A-LEVEL OCR ChEMISTRY NOTES
Basic Concepts of Organic Chemistry
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
- Saturated- containing only single bonds
Carbon Atoms in alkyl group |
Prefix |
1 |
Methyl |
2 |
Ethyl |
3 |
Propyl |
4 |
Butyl |
5 |
Pentyl |
6 |
Hexyl |
- 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.
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. They may contain the same functional group but on a different carbon atom, or different functional groups e.g. ketones and aldehydes with the same number of carbon atoms have the same molecular formula
Reaction Mechanisms
- Bond fission can be homolytic or heterolytic
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.
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.
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
- Bonds can also be formed when ions are attracted to dipoles, e.g. nucleophilic substitution reactions
