Organic: Structure and Nomenclature

Key Ideas

  • Homologous Series: Members of the same homologous series have a similar structure, a pattern to their physical properties, similar chemical properties and the same general formula
  • Saturated Molecules: Molecules which only contain single bonds
  • Non-Saturated Molecules: Molecules which can contain one or more double or triple bonds
Number of Carbon Atoms Prefix
1 Meth
2 Eth
3 Prop
4 But
5 Pent
6 Hex
7 Hept
8 Oct

Stability of Carbon Bonds

On your way to Biology mastery? Enrol in our 50 studyscore masterclass. Click here!

Bond energy is a measure of bond strength and is the amount of energy required to break the covalent bond. The higher the bond energy, the stronger the bond. Thus, a carbon-carbon triple bond is stronger than a carbon-carbon double bond as its bond energy is greater


Isomers are molecules that contain the same number and type of atoms, arranged in different ways. They have the same molecular formula, but can have different physical and chemical properties

Structural Isomers

These form when the atoms in molecules with the same molecular formula bond together in different arrangements. For example, when considering hexane, a carbon can be moved so that an alkyl group is formed. Or, if the molecule contains a function group, it can be moved to a different carbon and hence a different position forming an isomer.


Stereoisomers are isomers in which the atoms in two molecules are connected in the same order but have different arrangements in space. Essentially, they differ in their 3D shape and can have different chemical properties.

Optical Isomers

  • Optical isomers occur when there are four different groups attached to a carbon and therefore there is no symmetry in the molecule. Hence, they are said to be chiral – two objects that are mirror images and cannot be superimposed on each other. A pair of chiral molecules are called enantiomers.
  • A carbon atom attached to four different groups is called the chiral centre. Hence, a chiral molecule must contain at least one chiral centre and not be symmetrical overall

Geometric Isomers

  • Geometric isomers can occur when there is restricted rotation somewhere in a molecule. Therefore, they can occur around a carbon-carbon double bond or a ring.
  • cis-trans isomers can occur when there are two different groups attached to each carbon atom involved in the double bond. When the corresponding groups are on the same side, the isomer is called the cis isomer. If the groups are on opposite sides, the isomer is called the trans isomer.


Series General Formula Carbon-Carbon Bond
Alkanes Alkenes Alkynes
\( (C_nH_{2n+2})\) \( (C_nH_{2n})\) \( (C_nH_{2n-2})\)
Single Double Triple

  • Benzene is an unsaturated cyclic hydrocarbon molecule. Its structure consists of six carbon atoms arranged in a ring.

Functional Groups

Homologous Series Name Semi-structural formula Suffix
Carboxylic Acids Carboxyl \(-COOH\) -oic acid
Alcohols Hydroxyl \(-OH\) -ol
Amines (Primary) Amino \(-NH_2\) -amine
Alkenes Carbon-Carbon Double Bond \(-C=C-\) -ene
Alkynes Carbon-Carbon Triple Bond \(-C\equiv C-\) -yne
Haloalkanes Halo \(-X (X = F, Cl, Br, I) \) -ane
Amides (Primary) Amide \(-CONH_2\) -amide
Esters Ester \(-COOC-\) -yl -ate
(Carbonyl) Aldehydes Carbonyl \(-CHO\) -hyde
(Carbonyl) Ketones Carbonyl \(-CO-\) -one


  • The carbonyl group is always at the end of the hydrocarbon chain


  • Primary: The carbon bonded to the -OH group is only bonded to one alkyl group
  • Secondary: The carbon bonded to the -OH group is also bonded to two alkyl groups
  • Tertiary: The carbon bonded to the -OH group is also bonded to three alkyl groups


  • Primary: Bonded with two hydrogens and one alkyl group


  • The carbonyl group is always attached to other carbon atoms, never at the end


  • There are no spaces in a name, apart from the two-word names of esters and carboxylic acids
  • The longest carbon chain is used to derive the parent name. The longest chain must include the functional group for alkenes, alkynes, alcohols, amines, carboxylic acids and esters
  • The names and locations of branches and additional functional groups are added to this parent name
  • Numbers are used to identify the carbon atom that groups are attached to
  • Numbers and letters separated by dashes
  • Numbers are separated from other numbers by commas
  • The names of branch alkyl groups are added before the parent name
  • If there is more than one type of function group to be listed at the beginning of a name, they are listed in alphabetical order
  • If there is more than one of the same type of functional group, the prefixes ‘di’, ‘tri’, ‘tetra’ are used. Each group is still given a number to indicate its position on the carbon chain