Basic Principles of Organic Chemistry
The grammar of carbon — hybridisation and shape, the rules for naming, the electronic effects that push and pull electrons, and the intermediates and reagents behind every organic reaction
- Why carbon's tetravalence and catenation make millions of compounds possible.
- The sp³, sp², sp hybridisations and the shapes they dictate.
- Homologous series, functional groups, and the rules of IUPAC nomenclature.
- The electronic effects: inductive, resonance, electromeric, hyperconjugation.
- The four reaction intermediates and the order of their stability.
- Homolytic vs heterolytic fission, electrophiles vs nucleophiles, and reaction types.
Why Carbon Is Special
Organic chemistry is the chemistry of carbon compounds, and carbon earns that whole branch to itself for a handful of reasons. It is strictly tetravalent — forming four strong covalent bonds — and it excels at catenation, linking to itself in chains and rings of any length. Being small, its bonds (to itself, to H, O, N, halogens) are strong, and it readily forms single, double and triple bonds.
Together these let carbon build an essentially unlimited variety of stable skeletons — the structural basis of every fuel, plastic, drug and biomolecule.
Hybridisation & Shape
The geometry of any carbon depends on its hybridisation — how many \(\sigma\) bonds it forms. Each \(\pi\) bond uses an unhybridised \(p\) orbital, so the number of attached atoms fixes the shape.
| Hybridisation | σ / π bonds | Shape | Angle | Example |
|---|---|---|---|---|
| \(sp^3\) | 4 σ | tetrahedral | 109.5° | \(\ce{CH4}\) |
| \(sp^2\) | 3 σ + 1 π | trigonal planar | 120° | \(\ce{C2H4}\) |
| \(sp\) | 2 σ + 2 π | linear | 180° | \(\ce{C2H2}\) |
Classification & Homologous Series
Organic compounds are first split into acyclic (open-chain) and cyclic (ring) types, and cyclic ones into carbocyclic and heterocyclic. Compounds sharing the same functional group form a homologous series — members differing by a \(\ce{CH2}\) unit, sharing one general formula and similar chemistry, with a smooth gradation in physical properties.
| Functional group | Class | General formula |
|---|---|---|
| — | alkane | \(\ce{C_nH_{2n+2}}\) |
| \(\ce{C=C}\) | alkene | \(\ce{C_nH_{2n}}\) |
| \(\ce{-OH}\) | alcohol | \(\ce{C_nH_{2n+1}OH}\) |
| \(\ce{-CHO}\) | aldehyde | \(\ce{C_nH_{2n}O}\) |
| \(\ce{-COOH}\) | carboxylic acid | \(\ce{C_nH_{2n}O2}\) |
IUPAC Nomenclature
An IUPAC name is built in three parts: a word root (chain length), a primary suffix (saturation), and a secondary suffix (functional group), with prefixes for substituents and locants (numbers) chosen to be lowest. The longest chain bearing the principal functional group is the parent.
The highest-priority group becomes the suffix; the rest become prefixes. Roots: meth (1), eth (2), prop (3), but (4), pent (5), hex (6)… Suffixes: -ane / -ene / -yne for saturation.
| Structure | IUPAC name |
|---|---|
| \(\ce{CH3-CH2-OH}\) | ethanol |
| \(\ce{CH3-CHO}\) | ethanal |
| \(\ce{CH3-CO-CH3}\) | propanone |
| \(\ce{CH3-COOH}\) | ethanoic acid |
The Inductive Effect
The inductive effect is the permanent shift of \(\sigma\)-bond electrons towards a more electronegative atom. It is transmitted through the chain of \(\sigma\) bonds and weakens rapidly with distance. Electron-withdrawing groups show \(-I\); electron-donating groups (notably alkyls) show \(+I\).
| \(-I\) (withdrawing) | \(+I\) (donating) |
|---|---|
| \(\ce{-NO2},\ \ce{-CN},\ \ce{-COOH}\) | \(\ce{-CH3},\ \ce{-C2H5}\) (alkyl) |
| halogens (\(\ce{-F} > \ce{-Cl} > \ce{-Br}\)) | \(\ce{-O-}\) (alkoxide) |
Resonance & Hyperconjugation
The resonance (mesomeric) effect is the delocalisation of \(\pi\) electrons or lone pairs across a conjugated system. The real molecule is a resonance hybrid of contributing structures, and is more stable than any one of them. Groups that donate electrons by resonance are \(+M\) (\(+R\)); those that withdraw are \(-M\) (\(-R\)).
| \(+M\) / \(+R\) (donating) | \(-M\) / \(-R\) (withdrawing) |
|---|---|
| \(\ce{-OH},\ \ce{-NH2},\ \ce{-OR}\) | \(\ce{-NO2},\ \ce{-CN},\ \ce{-CHO}\) |
| halogens (lone pairs) | \(\ce{-COOH},\ \ce{>C=O}\) |
The more \(\alpha\)-hydrogens available, the more hyperconjugation — which is why a \(3^\circ\) carbocation (\(9\) \(\alpha\)-H) is far more stable than a methyl cation, and why more-substituted alkenes are more stable. There is also a temporary electromeric effect: the complete shift of a \(\pi\)-pair only in the presence of an attacking reagent.
Reaction Intermediates
Most organic reactions pass through short-lived intermediates. Four matter most — carbocations, carbanions, free radicals and carbenes — and knowing the order of their stability lets you predict the major product.
| Intermediate | Charge / electrons | Shape | Stability order |
|---|---|---|---|
| Carbocation \(\ce{R3C+}\) | +, six electrons | \(sp^2\), planar | \(3^\circ > 2^\circ > 1^\circ > \ce{CH3+}\) |
| Carbanion \(\ce{R3C^-}\) | −, lone pair | \(sp^3\), pyramidal | \(\ce{CH3^-} > 1^\circ > 2^\circ > 3^\circ\) |
| Free radical \(\ce{R3C.}\) | unpaired electron | \(sp^2\), planar | \(3^\circ > 2^\circ > 1^\circ\) |
| Carbene \(\ce{:CH2}\) | neutral, six electrons | bent / linear | singlet vs triplet |
Bond Fission & Reagents
A covalent bond can break in two ways. Homolytic fission splits the pair evenly, giving free radicals (favoured by heat/light in non-polar bonds). Heterolytic fission gives one atom both electrons, producing ions. The reagents that attack are likewise of two kinds.
| Reagent | Nature | Examples |
|---|---|---|
| Electrophile (\(\ce{E+}\)) | electron-loving, electron-deficient | \(\ce{H+},\ \ce{NO2+},\ \ce{AlCl3},\ \ce{BF3}\) |
| Nucleophile (\(\ce{Nu-}\)) | nucleus-loving, electron-rich | \(\ce{OH-},\ \ce{CN-},\ \ce{NH3},\ \ce{H2O}\) |
Types of Organic Reactions
Almost every organic transformation falls into one of four families. Recognising the family is the first step to writing a mechanism.
| Type | What happens | Typical of |
|---|---|---|
| Substitution | one atom/group replaced by another | alkanes, haloalkanes |
| Addition | atoms add across a multiple bond | alkenes, alkynes, carbonyls |
| Elimination | atoms removed to form a multiple bond | alcohols, haloalkanes |
| Rearrangement | atoms reorganise within the molecule | carbocation shifts |
Putting It to Work
Problem. State the hybridisation of each carbon in \(\ce{CH2=CH-C#N}\).
Solution. Count σ bonds per carbon: double-bond C's are \(sp^2\), the nitrile C is \(sp\):
Problem. Give the IUPAC name of \(\ce{CH3-CH2-CH2-COOH}\).
Solution. Four carbons + \(\ce{-COOH}\) (priority suffix):
Problem. Which is the stronger acid, \(\ce{CH3COOH}\) or \(\ce{ClCH2COOH}\)? Explain.
Solution. The \(-I\) chlorine stabilises the carboxylate anion:
Problem. Arrange \(\ce{CH3+},\ \ce{CH3CH2+},\ (\ce{CH3})3C+\) by stability.
Solution. More alkyl groups give more \(+I\) and hyperconjugation:
Problem. Classify \(\ce{OH-},\ \ce{NO2+}\) and \(\ce{BF3}\) as electrophile or nucleophile.
Solution. Electron-rich = nucleophile; electron-deficient = electrophile:
Problem. \(\ce{CH2=CH2 + Br2 -> CH2Br-CH2Br}\). What type of reaction is this?
Solution. Atoms add across the double bond, no atoms leave:
Chapter Summary
Tetravalence + catenation + small size + multiple bonding = millions of compounds.
\(sp^3\) tetrahedral, \(sp^2\) planar, \(sp\) linear — set by the σ-bond count.
Root + suffix + secondary suffix; highest-priority group becomes the suffix; lowest locants.
Inductive (σ), resonance (π), electromeric (temporary), hyperconjugation (no-bond).
Carbocation 3°>2°>1°; carbanion opposite; resonance beats everything.
Electrophiles vs nucleophiles; substitution, addition, elimination, rearrangement.
Problems
For each item, first decide which principle it tests — structure, naming, an electronic effect, or a reaction concept — then apply the relevant rule. Difficulty rises down the list.
- List four features of carbon that make organic chemistry so vast.
- Give the hybridisation, shape and bond angle for \(\ce{CH4},\ \ce{C2H4}\) and \(\ce{C2H2}\).
- Define a homologous series and state three of its characteristics.
- Name \(\ce{CH3-CO-CH2-CH3}\) and \(\ce{CH3-CH2-CHO}\) by IUPAC rules.
- Explain the inductive effect and arrange the acid strength of \(\ce{CH3COOH},\ \ce{ClCH2COOH},\ \ce{Cl2CHCOOH}\).
- What is the resonance effect? Distinguish \(+M\) from \(-M\) groups with examples.
- Explain hyperconjugation and use it to account for the stability order of carbocations.
- Arrange \(\ce{CH3+},\ \ce{(CH3)2CH+},\ \ce{(CH3)3C+}\) and the benzyl cation by stability.
- Distinguish homolytic and heterolytic bond fission and the species each produces.
- Define electrophile and nucleophile, giving two examples of each.
- Classify each as substitution, addition or elimination: \(\ce{CH4 + Cl2 -> CH3Cl + HCl}\); \(\ce{CH2=CH2 + H2 -> CH3CH3}\).
- Why is the allyl carbocation more stable than a simple primary carbocation?