1.5 – Introduction to Organic Chemistry – Glossary
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chain isomers – A category of structural isomerism in which molecules have the same molecular formula but have a different arrangement of the carbon chain. Chain
isomers belong to the same homologous series. For example, 3-methylpentane and 2,3-dimethylbutane are both chain isomers of hexane.
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cis–trans isomerism – A category of stereoisomerism in alkenes in which groups are attached to the carbon atoms involved in the double bond in a different way. In cis
isomers, the groups are on the same side of the double bond, whereas in trans isomers, the groups are on opposite sides. Also known as geometric isomerism.
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displayed formula – A chemical formula showing the arrangement of atoms and bonds in a molecule.
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empirical formula – A chemical formula showing the simplest ratio of atoms present in a molecule. For example, the empirical formula of hexane is C3H7.
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E–Z isomerism – A system of notation used for complex stereoisomers that cannot be identified as either cis or trans isomers.
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functional group – An atom or group of atoms present in a molecule that is responsible for its characteristic chemical properties.
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functional group isomerism – A category of structural isomerism in which molecules have the same molecular formula but contain different functional groups and so
belong to different homologous series. For example, both propanol (an alcohol) and methoxyethane (an ether) have the molecular formula C3H8O but contain different
functional groups.
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general formula – A chemical formula used to calculate the molecular formula of any member of a homologous series. For example, the general formula for alkanes is
CnH2n+2. The general formula of a specific alkane can be calculated by substituting n with the number of carbon atoms it contains.
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homologous series – A group of compounds that all have the same functional group but a different number of carbon atoms. Examples include alkanes, alkenes,
alcohols, amines and halogenoalkanes.
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isomersism – The existence of molecules that have the same molecular formula but a different arrangement of atoms. Two broad categories are structural isomerism
and stereoisomerism.
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molecular formula – A chemical formula showing the actual number of each type of atom in a molecule. For example, the molecular formula of hexane is C6H14.
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optical isomerism – A category of stereoisomerism, in which two molecules are non-superimposable, mirror images of each other, a feature known as chirality. Optical
isomers are also known as enantiomers.
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organic – Compounds that contain carbon atoms, which are usually bonded together to form chains or rings.
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positional isomerism – A category of structural isomerism in which molecules have the same molecular formula but in which the functional group is attached to a
different carbon atom. Positional isomers belong to the same homologous series.
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skeletal formula – A chemical formula showing the bonds between carbon atoms but, unlike in a displayed formula, the carbon and hydrogen atoms are not shown.
Other atoms are shown.
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stereoisomerism – A category of isomerism in which molecules have the same molecular formula but a different 3D arrangement of atoms. Two types are cis-trans
isomerism and optical isomerism.
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structural formula – A chemical formula showing how the atoms are arranged in a molecule and, in particular, showing what functional groups are present. Unlike in a
displayed formula, single bonds are not shown, but multiple bonds may be shown. For example, the structural formula of propane is CH3CH2CH3, and propene is
CH3CH2=CH2.
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structural isomerism – A category of isomerism in which molecules have the same molecular formula but a different structural formula. Three types are chain
isomerism, positional isomerism and functional group isomerism.