A-Level Chemistry AQA Notes

Nomenclature

Hydrocarbons can be:
- Aliphatic- carbon atoms form straight or branched chains
- Alicyclic- carbon atoms form a ring
- Aromatic - carbon atoms form a ring and have a delocalised electron system
Homologous series are compounds with the same functional group and similar chemical and physical properties. They differ by the number of repeating units they contain
A functional group is the group of atoms responsible for the characteristic reactions of a compound.
To name a compound:
- The stem is the main part of the name derived from the longest carbon chain.
- The suffix after the stem, comes from the most significant functional group
- The prefix before the stem comes from functional groups attached to the main carbon chain
- Numbers and hyphens indicating the position of functional groups on the carbon chain
- Functional groups are prioritised alphabetically

Formulae

General formula- the simplest algebraic formula for a homologous series
Structural formula gives the minimum detail on the arrangement of atoms in a molecule, without drawing any bonds
Molecular formula shows the number and types of atoms of each element in a compound. However, it does not give any information on how the molecule is bonded together.
Skeletal formula is a simplified formula used to represent organic molecules. Lines represent bonds between atoms, junctions are carbon atoms. Other labels are omitted.
Displayed formula shows the relative positioning of atoms and the bonds between them. All atoms and bonds are shown
Empirical formula the simplest whole-number ratio of each element present in a compound.

Reaction Mechanisms

Homolytic Fission
When the bond breaks, each electron in the bond goes to a different atom.

This results in the formation of highly reactive free radicals, each with an unpaired electron, represented by a dot.

Heterolytic Fission
When the bond breaks, both the electrons in the bond go to the same atom

This results in the formation of a positively charged cation and a negatively charged anion.

Bonds are formed on the collision of:
- Two free radials with unpaired electrons
- Oppositely charged ions

Isomerism

Isomers are compounds with the same molecular formula but a different arrangement of atoms
Structural isomers are compounds with the same molecular formula but a different structural formula
- Chain isomers- These are molecules with the same molecular formula but a different arrangement of the carbon chain. Chains can be straight or branched
- Position isomers- These are molecules with the same functional group attached to a different position on the carbon chain
- Functional group isomers- These are molecules with the same molecular formula but different functional groups
Stereoisomers are organic compounds with the same molecular and structural formulae but a different arrangement of atoms in space
E/Z isomerism is a type of stereoisomerism that can arise in alkenes due to the restricted rotation around the C=C bond.
If a carbon atom has two of the same substituent attached, it will not show E/Z isomerism
Substituents can be assigned priorities based on atomic mass using Cahn-Ingold-Prelog rules to name E/Z isomers. The greater the atomic mass, the higher the priority
When the highest priority groups are on different sides of the double bond, the isomer is an E-isomer.
When the highest priority groups are on the same side of the double bond, the isomer is a Z-isomer