Types of Reactions
Organic chemistry involves millions of known compounds, yet the reactions that transform them fall into just a handful of fundamental categories. Understanding reaction types is like learning the grammar of a language — once you know the rules, you can predict and explain an enormous variety of chemical behaviour. The four principal types of organic reactions are substitution, addition, elimination, and rearrangement, each defined by how bonds break and form during the transformation. Every organic reaction involves the movement of electrons, and chemists use curved arrow notation to track these movements through a mechanism. A mechanism is a step-by-step description of how reactants become products, showing which bonds break (and whether homolytically or heterolytically) and which new bonds form. Recognising the reaction type is the first step in working out any mechanism, because each type follows predictable patterns of electron flow. Reaction classification also has practical importance: it guides synthetic planning, helps predict by-products, and explains why certain reagents or conditions are needed. For example, knowing that HBr undergoes addition across a C=C double bond immediately tells you that a saturated product will form, while knowing that a secondary alkyl halide undergoes elimination under strong base tells you to expect an alkene. Mastering these four categories builds the foundation for all of organic chemistry.
- Substitution Reactions
- Addition Reactions
- Elimination Reactions
- Rearrangement Reactions
Substitution Reactions
In a substitution reaction, one atom or group attached to a carbon is replaced by another atom or group. The substrate loses a leaving group and gains a nucleophile (or, in radical substitution, a radical). The overall connectivity of the carbon skeleton is preserved. There are two ionic pathways: SN1 (unimolecular, proceeds via a carbocation intermediate, favoured by tertiary substrates and polar protic solvents) and SN2 (bimolecular, concerted backside attack, favoured by primary substrates and polar aprotic solvents). A classic example is the reaction of bromoethane with hydroxide ion: CH3CH2Br + OH⁻ → CH3CH2OH + Br⁻. In the SN2 mechanism, the nucleophile attacks the back face of the C–Br bond simultaneously as the bromide leaves, inverting the configuration (Walden inversion). Radical substitution (SR) also exists, as in the free-radical bromination of alkanes under UV light, where a bromine radical abstracts a hydrogen atom from the alkane.
Addition Reactions
Addition reactions occur when two reactants combine to form a single product, with no atoms lost. They require a substrate containing a multiple bond (C=C, C≡C, C=O) because the π bond provides the electrons needed to form a new bond with the incoming reagent, and the second new bond forms to the other atom of the multiple bond. Electrophilic addition to alkenes is the most common type at this level. In the addition of HBr to propene (CH3CH=CH2), the π electrons attack the electrophilic H⁺, forming the more stable secondary carbocation (Markovnikov's rule), which then captures Br⁻ to give 2-bromopropane. Syn addition (both groups added to the same face, as in catalytic hydrogenation) and anti addition (opposite faces, as in bromine addition) are important stereochemical distinctions. Nucleophilic addition occurs at carbonyl groups: a nucleophile attacks the electrophilic carbonyl carbon, breaking the π bond.
Elimination Reactions
Elimination reactions are in many ways the reverse of addition: atoms or groups are removed from adjacent carbons to form a new π bond, producing an alkene (or alkyne) and a small molecule such as HX (a hydrogen halide) or H2O. The two principal mechanisms are E1 (unimolecular, two-step, proceeds via a carbocation just like SN1) and E2 (bimolecular, concerted, requires antiperiplanar geometry of the H and leaving group). E2 is highly stereospecific because the C–H and C–LG bonds must be anti-periplanar for the orbitals to overlap correctly during the concerted bond reorganisation. Zaitsev's rule states that the predominant alkene product is the more substituted (more stable) alkene. For example, 2-bromobutane with KOH/ethanol gives mainly but-2-ene (Zaitsev product) rather than but-1-ene. Bulky bases like potassium tert-butoxide favour the Hofmann (less substituted) product by steric considerations.
Rearrangement Reactions
In a rearrangement reaction, the connectivity of a molecule changes — atoms migrate from one position to another, often producing a structural or skeletal isomer as the product. The most important rearrangements at this level involve carbocation intermediates. A 1,2-hydride shift occurs when a hydrogen atom (with its bonding pair of electrons) migrates from a carbon adjacent to a carbocation to the carbocation centre, shifting the positive charge. Similarly, a 1,2-methyl shift involves migration of a methyl group with its electrons. These shifts occur because they convert a less stable carbocation (primary or secondary) into a more stable one (secondary or tertiary). For example, the solvolysis of neopentyl bromide gives rearranged products because the initially formed primary carbocation undergoes a 1,2-methyl shift to give the more stable tertiary carbocation. Recognising potential rearrangements is essential when drawing mechanisms for reactions that go through carbocation intermediates (SN1, E1, electrophilic addition).
The Key Equation
- \ce{CH3CH2Br} — Bromoethane (substrate) (mol): The organic substrate; the C–Br bond is polarised with partial positive charge on carbon, making it susceptible to nucleophilic attack.
- \ce{OH^{-}} — Hydroxide ion (nucleophile) (mol): The nucleophile; donates a lone pair to the electrophilic carbon, forming the new C–O bond.
- \ce{Br^{-}} — Bromide ion (leaving group) (mol): The leaving group; departs with the bonding electron pair as the new C–O bond forms simultaneously. Br⁻ is a good leaving group because it is the conjugate base of a strong acid (HBr).
- \ce{CH3CH2OH} — Ethanol (product) (mol): The substitution product; an alcohol formed when the hydroxide displaces the bromide. In a chiral substrate, the product would show inverted configuration (Walden inversion).
Worked Example
Problem. Classify each of the following reactions as substitution, addition, elimination, or rearrangement, and identify the type of mechanism involved: (a) 2-methylpropene + HBr → 2-bromo-2-methylpropane; (b) 2-bromopropane + KOH/ethanol, heat → propene + KBr + H2O; (c) (CH3)3CBr + H2O → (CH3)3COH + HBr.
Answer. (a) Electrophilic addition (Markovnikov); (b) E2 elimination (anti-periplanar, Zaitsev product); (c) SN1 substitution via tertiary carbocation intermediate.
Origins · 1 minVladimir Vasilyevich Markovnikov and how this was discovered
Markovnikov formulated his famous rule in 1869, stating that in the addition of a hydrogen halide (HX) to an unsymmetrical alkene, the hydrogen atom adds to the carbon that already bears the greater number of hydrogen atoms (i.e., the less substituted carbon), while the halide adds to the more substituted carbon. This empirical observation, later explained by carbocation stability, is fundamental to predicting the regiochemistry of electrophilic addition reactions. Markovnikov was also one of the founders of the Moscow chemical school and contributed to petroleum chemistry. Markovnikov published his rule as part of his doctoral dissertation in 1869 at the age of 31, and he defended the superiority of Russian chemistry against the dominant German school of organic chemistry of his era. His rule was initially controversial but became universally accepted once electronic theory could explain the preferential formation of the more stable carbocation intermediate.
Related Concepts
- Nucleophiles and Electrophiles — Reaction type classification depends critically on identifying whether the reactive species is a nucleophile (electron pair donor) or an electrophile (electron pair acceptor); this distinction determines whether substitution or addition proceeds by SN1/SN2 or electrophilic/nucleophilic pathways.
- Carbocation Stability and Hyperconjugation — Carbocations are key intermediates in SN1, E1, rearrangement, and electrophilic addition reactions. Understanding why tertiary carbocations are more stable than primary ones explains Markovnikov's rule, rearrangement tendency, and the competition between SN1 and E1 pathways.
- Curved Arrow (Electron-Pushing) Notation — Curved arrows are the language used to represent electron flow in all four reaction types. Mastering this notation is essential for drawing mechanisms, tracking bond breaking and bond formation, and distinguishing homolytic from heterolytic cleavage.
- Stereochemistry: Inversion, Retention, and Syn/Anti Addition — Many reaction types produce specific stereochemical outcomes — SN2 gives inversion (Walden inversion), E2 requires anti-periplanar geometry, and catalytic hydrogenation gives syn addition. Understanding reaction types is inseparable from predicting stereochemical results.
- Reactions of Alkenes — Alkenes are the primary substrates for addition and elimination reactions. Studying alkene chemistry consolidates understanding of electrophilic addition (HX, X₂, H₂O, H₂) and the reverse process of elimination, as well as Markovnikov and anti-Markovnikov selectivity.