3.1.2Organic Chemistry
Reaction Mechanisms — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 3 of 55
Part 1 of 5Bond Fission
Every organic mechanism begins by deciding how a covalent bond breaks.
Homolytic fission
- Each atom takes one electron from the shared pair, producing two uncharged radicals;
- it is favoured by UV light or high temperature.
Heterolytic fission
- One atom takes both electrons of the bond, forming a positive ion and a negative ion;
- it is common with polar bonds.
Radical
- A species with an unpaired electron, written with a dot, such as Cl• or •CH₃, which makes it very reactive.
Carbocation
- A positive ion with a carbon atom carrying only three bonds, formed when a leaving group departs with the bonding pair.
Part 2 of 5Attacking Species
Attacking Species
Nucleophile
- An electron-pair donor, such as OH⁻, CN⁻ or NH₃, which is attracted to a carbon atom with a partial positive charge.
Electrophile
- An electron-pair acceptor, such as H⁺ or the δ+ hydrogen in HBr, attracted to a region of high electron density.
Polar bond
- Electronegativity differences make one atom δ+ and the other δ−, and this tells you where a nucleophile or electrophile will attack.
Lone pair
- A nucleophile always has a lone pair that forms a new covalent bond to the electron-poor carbon.
Part 3 of 5Curly Arrows
1
A curly arrow with a full head shows the movement of a pair of electrons, and it always starts from a lone pair or a bond.
2
The arrow ends where the electron pair arrives, either on an atom to make a lone pair or between atoms to make a new bond.
3
A half-headed (fishhook) arrow shows the movement of a single electron and is used only in free radical mechanisms.
4
Charges and partial charges are written on the structures, and the total charge must balance before and after each step.
Part 4 of 5Types of Reaction
Substitution
- One atom or group is replaced by another, for example chlorine replacing hydrogen in an alkane.
Addition
- Two reactants join to form one product, usually across a C=C double bond in an alkene.
Elimination
- A small molecule such as HBr or H₂O is removed from a molecule, leaving a double bond.
Hydrolysis
- A bond is broken by reaction with water or aqueous hydroxide, as in the hydrolysis of esters.
Oxidation and reduction
- Oxidation adds oxygen or removes hydrogen from the carbon; reduction does the reverse.
Part 5 of 5Exam Checklist & Tips
3.1.2 – Key Points for the Exam
- Radical A species with an unpaired electron.
- Nucleophile An electron-pair donor that attacks an electron-deficient carbon.
- Electrophile An electron-pair acceptor that attacks an electron-rich region.
- Heterolytic fission Bond breaking in which both electrons go to one atom, giving ions.
- Homolytic fission Bond breaking in which each atom takes one electron, giving radicals.
Common Mistakes
- Drawing a curly arrow that starts at an atom nucleus instead of at a bond or a lone pair.
- Using a full-headed arrow for radical steps, which must have half-headed arrows for single electrons.
- Describing a nucleophile as attracted to a positive charge only; it is attracted to a δ+ carbon atom as well.
- Forgetting to show the lone pair on the nucleophile before drawing its arrow.
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