Core organic reaction vocabulary

· Homologous series = family of organic compounds with the same functional group, similar chemical reactions, and successive members differing by CH₂.
· Saturated = contains single C–C bonds only; unsaturated = contains multiple bonds, usually C=C.
· Addition = two reactants combine to form one product, usually across a C=C.
· Substitution = one atom/group is replaced by another atom/group.
· Elimination = a small molecule is removed, forming a multiple bond such as C=C.
· Hydrolysis = bond breaking using water or aqueous acid/alkali.
· Condensation = two molecules join with loss of a small molecule, often H₂O or HCl.
· Oxidation in organic chemistry often means gain of O, loss of H, or increase in oxidation number.
· Reduction often means gain of H, loss of O, or decrease in oxidation number.
· In organic redox equations, [O] may represent one oxygen atom from an oxidising agent; [H] may represent one hydrogen atom from a reducing agent.

Bond fission: homolytic vs heterolytic

· Homolytic fission = covalent bond breaks evenly; each atom takes one electron.
· Homolytic fission forms free radicals, shown with a single dot for an unpaired electron.
· Heterolytic fission = covalent bond breaks unevenly; one atom takes both bonding electrons.
· Heterolytic fission forms ions, often a cation and anion.
· Exam tip: link homolytic fission to free-radical mechanisms and heterolytic fission to polar mechanisms involving nucleophiles and electrophiles.

Homolytic fission gives two radicals because each atom takes one electron. Heterolytic fission gives ions because both bonding electrons move to one atom. This distinction is essential for deciding whether a mechanism is radical or polar. Source

Free radicals and free-radical substitution

· A free radical is a species with an unpaired electron.
· Free-radical substitution involves replacement of an atom/group by another through radical steps.
· Initiation = radicals are first formed, usually by homolytic fission.
· Propagation = radicals react to form products and new radicals, allowing a chain reaction to continue.
· Termination = two radicals combine, removing radicals from the reaction mixture.
· Exam tip: always label radical mechanism stages as initiation, propagation, and termination.

Nucleophiles and electrophiles

· A nucleophile is an electron-pair donor attracted to an electron-deficient atom.
· Common nucleophile features: lone pair, negative charge, or high electron density.
· An electrophile is an electron-pair acceptor attracted to an electron-rich region.
· Common electrophile features: positive charge, partial positive charge δ+, or electron deficiency.
· Nucleophilic reactions involve attack by a nucleophile.
· Electrophilic reactions involve attack by an electrophile.
· Exam tip: identify the electron-rich species first; it usually attacks and donates the electron pair.

Curly arrows in organic mechanisms

· Curly arrows show the movement of electron pairs, not atoms.
· A curly arrow must start at a bond or a lone pair of electrons.
· A curly arrow points to where the electron pair moves: usually a new bond, an atom, or a bond being broken.
· Do not start curly arrows from positive charges or empty space.
· In mechanisms, charges and dipoles must be consistent with the direction of electron movement.

Electrophilic addition

· Electrophilic addition is typical of unsaturated compounds with an electron-rich C=C bond.
· The π bond in C=C is electron-rich and can attract an electrophile.
· First step usually forms a carbocation or a positively charged intermediate.
· Second step usually involves attack by a nucleophile to complete addition.
· Overall effect: the C=C double bond becomes a C–C single bond, and atoms/groups add across it.
· Exam tip: the first curly arrow usually starts from the π bond and points towards the electrophile.

The diagram shows electrophilic addition to an alkene. The electron-rich double bond reacts first, then bromide completes the addition. This is a useful model for recognising how C=C bonds undergo addition reactions. Source

Nucleophilic substitution

· Nucleophilic substitution = a nucleophile replaces another atom/group in a molecule.
· The atom/group being replaced is called the leaving group.
· The nucleophile donates an electron pair to an electron-deficient carbon.
· A polar bond such as C–X can make carbon δ+, allowing nucleophilic attack.
· Mechanism focus: show a curly arrow from the nucleophile’s lone pair to the carbon being attacked.
· Exam tip: if a group is substituted by OH⁻, CN⁻, or NH₃, think nucleophilic substitution.

This diagram models nucleophilic substitution using an SN2-style pathway. It shows the nucleophile forming a new bond as the leaving group leaves. It is useful for practising correct curly-arrow direction. Source

Nucleophilic addition

· Nucleophilic addition involves attack by a nucleophile followed by addition to a molecule.
· It is commonly associated with a polar C=O group, where carbon is δ+ and oxygen is δ−.
· The nucleophile attacks the electron-deficient carbonyl carbon.
· The π electrons in C=O move onto oxygen, forming an intermediate.
· A later step often adds H⁺ to the oxygen to form the final addition product.
· Exam tip: first curly arrow usually starts from the nucleophile lone pair and points to the carbonyl carbon.

Comparing the four key mechanism types

· Free-radical substitution: involves radicals, homolytic fission, and stages called initiation, propagation, termination.
· Electrophilic addition: involves attack of an electrophile on an electron-rich C=C bond.
· Nucleophilic substitution: a nucleophile replaces a leaving group.
· Nucleophilic addition: a nucleophile adds to an electron-deficient centre, often a polar C=O group.
· Exam tip: decide mechanism type by identifying the functional group, the attacking species, and whether the reaction is addition or substitution.