6A

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

Edexcel A-Level Chemistry molecular formula example of hexane C6H14

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

Example Hexane (C6H14) → C3H7

Edexcel A-Level Chemistry empirical formula example showing hexane simplified to C3H7

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

Example Alkanes → CnH2n+2

Edexcel A-Level Chemistry general formula example for the alkane series CnH2n+2

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

Example Hexane → CH3CH2CH2CH2CH2CH3

Edexcel A-Level Chemistry structural formula example for hexane showing CH3CH2 repeats

Displayed Formula – Shows all bonds and atoms.

Example Hexane

CIE A-Level Chemistry displayed formula diagram of hexane with all bonds shown

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

AQA A-Level Chemistry skeletal formula diagram of hexane as a zig-zag linePhoto of Matt

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.

CIE A-Level Chemistry tip image showing how to convert from displayed to skeletal by removing 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:

Homologous Series

General Formula

Example

Functional Group

Alkanes

CnH2n+2

Methane (CH4)

None

Alkenes

CnH2n

Ethene (C2H4)

C=C

Alcohols

CnH2n+1OH

Ethanol (C2H5OH)

–OH

Carboxylic Acids

CnH2nO2

Ethanoic Acid (CH3COOH)

–COOH

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

Edexcel A-Level Chemistry summary diagram of common organic functional groups and their symbols.

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)

Edexcel A-Level Chemistry example of IUPAC naming steps for 2-methylpentane with highlighted parent chain and substituent.

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)

Edexcel A-Level Chemistry diagram comparing straight-chain butane with branched 2-methylpropane for chain isomerism.

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)

Edexcel A-Level Chemistry examples showing 1-bromopentane, 2-bromopentane, and 3-bromopentane to illustrate position isomerism.

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)

Edexcel A-Level Chemistry comparison of propanal (aldehyde) and propanone (ketone) demonstrating functional group isomerism.

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.

Edexcel A-Level Chemistry schematic showing restricted rotation around C=C leading to different spatial arrangements.

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).

Edexcel A-Level Chemistry diagram distinguishing E and Z arrangements using CIP priority on a C=C bond.

  • 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

Edexcel A-Level Chemistry illustrations of E-but-2-ene and Z-but-2-ene showing group positions across the C=C.

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

Edexcel A-Level Chemistry comparison of cis-but-2-ene and trans-but-2-ene based on CH3 group positions.

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

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