13.2 Characteristic organic reactions
- Syllabus
- 9701–2028–2029
- Topic
- 13.2
- Level
- AS
Learning objectives
13.2.1Organic reaction terminology• Interpret/use: the following terminology associated with types of organic compounds and reactions:- (a) homologous series- (b) saturated and unsaturated- (c) homolytic and heterolytic fission- (d) free radical, initiation, propagation, termination- (e) nucleophile, electrophile, nucleophilic, electrophilic- (f) addition, substitution, elimination, hydrolysis, condensation- (g) oxidation and reduction (in equations for organic redox reactions, the symbol [O] can be used to represent one atom of oxygen- from an oxidising agent and the symbol [H] to represent one atom of hydrogen from a reducing agent)13.2.2Terminology associated with types• Understand and use the following terminology associated with types of organic mechanisms:- (a) free-radical substitution- (b) electrophilic addition- (c) nucleophilic substitution- (d) nucleophilic addition (in organic reaction mechanisms, the use of curly arrows to represent movement of electron pairs is expected; the arrow should begin at a bond or a lone pair of electrons)
Classify organic species, bond fission and reaction changes
| Term | Diagnostic meaning |
|---|---|
| homologous series | same functional group and general formula; similar chemical properties; successive members differ by CH₂ and show a gradual trend in physical properties |
| saturated | contains no carbon–carbon multiple bond |
| unsaturated | contains at least one carbon–carbon multiple bond |
Homolytic fission splits a covalent bond so that each bonded atom takes one electron, producing two radicals. Heterolytic fission moves both bonding electrons to one atom, producing oppositely charged ions.
A−BA⋅+⋅B
A−BAX++BX−
| Radical term | What happens to radicals |
|---|---|
| free radical | a species with one or more unpaired electrons |
| initiation | radicals are first generated |
| propagation | a radical is consumed and another radical is produced, continuing the chain |
| termination | two radicals combine so that no radical product remains |
A nucleophile donates an electron pair to form a bond; it has an available lone pair or electron-rich bond. An electrophile accepts an electron pair; it is positively charged or electron-deficient. Nucleophilic and electrophilic describe a species' role in a reaction.
| Reaction type | Structural change |
|---|---|
| addition | two reactants form one product, commonly by adding across a multiple bond |
| substitution | one atom or group is replaced by another |
| elimination | atoms or groups are removed and a multiple bond forms |
| hydrolysis | a bond is broken by reaction with water or aqueous hydroxide |
| condensation | two molecules join with elimination of a small molecule such as water |
| oxidation | in the organic patterns used here, oxygen is gained and/or hydrogen is lost |
| reduction | in the organic patterns used here, hydrogen is gained and/or oxygen is lost |
In an organic equation, [O] represents one oxygen atom supplied by an oxidising agent and [H] represents one hydrogen atom supplied by a reducing agent. The brackets are bookkeeping symbols for the agent's contribution, not formulas for free O or H atoms.
Identify four mechanisms from electron source, target and structural change
| Mechanism | Electron source and target | Net structural change |
|---|---|---|
| free-radical substitution | an unpaired electron participates in a radical chain | an atom, commonly H, is replaced |
| electrophilic addition | a C=C π bond donates an electron pair to an electrophile | two groups add across C=C |
| nucleophilic substitution | a nucleophile's lone pair attacks an electron-deficient carbon as the leaving-group bond breaks | one atom or group is replaced |
| nucleophilic addition | a nucleophile's lone pair attacks the δ+ carbon of C=O as the π pair moves to O | the carbonyl gains groups without loss of a leaving group |
A full curly arrow represents movement of an electron pair. Its tail must begin at the electron source—a bond or a lone pair—and its head must point to the atom where a new bond forms or to the atom that receives the pair when a bond breaks.
For nucleophilic substitution of a halogenoalkane, draw one arrow from the nucleophile's lone pair to the carbon bonded to X and a second arrow from the C–X bond to X. The two arrows account for bond formation and bond breaking.
For nucleophilic addition to C=O, draw an arrow from the nucleophile's lone pair to the carbonyl carbon and another from the C=O π bond to oxygen. For electrophilic addition, the first arrow instead begins at the C=C π bond and points to the electrophile.
Do not start a curly arrow at a positive charge or at an atom with no shown electron source, and do not use it as the overall reaction arrow. The mechanism name depends on both the attacking species and whether the net change is addition or substitution.