Difficulty: Introductory
Prerequisites: General chemistry (atomic structure, bonding, electronegativity). Familiarity with Lewis structures and functional groups will help.
This topic is the gateway to understanding how and why organic molecules react. Before you can predict products, propose synthesis routes, or interpret lab results, you need a working model of reaction types and the electron-level mechanics that drive them. The four reaction categories (addition, elimination, substitution, rearrangement) and the two bond-breaking modes (homolytic vs. heterolytic) form the scaffolding for nearly everything else in organic chemistry. If general chemistry gave you the atoms, this is where you learn what the atoms do when they meet.
Organic reactions fall into four types: addition, elimination, substitution and rearrangement. Bonds break either evenly (homolytic, producing radicals) or unevenly (heterolytic, producing nucleophiles and electrophiles). The stability of intermediates, especially carbocations, determines which products form and how fast, and energy diagrams map the whole journey from reactants to products.
Addition reaction
A reaction in which two reactants combine to form a single product, with no atoms lost. Think of it as: two molecules merging into one.
Elimination reaction
A reaction in which a single reactant loses atoms or groups to split into two products, often forming a double bond. In simple terms, this means a molecule sheds pieces to become simpler or more unsaturated.
Substitution reaction
A reaction in which two reactants exchange parts to form two new products. Think of it as: swapping one group for another on a molecule.
Rearrangement reaction
A reaction in which bonds within a single reactant reorganise to produce an isomeric product (same molecular formula, different structure). In simple terms, this means the molecule reshuffles its own atoms without gaining or losing any.
Homolytic cleavage
Bond breaking in which each fragment retains one electron from the shared pair, producing two radicals. Think of it as: splitting the bond down the middle, one electron each.
Heterolytic cleavage
Bond breaking in which both electrons from the shared pair go to one fragment, producing a cation and an anion. In simple terms, this means one side keeps both electrons and the other side walks away empty-handed.
Radical (free radical)
A highly reactive species with an unpaired electron, formed by homolytic cleavage. Think of it as: an atom or group that desperately wants to pair up its lone electron.
Nucleophile (Nu⁻)
An electron-rich species that donates an electron pair to form a new bond. In simple terms, this means the "electron giver" in a polar reaction.
Electrophile (E⁺)
An electron-poor species that accepts an electron pair to form a new bond. Think of it as: the "electron grabber" in a polar reaction.
Carbocation
A positively charged carbon intermediate. Stability increases with substitution: methyl < primary (1°) < secondary (2°) < tertiary (3°). In simple terms, this means a carbon atom carrying a positive charge because it has lost a bond and the electrons that went with it.
Hydride shift
Migration of a hydrogen atom (with its bonding electrons) from an adjacent carbon to a carbocation centre, increasing stability.
Alkyl (methyl) shift
Migration of an alkyl group (with its bonding electrons) from an adjacent carbon to a carbocation centre.
Activation energy (Eₐ)
The minimum energy required for reactants to reach the transition state and proceed to products. Think of it as: the energy hill you have to push the reaction over.
Gibbs free energy change (ΔG)
The difference in free energy between products and reactants. A negative ΔG means the reaction is thermodynamically favourable (exergonic); positive means unfavourable (endergonic).
Transition state
The highest-energy arrangement of atoms along the reaction pathway. It is not an isolable species, only a fleeting geometry at the top of the energy barrier.
Reaction intermediate
A transient species formed during a multi-step reaction that exists between two transition states. Unlike transition states, intermediates sit in energy wells and have a finite (though often very short) lifetime.
Addition: Two reactants → one product. Typical of alkenes and alkynes, where a pi bond breaks to accommodate new atoms.
Elimination: One reactant → two products. The reverse of addition: atoms or groups leave, often generating a new pi bond.
Substitution: Two reactants → two products. One group on a molecule is swapped for another. Common in alkyl halides.
Rearrangement: One reactant → one isomeric product. Bonds reorganise within the molecule. Driven by the formation of a more stable intermediate (usually a more substituted carbocation).
Every organic reaction begins with breaking a bond. The way the bond breaks determines the type of reactive species produced:
Homolytic cleavage splits the bonding pair evenly, one electron to each fragment. Both fragments become radicals. Drawn with fishhook (single-barbed) arrows.
Heterolytic cleavage gives both bonding electrons to one fragment. One fragment becomes a cation, the other an anion. Drawn with full (double-barbed) curved arrows.
Homolytic cleavage drives radical reactions. Heterolytic cleavage drives polar reactions. Knowing which type of cleavage is at work tells you immediately which mechanistic framework to apply.
Radical reactions proceed through three stages:
Initiation: Energy input (UV light, heat, or a chemical initiator such as a peroxide) breaks a bond homolytically to generate the first radicals.
Propagation: Each radical reacts with a stable molecule to form a new radical, creating a self-sustaining chain. This is where most of the product forms.
Termination: Two radicals combine, destroying the chain carriers and ending the reaction.
Radical reactions are chain reactions: once initiated, the propagation cycle can repeat thousands of times before termination occurs.
Polar reactions are the most common class of organic reactions. They arise from heterolytic cleavage and are governed by the interaction of two complementary species:
Nucleophiles are electron-rich. They have a lone pair or a pi bond to donate. Examples: OH⁻, NH₃, H₂O, CN⁻.
Electrophiles are electron-poor. They have an empty or partial-positive site that can accept electrons. Examples: H⁺, carbocations, carbonyl carbons.
The fundamental motion in every polar mechanism is the same: electrons flow from the nucleophile to the electrophile. Curved arrows in mechanisms always point from the electron source to the electron sink.
Four recurring patterns describe how electrons move in polar mechanisms:
Nucleophilic attack / addition: The nucleophile donates an electron pair to the electrophile, forming a new bond. The molecule gains atoms.
Loss of a leaving group / elimination: A group departs with the bonding electrons, breaking a bond. The molecule loses atoms and often gains unsaturation.
Substitution: A nucleophile attacks while a leaving group departs, effectively swapping one group for another in a single molecule.
Rearrangement: Atoms or groups migrate within the molecule (commonly a hydride or alkyl shift) to produce a more stable intermediate.
Most complex organic mechanisms are built from combinations of these four elementary patterns.
Rearrangements are driven by thermodynamics: carbocations rearrange to reach greater stability.
Carbocation stability order (least → most stable):
Methyl cation (CH₃⁺): least stable, rarely formed
Primary (1°): one alkyl group attached to the positive carbon
Secondary (2°): two alkyl groups
Tertiary (3°): three alkyl groups, most stable
Alkyl groups stabilise carbocations through hyperconjugation and inductive effects: more alkyl substituents means more electron density donated toward the positive centre.
Two common rearrangement shifts:
Hydride shift (1,2-H shift): A hydrogen migrates with its bonding electrons from an adjacent carbon to the carbocation carbon.
Alkyl (methyl) shift (1,2-alkyl shift): An alkyl group migrates in the same way.
Both shifts convert a less stable carbocation into a more stable one. They happen rapidly and without external reagents, so they can surprise you mid-mechanism if you forget to check whether the carbocation that forms could rearrange.
Many organic reactions proceed through one or more intermediates, transient species that form and then react further before the final product appears.
Energy diagrams plot free energy on the vertical axis against the reaction coordinate (progress of the reaction) on the horizontal axis. Two quantities to know:
ΔG (Gibbs free energy change): The energy difference between products and starting materials. Negative ΔG = exergonic (thermodynamically favourable). Positive ΔG = endergonic (thermodynamically unfavourable).
Eₐ (activation energy): The energy gap between the starting materials and the transition state. A lower Eₐ means a faster reaction (kinetically favourable).
Key features of an energy diagram:
Transition states sit at energy maxima (peaks). They cannot be isolated.
Intermediates sit at energy minima (valleys) between two transition states. They have a measurable, if brief, lifetime.
A multi-step reaction has one transition state per step and one intermediate between each pair of steps.
The rate-determining step is the step with the highest-energy transition state.
ΔG tells you whether a reaction will happen; Eₐ tells you how fast.
Radical chain reactions are the basis of industrial polymerisation: polyethylene, polystyrene and PVC are all made by radical addition across double bonds. Nucleophilic substitution is central to pharmaceutical synthesis, where chemists swap functional groups on drug candidates to fine-tune activity, solubility and metabolism.
Students often confuse homolytic and heterolytic cleavage. Homolytic = one electron each (radicals). Heterolytic = both electrons to one side (ions). The arrow type in the mechanism tells you which: fishhook arrows for homolytic, full curved arrows for heterolytic.
Students sometimes think nucleophiles must carry a formal negative charge. They do not. Neutral molecules with lone pairs (water, ammonia, alcohols) act as nucleophiles regularly.
A common error is forgetting to check for carbocation rearrangements. Whenever a mechanism produces a carbocation, ask whether a hydride or alkyl shift to an adjacent carbon would give a more stable cation. If it would, the rearrangement is likely.
Students occasionally treat the transition state and the intermediate as the same thing. A transition state is an energy maximum that cannot be isolated. An intermediate is an energy minimum between steps that does exist, briefly, as a real species.
⚠️ Be able to classify any given reaction as addition, elimination, substitution or rearrangement. This is a near-certain exam question.
⚠️ Know the difference between homolytic and heterolytic cleavage and which reactive species each produces.
⚠️ Be ready to identify nucleophiles and electrophiles in a reaction and draw curved arrows showing electron flow from nucleophile to electrophile.
⚠️ Expect questions on carbocation stability ranking (methyl < 1° < 2° < 3°) and when to apply a hydride or alkyl shift.
⚠️ Energy diagram interpretation: be able to label reactants, products, transition states, intermediates, ΔG and Eₐ, and identify the rate-determining step.
True or False: Homolytic cleavage produces ions.
False. It produces radicals (neutral species with unpaired electrons).
Fill in the blank: A species that donates an electron pair is called a ______.
Nucleophile.
True or False: A tertiary carbocation is less stable than a primary carbocation.
False. Tertiary is more stable than primary.
Fill in the blank: The three stages of a radical chain reaction are ______, ______ and ______.
Initiation, propagation, termination.
True or False: The transition state of a reaction can be isolated in the lab.
False. Transition states exist only at the energy maximum and cannot be isolated.
Q: What are the four main types of organic reactions?
A: Addition, elimination, substitution and rearrangement.
Q: Distinguish between homolytic and heterolytic bond cleavage. What species does each produce?
A: Homolytic cleavage splits the bonding pair evenly, giving each fragment one electron and producing radicals. Heterolytic cleavage gives both electrons to one fragment, producing a cation and an anion.
Q: In a polar reaction, what determines the direction of electron flow?
A: Electrons flow from the nucleophile (electron-rich) to the electrophile (electron-poor).
Q: Rank the following in order of increasing carbocation stability: tertiary, methyl, secondary, primary.
A: Methyl < primary (1°) < secondary (2°) < tertiary (3°).
Q: A secondary carbocation forms during a reaction. An adjacent carbon bears a hydrogen. What is likely to happen, and why?
A: A 1,2-hydride shift is likely. The hydrogen migrates with its bonding electrons to the carbocation centre, converting it to a more stable tertiary carbocation.
Q: On an energy diagram, how do you distinguish a transition state from a reaction intermediate?
A: A transition state is an energy maximum (peak on the diagram) and cannot be isolated. An intermediate is an energy minimum (valley) between two transition states and has a finite lifetime.
Q: What does a negative ΔG tell you about a reaction? What does a large Eₐ tell you?
A: Negative ΔG means the reaction is exergonic and thermodynamically favourable. A large Eₐ means the reaction is slow (kinetically unfavourable), regardless of the thermodynamics.
This material connects directly to nucleophilic substitution (SN1 and SN2) and elimination (E1 and E2) reactions, which are built on the nucleophile/electrophile framework and carbocation stability concepts introduced here. It also connects to alkene addition reactions, where the addition and rearrangement patterns recur with specific regiochemistry (Markovnikov's rule) and stereochemistry.
The energy diagram concepts here carry forward into kinetics and thermodynamics discussions later in the course, and into transition state theory in physical chemistry.
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