Last updated: 21 August 2026
Organic Chemistry can initially seem like a long collection of reactions, reagents and conditions to memorise. However, many of these reactions are built from the same underlying ideas. Once you can identify where electron density is concentrated, where it is deficient and how bonds can break or form, unfamiliar reactions become much easier to interpret.
This chapter introduces the common language used throughout A-Level Organic Chemistry: functional groups, degrees of substitution, bond fission, nucleophiles, electrophiles, reaction types, electronic and steric effects, delocalisation and curly arrows. These ideas will repeatedly reappear when individual homologous series and mechanisms are studied in greater detail.
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
- Instant explanation for every answer option
Functional groups: the reactive part of an organic molecule
An organic molecule may contain a large carbon skeleton, but its characteristic reactions are often concentrated around a particular atom, bond or group of atoms. This chemically distinctive part of the molecule is called the functional group.
Compounds containing the same functional group tend to undergo similar reactions even when their carbon skeletons differ. Identifying the functional group therefore gives an immediate clue to the likely chemistry of a molecule.
A functional group is an atom or group of atoms responsible for the characteristic chemical reactions of an organic compound.
Recognising common functional groups
| Homologous series | Functional group | Typical chemical behaviour |
|---|---|---|
| Alkene | C=C | Addition reactions occur across the carbon-carbon double bond. |
| Halogenoalkane | C–X | The polar C–X bond can undergo substitution or elimination. |
| Alcohol | –OH | Alcohols may undergo oxidation, substitution and elimination. |
| Carbonyl compound | C=O | The carbonyl carbon is electron-deficient because the bond is polar. |
| Carboxylic acid | –COOH | Carboxylic acids show acidic behaviour and can undergo condensation reactions. |
Primary, secondary and tertiary carbon atoms
The terms primary (1°), secondary (2°) and tertiary (3°) describe how many other carbon atoms are directly bonded to a particular carbon atom.
Primary carbon, 1°: directly bonded to one other carbon atom.
Secondary carbon, 2°: directly bonded to two other carbon atoms.
Tertiary carbon, 3°: directly bonded to three other carbon atoms.
Quaternary carbon, 4°: directly bonded to four other carbon atoms.
For an alcohol, classify the carbon directly bonded to the –OH group. For a halogenoalkane, classify the carbon directly bonded to the halogen.
How covalent bonds break: bond fission
A covalent bond contains a shared pair of electrons. When the bond breaks, the two electrons may be distributed between the atoms in two fundamentally different ways.
Homolytic fission
In homolytic fission, the bonding pair is divided equally. Each atom receives one electron from the original bond.
The products contain unpaired electrons and are called radicals.
Heterolytic fission
In heterolytic fission, both electrons from the original bond move to the same atom.
Homolytic fission: one bonding electron goes to each atom → radicals are formed.
Heterolytic fission: both bonding electrons go to one atom → ions are formed.
Carbocations
A positively charged organic species in which the positive charge is located on carbon is called a carbocation.
Carbocations are electron-deficient intermediates. Their relative stabilities can influence which reaction pathway is favoured.
Nucleophiles and electrophiles: following electron pairs
Many organic mechanisms can be interpreted by identifying an electron-rich region and an electron-deficient region. New covalent bonds form when an electron pair is donated from the former to the latter.
Nucleophiles
A nucleophile is an electron-pair donor.
A nucleophile possesses an available electron pair that can be used to form a new covalent bond. The species may be negatively charged or neutral.
Electrophiles
An electrophile is an electron-pair acceptor.
Electrophiles are electron-deficient. Some carry a full positive charge, while others contain an atom carrying a partial positive charge, δ⁺.
Nucleophile: electron-rich → donates an electron pair.
Electrophile: electron-deficient → accepts an electron pair.
Bond polarity can create an electrophilic carbon
In a halogenoalkane, the halogen is more electronegative than carbon. Electron density in the C–X bond is therefore drawn towards the halogen, leaving the carbon atom partially positive.
The carbon bonded to Br can therefore act as an electrophilic centre and be attacked by a nucleophile such as OH⁻.
Common types of organic reactions
Organic reactions are classified according to the overall structural change occurring between reactants and products.
| Reaction type | What happens? | Typical example |
|---|---|---|
| Addition | Atoms or groups are added across a multiple bond. | Hydration of an alkene to form an alcohol. |
| Substitution | One atom or group is replaced by another. | Replacement of Br by OH in a halogenoalkane. |
| Elimination | Atoms or groups are removed and a multiple bond is commonly formed. | Dehydration of an alcohol to form an alkene. |
| Condensation | Two molecules join and a small molecule is eliminated. | Formation of an ester and H₂O. |
| Hydrolysis | A bond is broken through reaction involving water. | Hydrolysis of an ester. |
| Oxidation | Often recognised by gain of oxygen and/or loss of hydrogen. | Oxidation of ethanol to ethanoic acid. |
| Reduction | Often recognised by gain of hydrogen and/or loss of oxygen. | Reduction of a carbonyl compound to an alcohol. |
Substitution
In substitution, one atom or group is replaced by another while the main carbon skeleton remains unchanged.
Addition and elimination
Addition and elimination provide a useful contrast. Addition commonly consumes a multiple bond, whereas elimination commonly produces one.
The conversion of ethanol to ethene is an elimination because atoms are removed overall and a C=C bond is formed.
Condensation and hydrolysis
In a condensation reaction, two molecules join while a small molecule such as H₂O is eliminated. Hydrolysis involves bond cleavage through reaction involving water.
Oxidation and reduction
In introductory organic chemistry, oxidation can often be recognised by gain of oxygen and/or loss of hydrogen. Reduction often involves gain of hydrogen and/or loss of oxygen.
Why are some organic species more stable or reactive?
Identifying nucleophiles and electrophiles tells us where electron-pair donation and acceptance may occur. To compare related species, we also need to consider how electron density is distributed and whether a reaction centre can be approached easily.
Electron-donating and electron-withdrawing effects
Groups attached to a reactive centre can change its electron density. Alkyl groups can donate electron density towards a nearby electron-deficient carbon, whereas electronegative atoms or groups can withdraw electron density.
When comparing related organic species, consider the direction in which electron density is shifted and how this affects the stability of the species.
Carbocation stability
A carbocation contains a positively charged and electron-deficient carbon centre. Alkyl groups can donate electron density towards this centre, reducing the concentration of positive charge.
At this level, this helps explain the general stability order:
Delocalisation
Delocalisation occurs when electron density is spread over more than two atoms rather than being confined to one atom or one bond.
Delocalisation generally stabilises a species because electron density or charge is spread over a larger framework.
Steric effects
Reacting species must be able to approach one another closely enough for effective orbital interaction and bond formation.
If a reaction centre is surrounded by bulky groups, these groups may obstruct the approach of another species. This is called steric hindrance.
Electronic effect: changes electron density or the stability of a species.
Steric effect: changes how easily another species can approach the reaction centre.
Curly arrows: the language of organic mechanisms
A reaction equation shows the overall conversion of reactants into products. A mechanism shows how bonds are broken and formed step by step. Curly arrows are used to track the electrons involved.
Full curly arrows
A full curly arrow represents movement of a pair of electrons.
Its tail must begin at the source of the electron pair — normally a lone pair or covalent bond — and its head points towards the destination of that pair.
A curly arrow starts from electrons, not merely from an atom or a charge.
Half-headed or fish-hook arrows
A half-headed curly arrow represents movement of one electron and is therefore used in radical mechanisms.
Full curly arrow: movement of two electrons.
Half-headed curly arrow: movement of one electron.
Curly arrows represent electrons, not atoms
An atom may appear in a different position in the product, but the arrow does not represent the physical path travelled by that atom. It represents the redistribution of electrons responsible for breaking or forming bonds.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Counting every carbon atom when classifying an alcohol as primary, secondary or tertiary. | The classification depends only on the carbon directly bonded to –OH. | Identify the carbon carrying –OH and count its directly bonded carbon neighbours. |
| Thinking homolytic fission produces ions. | Each atom receives one electron, producing radicals. | Associate equal splitting of the bonding pair with radical formation. |
| Thinking heterolytic fission gives one electron to each atom. | That describes homolytic fission. | Move the complete bonding pair to one atom. |
| Defining a nucleophile simply as a negative ion. | Neutral molecules such as NH₃ and H₂O can also donate electron pairs. | Use the definition: an electron-pair donor. |
| Assuming every electrophile carries a full positive charge. | An electrophilic centre may carry only δ⁺. | Look for an electron-deficient centre capable of accepting an electron pair. |
| Classifying reactions by counting reactant and product molecules. | Reaction type depends on the structural change. | Compare the bonds and functional groups before and after reaction. |
| Describing delocalised electrons as jumping rapidly between structures. | Delocalisation describes the actual spreading of electron density. | State that electron density is distributed over several atoms. |
| Using steric hindrance to describe electronic instability. | Steric hindrance concerns physical accessibility. | Ask whether bulky groups obstruct approach to the reaction centre. |
| Starting a curly arrow at a positive charge. | A positive charge is not an electron pair. | Start at the lone pair or covalent bond containing the electrons that move. |
| Using a full curly arrow for movement of one electron. | A full curly arrow represents two electrons. | Use a fish-hook arrow for a single electron. |
What you should be able to do
Recognise functional groups
Identify the part of an organic molecule responsible for its characteristic reactions.
Classify carbon centres
Distinguish primary, secondary, tertiary and quaternary carbon atoms.
Distinguish bond fission
Explain the difference between homolytic and heterolytic bond breaking.
Identify nucleophiles and electrophiles
Use electron-pair donation and acceptance rather than charge alone.
Classify organic reactions
Recognise addition, substitution, elimination, condensation, hydrolysis, oxidation and reduction.
Explain electronic effects
Relate electron donation and withdrawal to stability and reactivity.
Use delocalisation correctly
Relate spreading of electron density or charge to increased stability.
Recognise steric hindrance
Explain how bulky groups can make a reaction centre less accessible.
Interpret curly arrows
Distinguish movement of an electron pair from movement of one electron.
Start mechanism arrows correctly
Begin each arrow at the electrons that actually move.
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
Common questions about organic reaction terminology
These ideas recur throughout A-Level Organic Chemistry, so it is useful to make the distinctions clear before applying them to individual reaction mechanisms.
Does a nucleophile have to be negatively charged?
No. A nucleophile is an electron-pair donor. OH⁻ and CN⁻ are negatively charged nucleophiles, but neutral molecules such as NH₃ and H₂O can also act as nucleophiles because they contain available lone pairs.
Does an electrophile have to carry a full positive charge?
No. An atom carrying a partial positive charge, δ⁺, may also be electrophilic if it is sufficiently electron-deficient to accept an electron pair.
What is the easiest way to distinguish homolytic and heterolytic fission?
Follow the bonding electrons. Homolytic fission gives one electron to each atom and forms radicals. Heterolytic fission gives both electrons to one atom and forms ions.
What is the difference between an electronic effect and a steric effect?
An electronic effect changes the distribution of electron density or the stability of a species. A steric effect concerns physical crowding and how easily another species can approach the reaction centre.
How do I know where to start a curly arrow?
First identify the electrons that move. A full curly arrow usually starts at a lone pair or a covalent bond and points towards the destination of that electron pair.
These terms form the common language of A-Level Organic Chemistry. Instead of treating every reaction as a new fact to memorise, keep asking where the electrons are, which centre is electron-deficient, how bonds are breaking or forming and what factors stabilise the species involved. Those questions provide the foundation for understanding the mechanisms that follow.