Formulae and names - H1 Chemistry

An organic structure is a map of connections. A functional group is the atom or group of atoms responsible for a characteristic set of reactions. First find the functional group; then identify the carbon skeleton. The same molecular formula can describe compounds with different connections and therefore different chemistry.

Five ways to describe lactic acid
Representation	What it tells you
Empirical: CH₂O	The simplest whole-number atom ratio, C:H:O = 1:2:1.
Molecular: C₃H₆O₃	The actual numbers of atoms in one molecule.
Structural: CH₃CH(OH)CO₂H	The middle carbon bears OH; the final carbon belongs to COOH. Brackets keep the connectivity unambiguous.
Displayed	Every atom and bond is shown, including each C-H and O-H bond.
Skeletal	Carbon atoms are implicit at line ends and corners; enough attached hydrogen atoms are implied to give each carbon four bonds. Heteroatoms and their attached hydrogens, such as OH, are shown.

To turn a condensed formula into a displayed formula, draw the carbon skeleton first, then expand every bracketed group at its stated attachment point. Replace COOH by C(=O)-O-H and CHO by C(=O)-H. Finally add each C-H bond explicitly and check that each ordinary neutral carbon has four bonds, each oxygen two, and each hydrogen one. In a skeletal formula, apply the same valency check to recover the hidden C-H bonds.

A displayed formula keeps every atom visible

COOH contains a C=O bond and a C-O-H group; it is not a carbon joined to three oxygens.

The same molecule in skeletal form

Count a carbon at every unlabelled end or corner. Do not count the end labelled O or OH as another carbon.

Recognise the whole group, then name an example
Class and general structure	Example and naming cue
Alkane: C_nH_2n+2 for an acyclic saturated hydrocarbon	CH₃CH₃, ethane; suffix -ane.
Alkene: C_nH_2n for an acyclic hydrocarbon with one C=C	CH₂=CHCH₃, propene; locate the C=C and use -ene.
Benzene: C₆H₆, a six-carbon aromatic ring	Use a hexagon containing a circle to represent the delocalised ring. A fully displayed benzene ring is not required.
Halogenoalkane: R-X; C_nH_2n+1X for a saturated acyclic monohalogen compound	CH₃CH₂Br, bromoethane; fluoro-, chloro-, bromo- or iodo- gives the substituent.
Alcohol: R-OH; C_nH_2n+2O for a saturated acyclic monohydric alcohol	CH₃CH(OH)CH₃, propan-2-ol; number the OH position.
Aldehyde: R-CHO; C_nH_2nO for a saturated acyclic monoaldehyde	CH₃CHO, ethanal; the carbonyl carbon has at least one H, and its carbon counts in the chain.
Ketone: R-CO-R'; C_nH_2nO for a saturated acyclic monoketone	CH₃COCH₃, propanone; the carbonyl carbon is bonded to two carbon groups.
Carboxylic acid: R-COOH; C_nH_2nO₂ for a saturated acyclic monocarboxylic acid	CH₃COOH, ethanoic acid; the COOH carbon is carbon 1.
Ester: R-COO-R'; C_nH_2nO₂ for a saturated acyclic monoester	CH₃COOCH₂CH₃, ethyl ethanoate; name the O-attached alkyl group first, then the acid-derived -oate part.
Amine: R-NH₂ for a primary amine; N can also carry further carbon groups	CH₃CH₂NH₂, ethylamine (ethanamine). For a saturated acyclic monoamine, C_nH_2n+3N.
Amide: R-CONH₂; N-substituted forms include R-CONHR'	CH₃CONH₂, ethanamide; C=O is directly attached to N. A saturated acyclic monoamide has C_nH_2n+1NO.
Amino acid: contains both NH₂ and COOH; an alpha-amino acid is H₂N-CH(R)-COOH	H₂NCH₂COOH, aminoethanoic acid; the amino group is on the carbon next to COOH. For a saturated acyclic compound with one of each group: C_nH_2n+1NO₂.

Here R and R' represent carbon-containing groups; they need not be identical. In methanal and methanoic acid, the group attached to the carbonyl carbon can be H instead. General molecular formulae apply only with the stated restrictions: rings, extra double bonds or extra functional groups change them. A formula alone does not establish the family; for example, propanal and propanone both have C₃H₆O.

For straightforward names, choose a parent chain containing the principal functional group and number it to locate that group or the double bond with a low number. The prefixes meth-, eth-, prop-, but-, pent- and hex- indicate 1-6 carbons. Show branches and substituent positions: CH₃CH(CH₃)CH₂OH is 2-methylpropan-1-ol. Count the carbon attached to OH as carbon 1; the methyl branch is then on carbon 2.

Primary, secondary and tertiary classify the carbon carrying OH
Alcohol class	Carbon groups attached to the OH-bearing carbon	Example
Primary	One carbon group; methanol is conventionally treated with primary alcohols.	CH₃CH₂OH
Secondary	Two carbon groups.	CH₃CH(OH)CH₃
Tertiary	Three carbon groups.	(CH₃)₃COH

Check your understandingName CH₃CH₂COOCH₃ and identify its two carbon fragments.Think it through, then reveal the answer

Methyl propanoate. The O-attached CH₃ group gives methyl; CH₃CH₂COO^- contains three carbons including the carbonyl carbon, so it gives propanoate. Reversing these fragments would describe a different ester.

Read and name an organic structure

A displayed formula keeps every atom visible

The same molecule in skeletal form