What is organic chemistry?
Carbon is the only element that combines four bonds, moderate electronegativity, and a willingness to chain with itself into rings, branches, and frameworks — so much so that organic chemistry, the chemistry of carbon compounds, covers millions of known substances while all the other hundred-odd elements combined account for a small fraction of that. The name is a historical leftover: chemists once believed these compounds needed a “vital force” from living things, until Wöhler made the urinary compound urea from inorganic salts in 1828. The backbone of the subject is simple to state: a mostly-inert hydrocarbon skeleton carrying one or more functional groups that do the actual chemistry.
The one rule to remember: the functional group is the personality of a molecule. Every ethanol molecule behaves like every other ethanol molecule because the –OH group — not the two carbons behind it — is doing the reacting.
Functional groups: the reactive personality
The Functional Groups panel lists 18 groups by IUPAC naming priority. Priority here means nomenclature, not reactivity: when a molecule carries several groups, the highest-priority one claims the suffix and the rest are demoted to prefixes — lactic acid, CH3CH(OH)COOH, is officially 2-hydroxypropanoic acid: “oic acid” outranks “ol”. The general formulas use R (any carbon chain) as a spectator, which is exactly the point: replace the R and the group chemistry barely notices.
IUPAC naming is an algorithm
Naming runs in four fixed steps: (1) find the longest continuous chain containing the principal group, (2) number it so the principal group (or first substituent) gets the lowest locant, (3) list substituents alphabetically with position numbers, (4) attach the suffix of the winning group. The Alkane panel's example CH3CH(CH3)CH2CH3 walks the classic: five atoms in the longest chain? No — four, so it is 2-methylbutane, not a pentane. Two groups escape numbering entirely: the aldehyde and carboxyl carbons are always C1 by definition.
Structural isomers: same formula, different architecture
Formula alone determines nothing about structure. C4H10 is butane (straight chain) or 2-methylpropane (branched) — same atoms, different connectivity, different boiling points (−0.5 °C vs −11.7 °C). C2H6O is ethanol (liquid, hydrogen-bonded) or dimethyl ether (gas) — a functional group isomerism so drastic that the two barely resemble each other. The Isomer Types panel sorts all of it: chain, position, functional-group isomerism, plus tautomerism (keto ↔ enol), the dynamic equilibrium that biological chemistry constantly exploits.
Stereoisomers: same atoms, different arrangement in space
Connectivity can be identical and the molecule still has two versions. Geometric (cis/trans, E/Z) isomers need restricted rotation — a double bond or ring — so the pieces stay locked on their sides. Enantiomers are non-superimposable mirror images (your hands): they rotate polarized light in opposite directions and, crucially, can have different biological effects because enzymes and receptors are themselves chiral. Diastereomers are stereoisomers that are not mirror images, and meso compounds carry chiral centers yet cancel internally. Even rotation about single bonds creates conformational isomers — the chair and boat forms of cyclohexane are the famous pair. E/Z naming is the rigorous version of cis/trans: Z (zusammen) puts the high-priority groups on the same side, E (entgegen) opposite — essential when neither end carries identical groups.
Saturated vs unsaturated
Alkanes are saturated: CnH2n+2, every carbon full of hydrogens, chemically sluggish. Add a double or triple bond and the compound becomes unsaturated — CnH2n for one double bond — with a region of high electron density that invites attack. That one difference drives the reaction families: alkenes undergo addition (HBr adds across the double bond, Markovnikov placing H on the less-substituted carbon), while alkanes manage only radical substitution under UV light. Counting hydrogens against CnH2n+2 is the standard way to find rings and multiple bonds from a formula alone.
The mechanism families
Fourteen reaction cards reduce to a few recurring logic patterns. Substitution swaps a leaving group for a nucleophile by two rival routes: SN1 goes through a carbocation in two slow-then-fast steps with first-order kinetics, favored for tertiary substrates in polar protic solvents; SN2 is a one-step backside attack with second-order kinetics and full inversion of configuration, favored for primary substrates in polar aprotic solvents. Addition and elimination are inverses (make or destroy a double bond). Oxidation climbs a ladder — primary alcohol → aldehyde → carboxylic acid — while reduction (NaBH4, LiAlH4, H2/Pt) descends it, each rung a two-electron step in disguise (see Redox).
Common misconceptions
- Organic means natural or living. It means carbon-based. Plastics, gasoline, and aspirin are as organic as sugar; Wöhler's urea synthesis ended the vital-force idea in 1828.
- Isomers have similar properties. Ethanol boils at 78 °C; its isomer dimethyl ether at −24 °C. Different architecture means different physics.
- The priority table ranks reactivity. It ranks naming priority only — an acid chloride is far more reactive than a carboxylic acid despite sitting lower in nomenclature order.
- 2-methylbutane and pentane are the same. Longest continuous chain: the branch does not count. Pentane would need five connected carbons.
Related tools: Spectroscopy (how those functional groups are identified), Chemical Bonding (why C–C chains are stable), and Structure Editor for drawing them.
