● Organic compounds are broadly defined as chemical substances containing carbon–hydrogen bonds. Historically, chemists distinguished between "organic" substances derived from living organisms and "inorganic" substances derived from mineral sources. This outdated "vital force" viewpoint was overturned in the early 19th century when chemists synthesized urea (an animal waste product) from ammonium cyanate (an inorganic salt). Today, organic chemistry encompasses all carbon-containing compounds except for a handful of simple carbon oxides (carbon monoxide CO, carbon dioxide CO2), carbonates (CO3 2–), carbides, and cyanides, which are often classified under inorganic chemistry.

● Why carbon?
1. Tetravalency: Carbon’s atomic number is 6, with an electron configuration of 1s2 2s2 2p2. In its ground state, carbon has four valence electrons, allowing it to form up to four covalent bonds. This tetravalency leads to a vast array of bonding possibilities—single, double, and triple bonds; chains and rings; branched structures; and complex three-dimensional shapes.
2. C–C bond strength: C–C single bonds have a bond energy of roughly 348 kJ/mol, making them relatively strong compared to many other single bonds. Carbon–carbon double bonds (around 614 kJ/mol) and triple bonds (around 839 kJ/mol) are even stronger. This stability allows carbon frameworks to persist under a wide range of conditions.
3. Catenation: Carbon can bond to itself repeatedly, forming long chains (linear, branched) and cyclic structures. Few other elements exhibit catenation to the same extent (silicon does to some degree but yields fewer stable structures).
4. Variety of hybridization: Carbon can adopt sp3, sp2, and sp hybridization states, leading to tetrahedral, trigonal planar, and linear geometries. This flexibility generates diverse shapes and bonding patterns.
5. Compatibility with heteroatoms: Carbon forms stable bonds to elements such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, and halogens. In doing so, carbon-based molecules can incorporate electronegative atoms that introduce polarity, acidity, basicity, and reactivity.

● Classification of organic compounds
1. Aliphatic compounds: Compounds in which carbon atoms form open chains (straight or branched) rather than rings.
- Alkanes: Saturated hydrocarbons with only single C–C bonds (general formula CnH2n+2). Examples: methane (CH4), ethane (C2H6), isobutane (C4H10).
- Alkenes: Unsaturated hydrocarbons containing at least one C=C double bond (general formula CnH2n). Examples: ethene (C2H4), propene (C3H6), 1-butene (C4H8).
- Alkynes: Unsaturated hydrocarbons containing at least one C≡C triple bond (general formula CnH2n–2). Examples: ethyne (C2H2, acetylene), propyne (C3H4).
2. Alicyclic compounds: Carbon atoms closed into non-aromatic rings but with properties resembling aliphatic compounds. Examples: cyclohexane (C6H12), cyclopentane (C5H10).
3. Aromatic compounds: Compounds containing one or more benzene-like rings with delocalized pi electrons following Hückel’s rule (4n + 2 pi electrons). The prototypical example is benzene (C6H6), with six pi electrons delocalized over a planar hexagonal ring. Derivatives include toluene (methylbenzene), phenol (hydroxybenzene), and naphthalene (two fused benzene rings).
4. Heterocyclic compounds: Rings in which one or more carbon atoms are replaced by atoms such as oxygen, nitrogen, or sulfur. Examples: pyridine (C5H5N), furan (C4H4O), thiophene (C4H4S). Heterocycles are extremely important in pharmaceuticals and biological systems.
5. Functionalized hydrocarbons: Hydrocarbons bearing one or more functional groups (oxygen-, nitrogen-, sulfur-, or halogen-containing substituents) that confer distinct reactivity patterns. Functional groups will be covered in detail in Section 10.2. Examples include alcohols (–OH), aldehydes (–CHO), ketones (C=O in the middle of a chain), carboxylic acids (–COOH), amines (–NH2, –NHR, –NR2), and halides (–Cl, –Br, –I).