Introduction to Organic Chemistry

Full Notes

Hydrocarbons

A hydrocarbon is an organic compound made up only of carbon and hydrogen atoms.

Types of hydrocarbons:

• Alkanes (saturated, single bonds)
• Alkenes (contain double bonds)
• Alkynes (contain triple bonds)
• Arenes (contain benzene rings)

Types of Organic Formulas

Organic molecules can be written using different formula types and you need to be comfortable using and changing between the following:

Molecular Formula – Shows the actual number of atoms in a molecule.

ExampleHexane → C₆H₁₄

Empirical Formula – Shows the simplest whole number ratio of atoms in a compound.

Example Hexane (C₆H₁₄) → C₃H₇

General Formula – Represents a homologous series using an algebraic expression.

Example Alkanes → C_nH_2n+2

Structural Formula – Shows how atoms are arranged in a molecule without drawing bonds.

Example Hexane → CH₃CH₂CH₂CH₂CH₂CH₃

Displayed Formula – Shows all bonds and atoms.

Example Hexane

Skeletal Formula – Simplified representation where carbon atoms are at the ends and bends of lines, and hydrogens bonded to carbon atoms aren’t shown.

Example Hexane

Matt’s exam tip

Matt’s exam tip - if you find drawing skeletal formulas hard, start by drawing the displayed formula with a diagonal pattern between C-C bonds, then remove all the C-H bonds.

Homologous Series and Functional Groups

A homologous series is a family of organic compounds that:

• Have the same general formula.
• Contain the same functional group.
• Have similar chemical properties.
• Show a gradual change in physical properties (e.g., boiling points increase with chain length).

Examples of Homologous Series:

A functional group is an atom or group of atoms within a molecule that gives the molecule its reactivity and can determine its physical and chemical properties

IUPAC Naming Rules

IUPAC (International Union of Pure and Applied Chemistry) naming rules are used to help name compounds in chemistry, including:

• Chains and rings with up to six carbon atoms.
• Alkanes, alkenes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines, nitriles, arenes.

Steps for Naming Organic Compounds:

1. Find the longest carbon chain
   Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C), Hex- (6C).
2. Identify the main functional group (determines the suffix - see table above).
3. Number the carbon chain to give the lowest possible numbers to functional groups.
4. Identify and name side chains (alkyl groups) as prefixes.
5. Use commas between numbers and hyphens between letters and numbers.

Example 2-Methylpentane (C₆H₁₄)

Longest chain = pentane, with a methyl group at carbon 2.

Reaction Types

Reactions in organic chemistry can be classified into certain types. You should be confident identifying the following reaction types as the course progresses.

• Addition: two molecules combine to form one (common with alkenes)
• Substitution: one atom/group replaced by another (e.g. halogen swapped for –OH)
• Elimination: removal of a small molecule (e.g. H₂O from an alcohol to make an alkene)
• Hydrolysis: water breaks a bond (e.g. ester hydrolysis)
• Oxidation: gain of oxygen or loss of hydrogen
• Reduction: gain of hydrogen or loss of oxygen
• Polymerisation: small monomers join to make a long-chain polymer

Structural Isomerism

Structural isomers have the same molecular formula but different arrangements of atoms.

There are three types of structural isomerism you should know be able to recognise - chain isomerism, position isomerism and functional group isomerism.

Chain Isomerism

In chain isomers, the carbon chain is arranged differently.

Example C₄H₁₀ (butane, methylpropane)

Propane: Straight chain.

2-Methylpropane: One branch.

Position Isomerism

Position isomers have the same functional group in a different position on the chain.

Example C₅H₁₁Br (Bromopentane isomers)

1-Bromopentane: Br on carbon 1.

2-Bromopentane: Br on carbon 2.

3-Bromopentane: Br on carbon 3.

Functional Group Isomerism

Functional group isomers have different functional groups however the same molecular formula.

Example C₃H₆O (Aldehyde and Ketone Isomers)

Propanal (Aldehyde, –CHO)

Propanone (Ketone, –CO– group)

Stereoisomerism and E/Z Isomerism

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

There are different types of stereoisomer, however at this stage you need to be confident with stereoisomerism in alkenes.

E-Z Stereoisomerism

The C=C double bond in an alkene is unable to freely rotate, it has restricted rotation.

This means atoms and groups bonded to the carbons in a double bond are ‘locked’ into position and there are two possible ways they can be arranged.

If the two atoms or groups bonded to each carbon are different, stereoisomerism exists.

Example In 1-bromo-2-chloroethene the Br and Cl groups bonded to the C=C double bond can have two different relative positions.

To name these kinds of stereoisomers we use E and Z notation. The highest priority groups on each carbon determine the name using Cahn–Ingold–Prelog (CIP) rules (see below).

• If highest priority groups are on opposite sides = E Isomer
• If highest priority groups are on the same side = Z Isomer

Example But-2-ene

E-But-2-ene: CH₃ and H on opposite sides of C=C.

Z-But-2-ene: CH₃ and H on the same side of C=C.

Cahn–Ingold–Prelog (CIP) Priority Rules

• Step 1: Look at the atoms directly attached to the C=C bond.
• Step 2: Assign higher priority to the atom with the higher atomic number. If the highest priority atoms bonded to each carbon are the same (for example C) you then assign the highest priority of the groups bonded to that atom (for example CH₂CH₃ group has higher priority than CH₃ group).
• Step 3:
  • If the higher priority groups are on the same side, it is Z.
  • If the higher priority groups are on opposite sides, it is E.

Cis and Trans Isomerism

If two groups bonded to each carbon atom are the same, cis and trans notation can be used instead of E and Z.

• Cis = the same groups on each carbon are on the same side of the double bond
• Trans = the same groups on each carbon are on opposite sides of the double bond

Example But-2-ene

• Cis-but-2-ene: CH₃ groups on same side of C=C
• Trans-but-2-ene: CH₃ groups on opposite sides of C=C

Summary

• Hydrocarbons contain only carbon and hydrogen atoms.
• Organic formulas include empirical, molecular, general, structural, displayed and skeletal forms.
• Homologous series share a functional group and general formula and show trends in properties.
• IUPAC names are built from the longest chain, functional group suffixes and position numbers.
• Key reaction types include addition, substitution, elimination, condensation, hydrolysis, oxidation, reduction and polymerisation.
• Structural isomers differ by chain, position or functional group.
• E/Z and cis/trans arise from restricted rotation around C=C and are assigned using CIP priorities.