Carbon has 4 valence electrons and always forms exactly 4 covalent bonds. This tetravalency, combined with carbon's unique ability to bond to itself repeatedly (catenation), is what makes the vast diversity of organic compounds possible.
| Hybridisation | Geometry | Bond Angle | Example |
|---|---|---|---|
| sp³ | Tetrahedral | 109.5° | Methane, ethane |
| sp² | Trigonal planar | 120° | Ethene, benzene |
| sp | Linear | 180° | Ethyne |
| Category | Description | Example |
|---|---|---|
| Acyclic (open-chain) | Carbon atoms linked in a straight or branched chain, no ring | Ethane, isobutane |
| Alicyclic | Carbon atoms joined in a ring, non-aromatic | Cyclohexane |
| Aromatic (benzenoid) | Contains one or more benzene rings, following Hückel's rule of aromaticity | Benzene, naphthalene |
| Aromatic (non-benzenoid) | Aromatic character without a benzene ring itself | Tropolone |
| Heterocyclic | Ring containing at least one atom other than carbon (N, O, S) | Furan, pyridine |
An IUPAC name has three parts: word root (indicates the number of carbons in the longest continuous chain), suffix (indicates the principal functional group and saturation/unsaturation), and prefix (indicates substituent groups, listed alphabetically with locants).
| Number of Carbons | Word Root |
|---|---|
| 1 | Meth |
| 2 | Eth |
| 3 | Prop |
| 4 | But |
| 5 | Pent |
| 6 | Hex |
Give the IUPAC name of $\text{CH}_3-\text{CH(CH}_3\text{)}-\text{CH}_2-\text{CHO}$.
Longest chain containing the -CHO group: 4 carbons (butanal). Numbering from the CHO end (C1), the methyl substituent is on C3.
Name: 3-methylbutanal
| Type | Description | Example |
|---|---|---|
| Chain isomerism | Differ in the arrangement of the carbon skeleton (straight vs branched) | Butane and isobutane |
| Position isomerism | Same carbon skeleton and functional group, but different position of the group/substituent | Propan-1-ol and propan-2-ol |
| Functional isomerism | Same molecular formula, but different functional groups entirely | Ethanol (C₂H₆O) and dimethyl ether (C₂H₆O) |
| Metamerism | Same functional group, but different alkyl groups distributed on either side of it | Diethyl ether and methyl propyl ether (both C₄H₁₀O) |
Show that ethanol and dimethyl ether are functional isomers by writing their structures and molecular formula.
Ethanol: $\text{CH}_3\text{CH}_2\text{OH}$ — molecular formula $\text{C}_2\text{H}_6\text{O}$ (an alcohol).
Dimethyl ether: $\text{CH}_3-\text{O}-\text{CH}_3$ — molecular formula $\text{C}_2\text{H}_6\text{O}$ (an ether).
Same molecular formula, but entirely different functional groups — confirming functional isomerism.
A covalent bond breaks such that each atom retains one electron from the shared pair, producing two neutral species with unpaired electrons called free radicals. Typically favoured in the gas phase or under conditions like UV light/heat, especially for non-polar bonds.
$$\text{A:B} \rightarrow \text{A}^{\bullet} + \text{B}^{\bullet}$$A covalent bond breaks such that both electrons from the shared pair go to just one of the two atoms — the more electronegative atom typically keeps both electrons, becoming negatively charged, while the other becomes positively charged. Common in polar solvents, especially for already-polar bonds.
$$\text{A:B} \rightarrow \text{A}^+ + \text{B}^-$$| Species | Charge/Nature | Carbon Hybridisation | Stability Order |
|---|---|---|---|
| Carbocation | Positively charged (electron-deficient) | sp², planar | 3° > 2° > 1° > methyl |
| Carbanion | Negatively charged (lone pair present) | sp³, pyramidal | Methyl > 1° > 2° > 3° |
| Free radical | Neutral, unpaired electron | sp², planar (or near-planar) | 3° > 2° > 1° > methyl |
Both carbocations and free radicals are stabilised by electron-donating alkyl groups nearby (through the inductive effect and hyperconjugation), so more substituted (3°) versions are more stable. Carbanions, however, already have excess electron density (a lone pair) — electron-donating alkyl groups nearby actually destabilise a carbanion by adding even more electron density, so less substituted (methyl, 1°) carbanions are more stable.
Explain why a tertiary carbocation is more stable than a primary carbocation.
A tertiary carbocation is attached to 3 alkyl groups, each of which donates electron density towards the positively charged, electron-deficient carbon through the inductive effect and through hyperconjugation (donation from adjacent C-H sigma bonds) — this electron donation partly neutralises the positive charge, spreading it out and stabilising the cation. A primary carbocation has only 1 alkyl group providing this stabilisation, making it considerably less stable.
A species that is electron-deficient and seeks to accept a pair of electrons to form a new bond — behaves as a Lewis acid. Can be positively charged (e.g., $\text{H}^+$, carbocations) or neutral but electron-deficient (e.g., $\text{BF}_3$, $\text{AlCl}_3$).
A species that is electron-rich and donates a pair of electrons to form a new bond — behaves as a Lewis base. Can be negatively charged (e.g., $\text{OH}^-$, $\text{CN}^-$) or neutral with a lone pair (e.g., $\text{NH}_3$, $\text{H}_2\text{O}$).
Classify the following as electrophiles or nucleophiles: NH₃, BF₃, CN⁻, NO₂⁺.
NH₃ — nucleophile (nitrogen's lone pair can be donated).
BF₃ — electrophile (boron has an incomplete octet, electron-deficient).
CN⁻ — nucleophile (negatively charged, carbon lone pair available for donation).
NO₂⁺ — electrophile (positively charged, electron-deficient nitronium ion).
The inductive effect is the permanent, partial displacement of sigma (σ) bond electrons towards a more electronegative atom or group, which then weakly extends its influence along a chain of adjacent bonds — the effect weakens rapidly with distance and is negligible beyond about 3 carbons away.
−I effect (electron-withdrawing): groups/atoms more electronegative than carbon pull electron density away, e.g., −NO₂, −CN, halogens, −COOH. +I effect (electron-donating): alkyl groups push electron density towards the chain, e.g., −CH₃, −C₂H₅, tertiary butyl groups (strongest +I).
Explain why trichloroacetic acid (Cl₃C–COOH) is a much stronger acid than acetic acid (CH₃–COOH).
The three chlorine atoms in trichloroacetic acid exert a strong cumulative −I (electron-withdrawing) effect, pulling electron density away from the carboxylate group after it loses H⁺. This helps disperse and stabilise the negative charge on the resulting carboxylate ion, making the ion more stable and the parent acid more willing to release its proton — hence trichloroacetic acid is a much stronger acid. Acetic acid's methyl group, by contrast, has a +I (electron-donating) effect, which intensifies (destabilises) the negative charge on its conjugate base, making it a weaker acid.
When a molecule or ion can be represented by two or more valid Lewis structures differing only in the position of electrons (not atoms), the actual structure is a hybrid of all these resonance structures, generally more stable than any single one of them. Delocalisation of π electrons (or lone pairs adjacent to a π system) is the underlying cause.
The polarity introduced into a molecule due to the interaction between a lone pair (or π bond) and an adjacent π system, transmitted through delocalisation rather than through sigma bonds. Can be +M (electron-donating into the system, e.g., −NH₂, −OH, −OR when attached to a ring) or −M (electron-withdrawing from the system, e.g., −NO₂, −CHO, −COOH).
Explain why phenol (C₆H₅OH) is more acidic than a simple alcohol, using resonance.
After phenol loses a proton, the resulting phenoxide ion has its negative charge on oxygen delocalised into the benzene ring through resonance (the oxygen's lone pair overlaps with the ring's π system), spreading the negative charge over several ring carbons as well. This delocalisation stabilises the phenoxide ion significantly. A simple alcohol's alkoxide ion has no such delocalisation available — its negative charge stays fully localised on oxygen, making it less stable, and the parent alcohol correspondingly less acidic than phenol.
Hyperconjugation is the delocalisation of electrons from a $\sigma$ (C-H or C-C) bond adjacent to an electron-deficient centre (like a carbocation) or a π system (like an adjacent double bond), into an empty or partially-filled orbital/π system — sometimes called "no-bond resonance," since one resonance structure shows the C-H bond effectively broken.
More alkyl groups on a carbocation (or alkene carbon) mean more adjacent C-H bonds available to hyperconjugate — each such C-H bond provides some stabilisation, so a tertiary carbocation (with more adjacent C-H bonds via its 3 alkyl groups) is more stabilised by hyperconjugation than a primary carbocation (fewer adjacent C-H bonds), reinforcing the same stability order seen from the inductive effect.
Explain, using hyperconjugation, why propene (CH₃–CH=CH₂) is more stable than ethene (CH₂=CH₂).
In propene, the methyl group has 3 C-H sigma bonds adjacent to the double bond, which can hyperconjugate with (donate electron density into) the π system, providing extra stabilisation. Ethene has no such adjacent alkyl C-H bonds to hyperconjugate with the double bond, so it lacks this extra stabilisation — making propene the more stable (lower energy) alkene.
| Reaction Type | Description | Example |
|---|---|---|
| Substitution | One atom/group is replaced by another | $\text{CH}_4 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{HCl}$ |
| Addition | Atoms/groups add across a multiple bond, which becomes saturated | $\text{CH}_2=\text{CH}_2 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_3$ |
| Elimination | Atoms/groups are removed, typically forming a new multiple bond | $\text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_2=\text{CH}_2 + \text{HBr}$ |
| Rearrangement | Atoms within a molecule migrate, forming a structural isomer | Carbocation rearrangements (e.g., 1,2-hydride shift) |