Chemical reactions occur when chemical bonds break and new bonds form. To understand organic reaction mechanisms, we need to know how covalent bonds can break (bond fission), how to trace the movement of electrons using curly arrows, and how to classify the organic reagents and reaction pathways.
🔑 Key Principle
Curly arrows always show the movement of electrons (never atoms). A full-headed arrow represents a pair of electrons moving, while a single-headed arrow (fishhook) represents a single electron moving.
Bond Fission
A covalent bond consists of a shared pair of electrons. When this bond is broken, it can split in two ways:
1. Homolytic Fission
During homolytic fission, the shared pair of electrons in the covalent bond breaks evenly, with each bonded atom taking one electron from the shared pair. This process forms highly reactive neutral species containing an unpaired electron, called free radicals. We represent homolytic fission using single-headed curly arrows (fishhook arrows).
\( \text{X-Y} \rightarrow \text{X}^\bullet + \text{Y}^\bullet \)
2. Heterolytic Fission
During heterolytic fission, the shared pair of electrons in the covalent bond breaks unevenly, with one of the bonded atoms taking both of the shared electrons. This results in the formation of charged species: a positive ion (cation) and a negative ion (anion). We represent heterolytic fission using a standard full-headed curly arrow pointing to the more electronegative atom.
\( \text{X-Y} \rightarrow \text{X}^+ + \text{:Y}^- \)
The breaking of a covalent bond so that each bonded atom receives one electron from the bonded pair, forming two radicals.
The breaking of a covalent bond so that both bonded electrons go to one atom, forming a cation and an anion.
A highly reactive chemical species containing an unpaired valence electron.
Curly Arrow Notation
Curly arrows show the path of valence electrons during chemical processes. When drawing reaction mechanisms, ensure your arrows adhere strictly to these conventions:
- Start: The arrow tail must originate precisely at a site of electron density (a covalent bond, a lone pair on an atom, or a formal negative charge).
- Point: The arrow head must point exactly to the atom or bond region where the electrons are moving to form a new bond or charge.
- Full vs Half: Use full-headed arrows for ionic mechanisms (double-electron movement) and half-headed arrows (fishhook arrows) for radical mechanisms (single-electron movement).
Never start a curly arrow from a positive charge (like \( \text{H}^+ \)) or from an element symbol itself unless it has a drawn lone pair. This is a common error that instantly loses marks in mechanism questions. Always draw the lone pair of electrons (e.g. on \( \text{:OH}^- \) or \( \text{:NH}_3 \)) to show exactly where the arrow begins.
Classifying Reagents
In organic mechanisms, species attacking organic molecules are classified as:
Nucleophiles (Nucleus-loving)
An electron pair donor. They are electron-rich species containing a lone pair of electrons. They attack delta positive (\( \delta^+ \)) carbons.
Examples: \( \text{:OH}^- \), \( \text{:NH}_3 \), \( \text{H}_2\text{O:} \), \( \text{:CN}^- \)
Electrophiles (Electron-loving)
An electron pair acceptor. They are electron-deficient species (often positive ions or molecules with polar bonds) that attack electron-rich sites.
Examples: \( \text{NO}_2^+ \), \( \text{H}^+ \), polar molecules like \( \text{H-Br} \)
An electron pair donor that attacks a positively charged or electron-deficient carbon atom.
An electron pair acceptor that attacks an electron-rich site (such as a double bond or aromatic ring).
Reaction Types Overview
You must be able to classify organic reactions based on what happens to the reactants and products:
| Reaction Type | Description | General Form | Example |
|---|---|---|---|
| Addition | Two reactant molecules combine to form a single product. Typical of alkenes. | \( \text{A} + \text{B} \rightarrow \text{C} \) | \( \text{CH}_2\text{=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} \) |
| Substitution | An atom or group of atoms in a molecule is replaced by another atom or group. | \( \text{A-B} + \text{C} \rightarrow \text{A-C} + \text{B} \) | \( \text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^- \) |
| Elimination | A small molecule (e.g. \( \text{H}_2\text{O} \), \( \text{HCl} \)) is removed from a single reactant, forming a double bond. | \( \text{A} \rightarrow \text{B} + \text{C} \) | \( \text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_2\text{=CH}_2 + \text{H}_2\text{O} + \text{Br}^- \) |
| Oxidation | The addition of oxygen, removal of hydrogen, or loss of electrons. Represented as \( \text{[O]} \). | \( \text{Reactant} + \text{[O]} \) | \( \text{CH}_3\text{CH}_2\text{OH} + \text{[O]} \rightarrow \text{CH}_3\text{CHO} + \text{H}_2\text{O} \) |
| Hydrolysis | The breaking of a covalent bond using water or hydroxide ions. | \( \text{R-X} + \text{H}_2\text{O} \rightarrow \text{R-OH} + \text{HX} \) | \( \text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{CH}_3\text{OH} \) |
Worked Examples
- The homolytic fission of a chlorine molecule (\( \text{Cl}_2 \)) under UV light.
- The heterolytic fission of the C-Br bond in bromoethane (\( \text{CH}_3\text{CH}_2\text{Br} \)).
Solution:
- Homolytic fission of chlorine splits the bond evenly to form chlorine radicals: \[ \text{Cl}_2 \xrightarrow{\text{UV}} 2\text{Cl}^\bullet \]
- Heterolytic fission of bromoethane splits the polar bond unevenly, with the bromine atom taking both electrons because it is more electronegative: \[ \text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_3\text{CH}_2^+ + \text{Br}^- \]
- \( \text{CH}_2\text{=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} \)
- \( \text{CH}_4 + \text{Cl}^\bullet \rightarrow \text{CH}_3^\bullet + \text{HCl} \)
Solution:
- Two reactant molecules combine into one. This is an addition reaction. The alkene has an electron-rich double bond which is attacked by the electron-deficient \( \text{H}^{\delta+} \) of \( \text{H-Br} \), so the attacking reagent is an electrophile (specifically, this is electrophilic addition).
- An atom is substituted. This is a substitution reaction (propagation step). The attacking chlorine species Cl• has an unpaired electron, so it is a free radical (free radical substitution).
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