency and catenation of carbon

Two properties of carbon explain the huge number of organic compounds:

• Tetravalency (tetracovalency): carbon has four valence electrons ($2s^2 2p^2$) and forms four covalent bonds, giving stable, saturated structures.
• Catenation: carbon's unique ability to bond to other carbon atoms, forming long chains, branched chains and rings of almost unlimited length. The $\ce{C-C}$ bond is strong and stable, so these skeletons persist.

3. Classification of Organic Compounds

Organic compounds are broadly classified as:

• Acyclic (open-chain / aliphatic): straight or branched chains, e.g. $\ce{CH3-CH2-CH3}$ (propane).
• Cyclic (closed-chain):
  • Alicyclic: rings that behave like aliphatic compounds, e.g. cyclohexane.
  • Aromatic: compounds containing a benzene ring or resembling benzene, e.g. $\ce{C6H6}$.
  • Heterocyclic: rings containing an atom other than carbon (e.g. N, O, S), such as pyridine.