Introduction to organic chemistry

Organic chemistry begins with safe practice.

Key Definition A hazard is the inherent property of a substance to cause harm, while risk is the probability of that harm actually occurring under the specific conditions of use.

Risk assessments are essential because organic compounds are commonly flammable, toxic, corrosive or environmentally damaging.

Risk is reduced by:

• Working on a smaller scale
• Taking hazard-specific precautions (e.g. fume cupboard, gloves, eye protection)
• Substituting a less hazardous reagent or method

Organic compounds are organised into homologous series.

Members of a homologous series:

• Share the same general formula and functional group
• Differ from neighbouring members by –CH₂– (a relative molecular mass difference of 14)
• Undergo the same types of chemical reaction
• Show a gradual change (trend) in physical properties down the series

The functional group is the part of the molecule that determines its characteristic reactions and reactivity.

IUPAC nomenclature names compounds systematically using:

• Stem prefixes for the carbon chain length (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec- for 1 to 10 carbons)
• Suffixes (and sometimes prefixes) for the principal functional group
• Locants (numbers) and multiplying prefixes (di-, tri-) to show the positions and number of substituents

Compounds are drawn as structural, displayed or skeletal formulae depending on the level of detail required.

Reactions are classified into five categories:

• Addition
• Substitution
• Oxidation
• Reduction
• Polymerisation

Mechanisms depend on how the covalent bonds break.

Homolytic fission

• Produces two free radicals, with each atom taking one of the shared bonding electrons (giving each an unpaired electron)
• Is shown using single-headed (fish-hook) curly arrows, each representing the movement of one electron

Heterolytic fission

• Produces two oppositely charged ions, because one atom takes both of the shared bonding electrons
• Is shown using double-headed (full) curly arrows, each representing the movement of a pair of electrons

Two key reactive species recur throughout organic chemistry.

Key Definition A free radical is a species containing an unpaired electron; it forms via homolytic fission.

Key Definition An electrophile is an electron-pair acceptor that is attracted to regions of high electron density.

These definitions underpin every named mechanism studied later in the course, so precise wording matters in exam answers.