Fundamentals of Organic Chemistry

Organic chemistry is the chemistry of carbon compounds. Carbon is unique because it forms four strong covalent bonds and bonds to itself in endless chains and rings, so millions of compounds are possible. To keep them manageable, they are grouped into homologous series — families sharing the same functional group (the reactive part) and a general formula, with each member differing from the next by CH₂.

The functional group, not the carbon skeleton, dictates a molecule's reactions — so learning families (alkanes, alkenes, alcohols, carboxylic acids…) means learning chemistry once and applying it to thousands of compounds.

Systematic names are built from three parts: a stem for the number of carbons (meth-1, eth-2, prop-3, but-4, pent-5…), a suffix for the functional group (-ane, -ene, -ol, -oic acid…), and prefixes with locant numbers for branches and substituents. So 2-methylbutan-1-ol tells you the chain length, where the branch is, and which functional group sits where.

Isomers share a molecular formula but differ in structure. Structural isomers differ in how the atoms are connected — a different chain, branch position, or functional group (butane vs 2-methylpropane). Stereoisomers have the same connectivity but a different arrangement in space; the common school case is geometric (cis–trans) isomerism around a C=C double bond, which cannot rotate freely.

Isomerism matters because isomers can behave completely differently despite the same formula.

Most organic reactions fall into a few types: addition (atoms add across a double bond), substitution (one group replaces another), elimination (a small molecule is removed to form a double bond), and oxidation/reduction.

How a bond breaks decides the mechanism. Homolytic fission splits a bond evenly, giving free radicals (each atom keeps one electron); heterolytic fission splits it unevenly, giving ions — a nucleophile (electron-pair donor, attacks positive centres) or an electrophile (electron-pair acceptor, attacks negative centres).

A mechanism shows how a reaction happens, step by step, by tracking pairs of electrons. Bonds break either homolytically (one electron to each atom, making radicals — as in the chlorination of alkanes in UV light) or heterolytically (both electrons to one atom, making ions).

The attacking species has a name that matches the target: a nucleophile is electron-rich and attacks electron-poor carbon (as in the substitution of a haloalkane by OH⁻), while an electrophile is electron-poor and attacks electron-rich sites like the C=C double bond (as in the addition of bromine to an alkene). Curly arrows in a mechanism always show the movement of an electron pair.

Structural isomers differ in how atoms are connected; stereoisomers have the same connections but a different arrangement in space. Geometric (cis–trans, or E/Z) isomerism arises at a C=C double bond, because there is no rotation about it — so groups are locked on the same side (cis/Z) or opposite sides (trans/E).

Optical isomerism occurs when a carbon carries four different groups (a chiral centre): the molecule and its mirror image cannot be superimposed, like left and right hands. The two forms rotate plane-polarised light in opposite directions and can behave very differently in the body.

Real problems ask you to turn one compound into another, often in more than one step — so it pays to know the map of functional-group conversions. Alkenes are a common starting point: add water for an alcohol, or bromine for a dihalide. Alcohols are a hub too: oxidise a primary alcohol to an aldehyde and then a carboxylic acid, oxidise a secondary alcohol to a ketone, or dehydrate any alcohol back to an alkene.

Haloalkanes open still more doors — hydroxide gives an alcohol, cyanide adds a carbon to give a nitrile, ammonia gives an amine. Planning a synthesis means picking the shortest sequence of these steps, each with the right reagent and conditions.