Organic Reaction Mechanisms
Organic Reaction Mechanisms
Lesson duration is: 50 minItems in this lesson
Organic Reaction Mechanisms
Open by framing mechanisms as a way to explain how and why organic reactions happen at the particle level, not just what the products are.
Learning goal
I can use curly-arrow notation to explain substitution, addition, elimination and nucleophilic addition mechanisms.
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What words do you associate with curly-arrow mechanisms?
Use this to activate prior knowledge from bonding, polarity, electrophiles, nucleophiles and previous organic reactions. Invite students to notice whether their ideas name particles, bonds, charges or reaction types.
Lesson overview
This KS5 chemistry lesson explores Organic Reaction Mechanisms. Students use curly-arrow notation to explain substitution, addition, elimination and nucleophilic addition mechanisms. It is suitable for A level classroom teaching, independent study or exam revision.
This slide contains the required SEO introduction verbatim.
What is a reaction mechanism?
A reaction mechanism shows the step-by-step movement of electron pairs during a reaction.
It explains:
which bonds break
which bonds form
why particular products are made
Stress that curly arrows show movement of electron pairs, not movement of atoms. This is a common source of error.
The role of curly arrows
Curly arrows always start from a lone pair, a negative charge or a bond.
They point to where the electron pair goes:
to an atom to form a bond
to a bond position between atoms
onto an atom when a bond breaks
A full curly arrow shows the movement of two electrons.
You may briefly mention that fish-hook arrows are used for single electrons in radical chemistry, but they are not the focus here.
Electrophiles and nucleophiles
Mechanisms depend on electron-rich and electron-poor species.
Nucleophile: electron pair donor
Electrophile: electron pair acceptor
Polar bonds create partial charges that guide attack
Link this to prior knowledge from bond polarity and heterolytic fission. Students often remember the definitions better when tied to electron pair donation and acceptance.
Recognising the reacting species
OH⁻
CN⁻
NH₃
H₂O
the C=C bond in alkenes
δ⁺ carbon in C–Br
H⁺ from acids
Br₂ polarised by a double bond
δ⁺ carbon in C=O
Common nucleophiles
Common electrophiles
Clarify that a C=C bond acts as a nucleophile because it contains a region of high electron density.
Where should a full curly arrow start?
A lone pair
An atom label
A partial charge
A product
Checks the key misconception flagged on slides 3 to 4: arrows show electron pair movement, so they start from a bond, lone pair or negative charge, not from an atom symbol. Only one option here is always valid in the wording given.
Electron-rich and electron-poor species
A nucleophile donates an electron pair, while an electrophile accepts one. In alkenes, the C=C bond is a region of high electron density, so it can act as a nucleophile.
All three blanks come directly from slides 5 to 6 and reinforce the particle-level meaning of the terms.
Substitution: the core idea
A substitution reaction happens when one atom or group is replaced by another.
In A level organic chemistry, this often involves a halogenoalkane reacting with a nucleophile.
Typical pattern:
C–X bond present and X leaves as a halide ion.
Keep the focus on nucleophilic substitution rather than free-radical substitution, because the lesson objective centres on curly-arrow mechanisms.
Nucleophilic substitution in halogenoalkanes
Example: bromoethane + OH⁻ → ethanol + Br⁻
Key ideas:
the carbon bonded to Br is δ⁺
OH⁻ is attracted to that carbon
the C–Br bond breaks heterolytically
Mention that the electronegativity of bromine polarises the bond, making the carbon atom susceptible to nucleophilic attack.
How the arrows are drawn
For nucleophilic substitution:
lone pair on OH⁻ → carbon atom
C–Br bond → Br
This shows:
bond formation between O and C
bond breaking between C and Br
Br leaves with the electron pair
Remind students that the arrow must begin at the lone pair on the nucleophile, not at the atom symbol itself.
What affects substitution?
Substitution is favoured when:
a good leaving group is present
the carbon atom is accessible to attack
the nucleophile has a lone pair to donate
At A level, you mainly explain the mechanism by polarity and electron pair movement.
Avoid pushing beyond specification into detailed SN1/SN2 comparison unless the class is ready and it supports exam technique.
In bromoethane + OH⁻, what is the δ⁺ carbon attacked by?
Br⁻
OH⁻
H⁺
H₂O
Tests understanding of nucleophilic substitution rather than simple recall of products. H⁺ and Br⁻ are plausible confusions from other mechanisms.
Why does Br leave as Br⁻ in substitution?
Expected elements: one arrow from a lone pair on OH⁻ to the carbon attached to Br; one arrow from the C–Br bond to Br; explanation that oxygen forms a bond to carbon and bromide leaves with the electron pair.
Addition: the core idea
An addition reaction happens when atoms are added across a multiple bond.
In organic chemistry, the electron-rich C=C bond reacts with an electrophile.
The double bond is not 'attacking' as a whole bond line on paper; its electron pair moves towards the electrophile.
Students often memorise alkene reactions without understanding that the π bond is the source of electron density.
Electrophilic addition to alkenes
Example: ethene + HBr → bromoethane
the C=C bond donates electrons
H in HBr acts as the electrophile
Br⁻ forms when the H–Br bond breaks
State that this is electrophilic addition because the first species accepted by the alkene is an electrophile.
Arrow pattern for HBr addition
The mechanism can be summarised as:
C=C bond → H
H–Br bond → Br
then
Br⁻ → positive carbon
This forms the new C–H bond and then the C–Br bond.
If helpful, mention that the first step produces a carbocation intermediate, though students mainly need the arrow logic and product formation.
Why the double bond reacts
The π bond in a double bond is a region of high electron density.
That makes alkenes attractive to electrophiles.
Compared with a single bond, the π bond is more exposed, so it is easier for reagents to interact with those electrons.
This is the particle-level explanation students need for prediction questions about alkene reactivity.
Why does an alkene react with HBr?
The π bond is electron-rich
The σ bond is electron-poor
Bromide is a strong base
The carbonyl carbon is δ⁺
Draws on slides 11 to 14. Distractors reflect nearby confusions: σ instead of π, bromide's later role, and mixing alkenes with carbonyl chemistry.
Electrophilic addition to ethene
In the addition of HBr to ethene, the C=C bond donates electrons to H. The H–Br bond then breaks to form Br⁻, which can react with the positive carbon.
This anchors the taught arrow sequence without overcomplicating the carbocation step.
Elimination: the core idea
An elimination reaction forms a multiple bond when a small molecule is removed.
For A level organic chemistry, this commonly means removing HBr from a halogenoalkane or H₂O from an alcohol to make an alkene.
Connect elimination to the reverse of addition in terms of structural change, while noting that the mechanism and conditions differ.
Elimination of HBr from a halogenoalkane
Example: 2-bromopropane + OH⁻ → propene + H₂O + Br⁻
Key steps:
OH⁻ acts as a base
a hydrogen is removed from a carbon next to the C–Br carbon
the C–Br bond breaks as the C=C bond forms
Emphasise that in elimination, hydroxide is acting as a base rather than as a nucleophile.
Arrow pattern for elimination
In elimination, the arrows show a linked change:
lone pair on OH⁻ → H
C–H bond → C–C bond
C–Br bond → Br
The result is formation of the double bond.
Students often miss the arrow from the C–H bond to the C–C bond. That arrow is essential because it shows how the π bond forms.
Substitution or elimination?
The same halogenoalkane can give different outcomes.
A level conditions often favour:
aqueous OH⁻ → substitution
ethanolic OH⁻ → elimination
So conditions help determine which mechanism is more likely.
This is an important exam link: reagents and conditions are part of mechanism prediction, not just recall.
Under which conditions is elimination favoured?
Aqueous OH⁻
Ethanolic OH⁻
Dilute HBr
Water only
Checks the exam-relevant conditions contrast on slide 18.
Which part of elimination is easiest to miss when drawing arrows?
OH⁻ to H
C–H to C–C
C–Br to Br
State symbols
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How do conditions change the product?
Expected elements: aqueous OH⁻ tends to give substitution, ethanolic OH⁻ tends to give elimination; students should link the outcome to conditions, not just reagent identity.
Nucleophilic addition: the core idea
A nucleophilic addition reaction happens when a nucleophile adds to the δ⁺ carbon of a carbonyl group.
The C=O bond is polar, so the carbon atom is electrophilic.
This makes aldehydes and ketones react differently from alkenes.
Set up the contrast clearly: alkenes react because of an electron-rich π bond, while carbonyl compounds react because the carbonyl carbon is electron-poor.
Why carbonyl compounds react
In a carbonyl group:
oxygen is more electronegative than carbon
the C=O bond is polar
carbon becomes δ⁺
nucleophiles are attracted to that carbon atom
Tie this back to bond polarity and remind students that the same visual feature, a double bond, does not always react in the same way.
Example: addition of cyanide
Example reaction:
ethanal + CN⁻ → hydroxynitrile
Mechanism outline:
CN⁻ attacks the carbonyl carbon
the C=O electrons move to oxygen
a proton is then added to the oxygen
If naming the product, keep it simple and specification-friendly. The key aim is understanding the electron flow, not advanced nomenclature.
Arrow pattern for nucleophilic addition
The key electron movements are:
lone pair on CN⁻ → carbonyl carbon
C=O bond → O
This forms a new bond to carbon and gives oxygen a negative charge before protonation.
Students often forget the second arrow from the C=O bond to oxygen. Without it, the carbon atom would exceed its usual bonding.
Comparing the four mechanisms
Substitution: one group is replaced
Addition: atoms add across C=C
Elimination: a small molecule is removed to form C=C
Nucleophilic addition: a nucleophile adds to a carbonyl carbon
The mechanism depends on where the electron-rich and electron-poor regions are.
This summary slide helps students classify unfamiliar reactions by pattern before drawing arrows.
Writing stronger mechanism answers
For exam-quality mechanism work:
identify the nucleophile or electrophile first
show every relevant curly arrow clearly
include charges where needed
make sure arrows start and end in the correct place
check that atoms keep sensible bonding
Useful differentiation point: higher-attaining students can be pushed to justify each arrow in terms of electron density and bond polarity.
In nucleophilic addition, CN⁻ attacks which atom?
Carbonyl carbon
Carbonyl oxygen
Hydrogen atom
Bromine atom
Tests the contrast set up on slides 19 to 22: carbonyl compounds react at the δ⁺ carbon because the C=O bond is polar.
Applying nucleophilic addition ideas
In ethanal, the C=O bond is polar, so the carbon atom is δ⁺. When CN⁻ reacts, electrons move from the nucleophile to carbon and the bond electrons move to oxygen before protonation.
This is the applied task for the final mechanism block, using the exact ethanal/cyanide context already taught.
Which mechanism do you now explain best?
Expected elements: substitution replaces one group; addition adds across C=C; elimination removes a small molecule to form C=C; nucleophilic addition adds a nucleophile to a carbonyl carbon. Strong answers may also mention electron-rich and electron-poor regions.
What part of mechanisms still feels hardest?
Use responses to identify whether students found arrow starts/ends, charges, substitution vs elimination conditions, or carbonyl mechanisms most challenging.
One thing you will remember from this lesson
Encourage students to summarise a single durable takeaway, such as 'arrows start at electrons', 'aqueous vs ethanolic', or 'δ⁺ carbonyl carbon'.
Glossary
reaction mechanism - a step-by-step representation of how a reaction happens using electron pair movement
curly arrow - a symbol showing the movement of a pair of electrons
nucleophile - a species that donates a pair of electrons
electrophile - a species that accepts a pair of electrons
heterolytic fission - bond breaking in which both bonding electrons go to one atom
partial charge - a small charge caused by unequal sharing of electrons
leaving group - an atom or group that departs with an electron pair during a reaction
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Glossary
substitution - a reaction in which one atom or group is replaced by another
addition - a reaction in which atoms are added across a multiple bond
elimination - a reaction in which a small molecule is removed to form a multiple bond
nucleophilic addition - addition of a nucleophile to a carbonyl group
carbonyl group - the C=O functional group found in aldehydes and ketones
π bond - the part of a double bond with electron density above and below the bonding axis
carbocation - a positively charged carbon intermediate formed in some mechanisms
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