AS Chemistry: Introduction to Organic Chemistry — Revision Notes

Condensed for the final weeks. For the full explanation, use the Introduction to Organic Chemistry study guide.

Formulae

Type	Shows
Empirical	Simplest whole-number ratio of atoms
Molecular	Actual number of each atom
Structural	Arrangement, written on one line — CH₃CH₂OH
Displayed	Every atom and every bond drawn
Skeletal	Carbon backbone as lines; carbons and their hydrogens implied

In skeletal formulae, each vertex and each line end is a carbon, with enough hydrogens to complete four bonds. Forgetting the implied hydrogens is the most common reading error.

Shape and hybridisation

The same VSEPR and hybridisation ideas apply directly to organic molecules: an sp³ carbon is tetrahedral, an sp² carbon is trigonal planar, and an sp carbon is linear.

Ethene (C₂H₄) is planar because both carbons are sp² hybridised — all six atoms in the molecule lie in a single flat plane, which is exactly the geometry that allows its π bond to form by sideways overlap above and below that plane.

Nomenclature

Root = longest continuous carbon chain. Suffix = principal functional group. Prefix = substituents, in alphabetical order.

Number from the end giving the lowest possible locants to the principal group.

Use di-, tri-, tetra- for repeats, and separate numbers with commas, numbers from letters with hyphens.

Isomerism

Structural isomerism — same molecular formula, different atom connectivity:

• Chain — different carbon skeleton (butane / methylpropane).
• Position — same group in a different place (propan-1-ol / propan-2-ol).
• Functional group — different group entirely (propanal / propanone).

Stereoisomerism — same connectivity, different spatial arrangement:

Cis-trans / E-Z requires both conditions:

1. A C=C double bond, which prevents rotation.
2. Two different groups on each of the double-bonded carbons.

Giving only one condition is a half-answer. If either carbon carries two identical groups, no E/Z isomerism exists.

Optical isomerism requires a chiral centre — a carbon with four different groups. The two enantiomers are non-superimposable mirror images and rotate plane-polarised light in opposite directions.

A racemic mixture is optically inactive, because equal amounts of the two enantiomers rotate the light equally in opposite directions and cancel.

Reaction types

Separate from how a bond breaks is what happens overall to the molecule:

Type	What happens
Addition	Two reactants become one product; nothing is lost
Substitution	One atom or group replaces another
Elimination	A small molecule is lost, typically forming a double bond
Hydrolysis	Breakdown by reaction with water
Condensation	Two molecules join with loss of a small molecule, often water

Naming a mechanism combines both vocabularies: electrophilic addition is addition initiated by an electrophile attacking an electron-rich double bond; nucleophilic substitution is substitution initiated by a nucleophile attacking an electron-poor carbon.

Bond fission

Homolytic — the bond breaks evenly, each atom taking one electron, forming two radicals. Shown with half-arrows (fish hooks). Occurs with non-polar bonds under UV light.

Heterolytic — the bond breaks unevenly, one atom taking both electrons, forming a cation and an anion. Shown with full curly arrows. Occurs with polar bonds.

Reactive species and mechanism types

Species	Definition
Nucleophile	Electron-pair donor, attracted to δ+
Electrophile	Electron-pair acceptor, attracted to δ−
Radical	Has an unpaired electron

Curly arrow rules — non-negotiable:

1. A full arrow shows a pair of electrons; a half-arrow shows one.
2. It starts from a bond or a lone pair — never from an atom, never from a charge.
3. It ends where the electrons go.
4. Charges must balance on both sides.

Four mechanisms and their substrates:

Mechanism	Substrate
Free radical substitution	Alkanes + halogen, UV
Electrophilic addition	Alkenes
Nucleophilic substitution	Halogenoalkanes
Nucleophilic addition	Carbonyl compounds (aldehydes/ketones)

Addition and substitution each come in an electrophilic and a nucleophilic form: electrophilic addition adds an electrophile across an alkene’s electron-rich C=C bond, while nucleophilic addition adds a nucleophile (e.g. CN⁻, from HCN) across a carbonyl’s electron-poor C=O bond — the polarity of both the substrate and the attacking species simply flips between the two.

Exam traps

• Forgetting implied hydrogens in skeletal formulae.
• Giving only one condition for E/Z isomerism.
• Arrows starting at an atom or a positive charge.
• Using full arrows in a radical mechanism.
• Saying a racemic mixture rotates light.
• Numbering from the wrong end of the chain.
• Confusing elimination (losing a small molecule) with substitution (swapping one group for another).
• Treating hybridisation labels as unrelated to shape — sp² always means trigonal planar, sp³ always tetrahedral.

Self-test

1. What is implied at each vertex of a skeletal formula?
2. Give both conditions for E/Z isomerism.
3. What is a chiral centre, and why is a racemate optically inactive?
4. Distinguish homolytic from heterolytic fission and the arrows used for each.
5. Why does an alkene react with an electrophile by addition, while a halogenoalkane reacts with a nucleophile by substitution?
6. State the hybridisation and shape of the carbon atoms in ethene, and explain why this geometry allows the π bond to form.
7. Distinguish elimination from substitution, and give the term for a mechanism combining “electrophilic” with “addition”.

Answers: 1. A carbon atom, with enough hydrogen atoms to give it four bonds. 2. A C=C double bond preventing rotation, and two different groups attached to each of the double-bonded carbons. 3. A carbon bonded to four different groups; a racemate contains equal amounts of both enantiomers, whose equal and opposite rotations cancel. 4. Homolytic fission splits the pair evenly to give two radicals and uses half-arrows; heterolytic fission gives an ion pair and uses full curly arrows. 5. An alkene has no leaving group, so an electrophile can only add across the π bond, breaking it to form two new σ bonds; a halogenoalkane’s halogen is a leaving group, so a nucleophile substitutes it directly rather than adding to the molecule. 6. Both carbons are sp² hybridised, giving a trigonal planar shape; this places all six atoms in a single flat plane, allowing the π bond to form by sideways overlap of the p orbitals above and below that plane. 7. Elimination removes a small molecule, typically forming a double bond, while substitution replaces one atom or group with another; electrophilic addition describes an electrophile attacking an electron-rich double bond so that two reactants become one product.