Characteristic Organic Reactions

Specification Reference Organic Chemistry, An introduction to AS level organic chemistry 13.2

Quick Notes

Homologous series: A family of compounds with the same functional group and general formula.
Saturated hydrocarbon = only single carbon–carbon bonds
Unsaturated hydrocarbon = contains double/triple carbon–carbon bonds.
Homolytic fission = bond splits evenly and free radicals formed
Heterolytic fission = bond splits unevenly and ions get formed
Free radical reactions involve free radicals (species with an unpaired electron) and have initiation, propagation and termination steps
- Initiation: Free radicals are formed from non-radicals
- Propagation: Chain reaction continues (a radical and non-radical react to form a new radical and non-radical)
- Termination: Radicals combine and stop the reaction
Electrophiles = electron pair acceptors
Nucleophiles = electron pair donors
Reaction types:
- Addition: Two molecules combine to form one product.
- Substitution: One atom or group replaces another in a molecule.
- Elimination: Atoms/groups removed to form a double bond.
- Condensation: Two molecules join, losing a small molecule (e.g. water).
- Hydrolysis: Water breaks a bond in a molecule.
- Oxidation/Reduction: Gain/loss of electrons or oxygen/hydrogen.
Mechanisms:
- Free radical substitution: Involves radicals replacing atoms in molecules (e.g. halogenation of alkanes).
- Electrophilic addition: Electrophile adds to a double bond (e.g. alkenes + HBr).
- Nucleophilic substitution: Nucleophile replaces a leaving group (e.g. halogenoalkanes + OH⁻).
- Nucleophilic addition: Nucleophile adds to a polar double bond (e.g. aldehydes + H⁻).
Curly arrows (↷) are used in mechanisms to show electron pair movement.

Full Notes

Key Terms in Organic Chemistry

A homologous series is a family of organic compounds that:

Have the same general formula.
Contain the same functional group.
Have similar chemical properties.
Show a gradual change in physical properties (e.g., boiling points increase with chain length).

Examples of Homologous Series:

Homologous series	Functional group	General formula	Example (formula)	Example (name)
Alkanes	None	C_nH_2n+2	C₄H₁₀	Butane
Alkenes	C=C	C_nH_2n	C₃H₆	Propene
Alcohols	–OH	C_nH_2n+2O	C₂H₆O	Ethanol
Halogenoalkanes	–X (Cl, Br, I)	C_nH_2n+1X	C₂H₅Cl	Chloroethane
Aldehydes	–CHO	C_nH_2nO	C₂H₄O	Ethanal
Ketones	>C=O	C_nH_2nO	C₃H₆O	Propanone
Carboxylic acids	–COOH	C_nH_2nO₂	C₂H₄O₂	Ethanoic acid
Esters	–COO–	C_nH_2nO₂	C₃H₆O₂	Methyl ethanoate
Amines	–NH₂	C_nH_2n+3N	C₂H₇N	Ethylamine
Nitriles	–C≡N	C_nH_2n−1N	C₃H₅N	Propanenitrile

Saturated and Unsaturated

Diagram contrasting saturated (single bonds) vs unsaturated (double/triple bonds) hydrocarbons.

Saturated: Only single bonds between carbon atoms (e.g. alkanes).
Unsaturated: Contains double or triple bonds between carbon atoms (e.g. alkenes, alkynes).

Bond Fission

Covalent bonds can break in two ways.

Homolytic fission: Each atom takes one electron from the bond and free radicals get formed (species with an unpaired electron).

Homolytic fission forming free radicals.

Heterolytic fission: One atom takes both electrons and ions get formed.

Free Radical Reactions

Free radical substitution has been covered in more detail here and below

Free radical substitution involves Free Radicals (species with an unpaired electron) and occurs in three stages:

Initiation: Free radicals are formed from non-radicals
Propagation: Chain reaction continues (a radical and non-radical react to form a new radical and non-radical)
Termination: Radicals combine and stop the reaction

Nucleophiles and Electrophiles

Nucleophile: Electron pair donor (has a lone pair of electrons and is often negatively charged)
Examples: OH⁻, NH₃, CN⁻

Electrophile: Electron pair acceptor (positive or δ⁺)
Examples: H⁺, Br⁺, NO₂⁺

Reaction Types

Addition: Two molecules combine to form one
(e.g. alkene + Br₂ → dibromoalkane)
Substitution: One atom/group replaces another
(e.g. halogenoalkane + OH⁻ → alcohol + halide)
Elimination: One molecule splits into two, forming a small molecule like H₂O or HBr
Hydrolysis: Reaction with water to break bonds
(e.g. ester + H₂O → carboxylic acid + alcohol)
Condensation: Two molecules join with loss of water
Oxidation/Reduction in organic chemistry: Use [O] to show addition of oxygen from an oxidising agent; use [H] to show addition of hydrogen from a reducing agent

Organic Mechanisms

How bonds are broken and formed in reactions can be represented using curly arrows in models called mechanisms. Curly arrows show the movement of electron pairs.

The formation of a covalent bond is shown by a curly arrow that starts from a lone pair of electrons or from the middle of a bond.
The breaking of a covalent bond is shown by a curly arrow starting from the middle of the bond.

Example: Nucleophilic Substitution (Halogenoalkanes and OH⁻)

Curly arrow mechanism: nucleophilic substitution of a halogenoalkane with OH−.

Curly arrow shows the C–Br bond breaking, with Br⁻ leaving. Nucleophile (OH⁻) attacks positively charged carbon. Curly arrow shows the lone pair forming a new bond.

Free-Radical Substitution

Free radical species gets ‘substituted’ into a molecule, replacing another atom or group.

Example: Chlorination of alkanes

Chlorination of methane via free-radical substitution.

Initiation, propagation, termination steps in free-radical substitution.

Electrophilic Addition

Electrophile gets ‘added’ into a molecule.

Example: Electrophilic addition of hydrogen halides to alkenes.

Electrophilic addition of HBr to an alkene with curly arrows.

Double bond attacks electrophile (e.g. H⁺), followed by addition of Br⁻.

Nucleophilic Substitution

Nucleophile species gets ‘substituted’ into a molecule by replacing another atom or group bonded to an electron deficient carbon atom.

Example: Nucleophilic substitution of hydroxide ions with halogenoalkanes.

Nucleophilic substitution of a halogenoalkane by hydroxide.

Lone pair from OH⁻ attacks δ⁺ carbon; Br⁻ leaves.

Nucleophilic Addition

Nucleophile species gets ‘added’ into a molecule by bonding to an electron deficient carbon atom.

Example: Nucleophilic addition of a hydride ion to an aldehyde.

Nucleophilic addition of hydride to an aldehyde.

Nucleophile (H⁻) adds to carbonyl carbon.

Summary

Homologous series share a functional group and general formula; properties change gradually with chain length.
Saturated = only single C–C bonds; unsaturated = contains C=C/C≡C.
Homolytic vs heterolytic fission form radicals vs ions, respectively.
Free-radical reactions proceed via initiation, propagation, termination.
Nucleophiles donate pairs; electrophiles accept pairs.
Key reaction types: addition, substitution, elimination, condensation, hydrolysis, redox.
Mechanisms you must know: free-radical substitution, electrophilic addition, nucleophilic substitution, nucleophilic addition — shown with curly arrows (↷).