OCR A LEVEL MODULE 4: ORGANIC CHEMISTRY
OCR A 4.1.1 Basic concepts of organic chemistry
REVISION
🧪 Types of Chemical Formulae
General Formula
Algebraic expression that applies to every member of a homologous family.
Alkanes:
Alkenes (1 double bond):
Cycloalkanes:
Alcohols: (written as in transcript)
= number of carbon atoms.
Molecular Formula
Actual count of atoms in one molecule.
Example: ethane =
Empirical Formula
Simplest whole-number ratio of atoms in a compound.
Example:
Molecular formula → Empirical formula
(Do not worry if it looks chemically odd; it is only a ratio.)
Structural Formula
Shows how atoms are grouped without drawing every bond.
Example: butan-1-ol
(chunked notation, no C–C or C–H bonds shown)
Skeletal Formula
Zig-zag lines represent the carbon backbone; C and H atoms omitted, functional groups shown.
Each vertex = carbon
butan-1-ol skeleton: 4 vertices (carbons) with –OH on the first carbon
Displayed Formula
Every atom and bond drawn.
Example: 2-chloropropane
Full Lewis structure with all C–H, C–C, and C–Cl bonds displayed
,🧬 Homologous Series & Naming Patterns
A homologous series is a family of compounds with the same functional group and same general formula;
successive members differ by .
🌲
Carbon Skeleton Types
1. Aromatic
Contains benzene ring (hexagon with circle inside or three double bonds)
May carry alkyl or functional-group substituents.
2. Aliphatic
Straight or branched chains; no rings.
3. Alicyclic
Ring structures that are NOT aromatic (no delocalised circle).
Example: cyclohexane
✏️IUPAC Naming Protocol
International Union of Pure & Applied Chemistry rules:
1. Find longest continuous carbon chain → gives stem (meth-, eth-, prop-,
but-, pent-, hex-, hept-, oct-, non-, dec-)
2. Identify principal functional group → provides suffix
3. Number chain so functional group sits on lowest possible carbon
4. Locate & alphabetise substituents as prefixes (di-, tri-… for multiples)
Example:
Three methyl, four-carbon chain, two –OH on C1 → 3-methylbutane-1,1-diol
, ↩️Reaction Mechanisms & Curly Arrows
A mechanism depicts the movement of electron pairs during reaction.
Curly arrow () starts from electron-rich source (lone pair or π-bond) and
ends where new bond forms
Double-headed arrow = pair of electrons
Example: OH⁻ + bromoethane
Lone pair on O⁻ attacks Cδ⁺ of C–Br
Simultaneously C–Br bond pair moves onto Br
Products: ethanol + Br⁻
Do NOT use curly arrows for free-radical steps.
⚡ Bond Fission Types
🪞
Structural Isomerism
Same molecular formula, different connectivity.
1. Chain Isomerism
Different carbon skeleton branching
Example: pentane vs 2-methylbutane
2. Positional Isomerism
Same skeleton, functional group in different position
Example: pentan-1-ol vs pentan-2-ol
3. Functional-Group Isomerism
Same atoms, different functional group
Example: pent-1-ene () vs cyclopentane ()
Reminder:
When drawing, rotate or bend a chain does NOT create a new isomer—longest
continuous chain must change or substituent position must differ.
OCR A 4.1.2 Alkanes REVISION
OCR A 4.1.1 Basic concepts of organic chemistry
REVISION
🧪 Types of Chemical Formulae
General Formula
Algebraic expression that applies to every member of a homologous family.
Alkanes:
Alkenes (1 double bond):
Cycloalkanes:
Alcohols: (written as in transcript)
= number of carbon atoms.
Molecular Formula
Actual count of atoms in one molecule.
Example: ethane =
Empirical Formula
Simplest whole-number ratio of atoms in a compound.
Example:
Molecular formula → Empirical formula
(Do not worry if it looks chemically odd; it is only a ratio.)
Structural Formula
Shows how atoms are grouped without drawing every bond.
Example: butan-1-ol
(chunked notation, no C–C or C–H bonds shown)
Skeletal Formula
Zig-zag lines represent the carbon backbone; C and H atoms omitted, functional groups shown.
Each vertex = carbon
butan-1-ol skeleton: 4 vertices (carbons) with –OH on the first carbon
Displayed Formula
Every atom and bond drawn.
Example: 2-chloropropane
Full Lewis structure with all C–H, C–C, and C–Cl bonds displayed
,🧬 Homologous Series & Naming Patterns
A homologous series is a family of compounds with the same functional group and same general formula;
successive members differ by .
🌲
Carbon Skeleton Types
1. Aromatic
Contains benzene ring (hexagon with circle inside or three double bonds)
May carry alkyl or functional-group substituents.
2. Aliphatic
Straight or branched chains; no rings.
3. Alicyclic
Ring structures that are NOT aromatic (no delocalised circle).
Example: cyclohexane
✏️IUPAC Naming Protocol
International Union of Pure & Applied Chemistry rules:
1. Find longest continuous carbon chain → gives stem (meth-, eth-, prop-,
but-, pent-, hex-, hept-, oct-, non-, dec-)
2. Identify principal functional group → provides suffix
3. Number chain so functional group sits on lowest possible carbon
4. Locate & alphabetise substituents as prefixes (di-, tri-… for multiples)
Example:
Three methyl, four-carbon chain, two –OH on C1 → 3-methylbutane-1,1-diol
, ↩️Reaction Mechanisms & Curly Arrows
A mechanism depicts the movement of electron pairs during reaction.
Curly arrow () starts from electron-rich source (lone pair or π-bond) and
ends where new bond forms
Double-headed arrow = pair of electrons
Example: OH⁻ + bromoethane
Lone pair on O⁻ attacks Cδ⁺ of C–Br
Simultaneously C–Br bond pair moves onto Br
Products: ethanol + Br⁻
Do NOT use curly arrows for free-radical steps.
⚡ Bond Fission Types
🪞
Structural Isomerism
Same molecular formula, different connectivity.
1. Chain Isomerism
Different carbon skeleton branching
Example: pentane vs 2-methylbutane
2. Positional Isomerism
Same skeleton, functional group in different position
Example: pentan-1-ol vs pentan-2-ol
3. Functional-Group Isomerism
Same atoms, different functional group
Example: pent-1-ene () vs cyclopentane ()
Reminder:
When drawing, rotate or bend a chain does NOT create a new isomer—longest
continuous chain must change or substituent position must differ.
OCR A 4.1.2 Alkanes REVISION
