Introduction to Organic Chemistry

Organic chemistry is the study of carbon-based compounds, which form the basis of all life on Earth. Carbon's unique ability to form four strong, stable covalent bonds with itself and other elements leads to a vast array of molecules with diverse structures and functions.

### Homologous Series and Nomenclature

A homologous series is a family of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a -CH₂- group. For example, the alkanes (CₙH₂ₙ₊₂) and alcohols (CₙH₂ₙ₊₁OH) are homologous series. As the carbon chain length increases within a series, the boiling points generally increase due to stronger instantaneous dipole-induced dipole forces between molecules.

To name these compounds systematically, we use IUPAC nomenclature. The name is based on the longest continuous carbon chain (the parent alkane), with prefixes and suffixes indicating the position and type of functional groups or side chains. For example, butan-2-ol indicates a four-carbon chain (butan-) with an -OH group (-ol) on the second carbon atom.

### Representing Organic Molecules

We use several types of formulas to represent organic molecules:

* Molecular Formula: Shows the actual number of atoms of each element (e.g., C₄H₁₀).

* Empirical Formula: The simplest whole-number ratio of atoms (e.g., C₂H₅ for C₄H₁₀).

* Structural Formula: Shows the arrangement of atoms. This can be a displayed formula (showing every atom and bond, like H₃C-CH₂-CH₃) or a condensed structural formula.

* Skeletal Formula: A simplified representation where carbon atoms are at the vertices and ends of lines, and hydrogen atoms attached to carbon are omitted for clarity. It is the quickest way to draw complex molecules.

### Bonding and Shape

Covalent bonds in organic molecules are formed by the overlap of atomic orbitals.

* A sigma (σ) bond is formed by the direct, head-on overlap of orbitals, resulting in a single bond with free rotation. For example, in methane (CH₄), carbon undergoes sp³ hybridisation, forming four σ bonds arranged in a tetrahedral shape with bond angles of 109.5°.

* A pi (π) bond is formed by the sideways overlap of parallel p-orbitals, above and below the plane of the σ bond. A C=C double bond, as in ethene (C₂H₄), consists of one σ bond and one π bond. The carbons in ethene are sp² hybridised, leading to a trigonal planar geometry around each carbon with bond angles of approximately 120°. The presence of the π bond prevents rotation around the C=C axis.

### Isomerism

Isomerism occurs when two or more compounds have the same molecular formula but different arrangements of atoms.

* Chain Isomerism: Different arrangements of the carbon skeleton. E.g., butane and methylpropane (both C₄H₁₀).

* Positional Isomerism: The functional group is attached to a different carbon atom. E.g., propan-1-ol and propan-2-ol (both C₃H₈O).

* Functional Group Isomerism: Same molecular formula but different functional groups. E.g., propanal (an aldehyde) and propanone (a ketone) (both C₃H₆O).

* Geometrical (Cis-Trans) Isomerism: Occurs in compounds with restricted rotation around a bond (typically C=C) and where each carbon of the double bond is attached to two different groups. The *cis* isomer has identical groups on the same side of the double bond, while the *trans* isomer has them on opposite sides. E/Z nomenclature is a more rigorous system based on priority rules for the attached groups.

### Introduction to Reaction Mechanisms

A reaction mechanism is a step-by-step sequence of elementary reactions by which an overall chemical change occurs. We use curly arrows to show the movement of a pair of electrons, typically from an area of high electron density (a lone pair or a bond) to an area of low electron density (an atom with a partial or full positive charge).

Bond breaking, or fission, can occur in two ways:

* Homolytic Fission: Each atom in the bond receives one electron from the shared pair, forming two highly reactive free radicals (species with an unpaired electron).

* Heterolytic Fission: One atom takes both electrons from the shared pair, forming a positive ion (cation) and a negative ion (anion).

Key reacting species include:

* Electrophile: An 'electron-loving' species that is an electron pair acceptor, attracted to electron-rich areas (e.g., H⁺, NO₂⁺).

* Nucleophile: A 'nucleus-loving' species that is an electron pair donor, attracted to electron-deficient areas (e.g., OH⁻, NH₃, CN⁻).