Organic Chemistry – Some Basic Principles and Techniques · Lesson 7 of 15
Bond Breaking and Reactive Species
“Follow electrons through bond cleavage and identify the species that begin organic reactions.”
• Explain substrate, reagent, intermediate, and reaction mechanism. • Distinguish heterolytic and homolytic bond cleavage using arrows. • Identify carbocations, carbanions, and free radicals and describe their local shapes. • Recognise nucleophiles, electrophiles, and the centres they attack.
A reaction mechanism follows the changes
A chemical equation shows the overall conversion from reactants to products. A reaction mechanism explains the sequence: which bond breaks, which bond forms, where electrons move, and what temporary intermediates appear. It also connects those steps with their energetics and rates. Understanding this sequence helps us explain reactivity and plan a synthesis rather than memorising an isolated product.
The organic molecule under attention is called the substrate, and the reacting attacking species is called the reagent. In many examples the substrate supplies the carbon involved in the new bond. If both reactants supply carbon, the choice depends on which molecule is being examined. Ethene is the substrate in its reaction with bromine; a benzene molecule can be the substrate when an alkyl group is introduced into its ring.
A short-lived species formed in one step of a reaction and used up in a later step. It is neither simply the initial reactant nor the final stable product.
Two ways to divide a shared electron pair
A covalent bond contains a shared pair of electrons. If both electrons go to the same atom when the bond breaks, the process is heterolytic cleavage or heterolysis. The two fragments acquire opposite charges in a neutral starting example. If each atom takes one electron, cleavage is homolytic or homolysis, producing two radicals. The distinction is about electron ownership, not whether the drawn line happens to break in the middle.
For example, heterolysis of bromomethane with the pair going to bromine gives and . Homolysis of an alkyl halide, often promoted by heat or light, gives and . Reactions involving ionic intermediates are called polar or heteropolar reactions; those involving homolysis are described as free-radical or homopolar reactions.
Carbon intermediates differ in charge and electron count
A carbocation has a positively charged carbon with only six electrons in its valence shell in the simple alkyl examples. The methyl carbocation, , uses three orbitals for sigma bonds and has an empty p orbital perpendicular to its trigonal plane. A carbanion has a negatively charged carbon with a lone pair. A simple methyl carbanion is commonly described as with a pyramidal atomic arrangement and a tetrahedral electron-pair arrangement.
A carbon radical has an unpaired electron and need not carry a charge. It is therefore different from both a carbocation and a carbanion. Primary, secondary, and tertiary labels count the carbon atoms directly attached to the reactive carbon: one, two, or three. They do not count every carbon in the molecule, nor the number of hydrogens that were originally attached.
| Species | Simple example | Important electronic feature | Local structural idea |
|---|---|---|---|
| Carbocation | Electron deficient; six valence electrons | Trigonal planar with empty p orbital | |
| Carbanion | Lone pair on negative carbon | Usually pyramidal for a simple alkyl example | |
| Free radical | One unpaired electron | Reactive neutral radical in this example |
For simple alkyl carbocations, the observed stability order is tertiary > secondary > primary > methyl. Simple alkyl radicals follow the same broad order. Nearby alkyl groups help distribute the electronic disturbance. Inductive effects and hyperconjugation will explain the carbocation trend in the following lessons. Do not transfer this order automatically to every charged or resonance-stabilised species.
Problem
Break the indicated C–S, C–CN, and C–Cu bonds in , , and heterolytically.
- 1.For , move the shared pair to sulphur. The fragments are and .
- 2.For , moving the pair to the CN fragment gives and . The C–C connection breaks; the nitrile triple bond stays intact.
- 3.For , move the shared pair towards carbon, giving and . Carbon can therefore emerge as either cation or anion, depending on which fragment receives the pair.
Donors seek electron-poor centres
A nucleophile can donate an electron pair to form a new bond. It may be negative, such as hydroxide, cyanide, hydrogensulphide, or an alkoxide, but it can also be neutral, such as water, ammonia, or an amine with a lone pair. The defining feature is an available pair, not merely a negative sign. A nucleophile attacks an electrophilic centre: a site able to accept electron density.
An electrophile accepts an electron pair. Carbocations are clear examples, but a neutral species can also be electrophilic. Boron in has an incomplete octet. The carbonyl carbon in an aldehyde or ketone is partially positive because oxygen draws electron density. Similarly, the carbon bonded to a halogen in an alkyl halide can act as an electrophilic centre.
Problem
Classify , , , , and .
- 1.HS⁻ and ethoxide have lone pairs and negative charge; both can donate a pair and act as nucleophiles.
- 2.Trimethylamine is neutral but nitrogen has a lone pair, so it can also act as a nucleophile.
- 3.BF₃ can accept a pair at boron, and the nitronium ion can act as an electron-pair acceptor. These are electrophiles. Neutrality by itself does not decide the category.
Problem
Identify the electrophilic carbon in , , and .
- 1.In the aldehyde, oxygen polarises C=O, leaving the carbonyl carbon partially positive.
- 2.In the nitrile, nitrogen polarises the C≡N bond, so the nitrile carbon is the electron-poor centre.
- 3.In iodomethane, the carbon bonded to iodine is the electrophilic centre. A nucleophile can donate a pair towards this carbon as the C–I bond breaks.
Arrows start at electrons
A full curved arrow tracks a pair of electrons. Its tail begins at a lone pair or bond and its head ends at the atom or bond position receiving the pair. A half-headed or fish-hook arrow tracks one electron. These are different from the straight arrow between reactants and products, which indicates overall reaction direction.
In , draw one full curved arrow from an oxygen lone pair to carbon and another from the C–Br bond to bromine. This shows both bond formation and bond breaking. In homolysis of chloromethane, two fish-hook arrows from the C–Cl bond show one electron moving to carbon and one to chlorine.
Do not draw an electron-flow arrow from a positive charge towards a nucleophile. The arrow starts at the electrons being moved. Also distinguish a negative ion with an electron pair from a neutral radical with one unpaired electron.
Quiz
What distinguishes heterolysis from homolysis?
Which species can be a neutral nucleophile?
Which carbon species has an empty p orbital in the simple methyl case?
Where should the tail of a full curved electron-flow arrow begin?
Which is the most stable simple alkyl carbocation among the following?
Practice Problems
- Use full curved arrows to show heterolysis of bromomethane and identify the electron-pair recipient.
- Use two fish-hook arrows to show homolysis of an alkyl halide and label the two radicals.
- Draw the local shapes of methyl carbocation and methyl carbanion, including the empty orbital or lone pair.
- Classify , , , and as electron-pair donors or acceptors.
- Locate the electrophilic carbon in , , and .
- Explain why the substrate–reagent distinction can be a choice of focus when both reactants supply carbon to a newly formed bond.
Key Takeaways
• Mechanisms account for bond changes and electron movement step by step. • Heterolysis moves a pair together; homolysis separates the pair. • Carbocations, carbanions, and radicals differ in electron count and charge. • Nucleophiles donate pairs to electrophilic centres; arrows begin at electrons.