These reactions are classified by chemists as substitution, addition, elimination, and rearrangement.
Types of Organic Reactions: Substitution, Addition, Elimination & Rearrangement Explained
Organic chemistry may seem overwhelming, but it turns out to be a lot simpler when you realise that almost all reactions fall into just four basic categories. These reactions are classified by chemists as substitution, addition, elimination, and rearrangement. Because of this, students who grasp these four kinds of organic reactions can confidently predict the products, draw out the mechanisms, and tackle Class 11 and Class 12 problems. The NCERT Class 11 syllabus actually introduces these reaction types at an early stage since they form the basis for all the later topics, ranging from haloalkanes to biomolecules. For this reason, the present guide breaks down the four reaction types step by step, including mechanisms, worked examples, and a comparison table. You will thereby be able to identify the type of any given reaction, since this is precisely what examiners assess. Let’s now look at how bonds break and form.
Key Takeaways: Types of Organic Reactions
- There are four basic kinds of organic reactions: substitution, addition, elimination, and rearrangement.
- Substitution involves replacing one atom or group with another, for example in SN1 and SN2 reactions.
- Addition reactions join two molecules and generally involve the breaking of a π bond, as is the case with alkene additions.
- Reactions involving elimination cause atoms or groups to be remove and as a result form double or triple bonds.
What are Organic Reactions?
In organic reactions old bonds are broken down and new ones are forms. Additionally, whether a reaction takes place via free radicals or ions depends on the way a bond breaks. For this reason, prior to looking at the four types of reaction you should understand the two different kinds of bond cleavage.
Homolytic vs Heterolytic Bond Cleavage
In homolytic cleavage the covalent bond breaks evenly and so each atom retains one electron; this leads to the formation of neutral free radicals. For example, chlorine gas (Cl₂) splits into two chlorine radicals when it is exposes to ultraviolet light. On the other hand, in heterolytic cleavage the bond breaks unevenly with both electrons going to one atom and thus a cation and an anion are produces. For instance, when HBr ionizes, bromine takes both of the bonding electrons and leaves behind a proton (H⁺). Homolysis is therefore typical of free radical reactions while heterolysis is the basis of ionic mechanisms such as SN1, SN2, E1 and E2.
Electrophiles vs Nucleophiles
An electron-poor species is known as an electrophile since it tends to gain electrons; on the other hand, an electron-rich species is a nucleophile and donates its electrons. For instance, H⁺ and Br⁺ are electrophiles, whereas OH⁻ and NH₃ are nucleophiles. It is for this reason that substitution and addition mechanisms are refer to as electrophilic or nucleophilic according to which species carries out the attack first.
The 4 Fundamental Types of Organic Reactions
It is at this stage useful to first summarise all four types before looking at each one in detail. The comparison table given below therefore includes, for each reaction type, the bonds broken, the bonds formed, the typical reagents, and one characteristic feature.
| Reaction Type | Bonds Broken | Bonds Formed | Typical Reagent | Example | Identifying Feature |
| Substitution | 1 σ bond (C–X) | 1 σ bond (C–Y) | NaOH, Cl₂/UV, HNO₃/H₂SO₄ | CH₃Br + NaOH → CH₃OH + NaBr | One atom or group is replaced by another |
| Addition | 1 π bond | 2 σ bonds | HBr, H₂/Pt, H₂O/H⁺, Br₂ | CH₂=CH₂ + HBr → CH₃CH₂Br | Two reactants form one product |
| Elimination | 2 σ bonds | 1 π bond | Alcoholic KOH, conc. H₂SO₄ | C₂H₅OH → CH₂=CH₂ + H₂O | A small molecule is removed |
| Rearrangement | 1 migrating σ bond | 1 σ bond (new position) | Heat, acid, or Lewis acid | Pinacol → Pinacolone | The same atoms rearrange into an isomer |
Reactions involving substitution (SN1, SN2, electrophilic, free radical)
A substitution reaction involves replacing one atom or group in a molecule by another atom or group, so that the general formula is R–X + Y → R–Y + X. In addition, substitution reactions are classified as nucleophilic, electrophilic, or free radical according to the species which is attacking.
Nucleophilic Substitution: SN1 vs SN2
Nucleophilic substitution takes place when a nucleophile replaces a leaving group, for instance a halide in a haloalkane. There are two primary mechanisms. The SN2 mechanism is bimolecular and happens in a single step, involving a strong nucleophile attacking the carbon from the back side and kicking out the leaving group. Therefore, SN2 reactions exhibit second-order kinetics and result in inversion of stereochemistry (known as Walden inversion). An example of an SN2 reaction is CH₃Br reacting with NaOH to give CH₃OH and NaBr. On the other hand, the SN1 mechanism is unimolecular and occurs in two steps: first, the leaving group leaves to produce a carbocation, followed by the nucleophile attacking the planar cation. As a result, SN1 reactions show first-order kinetics and usually yield a racemic mixture since the nucleophile can attack from either side.
Electrophilic Aromatic Substitution
On the other hand, electrophilic aromatic substitution (EAS) replaces a hydrogen on a benzene ring with an electrophile. For instance, nitration uses HNO₃/H₂SO₄ to add an –NO₂ group, whereas halogenation uses Cl₂ or Br₂ with a Lewis-acid catalyst. In addition, Friedel–Crafts alkylation and acylation attach alkyl or acyl groups to the ring.
Free Radical Substitution: Types of Organic Reactions
In contrast, free radical substitution involves replacing a hydrogen atom in an alkane with a halogen; the chlorination of methane takes place in three stages: first, initiation (in which Cl₂ splits into radicals as a result of UV light), then propagation (in which a chlorine radical removes a hydrogen atom and the methyl radical then reacts with further Cl₂), and finally termination (when two radicals combine). The reaction is thus driven by light and proceeds via free radical intermediates.
Solved Examples
The reaction between CH₃Br and NaOH gives CH₃OH and NaBr; this is an example of an SN2 nucleophilic substitution.
Benzene reacts with a mixture of nitric acid and sulphuric acid to give nitrobenzene; this is an electrophilic aromatic substitution reaction.
The reaction between methane and chlorine in the presence of ultraviolet light gives methylene chloride and hydrogen chloride; it is a free radical substitution reaction.
Reactions involving addition (electrophilic, nucleophilic, free radical)
An addition reaction joins two reactants into a single product, and it almost always breaks a π bond while forming two new σ bonds. Consequently, addition reactions are typical of alkenes, alkynes, and carbonyl compounds.
Electrophilic Addition to Alkenes
Electrophilic addition begins when an electrophile attacks the electron-rich π bond of an alkene. For example, HBr adds to ethene in two steps; first, H⁺ attacks the double bond to form the more stable carbocation, and then Br⁻ attaches to that cation. Therefore, Markovnikov’s rule predicts that hydrogen adds to the carbon with more hydrogens, and the halogen adds to the more substituted carbon. Likewise, hydration (H₂O/H⁺) follows the same rule.
Nucleophilic Addition to Carbonyls
In contrast, nucleophilic addition takes place at the polar C=O bond found in aldehydes and ketones; specifically, a nucleophile such as a Grignard reagent or cyanide attacks the carbonyl carbon and forms a new σ bond. This leads to the formation of alcohols, cyanohydrins, and aldol products.
Free Radical Addition (Anti-Markovnikov)
Free radical addition yields the anti-Markovnikov product when peroxides are present. For instance, HBr adds to an alkene in the presence of peroxides so that bromine attaches to the less substituted carbon; this is called the peroxide or Kharasch effect. Hence, the radical pathway reverses the regiochemistry of ordinary electrophilic addition.
Hydrogenation and Halogenation
Hydrogenation adds H₂ across a double bond with a metal catalyst such as Pt, Pd, or Ni. Similarly, halogenation adds Br₂ or Cl₂ across the double bond to give a vicinal dihalide. Therefore, both are simple two-reactant, one-product additions that confirm the addition pattern.
Reactions involving elimination (E1, E2, dehydration, dehydrohalogenation)
Elimination is the opposite of addition since it involves the removal of two atoms or groups from adjacent carbons and results in the formation of a new double or triple bond. As a result, elimination reactions introduce unsaturation and can often turn a haloalkane into an alkene.
E1 vs E2 Mechanism Comparison
E2 elimination is bimolecular and occurs in a single step; in other words, a base removes a β-hydrogen while the leaving group departs at the same time, and the π bond forms. Therefore, E2 shows second-order kinetics and works best with a strong base. In contrast, E1 elimination is unimolecular and occurs in two steps; first, the leaving group leaves to form a carbocation, and then a base removes a proton. Consequently, E1 follows first-order kinetics and favors tertiary substrates. Moreover, Saytzeff’s rule predicts the more substituted alkene as the major product, whereas the Hofmann rule favors the less substituted alkene when the base is bulky.
Dehydration of Alcohols
Water is removed from an alcohol by means of concentrated H₂SO₄ at a temperature of about 170°C; for instance, ethanol produces ethene when the conditions are applied. This process results in the formation of an alkene and water.
Dehydrohalogenation
Dehydrohalogenation is a process in which HX is removed from an alkyl halide by means of alcoholic KOH; for example, bromoethane reacts with alcoholic KOH to give ethene, potassium bromide, and water; it is therefore a standard method for the preparation of alkenes.
Rearrangement Reactions
In a rearrangement reaction the atoms of a molecule are reorganized so that an isomeric product is formed, with no atoms being added or remove. The molecular formula therefore remains unchange even though the way the atoms are connected changes. Indeed, a rearrangement usually results in a structural isomer of the original compound.
Carbocation Rearrangements (1,2-Hydride and 1,2)
A carbocation rearrangement involves the movement of a hydrogen or an alkyl group from one adjacent carbon atom to the carbocation carbon. For instance, a secondary carbocation can shift a hydride in order to form a more stable tertiary carbocation. As a result, rearrangements often occur in SN1 and addition mechanisms when a more stable intermediate can be form is possible.
Beckmann, Pinacol–Pinacolone, and Hofmann Rearrangements
In addition, several named rearrangements are important. Specifically, the Beckmann rearrangement converts an oxime into an amide, the Pinacol–Pinacolone rearrangement converts a 1,2-diol into a ketone, and the Hofmann rearrangement converts a primary amide into an amine. Therefore, students meet these transformations again under named reactions.
How to Identify Reaction Types from a Given Equation
You can identify any reaction type using a simple four-step decision tree. First, count the reactants; if there is only one reactant, it is a rearrangement. Second, if there are two or more reactants, check whether a π bond is present; if yes, it is an addition. Third, if no π bond is present, check whether a small molecule such as water or HX is lost; if yes, it is an elimination. Fourth, if a single atom or group is replace, it is a substitution. Thus, this flowchart works for nearly every Class 11–12 problem.
Practice — Classify These Example Reactions: Types of Organic Reactions
CH₂=CH₂ + H₂O/H⁺ → CH₃CCH₃CH₂Br reacts with KOH(alc.). This forms CH₂=CH₂. This is an elimination reaction.CH₂ → Elimination
CH₃Cl + KOH(aq.) → CH₃OH → Substitution
Pinacol → Pinacolone → Rearrangement
CH₃CH=CH₂ + HBr + peroxide → CH₃CH₂CH₂Br → Addition (anti-Markovnikov)
Reaction Types Comparison Table
The one-page summary table given below combines all four types for revision.
| Type | Bonds Broken | Bonds Formed | Reagent | Product | Example |
| Substitution | 1 σ | 1 σ | NaOH, Cl₂/UV | One group swapped | CH₃Br + NaOH → CH₃OH |
| Addition | 1 π | 2 σ | HBr, Br₂, H₂/Ni | One product | CH₂=CH₂ + HBr → CH₃CH₂Br |
| Elimination | 2 σ | 1 π | KOH(alc.), H₂SO₄ | Alkene + small molecule | C₂H₅OH → CH₂=CH₂ + H₂O |
| Rearrangement | 1 migrating σ | 1 σ (new site) | Heat / acid | Isomer | Pinacol → Pinacolone |
Frequently Asked Questions : Types of Organic Reactions
What are the four primary types of organic reactions?
The four main types of organic reactions are substitution, addition, elimination, and rearrangement. Specifically, substitution swaps a group, addition joins two molecules, elimination removes atoms to form a multiple bond, and rearrangement reorganizes atoms into an isomer.
How can I tell the difference between substitution and elimination?
Consider what occurs to the molecule. In the case of substitution, one group is replaces and the carbon skeleton remains unchange. On the other hand, elimination involves the removal of two groups and results in the formation of a double or triple bond. As a result, whenever a C=C or C≡C bond is present, the reaction must be elimination.
What is Saytzeff’s rule?
According to Saytzeff’s rule the more substituted alkene is the major product of an elimination reaction and so the hydrogen is removes from the carbon which has the fewer hydrogens to give the more stable alkene.
Does Markovnikov’s rule apply to addition or to elimination?
Markovnikov’s rule relates to addition, more precisely to electrophilic addition across a double bond; in this case, the hydrogen bonds to the carbon which has more hydrogens, and the electrophile attaches to the more substituted carbon.
How does the nature of a reaction relate to its mechanism?
A reaction’s mechanism refers to the step-by-step process involved, while its reaction type is the general category it falls into. As a result, each reaction type has a number of associated mechanisms; for instance, substitution encompasses both SN1 and SN2, and elimination includes both E1 and E2.
References
- Determination of reaction kinetics in three phase CO2 methanation” by Mathias Monning, Asad Asadli, Siegfried Bajohr, Moritz Wolf, and Thomas Kolb is associated withis associated with https://doi.org/10.1039/D5RE00337G
This article was written by Juveriya Khan and reviewed for editorial accuracy by our editorial team. It has not yet undergone independent review by a professional.
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