Introduction to organic chemistry - Chemistry (9620) - Oxford AQA A-Level

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Welcome to the World of Organic Chemistry!

Hello future chemist! This chapter is your essential starting block for understanding organic chemistry — the fascinating study of carbon compounds. Why is carbon so special? Because it can form millions of different compounds, making up everything from the fuels that power our world (like petrol and gas) to the complex molecules of life (like DNA and proteins).

In these notes, we will break down the language, drawing rules, and fundamental concepts that govern all organic molecules. Don't worry if this seems tricky at first; learning organic chemistry is like learning a new language — once you master the basics (like the alphabet and grammar), you can build endless complex sentences!

3.3.1 Nomenclature and Formulas: Speaking the Organic Language

Before we can talk about reactions, we need to know how to name and draw the molecules. Organic chemists use several types of formulas to represent a single compound, depending on how much detail they need to show.

Different Ways to Represent Organic Compounds

These six types of formulae allow chemists to communicate the structure of a molecule precisely. Let’s take ethanol (drinking alcohol) as an example:

Empirical Formula: The simplest whole number ratio of atoms of each element in a compound.
$C_2H_6O \longrightarrow CH_3O$
Molecular Formula: The actual number of atoms of each element in a compound.
$C_2H_6O$
General Formula: A formula representing any member of a homologous series.
For alkanes: $C_nH_{2n+2}$ (e.g., $n=2$, $C_2H_6$)
For alcohols (like ethanol): $C_nH_{2n+1}OH$
Structural Formula: Shows the minimal detail necessary to identify the arrangement of atoms, especially functional groups.
Example: $CH_3CH_2OH$
Displayed Formula: Shows every atom and every single covalent bond. This is the most detailed 2D representation.
(Cannot be drawn using text tags, but imagine C-C, C-H, and C-O bonds explicitly drawn out.)
Skeletal Formula: A highly simplified structure used mainly for carbon chains.
- Carbon atoms are assumed to be at the ends and corners of lines (zig-zags).
- Hydrogen atoms attached to carbon are omitted (assumed to be there to satisfy carbon's four bonds).
- All other atoms (like O, N, halogens) and their attached H atoms must be shown.

Homologous Series

A homologous series is a family of organic compounds that share key characteristics:

They contain the same functional group (the part of the molecule that determines its chemical reactions).
Each member differs from the next by a constant unit, usually $\text{CH}_2$.
They can be represented by the same general formula.
Their physical properties (like boiling point) show a gradual trend as the chain length increases.

Did you know? The prefix "homo" means "same." So, a homologous series is a "same type" series!

IUPAC Nomenclature Rules (Naming)

The International Union of Pure and Applied Chemistry (IUPAC) provides rules to ensure every compound has a unique, systematic name. For AS level, you must be able to name chains and rings with up to six carbon atoms.

The name is based on three parts: Prefix - Root - Suffix.

1. Root (Parent Chain Length):

1 Carbon	Meth-
2 Carbons	Eth-
3 Carbons	Prop-
4 Carbons	But-
5 Carbons	Pent-
6 Carbons	Hex-

2. Suffix (Functional Group): This tells you the primary family it belongs to.

Alkanes (single C-C bonds): -ane
Alkenes (C=C double bonds): -ene
Alcohols (-OH group): -ol

3. Prefix (Substituents/Side Chains): Anything hanging off the main chain (e.g., halogens, alkyl groups).

The Naming Steps (Simplified):

Find the longest continuous carbon chain that contains the functional group (this gives you the Root).
Identify the functional group (this gives you the Suffix).
Number the chain so that the functional group gets the lowest possible number.
Identify any side chains/substituents (this gives you the Prefix). Number these substituents based on their attachment point.
Assemble the name: (Number and Prefix)-(Root)-(Number and Suffix).

Example: $\text{CH}_3\text{CH}(\text{OH})\text{CH}_3$ $\rightarrow$ Prop-an-2-ol (a 3-carbon chain (Prop), saturated (an), with an alcohol group on carbon 2 (-2-ol)).

Key Takeaway: Formulas and Nomenclature

Structure is everything in organic chemistry. You must be able to fluently move between drawing displayed, structural, and skeletal formulae, and apply IUPAC rules consistently to chains up to C6.

3.3.1.2 Reaction Mechanisms: Tracking Electron Movement

A reaction mechanism is a step-by-step description of how bonds are broken and formed during a chemical reaction. Understanding mechanisms helps predict products!

The Language of Curly Arrows

When electrons move, we draw curly arrows. This is critical for understanding all organic mechanisms (except free-radical ones).

The golden rule for curly arrows: They show the movement of an electron PAIR.

Formation of a covalent bond: The curly arrow starts from a lone electron pair (on an atom) or from a covalent bond (e.g., a pi bond) and points towards the atom that will accept the bond.
Breaking of a covalent bond: The curly arrow starts from the middle of the covalent bond and points towards the atom that is taking both electrons from the bond.

Analogy: Think of a curly arrow as a path taken by a pair of dancers (the electrons). The arrow shows where they start and where they end up.

Free-Radical Mechanisms

Some reactions involve highly reactive species called free radicals.

A free radical is a species with an unpaired electron.
The unpaired electron is represented by a single dot ($\cdot$). Example: The chlorine free radical is $\text{Cl}\cdot$.

Unlike other mechanisms, you are not required to use curly arrows for free-radical mechanisms, but you must write balanced equations for the steps:

Initiation: Creates radicals (usually using UV light or heat to break a bond homolytically).
Propagation: Radicals react with stable molecules to form new stable molecules and new radicals (a chain reaction).
Termination: Two radicals combine to form a stable, non-radical molecule, ending the chain.

Common Mistake Alert!

When drawing mechanisms that involve ions (like nucleophilic substitution), always make sure your curly arrows start precisely from the lone pair or the bond, and not just randomly pointing from the atom itself.

3.3.1.3 Isomerism: Different Structures, Same Ingredients

Isomers are compounds that have the same molecular formula but different arrangements of atoms.

Structural Isomerism

Structural isomers have the same molecular formula but different structural formulas (they are connected differently).

There are three main types of structural isomerism:

Chain Isomerism: Different arrangements of the carbon backbone (straight chain vs. branched chain).
Example: Butane ($\text{C}_4\text{H}_{10}$) vs. Methylpropane ($\text{C}_4\text{H}_{10}$)
Position Isomerism: The functional group is attached at a different position on the carbon chain.
Example: Pentan-1-ol vs. Pentan-2-ol
Functional Group Isomerism: The atoms are arranged into completely different functional groups.
Example: Ethanol ($\text{C}_2\text{H}_6\text{O}$, an alcohol) vs. Methoxymethane ($\text{C}_2\text{H}_6\text{O}$, an ether)

Stereoisomerism

Stereoisomers have the same structural formula (same connectivity) but the atoms are arranged differently in 3D space. The only form you need to study in detail here is E-Z isomerism.

E-Z Isomerism (Geometric Isomerism)

This type of stereoisomerism occurs when there is restricted rotation around a carbon-carbon double bond ($C=C$).

For E-Z isomerism to occur, each carbon atom in the double bond must be attached to two different groups.

We use the Cahn-Ingold-Prelog (CIP) Priority Rules to determine if an isomer is E or Z.

CIP Priority Rule:
Priority is given based on the atomic number of the atom directly attached to the double-bonded carbon.

Higher atomic number = Higher priority.
If the atoms are the same (e.g., both are C), you move out to the next atoms until you find a difference.

E and Z Designation:

Z (Zusammen, "together"): The two highest-priority groups are on the same side of the double bond (either both up or both down).
E (Entgegen, "opposite"): The two highest-priority groups are on opposite sides of the double bond.

Memory Aid: Z is for Zame Side (Zusammen/Same).

Example: Consider 1-bromo-1-chloroethene.
Carbon 1 is attached to Br and Cl. Br (Z=35) has higher priority than Cl (Z=17).
Carbon 2 is attached to H and C. C (Z=6) has higher priority than H (Z=1).
If the Br and the C groups are on opposite sides, it is the (E) isomer. If they are on the same side, it is the (Z) isomer.

Quick Review: Isomerism

Structural Isomers: Different bonding pattern (e.g., but-1-ene and methylpropene).
Stereoisomers: Same bonding pattern, different 3D shape (e.g., (E)-but-2-ene and (Z)-but-2-ene).
E/Z condition: Each C of the double bond must be attached to two different groups.

Congratulations! You have covered the foundational concepts of representation, naming, and structural rules in organic chemistry. This framework is essential for every chapter that follows.