Welcome to AS Level Organic Chemistry!

Hey there! Getting started with organic chemistry can feel like learning a whole new language, but don't worry—it’s just the chemistry of carbon compounds, and carbon is an incredibly sociable atom!

This chapter, "An introduction to AS Level organic chemistry," is your foundation. We’ll cover how to name these molecules, what makes them react, how they are shaped, and why sometimes two different molecules can share the exact same chemical formula. Once you master these basics, the rest of the organic syllabus will make much more sense. Let’s dive in!

1. Formulas, Functional Groups, and Naming (13.1)

1.1 The Basics: Hydrocarbons and Alkanes

First, let's define the fundamental building blocks:

  • Hydrocarbon: A compound made up of Carbon (C) and Hydrogen (H) atoms only.
  • Alkane: These are the simplest hydrocarbons. They contain only single bonds (C–C) and C–H bonds. They have no functional group and are therefore quite unreactive.

1.2 Functional Groups: The Personality of a Molecule

The chemical and physical properties of an organic molecule are determined by its 'special part'—the functional group.

Analogy: Think of a functional group like the engine of a car. You can have the same body (the rest of the carbon chain), but the type of engine (functional group) dictates whether it drives fast (reactive) or slow (unreactive).

You must be able to recognise the following functional groups (referencing the syllabus table on pages 20-21):

  • Alkenes (\(\text{C=C}\) bond)
  • Halogenoalkanes (\(\text{R-X}\))
  • Alcohols (\(\text{R-OH}\))
  • Aldehydes (\(\text{R-CHO}\))
  • Ketones (\(\text{R-COR'}\))
  • Carboxylic acids (\(\text{R-COOH}\))
  • Esters (\(\text{R-COOR'}\))
  • Amines (\(\text{R-NH}_2\)) - primary only at AS level
  • Nitriles (\(\text{R-C\equiv N}\))

1.3 Representing Organic Molecules

You need to interpret and use four main types of formula:

1. General Formula: Shows the ratio of atoms in a homologous series.
Example: Alkanes: \(\text{C}_n\text{H}_{2n+2}\)

2. Molecular Formula: Shows the actual number of atoms of each element present.
Example: Propane: \(\text{C}_3\text{H}_8\)

3. Empirical Formula: Shows the simplest whole-number ratio of atoms present.
Example: Glucose is \(\text{C}_6\text{H}_{12}\text{O}_6\); its Empirical Formula is \(\text{CH}_2\text{O}\).

4. Structural Formula: Shows the minimal detail necessary to indicate how atoms are arranged.
Example: Propan-1-ol: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\)

5. Displayed Formula: Shows all the atoms and all the bonds (single, double, or triple) explicitly.

6. Skeletal Formula: This is the quick drawing method (used mostly in higher chemistry):

  • Carbon chains are represented by lines (ends or vertices are C atoms).
  • H atoms attached to C atoms are omitted (assumed to be there to complete the valency of four).
  • Functional groups and their attached H atoms (like OH or \(\text{NH}_2\)) must be shown.
Quick Review: Formulas

If you see a zig-zag line, you are looking at a skeletal formula. Each point is a carbon atom!

1.4 Systematic Nomenclature (Naming)

Organic compounds are named using IUPAC rules. The name tells you:

  1. The length of the longest carbon chain (e.g., Meth-, Eth-, Prop-, But-, Pent-, Hex-).
  2. The type of bonding (e.g., -an- for single bonds, -en- for a double bond).
  3. The functional group (suffix or prefix, e.g., -ol for alcohol, -al for aldehyde).

You must be able to name simple aliphatic (non-cyclic, non-aromatic) compounds up to six carbon atoms. (For esters and nitriles, straight chains only).

Key Takeaway (1.0)

Know your functional groups! They are the key to predicting a molecule's reactivity and determining its name.

2. Characteristic Organic Reactions and Terminology (13.2)

2.1 Homologous Series and Saturation

Homologous Series: A family of compounds having the same general formula, similar chemical properties (due to the same functional group), and successive members differ only by a \(\text{CH}_2\) unit (known as a methylene group).

Saturated: Contains only single C-C bonds (e.g., alkanes).

Unsaturated: Contains at least one double (\(\text{C=C}\)) or triple bond (\(\text{C\equiv C}\)) (e.g., alkenes).

2.2 Bond Fission: How Bonds Break

When a covalent bond breaks, it can happen in two ways:

1. Homolytic Fission:

  • The bond breaks evenly, with one electron going to each atom.
  • This creates two uncharged species called Free Radicals.
  • Free Radical: A species with one or more unpaired electrons (highly reactive).
  • Mnemonic: Homo means 'the same' – they split the electrons equally.

2. Heterolytic Fission:

  • The bond breaks unevenly. Both electrons from the bond go to one of the atoms.
  • This creates a positive ion (cation) and a negative ion (anion).
  • Mnemonic: Hetero means 'different' – one atom gets both electrons (is greedy).

2.3 Electron-Rich vs. Electron-Poor Species

Understanding these species is crucial for mechanisms:

  • Nucleophile: A species that is attracted to positive charges (the nucleus, hence 'nucleo-phile' = nucleus lover). They are electron-rich (have a lone pair or negative charge) and donate electrons.
    Examples: \(\text{OH}^-\), \(\text{CN}^-\), \(\text{NH}_3\).
  • Electrophile: A species that is attracted to negative charges/electron-rich areas (hence 'electro-phile' = electron lover). They are electron-poor (have a positive charge or partial positive charge) and accept electrons.
    Examples: \(\text{H}^+\), \(\text{NO}_2^+\), \(\text{Br}_2\) (due to polarisation).

2.4 Types of Organic Reactions

Organic reactions are classified by what happens to the functional group:

  1. Addition: Two reactants combine to form a single product. Usually involves breaking a \(\text{\(\pi\)}\) bond in an unsaturated molecule (like an alkene).
  2. Substitution: An atom or group is replaced by another atom or group.
  3. Elimination: A small molecule (like \(\text{H}_2\text{O}\) or \(\text{HBr}\)) is removed from a larger molecule, usually forming a double bond.
  4. Hydrolysis: A molecule is broken down by reaction with water (or aqueous acid/alkali).
  5. Condensation: Two molecules join together, releasing a small molecule (like \(\text{H}_2\text{O}\) or \(\text{HCl}\)).
  6. Oxidation and Reduction:
    • In organic chemistry, Oxidation often means adding oxygen or removing hydrogen. Represented as \([\text{O}]\).
    • Reduction often means adding hydrogen or removing oxygen. Represented as \([\text{H}]\).

2.5 Mechanisms: Showing the Steps

A reaction mechanism shows the detailed, step-by-step path a reaction takes. You must be able to use curly arrows in mechanisms to show the movement of a pair of electrons. The arrow always starts at a bond or a lone pair and points to where the new bond or lone pair forms.

  • Free-radical substitution: Involves free radicals (e.g., reaction of alkanes with halogens). This occurs in three phases: Initiation, Propagation, and Termination.
  • Electrophilic addition: Occurs in alkenes where the \(\text{\(\pi\)}\) bond attracts an electrophile (e.g., addition of \(\text{Br}_2\) to ethene).
  • Nucleophilic substitution: Occurs in halogenoalkanes where a nucleophile replaces the halogen atom.
  • Nucleophilic addition: Occurs in carbonyl compounds (aldehydes/ketones).
Did you know?

Most organic reactions in the lab involve heterolytic fission (ions and polar species). Reactions involving free radicals are typically high-energy reactions, often needing UV light or high heat.

Key Takeaway (2.0)

Master the four mechanisms (Free-radical substitution, Electrophilic addition, Nucleophilic substitution, Nucleophilic addition) and the definitions of nucleophiles and electrophiles.

3. Shapes, Hybridisation, $\sigma$ and $\pi$ Bonds (13.3)

3.1 Carbon Structure and Classification

Organic molecules can be described structurally as:

  • Straight-chained: Carbons are connected in a continuous line.
  • Branched: Side chains of carbon atoms attached to the main chain.
  • Cyclic: Carbon atoms arranged in a ring.

3.2 The Chemistry of Hybrid Orbitals

Hybridisation is the mixing of atomic orbitals (s and p) to form new, identical orbitals (hybrid orbitals) suitable for bonding. This process explains the observed shapes and bond angles in organic molecules.

1. \(\mathbf{sp^3}\) Hybridisation (The Single Bond):

  • Involves one s and three p orbitals mixing.
  • Creates 4 equivalent \(\text{sp}^3\) orbitals.
  • Shape: Tetrahedral.
  • Bond Angle: \(109.5^\circ\).
  • Found in: Alkanes (e.g., Methane, \(\text{CH}_4\)).

2. \(\mathbf{sp^2}\) Hybridisation (The Double Bond):

  • Involves one s and two p orbitals mixing, leaving one unhybridised p orbital.
  • Creates 3 equivalent \(\text{sp}^2\) orbitals.
  • Shape: Trigonal Planar (all atoms surrounding the C=C bond lie in the same plane).
  • Bond Angle: \(120^\circ\).
  • Found in: Alkenes (e.g., Ethene, \(\text{C}_2\text{H}_4\)).

3. \(\mathbf{sp}\) Hybridisation (The Triple Bond):

  • Involves one s and one p orbital mixing, leaving two unhybridised p orbitals.
  • Creates 2 equivalent \(\text{sp}\) orbitals.
  • Shape: Linear.
  • Bond Angle: \(180^\circ\).
  • Found in: Alkynes (e.g., Ethyne).

3.3 Sigma (\(\sigma\)) and Pi (\(\pi\)) Bonds

All covalent bonds rely on the overlap of atomic orbitals, forming one of two types of bonds:

1. Sigma (\(\sigma\)) Bond:

  • Formed by the direct (end-on) overlap of orbitals (either s-s, s-p, or hybrid-hybrid).
  • All single bonds are \(\sigma\) bonds.
  • They allow free rotation around the bond axis.

2. Pi (\(\pi\)) Bond:

  • Formed by the sideways overlap of adjacent unhybridised p orbitals.
  • A double bond consists of one \(\sigma\) bond and one \(\pi\) bond.
  • A triple bond consists of one \(\sigma\) bond and two \(\pi\) bonds.
  • \(\pi\) bonds restrict rotation, which is key to understanding stereoisomerism (see 13.4).

Example: Ethene (\(\text{C}_2\text{H}_4\))
Each carbon atom is \(\text{sp}^2\) hybridised. The \(\text{C-C}\) bond is made of one \(\sigma\) bond (from \(\text{sp}^2\)-\(\text{sp}^2\) overlap) and one \(\pi\) bond (from the sideways overlap of the remaining p orbitals). This structure forces the ethene molecule to be planar.

Key Takeaway (3.0)

Hybridisation dictates shape. \(\text{sp}^3\) (single bond) means tetrahedral. \(\text{sp}^2\) (double bond) means planar and restricts rotation.

4. Isomerism: Structural and Stereo (13.4)

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

4.1 Structural Isomerism

Structural isomers have the same molecular formula but differ in the way the atoms are connected (they have different structural formulas).

1. Chain Isomerism: Different arrangements of the carbon skeleton (straight vs. branched).

2. Positional Isomerism: The functional group is attached at a different position on the carbon chain.
Example: Propan-1-ol (\(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\)) vs. Propan-2-ol (\(\text{CH}_3\text{CH}(\text{OH})\text{CH}_3\)).

3. Functional Group Isomerism: The atoms are arranged to give different functional groups entirely.
Example: Ethanol (\(\text{C}_2\text{H}_6\text{O}\), an alcohol) and Methoxymethane (\(\text{C}_2\text{H}_6\text{O}\), an ether).

4.2 Stereoisomerism

Stereoisomers have the same structural formula but the atoms are arranged differently in space.

A. Geometrical Isomerism (cis/trans or E/Z)

This type of isomerism occurs in alkenes (and some cyclic compounds) because of the restricted rotation around the \(\mathbf{C=C}\) double bond (due to the \(\pi\) bond).

For geometrical isomers to exist, each carbon atom in the double bond must be attached to two different groups.

  • cis isomer: The identical groups are on the same side of the double bond.
  • trans isomer: The identical groups are on opposite sides of the double bond.

Analogy: Imagine a traffic barrier (\(\text{C=C}\) bond). If the barrier is up, groups can swap sides (rotation). Since the \(\pi\) bond keeps the barrier down, the groups are fixed in position, leading to two distinct isomers.

B. Optical Isomerism (Chirality)

This arises when a molecule cannot be superimposed on its mirror image, like your left and right hands.

  • Chiral Centre: A carbon atom attached to four different atoms or groups.
  • A molecule with one chiral centre exists as two non-superimposable mirror images called enantiomers (optical isomers).
  • Optically Active: Enantiomers rotate the plane of plane-polarised light by equal amounts but in opposite directions.

Relevance to Drugs:

Often, only one enantiomer of a drug is biologically active (fits the receptor site in the body) while the other may be inactive or even harmful. Synthesising a single, pure optical isomer (rather than a mixture, called a racemic mixture) is vital in the pharmaceutical industry, often requiring expensive chiral catalysts.

Quick Review: Isomerism
  • Structural = Different connectivity.
  • Geometrical = Restricted rotation around \(\text{C=C}\).
  • Optical = Chiral carbon (four different groups).

You need to be able to identify chiral centres and geometrical isomerism in given structural formulas, and deduce the possible number of isomers for a given molecular formula.