Introduction
Organic chemistry — the chemistry of carbon compounds — occupies the second half of this subject. Carbon’s ability to form four strong covalent bonds, including to itself in chains and rings, produces an enormous diversity of compounds, but this diversity is governed by a manageable set of underlying principles covered in this foundational chapter.
1. Hybridization, bonding, and molecular shape
Carbon’s bonding geometry is explained by orbital hybridization: mixing one s and three p orbitals into four equivalent sp³ orbitals gives tetrahedral geometry (single bonds, as in alkanes); mixing one s and two p orbitals into three sp² orbitals (with one unhybridized p orbital left over to form a π bond) gives trigonal planar geometry (double bonds, as in alkenes); mixing one s and one p orbital into two sp orbitals (with two unhybridized p orbitals for two π bonds) gives linear geometry (triple bonds, as in alkynes). A single bond is one σ bond; a double bond is one σ plus one π bond; a triple bond is one σ plus two π bonds — π bonds, formed from sideways overlap of unhybridized p orbitals, are weaker than σ bonds and are the site of most addition reactions (Chapter 12).
2. Electronic effects: inductive, resonance, hyperconjugation
Inductive effects are the shifting of electron density through σ bonds due to electronegativity differences, weakening with distance from the electronegative atom. Resonance (mesomeric) effects involve delocalisation of π electrons or lone pairs across adjacent p orbitals, described by drawing multiple contributing structures (resonance structures) whose “average” better represents the real, delocalised electron distribution than any single structure alone. Hyperconjugation is a weaker stabilising interaction between a σ bond (typically C–H) and an adjacent empty or partially empty p orbital or π system, contributing to the observed stability order of carbocations and alkenes.
3. Acidity and basicity in organic molecules
Organic acidity/basicity (Brønsted–Lowry framework, Chapter 9) depends heavily on how well the resulting conjugate base can stabilise its negative charge — through electronegativity of the attached atom, resonance delocalisation, or inductive electron withdrawal by nearby groups. This is why carboxylic acids (resonance-stabilised conjugate base) are far more acidic than alcohols (no such stabilisation), despite both having an O–H bond.
4. Electrophiles, nucleophiles, radicals, and intermediates
An electrophile (“electron-loving”) is electron-deficient and seeks electron density (e.g., a carbocation or a positively polarised carbon). A nucleophile (“nucleus-loving”) is electron-rich and donates electron density (e.g., a lone pair or π bond). A radical has an unpaired electron and reacts by forming a new bond using that single electron. Reaction intermediates — carbocations, carbanions, radicals — are short-lived, high-energy species formed partway through a mechanism, whose relative stability (influenced by inductive, resonance, and hyperconjugative effects together) determines which reaction pathway is favoured.
5. Basic mechanism language and structural representation
Organic mechanisms are communicated using curved arrows, each representing the movement of a pair of electrons (or, for radical mechanisms, a single electron with a half-arrow) from an electron source to an electron sink. Correct mechanism-drawing requires identifying electron-rich and electron-poor sites in a structure first — a skill this chapter’s concepts (electronic effects, electrophile/nucleophile identification) directly support, and which underlies every mechanism in the remaining organic chapters.
Key Takeaways
- Hybridization (sp³, sp², sp) determines molecular geometry and bond type (σ only, or σ plus one or two π bonds).
- Inductive, resonance, and hyperconjugative effects together determine how electron density is distributed and, consequently, a molecule’s or intermediate’s relative stability.
- Organic acidity/basicity depends on how well the conjugate base or acid stabilises its charge — resonance stabilisation is usually the dominant factor.
- Electrophiles, nucleophiles, and radicals are the fundamental reactive species of organic chemistry; curved-arrow notation tracks electron movement through every reaction mechanism.