Core organic terms

· Hydrocarbon = compound made up of carbon and hydrogen atoms only.
· Alkanes = simple hydrocarbons with no functional group.
· Functional group = atom/group of atoms that dictates physical and chemical properties of an organic compound.
· Aliphatic organic molecules = organic compounds not based on benzene rings.
· R and R′ represent alkyl groups; they may be the same or different.
· X represents a halogen atom in organic formulas.

This image helps students recognise functional groups quickly from their structural formulas. Use it to practise linking each functional group to its homologous series and naming pattern. Source

Formula types you must interpret

· General formula = simplified formula showing the functional group pattern, e.g. R–OH for alcohols.
· Structural formula = shows the arrangement of atoms/groups, e.g. CH₃CH₂OH.
· Displayed formula = shows all atoms and all bonds.
· Skeletal formula = carbon chain drawn as lines; each line end or vertex = carbon atom unless another atom is shown.
· In skeletal formulas, hydrogen atoms bonded to carbon are not shown but are assumed to complete carbon’s four bonds.
· You must be able to convert between structural, displayed and skeletal formulas.

AS functional groups to know

· Alkene: functional group = C=C bond; example name ending = -ene, e.g. propene.
· Halogenoalkane: functional group = halogen, X; named using prefixes such as chloro-, bromo-, iodo-.
· Alcohol: functional group = hydroxyl, –OH; suffix = -ol, e.g. propan-1-ol.
· Aldehyde: functional group = carbonyl, –CHO; suffix = -al, e.g. propanal.
· Ketone: functional group = carbonyl, >C=O within the chain; suffix = -one, e.g. propanone.
· Carboxylic acid: functional group = carboxyl, –COOH; suffix = -oic acid, e.g. propanoic acid.
· Ester: functional group = –COO–; named as alkyl alkanoate, e.g. methyl propanoate.
· Primary amine: functional group = –NH₂; suffix often -amine, e.g. propylamine.
· Nitrile: functional group = –C≡N; suffix = -nitrile, e.g. propanenitrile.

This page supports recognition of functional groups by structure. It is especially useful for comparing similar groups such as aldehydes and ketones, or carboxylic acids and esters. Source

Primary, secondary and tertiary classification

· Halogenoalkanes can be primary, secondary or tertiary depending on how many carbon groups are attached to the carbon bonded to X.
· Alcohols can be primary, secondary or tertiary depending on how many carbon groups are attached to the carbon bonded to –OH.
· Primary amines only are required here at AS introductory level: R–NH₂.
· Do not confuse primary amine classification with primary alcohol/halogenoalkane classification: the atoms being classified are different.

Naming simple organic compounds: exam method

· Find the longest carbon chain containing the main functional group.
· Choose the correct root: meth- = 1C, eth- = 2C, prop- = 3C, but- = 4C, pent- = 5C, hex- = 6C.
· Number the chain to give the functional group the lowest possible number.
· Add position numbers for functional groups, halogens and C=C bonds where needed.
· Use the correct suffix: -ene, -ol, -al, -one, -oic acid, -nitrile, -amine.
· For halogenoalkanes, use prefixes such as chloro-, bromo-, iodo-.
· For esters, name the alcohol-derived part first, then the acid-derived part: alkyl alkanoate.
· CIE scope: simple aliphatic molecules with listed functional groups, up to six carbon atoms; for esters, up to six + six carbons; esters and nitriles are straight-chain only.

Formula deduction skills

· Molecular formula = actual number of each atom in one molecule, e.g. C₃H₈O.
· Empirical formula = simplest whole-number ratio of atoms, e.g. C₂H₄O₂ → CH₂O.
· From a displayed formula, count every atom shown.
· From a structural formula, expand groups carefully, e.g. CH₃CH₂CHO = C₃H₆O.
· From a skeletal formula, count each line end/vertex as carbon and add enough hydrogens to give each carbon four bonds.
· Remember: atoms such as O, N, halogens and their attached hydrogens are normally shown explicitly in skeletal formulas.

This image is a simple example of a skeletal formula containing an alcohol functional group. Students can practise converting it to displayed, structural and molecular formulas. Source

High-yield naming examples

· CH₃CH₂CH₃ = propane: alkane, no functional group.
· CH₃CH=CH₂ = propene: alkene, C=C included in the chain.
· CH₃CH₂CH₂Cl = 1-chloropropane: halogen prefix and position number.
· CH₃CH₂CH₂OH = propan-1-ol: alcohol suffix with position number.
· CH₃CH₂CHO = propanal: aldehyde carbon is automatically carbon 1.
· CH₃COCH₃ = propanone: ketone with carbonyl inside the chain.
· CH₃CH₂COOH = propanoic acid: carboxylic acid carbon included in the chain.
· CH₃CH₂COOCH₃ = methyl propanoate: methyl from alcohol part, propanoate from acid part.
· CH₃CH₂CH₂NH₂ = propylamine: primary amine.
· CH₃CH₂CN = propanenitrile: nitrile carbon is included in the carbon count.

Common exam traps

· Do not call alkanes a functional group family: alkanes have no functional group.
· Do not forget the carbonyl carbon when naming aldehydes, ketones, carboxylic acids, esters and nitriles.
· Do not name esters as one continuous chain; use alkyl alkanoate.
· Do not omit position numbers when there is ambiguity, e.g. propan-1-ol vs propan-2-ol.
· Do not add hydrogens randomly in skeletal formulas: carbon must have four bonds.
· Do not confuse aldehyde –CHO with ketone >C=O.
· Do not confuse carboxylic acid –COOH with ester –COO–.