Introduction to organic chemistryEdexcel A-Level Chemistry: Revision notes

A hydrocarbon is a compound of hydrogen and carbon only. Organic molecules can be represented in several ways.

• Empirical formula: simplest whole-number ratio of atoms, e.g. CH₂ for C₆H₁₂.
• Molecular formula: actual number of atoms of each element, e.g. C₄H₁₀.
• General formula: formula of a whole homologous series, e.g. CₙH₂ₙ₊₂ for alkanes.
• Structural formula: shows the arrangement of atoms without every bond, e.g. CH₃CH₂CH₂CH₃ or (CH₃)₂CHCH₃.
• Displayed formula: shows every atom and every bond.
• Skeletal formula: a zigzag of bonds with a carbon at each vertex and end, with hydrogen atoms on carbon omitted, but functional groups such as –OH or –Br shown.

Worked example: a hydrocarbon with 85.7% C and 14.3% H: ratio 7.14 : 14.3 = 1 : 2, so the empirical formula is CH₂; if M = 84.0, the molecular formula is C₆H₁₂.

A homologous series is a family of compounds with the same functional group and general formula, in which each member differs from the next by CH₂. Members show similar chemical properties and a gradual change in physical properties.

The functional group is the atom or group of atoms that determines a compound's characteristic reactions.

• Alkane: only C–C single bonds; suffix -ane; CₙH₂ₙ₊₂.
• Alkene: C=C; suffix -ene; CₙH₂ₙ.
• Haloalkane: –Cl, –Br or –I; prefix chloro-, bromo- or iodo-; CₙH₂ₙ₊₁X.
• Alcohol: –OH; suffix -ol; CₙH₂ₙ₊₁OH.

Saturated compounds contain only single C–C bonds; unsaturated compounds contain at least one C=C.

The prefixes for the number of carbon atoms in the main chain are: meth- 1, eth- 2, prop- 3, but- 4, pent- 5, hex- 6, hept- 7, oct- 8, non- 9, dec- 10.

1. Find the longest carbon chain containing the functional group; this gives the stem.
2. Number the chain from the end that gives the functional group the lowest number (then the lowest numbers to side groups).
3. Name side groups (methyl, ethyl, bromo-, chloro-) with their position numbers.
4. Use di-, tri- for repeated groups, e.g. 2,2-dimethyl.
5. List prefixes in alphabetical order (bromo before methyl).
6. Put commas between numbers and hyphens between numbers and letters.

• CH₃CH₂CH₂CH₃ is butane; (CH₃)₂CHCH₃ is 2-methylpropane.
• CH₂=CHCH₂CH₃ is but-1-ene; CH₃CH=CHCH₃ is but-2-ene.
• CH₃CH₂CH₂Br is 1-bromopropane; CH₃CHBrCH₃ is 2-bromopropane; (CH₃)₃CCl is 2-chloro-2-methylpropane.
• CH₃CH₂CH₂OH is propan-1-ol; CH₃CH(OH)CH₃ is propan-2-ol.

For a double bond the number is placed before -ene (pent-2-ene), and for an alcohol before -ol (butan-2-ol). To go from a name to a formula, draw the longest chain, add the functional group at the stated position, then attach the substituents.

• Addition: two molecules join to form one, usually across a C=C, which becomes a single bond, e.g. CH₂=CH₂ + HBr → CH₃CH₂Br.
• Elimination: a small molecule (such as HBr or H₂O) is removed from a saturated molecule to form a C=C.
• Substitution: an atom or group is replaced by another, e.g. C₂H₆ + Cl₂ → C₂H₅Cl + HCl.
• Oxidation: gain of oxygen or loss of hydrogen, e.g. ethanol to ethanal.
• Reduction: gain of hydrogen or loss of oxygen, e.g. ethanal to ethanol.
• Hydrolysis: a bond is broken by reaction with water or aqueous hydroxide ions, e.g. a haloalkane to an alcohol.
• Polymerisation: many small molecules (monomers), usually alkenes, join into a long chain.