Last updated: 21 August 2026
Organic Chemistry can initially look like a long list of reagents, conditions and products to memorise. Reaction mechanisms provide the connection between them. Instead of learning every reaction as an isolated fact, we ask where the electrons are concentrated, where electrons are deficient, and how those electrons can move when two species meet.
This way of thinking explains why an alkene reacts readily with an electrophile while an alkane does not, why benzene undergoes substitution rather than normal addition, and why the same nucleophile may react very differently with a primary halogenoalkane, a tertiary halogenoalkane and chlorobenzene.
Try these five questions before reading the chapter. Use the structures and reaction schemes carefully — Organic Chemistry often becomes much easier once you learn to read what is happening to the molecule.
- 5 diagnostic questions
- Structures and reaction schemes where useful
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The language of organic reaction mechanisms
A reaction mechanism describes the sequence of elementary steps by which reactants are converted into products. The important question is not merely which bonds disappear and which bonds appear. A mechanism also explains where the electrons responsible for those changes come from.
This makes electron density the natural starting point for understanding organic reactivity. Regions containing readily available electrons can donate electron pairs, whereas electron-deficient regions can accept them.
Think electron-rich → electron-poor. In many ionic organic mechanisms, an electron-rich species forms a bond to an electron-deficient centre. The curly arrow starts where the electrons are and points towards where those electrons will be used.
Nucleophiles and electrophiles
A nucleophile is an electron-pair donor. Nucleophiles therefore contain an available source of electron density, commonly a lone pair or a negative charge. Examples include OH⁻, CN⁻, NH₃ and H₂O.
Nucleophile: donates an electron pair to an electron-deficient centre. Look for lone pairs, negative charges or other accessible electron-rich regions.
Electrophile: accepts an electron pair. Look for positive charges or atoms carrying a sufficiently large δ⁺ partial charge.
Free radicals
A free radical is a species containing an unpaired electron. Radicals occur in mechanisms such as the photochemical halogenation of alkanes.
A chlorine radical is written Cl·. The dot represents the unpaired electron. Because radical reactions involve movement of individual electrons, their mechanisms use half-headed arrows rather than the full curly arrows used for electron pairs.
Homolytic and heterolytic bond fission
When a covalent bond breaks, the two electrons that originally formed the bond must go somewhere. The way those electrons are distributed determines whether the process is homolytic or heterolytic.
In homolytic fission, the bonding electron pair is divided equally. Each bonded atom receives one electron, so two radicals are formed.
Heterolytic fission
In heterolytic fission, both electrons from the original covalent bond move to the same atom. Ionic species are therefore produced.
| Bond fission | Distribution of bonding electrons | Typical products |
|---|---|---|
| Homolytic | One electron goes to each atom | Radicals |
| Heterolytic | Both electrons go to one atom | Ions |
| Arrow notation | Half-headed arrows for individual electrons | Full curly arrow for an electron pair |
Primary, secondary and tertiary carbon centres
Several organic mechanisms depend on the degree of substitution of a particular carbon atom. The classification is based on the number of other carbon atoms directly attached to that carbon.
How to classify a carbon centre
A primary carbon is attached to one other carbon, a secondary carbon to two other carbons, and a tertiary carbon to three other carbons. A quaternary carbon is attached to four other carbon atoms.
In 2-methylpropane above, the central carbon is bonded directly to three other carbon atoms. It is therefore a tertiary carbon.
Always classify the carbon involved in the reaction. Do not describe an entire molecule as “primary” or “tertiary” without first identifying the particular carbon centre being discussed.
Why alkanes are comparatively unreactive
Alkanes contain only C—C and C—H σ bonds. These bonds are relatively strong, and they do not normally contain strongly electron-rich or electron-poor centres.
A normal nucleophile therefore has no obvious electron-deficient carbon to attack, while a normal electrophile has no exposed electron-rich π cloud to attack. This is why alkanes are comparatively unreactive towards many laboratory reagents.
Free-radical substitution provides a different pathway
Alkanes can nevertheless react with halogens when ultraviolet radiation is used. The radiation generates highly reactive radicals, allowing the reaction to proceed through free-radical substitution.
The overall equation hides the fact that the reaction actually occurs through a radical chain mechanism consisting of initiation, propagation and termination steps.
Free-radical substitution of alkanes
Chlorination of methane provides the simplest model for understanding a radical chain reaction.
Initiation
Ultraviolet radiation provides sufficient energy for homolytic fission of the Cl—Cl bond.
This produces the radicals required to begin the chain reaction.
Propagation
A chlorine radical removes a hydrogen atom from methane:
The methyl radical then reacts with another chlorine molecule:
Notice that a chlorine radical has been regenerated. The chain can therefore continue through repeated propagation steps.
Termination
Termination occurs when two radicals collide and combine. Because no new radical is produced, the radical chain ends for those species.
Why alkenes undergo electrophilic addition
A carbon–carbon double bond consists of one σ bond and one π bond. The π electron cloud lies above and below the plane of the carbon framework and is comparatively exposed.
Propane contains only σ bonds between its carbon atoms. Propene contains both a σ bond and a π bond between the two alkene carbon atoms. The π electrons are more accessible to an approaching reagent.
The C=C region is therefore electron-rich and attracts electrophiles. The π bond can break while new σ bonds form, producing the characteristic electrophilic addition reactions of alkenes.
The reactive feature of an alkene is the π bond. Saying merely that “a double bond is reactive” does not explain why. The important point is that its π electrons are relatively accessible to electron-deficient species.
Addition of hydrogen halides
Consider the reaction between an alkene and HBr. The H—Br bond is polarised, so hydrogen carries δ⁺ character. The alkene π electrons are attracted to this electrophilic hydrogen.
For an unsymmetrical alkene, more than one carbocation intermediate may appear possible. The more stable carbocation is formed preferentially, so the relative stability of the intermediates helps determine the major organic product.
When propene reacts with HBr, formation of the secondary carbocation is favoured over formation of the primary carbocation. Br⁻ subsequently attacks the secondary carbocation, so 2-bromopropane is the major product.
Carbocation stability and electronic effects
A carbocation contains a positively charged carbon atom. Alkyl groups have an electron-releasing effect that helps reduce the electron deficiency at that carbon centre.
This stability order matters whenever carbocation formation is involved in a reaction mechanism. It can affect both the preferred reaction pathway and the major product formed.
Steric effects
Electronic effects are not the only structural consideration. Large groups around a reaction centre can physically obstruct the approach of a reagent. This is known as steric hindrance.
Electronic and steric effects may therefore favour different pathways. Increasing substitution stabilises a carbocation, but it also makes direct attack at that carbon more crowded.
Why benzene undergoes electrophilic substitution
Benzene contains an electron-rich π system, so electrophiles can be attracted to the ring. However, its six π electrons are delocalised around the ring rather than being confined to three independent C=C bonds.
This delocalisation gives the aromatic system additional stability. A permanent addition reaction would destroy that delocalised structure.
Alkene: electrophilic addition consumes the localised π bond and forms new σ bonds.
Benzene: electrophilic substitution allows the delocalised π system to be restored after the electrophile has been introduced.
Nitration of benzene
In nitration, the attacking electrophile is the nitronium ion, NO₂⁺. It is generated using concentrated nitric acid and concentrated sulfuric acid.
The benzene π system attacks NO₂⁺, temporarily disrupting the aromatic delocalisation. Loss of H⁺ then allows the delocalised π system to reform, producing nitrobenzene.
Substitution preserves aromatic stability. The ring is temporarily disturbed during the mechanism, but the final step restores the delocalised π system.
Why halogenoalkanes undergo nucleophilic substitution
Halogens are more electronegative than carbon, so the C—X bond in a halogenoalkane is polarised.
The carbon bonded to the halogen is therefore electron-deficient. A nucleophile can donate an electron pair to this carbon while the halogen leaves.
In this example, OH⁻ acts as the nucleophile and the bromine-containing group is replaced, converting bromoethane into ethanol.
SN2 substitution
In an SN2 mechanism, nucleophilic attack and departure of the leaving group occur in one concerted step. There is no carbocation intermediate.
The nucleophile approaches from the side opposite the leaving group. This backside attack requires physical access to the carbon centre, so steric hindrance is important.
Primary halogenoalkanes: relatively accessible carbon centres make backside nucleophilic attack easier.
Tertiary halogenoalkanes: bulky alkyl groups surround the carbon centre and strongly hinder direct backside attack.
SN1 substitution
In an SN1 mechanism, the C—X bond first undergoes heterolytic fission, producing a carbocation intermediate. The nucleophile attacks the carbocation in a later step.
Carbocation stability is therefore crucial. Tertiary halogenoalkanes can form substantially more stable carbocations than primary halogenoalkanes, so the SN1 pathway becomes much more favourable for tertiary substrates.
| Feature | SN1 | SN2 |
|---|---|---|
| Mechanistic pattern | Leaving group departs before nucleophile attacks | Attack and leaving-group departure occur together |
| Carbocation intermediate | Present | Absent |
| Favoured structural feature | Stable carbocation | Accessible carbon centre |
| Stereochemical consequence at a chiral reacting carbon | Attack can occur from either face of the planar carbocation | Backside attack gives inversion of configuration |
Stereochemical consequences of SN1 and SN2
The different geometries of the two mechanisms lead naturally to different stereochemical outcomes.
Inversion in SN2
Because an SN2 nucleophile approaches from the side opposite the leaving group, the three-dimensional arrangement at the reacting carbon is inverted during substitution.
Attack from either face in SN1
The carbocation intermediate in an SN1 mechanism is approximately planar around the positively charged carbon. A nucleophile can therefore attack from either side.
When this centre is stereogenic, attack from the two faces can produce both configurations, leading towards racemisation rather than the single inversion characteristic of an idealised SN2 reaction.
Why carbonyl compounds undergo nucleophilic addition
Aldehydes and ketones contain a strongly polar C=O bond. Oxygen is more electronegative than carbon and draws electron density towards itself.
The carbonyl carbon is therefore electron-deficient and can be attacked by a nucleophile.
Addition of HCN
In the reaction with HCN, CN⁻ acts as the nucleophile. The carbon atom of CN⁻ donates an electron pair to the δ⁺ carbonyl carbon.
At the same time as CN⁻ attacks, the π electron pair of the C=O bond moves towards oxygen. The negatively charged oxygen formed is subsequently protonated, producing a hydroxynitrile.
Attack occurs at carbon, not oxygen. The carbonyl oxygen is electron-rich and carries δ⁻ character. The carbonyl carbon carries δ⁺ character and is therefore the electrophilic centre attacked by CN⁻.
This reaction is synthetically useful because the carbon atom of CN⁻ becomes part of the product, increasing the carbon skeleton by one carbon atom.
Why chlorobenzene is much less reactive than chloroalkanes
It is tempting to see the C—Cl bond in chlorobenzene and assume that it should behave like the C—Cl bond in a chloroalkane. The bonding environment is actually very different.
Partial double-bond character
A lone pair on chlorine can interact with the delocalised π system of the benzene ring. As a result, the carbon–chlorine bond has some partial double-bond character.
The C—Cl bond in chlorobenzene is consequently shorter and stronger than a typical C—Cl bond in a chloroalkane, making bond cleavage more difficult.
Chloroalkane: the C—Cl bond is attached to an sp³ carbon and can undergo ordinary nucleophilic substitution under suitable conditions.
Chlorobenzene: the C—Cl bond is attached directly to an aromatic sp² carbon and has partial double-bond character, making normal nucleophilic substitution much more difficult.
The poor reactivity of chlorobenzene should therefore not be explained simply by saying that the benzene ring “blocks” the nucleophile. The strength and character of the C—Cl bond are central to the explanation.
Connecting the characteristic mechanisms
Although the mechanism names are different, the same reasoning can be used throughout Organic Chemistry: identify where electrons are concentrated, identify the electron-deficient centre, and then ask whether the reaction replaces a group, adds across a multiple bond or proceeds through radicals.
| Organic class | Characteristic mechanism | Structural reason |
|---|---|---|
| Alkanes | Free-radical substitution | Radicals provide a reactive pathway despite strong, largely non-polar σ bonds |
| Alkenes | Electrophilic addition | Electron-rich localised π bond attracts electrophiles |
| Arenes | Electrophilic substitution | Substitution allows aromatic delocalisation to be restored |
| Halogenoalkanes | Nucleophilic substitution | Polar C—X bond produces an electron-deficient carbon |
| Aldehydes and ketones | Nucleophilic addition | Polar C=O bond produces an electron-deficient carbonyl carbon |
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Starting a curly arrow from a positive charge. | A curly arrow represents electrons, so it must begin where the electron pair actually exists. | Start from a lone pair, negative charge or covalent bond containing the electrons being moved. |
| Defining a nucleophile simply as a negatively charged species. | Neutral molecules such as NH₃ and H₂O can also donate electron pairs. | Define a nucleophile as an electron-pair donor. |
| Confusing homolytic and heterolytic fission. | They differ in what happens to the two electrons from the broken covalent bond. | Track each bonding electron: one to each atom means homolytic; both to one atom means heterolytic. |
| Saying alkenes react readily merely because they contain a double bond. | This names the structural feature without explaining its electronic significance. | Explain that the π electron cloud is comparatively exposed and attracts electrophiles. |
| Predicting the major product of HX addition without comparing carbocations. | Different protonation routes may produce carbocations of different stability. | Identify the possible carbocation intermediates and favour formation of the more stable one. |
| Saying benzene undergoes substitution because addition is impossible. | Addition would destroy aromatic delocalisation; the issue is energetic favourability rather than absolute impossibility. | Explain that substitution allows the delocalised π system to be restored. |
| Assuming all compounds containing C—Cl undergo nucleophilic substitution equally readily. | The C—Cl bond in chlorobenzene has partial double-bond character and differs from that in a chloroalkane. | Compare the bonding environment of the carbon–halogen bond. |
| Drawing CN⁻ attacking the oxygen of a carbonyl group. | The oxygen carries δ⁻ character and is already electron-rich. | Direct nucleophilic attack towards the δ⁺ carbonyl carbon. |
| Associating inversion with SN1. | SN1 proceeds through a planar carbocation and can be attacked from either face. | Associate inversion specifically with backside SN2 attack. |
What you should be able to do
Identify reactive species
Recognise nucleophiles, electrophiles and radicals from their electron structures, charges and available electron pairs.
Interpret electron movement
Distinguish full curly arrows from half-headed arrows and identify the correct source and destination of moving electrons.
Explain characteristic reactivity
Relate alkane, alkene, arene, halogenoalkane and carbonyl reactivity to their structures and bonding.
Compare SN1 and SN2
Use carbocation stability and steric hindrance to explain why different substrates favour different nucleophilic substitution pathways.
Predict products mechanistically
Use electron density and intermediate stability rather than memorised product patterns alone to predict likely organic products.
Compare similar-looking compounds
Explain why superficially similar bonds, such as C—Cl in chloroethane and chlorobenzene, can show very different chemical behaviour.
These ten questions test whether you can apply the chapter to structures, reactions and unfamiliar examples.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Questions students often ask about organic reaction mechanisms
Once you connect mechanism names to electron movement, the most common questions are usually about recognising the attacking species and explaining why one pathway is favoured over another.
How do I know whether a species is a nucleophile or an electrophile?
Ask whether it can donate or accept an electron pair. A nucleophile donates an electron pair, so lone pairs and negative charges are useful clues. An electrophile accepts an electron pair and is therefore electron-deficient. It may carry a positive charge or simply contain an atom with δ⁺ character.
Where should a curly arrow begin?
A full curly arrow begins at the electron pair that is moving. This may be a lone pair, a negative charge-associated electron pair or a covalent bond. The arrow should never begin at a positive charge simply because that is the atom being attacked.
Why are alkenes much more reactive than alkanes towards electrophiles?
An alkene contains a π electron cloud above and below the carbon framework. These electrons are relatively exposed and provide an electron-rich region that attracts electrophiles. Alkanes contain only relatively strong σ bonds and lack this accessible π electron cloud.
What is the most important difference between SN1 and SN2?
SN1 proceeds through a carbocation intermediate: the leaving group departs before the nucleophile attacks. SN2 is concerted: nucleophilic attack and leaving-group departure occur in the same step. This is why carbocation stability is especially important for SN1, while steric accessibility is especially important for SN2.
Why is chlorobenzene much less reactive than chloroethane towards nucleophiles?
In chlorobenzene, a chlorine lone pair can interact with the aromatic π system, giving the C—Cl bond partial double-bond character. The bond is consequently shorter and stronger than the C—Cl bond in a typical chloroalkane. The chlorine-bearing carbon is also part of an sp² aromatic system rather than an sp³ carbon centre suited to normal SN2 attack.
Organic mechanisms become far less intimidating once you stop treating them as unrelated arrow diagrams. Structure controls electron density, electron density determines which species attack, and the stability and accessibility of the resulting intermediates determine which pathways are favoured. That same reasoning will continue to connect the reactions of individual functional groups throughout A-Level Organic Chemistry.