Welcome to Organic Chemistry: Nomenclature, Structure and Isomerism
Welcome to Unit A2 2: Organic Chemistry! Organic chemistry is the study of carbon-based molecules, which form the chemical backbone of all living systems and pharmaceuticals. While organic structures can look like a maze of lines and letters at first, they follow a logical set of international rules known as IUPAC nomenclature. In this chapter, you will learn how to represent molecules, name them systematically, classify them, and explore how molecules with the exact same chemical formula can take on completely different shapes and properties through isomerism.
Don't worry if this seems tricky at first! Once you learn the basic building blocks and naming steps, you will be able to decode even complex-looking organic structures with confidence.
---1. Types of Chemical Formulae & Key Terminology
In organic chemistry, we use several types of formulae depending on how much detail we need to show:
1. Empirical Formula: The simplest whole-number ratio of atoms of each element present in a compound.
Example: The empirical formula of butane (\(\text{C}_4\text{H}_{10}\)) is \(\text{C}_2\text{H}_5\).
2. Molecular Formula: The actual number of atoms of each element present in one molecule of a compound.
Example: Butane has the molecular formula \(\text{C}_4\text{H}_{10}\).
To calculate the molecular formula from the empirical formula, use the relationship:
\(\text{Molecular Formula} = (\text{Empirical Formula})_n\), where \(n = \frac{\text{M}_r\text{ (or RFM)}}{\text{Empirical Formula Mass}}\).
3. General Formula: An algebraic formula representing the composition of any member of an entire homologous series.
Examples:
• Alkanes: \(\text{C}_n\text{H}_{2n+2}\)
• Alkenes: \(\text{C}_n\text{H}_{2n}\)
• Alcohols: \(\text{C}_n\text{H}_{2n+1}\text{OH}\)
4. Structural Formula: Shows how atoms are arranged in a molecule without drawing every individual single bond.
Example: Propan-1-ol is written as \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\).
5. Displayed (Full Structural) Formula: A 2D drawing showing all atoms and all covalent bonds explicitly between them.
6. Skeletal Formula: A simplified line-structure representation where carbon-carbon bonds are drawn as lines. Carbon atoms and attached hydrogen atoms are implied at vertices and ends of lines, while functional groups and heteroatoms (like \(\text{O}\), \(\text{Cl}\), \(\text{Br}\)) are drawn explicitly.
Core Definitions to Memorise
• Homologous Series: A series of organic compounds with the same functional group, identical general formula, similar chemical properties, and a gradation in physical properties, where each successive member differs by a \(-\text{CH}_2-\) unit.
• Functional Group: An atom or group of atoms responsible for the characteristic chemical properties and reactions of a particular compound.
Key Takeaway: Always read exam questions carefully: if an examiner asks for a displayed formula, you must show every single bond explicitly—including the bond between oxygen and hydrogen in \(-\text{O}-\text{H}\)!
---2. IUPAC Nomenclature: Systematic Naming
Naming organic compounds is like following a recipe. We find the longest carbon chain (the stem/root), identify the functional group (the suffix), and add any branching side-chains or halogens (the prefixes).
Carbon Chain Root (Stem) Prefixes
The stem tells you how many carbon atoms are in the continuous main chain:
• \(1\text{C}\): meth-
• \(2\text{C}\): eth-
• \(3\text{C}\): prop-
• \(4\text{C}\): but-
• \(5\text{C}\): pent-
• \(6\text{C}\): hex-
Memory Trick: Monkeys Eat Peanut Butter (Meth-, Eth-, Prop-, But-).
Homologous Series & Suffixes for A2 2
• Alkanes: Contain only \(\text{C}-\text{C}\) single bonds \(\rightarrow\) Suffix: -ane (e.g., \(\text{ethane}\)).
• Alkenes: Contain a \(\text{C}=\text{C}\) double bond \(\rightarrow\) Suffix: -ene. Include the locant number for the double bond where needed (e.g., \(\text{but-1-ene}\), \(\text{but-2-ene}\)).
• Alcohols: Contain a hydroxyl group (\(-\text{OH}\)) \(\rightarrow\) Suffix: -ol (e.g., \(\text{propan-1-ol}\), \(\text{propan-2-ol}\)).
• Carboxylic Acids: Contain a carboxyl group (\(-\text{COOH}\)) \(\rightarrow\) Suffix: -oic acid (e.g., \(\text{ethanoic acid}\)). The carboxyl carbon is always carbon 1 (\(\text{C-1}\)), so no number is needed.
• Aldehydes: Contain a carbonyl group at the end of the chain (\(-\text{CHO}\)) \(\rightarrow\) Suffix: -al (e.g., \(\text{ethanal}\)).
• Ketones: Contain a carbonyl group (\(\text{C}=\text{O}\)) non-terminally within the chain \(\rightarrow\) Suffix: -one (e.g., \(\text{propanone}\), \(\text{pentan-2-one}\)).
• Halogenoalkanes: Prefix denoting the halogen: fluoro-, chloro-, bromo-, iodo- preceded by the position number (e.g., \(\text{2-chloropropane}\)).
• Alkyl Branches: Side chains such as methyl (\(-\text{CH}_3\)) and ethyl (\(-\text{C}_2\text{H}_5\)). If there are multiple identical side groups, use prefixes: di- (2), tri- (3), tetra- (4).
Punctuation Rules in IUPAC Naming
• Separate numbers from letters with a hyphen (e.g., \(\text{2-methylbutane}\)).
• Separate numbers from numbers with a comma (e.g., \(\text{2,2-dimethylpropane}\)).
• Do not leave spaces between words (except for carboxylic acids, e.g., \(\text{propanoic acid}\)).
Step-by-Step Guide: How to Name a Molecule
1. Find the longest continuous carbon chain: This gives the root name.
2. Identify the principal functional group: This gives the suffix.
3. Number the chain: Start from the end that gives the principal functional group (or substituents) the lowest possible number.
4. Identify side chains/substituents: List them alphabetically with their locant numbers at the front of the name.
3. Classification of Alcohols and Halogenoalkanes
Alcohols and halogenoalkanes are classified as primary (\(1^\circ\)), secondary (\(2^\circ\)), or tertiary (\(3^\circ\)) depending on the environment of the carbon atom directly bonded to the functional group:
• Primary (\(1^\circ\)): The carbon bearing the \(-\text{OH}\) or halogen is bonded directly to 0 or 1 other carbon atom.
Examples: Propan-1-ol (\(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\)), 1-chloropropane (\(\text{CH}_3\text{CH}_2\text{CH}_2\text{Cl}\)).
• Secondary (\(2^\circ\)): The carbon bearing the \(-\text{OH}\) or halogen is bonded directly to 2 other carbon atoms.
Examples: Propan-2-ol (\(\text{CH}_3\text{CH(OH)CH}_3\)), 2-chloropropane (\(\text{CH}_3\text{CHClCH}_3\)).
• Tertiary (\(3^\circ\)): The carbon bearing the \(-\text{OH}\) or halogen is bonded directly to 3 other carbon atoms.
Examples: 2-methylpropan-2-ol (\((\text{CH}_3)_3\text{COH}\)), 2-chloro-2-methylpropane (\((\text{CH}_3)_3\text{CCl}\)).
Quick Check: Look directly at the carbon holding the functional group. Count how many carbons are attached to it: \(1 \rightarrow 1^\circ\), \(2 \rightarrow 2^\circ\), \(3 \rightarrow 3^\circ\).
---4. Isomerism in Organic Compounds
Isomers are molecules that have the same molecular formula but a different arrangement of atoms. Isomerism is split into two major categories: Structural Isomerism and Stereoisomerism.
A. Structural Isomerism
Structural isomers have the same molecular formula but different structural formulae. There are three sub-types:
1. Chain Isomerism:
Molecules have the same functional group and molecular formula, but the carbon skeleton is arranged differently (branched vs straight chain).
Example: Molecular formula \(\text{C}_4\text{H}_{10}\) can be butane (\(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3\)) or 2-methylpropane (\(\text{CH}_3\text{CH(CH}_3)\text{CH}_3\)).
2. Positional Isomerism:
The carbon skeleton and functional group remain the same, but the functional group is attached at different positions along the chain.
Examples:
• Propan-1-ol and propan-2-ol (\(\text{C}_3\text{H}_8\text{O}\))
• But-1-ene and but-2-ene (\(\text{C}_4\text{H}_8\))
3. Functional Group Isomerism:
Molecules have the same molecular formula, but the atoms are rearranged into entirely different functional groups, giving them different chemical properties.
Examples:
• Alcohols and Ethers: \(\text{C}_2\text{H}_6\text{O}\) can be ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)) or methoxymethane (\(\text{CH}_3\text{OCH}_3\)).
• Alkenes and Cycloalkanes: \(\text{C}_3\text{H}_6\) can be propene (\(\text{CH}_3\text{CH}=\text{CH}_2\)) or cyclopropane.
B. Stereoisomerism: \(E/Z\) (Geometric) Isomerism
Stereoisomers have the same molecular and structural formula, but a different 3D spatial arrangement of their atoms in space.
Conditions Required for \(E/Z\) Isomerism:
1. Restricted rotation around a carbon-carbon double bond (\(\text{C}=\text{C}\)). The double bond cannot twist freely without breaking the pi bond.
2. Two different groups/atoms attached to each carbon atom of the \(\text{C}=\text{C}\) double bond.
Distinguishing Between \(E\) and \(Z\) Isomers:
• \(Z\)-Isomer (Zusammen = "together"): The higher-priority or identical groups are on the same side of the double bond (both "above" or both "below").
• \(E\)-Isomer (Entgegen = "opposite"): The higher-priority or identical groups are on opposite sides across the double bond.
Memory Aid: Think \(Z\) = Zame Zide (same side)!
Example: But-2-ene (\(\text{CH}_3\text{CH}=\text{CHCH}_3\)):
• In \(Z\)-but-2-ene, both methyl (\(-\text{CH}_3\)) groups are on the same side of the double bond.
• In \(E\)-but-2-ene, the two methyl groups are on opposite sides.
Important Test: Why doesn't but-1-ene (\(\text{CH}_2=\text{CHCH}_2\text{CH}_3\)) show \(E/Z\) isomerism? Because the first carbon has two identical hydrogen atoms bonded to it (\(\text{C-1}\) has \(-\text{H}\) and \(-\text{H}\)). Swapping them creates the exact same molecule!
---5. Examiner Pitfalls & Common Mistakes Checklist
Review this checklist before your examinations to avoid dropping easy marks:
• Displayed Formula Incompleteness: In a displayed formula question, writing \(-\text{OH}\) or \(-\text{CHO}\) loses marks. You must show the single bond between \(\text{O}\) and \(\text{H}\) as \(-\text{O}-\text{H}\).
• Missing Punctuation: Writing 2 methylbutane or 2 2-dimethylpropane is incorrect. Always write 2-methylbutane and 2,2-dimethylpropane.
• Numbering from the Wrong End: Always ensure the functional group gets the lowest possible locant number (e.g., \(\text{butan-2-ol}\), NOT \(\text{butan-3-ol}\)).
• Confusing "-ol" and "-al": Ensure clear spelling. Examiners frequently penalise ambiguous handwriting between alcohols (-ol) and aldehydes (-al).
• \(E/Z\) Eligibility: Before naming an isomer as \(E\) or \(Z\), check that both carbons on either side of the \(\text{C}=\text{C}\) bond hold two different groups.
6. Chapter Summary & Quick Review
• Empirical vs Molecular: Empirical is the simplest whole-number ratio; Molecular is the actual number of atoms.
• Homologous Series: Families of compounds with the same functional group and general formula, differing by \(-\text{CH}_2-\).
• Naming: Stem (number of carbons) + Suffix (main functional group) + Prefixes (branches/halogens).
• Classification: Primary (\(1^\circ\)), secondary (\(2^\circ\)), and tertiary (\(3^\circ\)) describe whether the functional carbon is attached to 0–1, 2, or 3 other carbon atoms.
• Structural Isomers: Same molecular formula, different structural formulae (Chain, Positional, Functional Group).
• Stereoisomers (\(E/Z\)): Same structural formula, different 3D arrangement due to restricted rotation around \(\text{C}=\text{C}\) and two different groups on each double-bonded carbon.