Introduction to Organic Nomenclature and Isomerism

Welcome to organic chemistry! If you have ever felt overwhelmed by long chemical names or intricate structures, do not worry — organic chemistry is very much like learning a logical language or playing with building blocks. Once you understand a few foundational rules, you will be able to name, draw, and understand almost any organic molecule with confidence.

In this chapter, you will learn how chemists communicate structures clearly using different types of chemical formulas, how to name compounds systematically using the IUPAC (International Union of Pure and Applied Chemistry) rules, and how molecules with the exact same chemical formula can arrange their atoms in surprisingly different ways (isomerism).

Did you know? There are over 100 million known organic compounds! Without a universal, systematic naming system, scientists around the world would never be able to keep track of them all.

1. Types of Chemical Formulas

Chemists use several different ways to represent the same organic molecule, depending on how much detail is needed. Let us explore the six key types of formulas you need to master for your CCEA AS chemistry exam.

A. Empirical Formula

The empirical formula gives the simplest whole number ratio of the atoms of each element present in a compound.
• Example: For butane (\( \text{C}_4\text{H}_{10} \)), divide the subscripts by their greatest common divisor (2) to get \( \text{C}_2\text{H}_5 \).

B. Molecular Formula

The molecular formula gives the actual number of atoms of each element present in one molecule of a compound.
• Example: For butane, the molecular formula is \( \text{C}_4\text{H}_{10} \). For ethanol, it is \( \text{C}_2\text{H}_6\text{O} \).

C. General Formula

An algebraic formula that represents an entire class or homologous series of compounds.
• Example for alkanes: \( \text{C}_n\text{H}_{2n+2} \)
• Example for alkenes: \( \text{C}_n\text{H}_{2n} \)
• Example for alcohols: \( \text{C}_n\text{H}_{2n+1}\text{OH} \)

D. Structural Formula

The structural formula shows the arrangement of atoms in a molecule carbon by carbon, without drawing all the individual chemical bonds.
• Example: Butan-1-ol can be written as \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH} \) or \( \text{CH}_3(\text{CH}_2)_3\text{OH} \).

E. Displayed Formula

The displayed formula shows every single atom and every single bond in the molecule. Covalent bonds are shown as solid lines (e.g., \( \text{C}-\text{H} \), \( \text{C}=\text{C} \), \( \text{O}-\text{H} \)).
Exam Tip: When an exam question asks for a displayed formula, make sure you show the bond between \( \text{O} \) and \( \text{H} \) in an alcohol (\( -\text{O}-\text{H} \)), not just \( -\text{OH} \)!

F. Skeletal Formula

A simplified representation where carbon-carbon bonds are drawn as lines in a zig-zag pattern. Carbon atoms and hydrogen atoms directly bonded to carbons are omitted:
• Each vertex (corner) and line end represents a carbon atom with its associated hydrogen atoms.
• Heteroatoms (like \( \text{O} \), \( \text{N} \), \( \text{Cl} \), \( \text{Br} \)) and any hydrogen atoms bonded to them (like \( -\text{OH} \)) must be shown.

Quick Summary Table of Formulas (Using Propane):
• Empirical: \( \text{C}_3\text{H}_8 \)
• Molecular: \( \text{C}_3\text{H}_8 \)
• General: \( \text{C}_n\text{H}_{2n+2} \)
• Structural: \( \text{CH}_3\text{CH}_2\text{CH}_3 \)
• Displayed: Shows all 3 carbons and 8 hydrogens with explicit single lines for all bonds.
• Skeletal: A two-segment zig-zag line (\( \wedge \)).

Key Takeaway: Different formulas serve different purposes. Structural formulas are quick to type, displayed formulas show every bond clearly, and skeletal formulas provide a clean look for larger structures.

2. Homologous Series and Functional Groups

What is a Functional Group?

A functional group is an atom or group of atoms responsible for the characteristic chemical reactions of a particular compound. For example, the double bond (\( \text{C}=\text{C} \)) in alkenes or the hydroxyl group (\( -\text{OH} \)) in alcohols.

What is a Homologous Series?

A homologous series is a family of organic compounds that share:
1. The same functional group.
2. The same general formula.
3. Similar chemical properties.
4. A gradual trend in physical properties (e.g., boiling point increases as chain length increases due to stronger London dispersion forces).
5. Successive members differing by a \( -\text{CH}_2- \) unit (relative formula mass increases by \( 14 \)).

Common Functional Groups in AS Chemistry:

• Alkane: \( \text{C}-\text{C} \) single bonds only (Suffix: -ane, e.g., Ethane \( \text{CH}_3\text{CH}_3 \))
• Alkene: \( \text{C}=\text{C} \) double bond (Suffix: -ene, e.g., Ethene \( \text{CH}_2=\text{CH}_2 \))
• Halogenoalkane: Carbon bonded to halogen \( -\text{F} \), \( -\text{Cl} \), \( -\text{Br} \), \( -\text{I} \) (Prefix: fluoro-, chloro-, bromo-, iodo-, e.g., Chloroethane \( \text{CH}_3\text{CH}_2\text{Cl} \))
• Alcohol: Hydroxyl group \( -\text{OH} \) (Suffix: -ol, Prefix if needed: hydroxy-, e.g., Ethanol \( \text{CH}_3\text{CH}_2\text{OH} \))
• Aldehyde: Carbonyl group at the end of a chain \( -\text{CHO} \) (Suffix: -al, e.g., Ethanal \( \text{CH}_3\text{CHO} \))
• Ketone: Carbonyl group in the middle of a chain \( -\text{C}(=\text{O})- \) (Suffix: -one, e.g., Propan-2-one \( \text{CH}_3\text{COCH}_3 \))
• Carboxylic Acid: Carboxyl group \( -\text{COOH} \) (Suffix: -oic acid, e.g., Ethanoic acid \( \text{CH}_3\text{COOH} \))

Key Takeaway: The functional group determines the chemical reactivity of the molecule, while the hydrocarbon chain length mainly influences physical properties like boiling point and solubility.

3. Systematic IUPAC Nomenclature

Naming organic molecules is easy when you follow a step-by-step recipe. The systematic IUPAC name consists of three parts:
Prefix (Substituents/Branches) + Root (Longest Carbon Chain) + Suffix (Principal Functional Group)

Step 1: Learn the Carbon Chain Roots

The root name depends on the number of carbon atoms in the longest continuous carbon chain:
• \( 1\text{ Carbon} \): meth-
• \( 2\text{ Carbons} \): eth-
• \( 3\text{ Carbons} \): prop-
• \( 4\text{ Carbons} \): but-
• \( 5\text{ Carbons} \): pent-
• \( 6\text{ Carbons} \): hex-
• \( 7\text{ Carbons} \): hept-
• \( 8\text{ Carbons} \): oct-

Memory Trick: Monkeys Eat Peeled Bananas (Meth-, Eth-, Prop-, But-).

Step 2: Identify the Longest Continuous Carbon Chain

Find the longest continuous chain of carbon atoms. Be careful! The longest chain might bend around a corner on paper. It must contain the principal functional group if one is present.

Step 3: Number the Carbon Chain

Number the carbon atoms starting from the end that gives the principal functional group the lowest possible number (locant). If only alkyl branches or halogens are present, number from the end that gives these substituents the lowest set of numbers.

Step 4: Identify and Name the Branches and Substituents

• Alkyl side-chains: \( -\text{CH}_3 \) (methyl), \( -\text{CH}_2\text{CH}_3 \) (ethyl), \( -\text{CH}_2\text{CH}_2\text{CH}_3 \) (propyl)
• Halogens: \( -\text{F} \) (fluoro), \( -\text{Cl} \) (chloro), \( -\text{Br} \) (bromo), \( -\text{I} \) (iodo)
• If the same substituent appears more than once, use multiplying prefixes: di- (2), tri- (3), tetra- (4). Note: These prefixes do not count when alphabetising!

Step 5: Assemble the Name

• Put substituents in alphabetical order (e.g., bromo- comes before chloro-, ethyl- comes before methyl-).
• Use hyphens between numbers and letters (e.g., 2-chlorobutane).
• Use commas between numbers (e.g., 2,2-dimethylpropane).
• When adding a suffix that starts with a vowel (like -ol, -al, -one, -oic acid), drop the terminal 'e' from the alkane root (e.g., propan-1-ol, not propane-1-ol). Keep the 'e' if the suffix starts with a consonant (e.g., propane-1,2-diol).

Punctuation and Worked Example:

Let us name: \( \text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}(\text{Cl})-\text{CH}_3 \)
1. Longest continuous chain = 4 carbons \( \rightarrow \) butane.
2. Number from right-to-left to give substituents the lowest numbers: \( \text{C}2 \) gets the chloro group, \( \text{C}3 \) gets the methyl group. (Left-to-right would give 2,3 as well, but alphabetical priority gives \( \text{C}2 \) to chloro).
3. Alphabetical order: 'c' before 'm' \( \rightarrow \) 2-chloro-3-methylbutane.

Key Takeaway: Always find the longest carbon chain first, number to give functional groups the lowest locants, and list substituents alphabetically ignoring multiplying prefixes.

4. Structural Isomerism

Isomers are molecules that have the same molecular formula but a different arrangement of atoms.

Structural isomerism occurs when compounds have the same molecular formula but different structural formulas (the atoms are connected in a different order). There are three types you need to know:

A. Chain Isomerism

Chain isomers have the same molecular formula, but their carbon skeleton is arranged differently (e.g., straight chain vs branched chain).
• Example (\( \text{C}_4\text{H}_{10} \)):
1. Butane: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \)
2. 2-Methylpropane: \( \text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_3 \)
• Physical properties: Branched chain isomers have lower boiling points than straight-chain isomers because the molecules cannot pack as closely together, resulting in weaker London dispersion forces.

B. Positional Isomerism

Positional isomers have the same carbon skeleton and the same functional group, but the functional group is attached to a different position on the carbon chain.
• Example 1 (\( \text{C}_3\text{H}_7\text{Br} \)):
1. 1-Bromopropane: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} \)
2. 2-Bromopropane: \( \text{CH}_3\text{CH}(\text{Br})\text{CH}_3 \)
• Example 2 (\( \text{C}_4\text{H}_8 \)):
1. But-1-ene: \( \text{CH}_2=\text{CH}-\text{CH}_2-\text{CH}_3 \)
2. But-2-ene: \( \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 \)

C. Functional Group Isomerism

Functional group isomers have the same molecular formula, but the atoms are arranged into completely different functional groups. Consequently, they have vastly different physical and chemical properties.
• Pair 1: Alkenes and Cycloalkanes (\( \text{C}_n\text{H}_{2n} \))
\( \text{C}_3\text{H}_6 \): Propene (\( \text{CH}_2=\text{CH}-\text{CH}_3 \)) and Cyclopropane (a 3-carbon ring).
• Pair 2: Alcohols and Ethers (\( \text{C}_n\text{H}_{2n+2}\text{O} \))
\( \text{C}_2\text{H}_6\text{O} \): Ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)) and Methoxymethane (\( \text{CH}_3-\text{O}-\text{CH}_3 \)).
• Pair 3: Aldehydes and Ketones (\( \text{C}_n\text{H}_{2n}\text{O} \))
\( \text{C}_3\text{H}_6\text{O} \): Propanal (\( \text{CH}_3\text{CH}_2\text{CHO} \)) and Propanone (\( \text{CH}_3\text{COCH}_3 \)).
• Pair 4: Carboxylic Acids and Esters (\( \text{C}_n\text{H}_{2n}\text{O}_2 \))
\( \text{C}_2\text{H}_4\text{O}_2 \): Ethanoic acid (\( \text{CH}_3\text{COOH} \)) and Methyl methanoate (\( \text{HCOOCH}_3 \)).

Key Takeaway: Structural isomers have identical molecular formulas. Chain isomers differ in their carbon backbone, position isomers differ in the placement of the functional group, and functional group isomers belong to entirely different families.

5. Stereoisomerism: \( E \)/\( Z \) and cis/trans Isomerism

What is Stereoisomerism?

Stereoisomers have the same molecular formula and the same structural formula (the atoms are joined in the same order), but their atoms are arranged differently in three-dimensional space.

Why Does \( E \)/\( Z \) Isomerism Occur?

\( E \)/\( Z \) isomerism is a form of stereoisomerism found in alkenes. It requires two specific conditions:
1. Restricted rotation around the \( \text{C}=\text{C} \) double bond (due to the presence of the \( \pi \) (pi) bond formed by the sideways overlap of p-orbitals).
2. Two different groups attached to each carbon atom of the \( \text{C}=\text{C} \) double bond.

Analogy: Imagine holding two pencils parallel in your hands. You cannot twist one hand without breaking the pencils! That rigid double bond prevents rotation, locking substituents in place on either side.

Important Non-Example: Propene (\( \text{CH}_2=\text{CH}-\text{CH}_3 \)) does not show stereoisomerism because carbon-1 is bonded to two identical atoms (two hydrogens: \( \text{H} \) and \( \text{H} \)).

The Cahn-Ingold-Prelog (CIP) Priority Rules

To assign \( E \) or \( Z \) nomenclature systematically:
1. Look at the two atoms directly attached to the left-hand carbon of the \( \text{C}=\text{C} \) bond. Assign higher priority to the atom with the higher atomic number (\( Z \)). If there is a tie, move along the chain until the first point of difference.
2. Look at the two atoms directly attached to the right-hand carbon of the \( \text{C}=\text{C} \) bond. Assign priority in the exact same way.
3. Compare the positions of the two highest-priority groups:
• \( Z \)-isomer (Zusammen = "together"): The highest-priority groups are on the same side of the double bond (both 'top' or both 'bottom').
• \( E \)-isomer (Entgegen = "opposite"): The highest-priority groups are on opposite sides across the double bond (one 'top', one 'bottom').

Memory Aids:
• \( Z \) = Zame Zide (Same Side).
• \( E \) = Enemies on opposite sides (or Across).

cis / trans vs. \( E \)/\( Z \)

• The cis/trans notation is an older system that is only unambiguous when each carbon of the double bond has an identical group (usually a hydrogen atom).
• cis-isomer: Identical groups on the same side.
• trans-isomer: Identical groups on opposite sides.
• Example: In but-2-ene:
- cis-but-2-ene is \( (Z) \)-but-2-ene (both \( -\text{CH}_3 \) groups on the same side).
- trans-but-2-ene is \( (E) \)-but-2-ene (the \( -\text{CH}_3 \) groups are on opposite sides).
• Note: For complex molecules where four different groups are attached to the double bond (e.g., 1-bromo-2-chloro-1-fluoroethene), the cis/trans system fails, and you must use the IUPAC \( E \)/\( Z \) CIP priority rules.

Key Takeaway: \( E \)/\( Z \) isomerism arises from restricted rotation around the \( \text{C}=\text{C} \) bond and requires two different groups on each carbon of the double bond. Prioritise groups using atomic numbers (CIP rules).

Common Mistakes to Avoid in Exams

• Missing the \( -\text{O}-\text{H} \) bond in displayed formulas: Remember that displayed formulas must show all bonds. Writing \( -\text{OH} \) loses marks; you must draw \( -\text{O}-\text{H} \).
• Forgetting alphabetical order: Naming a compound 2-methyl-3-bromobutane instead of 2-bromo-3-methylbutane.
• Incorrect locant numbering: Always ensure the principal functional group has the lowest locant number possible.
• Confusing structural isomers and stereoisomers: Structural isomers have different bonding connections; stereoisomers have identical connections but different 3D spatial orientations.
• Assuming all alkenes have \( E \)/\( Z \) isomers: Always check both carbons of the double bond. If either carbon has two identical attached groups (like \( -\text{CH}_2 \)), it cannot exhibit \( E \)/\( Z \) isomerism.

Quick Review Checklist

Before moving on to the next chapter, make sure you can:
1. Draw empirical, molecular, general, structural, displayed, and skeletal formulas.
2. State the defining characteristics of a homologous series.
3. Apply IUPAC rules to name alkanes, alkenes, halogenoalkanes, and alcohols accurately.
4. Distinguish between chain, positional, and functional group isomers.
5. State the two conditions required for \( E \)/\( Z \) stereoisomerism.
6. Apply Cahn-Ingold-Prelog (CIP) priority rules to classify an alkene as \( (E) \) or \( (Z) \).