Substitution Reactions: Swapping Atoms in Molecules
Learn how substitution replaces one atom or group with another: free-radical chlorination of methane in UV light, and nucleophilic substitution in haloalkanes.
Quick answer
In a substitution reaction, an atom or group of atoms in a molecule is replaced by a different atom or group. Alkanes undergo free-radical substitution with halogens in ultraviolet light, while haloalkanes undergo nucleophilic substitution, where a nucleophile displaces the halogen.
Example: Methane reacts with chlorine in ultraviolet light to give chloromethane and hydrogen chloride: CH4 + Cl2 → CH3Cl + HCl. One hydrogen atom has been substituted by chlorine — the defining pattern of this reaction family.
What you will learn
- Define substitution and write the general pattern of the reaction.
- Describe the initiation, propagation, and termination stages of free-radical chlorination.
- Explain why ultraviolet light is required for alkane halogenation.
- Describe nucleophilic substitution of a haloalkane by hydroxide ions.
What substitution means
In a substitution reaction, an atom or group of atoms in a molecule is replaced by a different atom or group. The general pattern is R–X + Y → R–Y + X: something leaves, something takes its place.
Substitution is one of the four great organic reaction families, alongside addition, elimination, and rearrangement. Recognizing which family a reaction belongs to is the first step in understanding it.
Two substitution mechanisms dominate introductory organic chemistry: free-radical substitution, which functionalizes unreactive alkanes, and nucleophilic substitution, which transforms haloalkanes into alcohols and other compounds.
Free-radical substitution of alkanes
Methane and chlorine do not react in the dark — but in ultraviolet light, substitution occurs:
CH4 + Cl2 → CH3Cl + HCl
The reaction proceeds as a chain of radical steps, best understood in three stages. In initiation, UV light splits a chlorine molecule into two chlorine radicals (Cl•), each with an unpaired electron. In propagation, a chlorine radical abstracts a hydrogen from methane, forming HCl and a methyl radical (•CH3); the methyl radical then reacts with Cl2 to give chloromethane and regenerate a chlorine radical, continuing the chain. In termination, two radicals combine — Cl• + Cl• → Cl2, for example — ending the chain.
This is a conceptual description of the mechanism, not a laboratory procedure: chlorine and UV chemistry demand professional handling far beyond this article's scope.
Why ultraviolet light is needed
The UV light is not a minor detail — it is the entire reason the reaction happens. In the dark, methane and chlorine simply coexist; no step of the mechanism can begin because the Cl–Cl bond stays intact.
Ultraviolet photons carry enough energy to break Cl–Cl into radicals, supplying the initiation step. Once a few radicals exist, the propagation cycle sustains itself: each cycle consumes one radical and produces one new radical.
This pattern — an energetic initiation producing reactive intermediates that then propagate a chain — recurs throughout radical chemistry, from polymerization to atmospheric reactions.
Further substitution is possible
The reaction does not conveniently stop at chloromethane. CH3Cl still has three hydrogens, and each can be substituted in turn:
CH3Cl → CH2Cl2 (dichloromethane) → CHCl3 (trichloromethane) → CCl4 (tetrachloromethane)
With excess chlorine, the fully substituted product dominates; with excess methane, the monosubstituted product is favored but never exclusive. In practice a mixture always forms.
This is an honest limitation worth remembering: free-radical substitution is excellent for introducing halogens but poor for making one pure product. Industrial processes manage the mixture by controlling reactant ratios and separating the products.
Nucleophilic substitution: the concept
Nucleophilic substitution is a different mechanism for a different substrate. In a haloalkane such as bromoethane, the C–Br bond is polarized — carbon carries a partial positive charge (δ+) because bromine is more electronegative.
A nucleophile — an electron-pair donor such as the hydroxide ion (OH−) — is attracted to this electron-poor carbon. It donates its electron pair to form a new C–O bond while the bromide ion departs:
CH3CH2Br + OH− → CH3CH2OH + Br−
The –OH group has substituted for the –Br atom. No light is needed and no radicals are involved; the driving force is the nucleophile's attraction to the polarized carbon and the stability of the departing halide ion.
Why substitution matters
Substitution reactions are the bridge from unreactive starting materials to useful products. Free-radical substitution is essentially the only way to coax alkanes into reacting, converting petroleum-derived alkanes into haloalkanes.
Nucleophilic substitution then transforms those haloalkanes: hydroxide gives alcohols, cyanide extends carbon chains, ammonia gives amines. Each substitution installs a new functional group with new chemistry.
Together, the two mechanisms form the opening chapters of organic synthesis — the art of building target molecules step by step. Almost every synthetic pathway you will meet begins with a substitution.
Common mistakes
Further substitution continues: chloromethane itself reacts with more chlorine to give dichloromethane, trichloromethane, and tetrachloromethane. In practice a mixture of products forms, which is why this reaction is a poor way to make a single pure product.
In substitution one group replaces another and a byproduct leaves (CH4 + Cl2 → CH3Cl + HCl). In addition two molecules combine into one with nothing left over (CH2=CH2 + Br2 → CH2BrCH2Br). Counting atoms in the products tells them apart.
The nucleophile attacks the carbon atom, which carries a partial positive charge because the electronegative halogen pulls electrons away. The halogen leaves as a halide ion. The site of attack is the electron-poor carbon, not the halogen.
Real-world applications
Chlorination produces chloromethane and related compounds used as industrial solvents and chemical intermediates.
Nucleophilic substitution converts haloalkanes into alcohols, a key step in manufacturing many organic chemicals.
The same substitution pattern appears in biochemistry, where enzymes displace leaving groups to build complex molecules.
Understanding substitution mechanisms lets chemists choose conditions that favor one product over competing reactions.
Frequently asked questions
What is a free radical?
A free radical is a species with an unpaired electron, such as a chlorine atom (Cl•). Radicals are highly reactive because pairing that electron releases energy. Ultraviolet light creates them by splitting Cl2 into two chlorine atoms, and each radical then drives a chain of propagation steps until two radicals combine and terminate the chain.
Why does the reaction need UV light?
Alkanes are unreactive toward chlorine in the dark because no step can get started — the Cl–Cl bond must first be broken into radicals, and that takes energy. Ultraviolet light provides exactly the energy needed to split Cl2, initiating the chain. Once started, the propagation steps sustain themselves.
What is a nucleophile?
A nucleophile ('nucleus-lover') is a species with an electron pair it can donate — either a negative ion like OH− or a neutral molecule with a lone pair like NH3. It is attracted to electron-poor (partially positive) atoms, especially carbon bonded to an electronegative halogen, where it forms a new bond as the halogen departs.
How is nucleophilic substitution different from free-radical substitution?
They differ in mechanism and conditions. Free-radical substitution uses neutral radicals, needs UV light, and occurs with unreactive alkanes. Nucleophilic substitution uses electron-pair donors, needs no light, and occurs at the polarized carbon–halogen bond of haloalkanes. Both replace one group with another, but the particles and pathways are completely different.