Structural & Molecular Formulae

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Molecular formulae show the TOTAL actual number of atoms in a single molecule. It isn’t to be confused with the empirical formula which is the atoms in their simplest ratio.


Molecular formulae group the same atoms together, so avoid the following:


Molecular formula of propan-1-ol is C3H8O




Structural formulae show how they are grouped together in a molecule.


The easiest structural formulae are for Alkanes:


Butane            CH3CH2CH2CH3                  


Pentane          CH3CH2CH2CH2CH3


As for Alkenes, they are also easy, but remember to include the double bond:


But-1-ene       CH2=CHCH2CH3


But-2-ene        CH3CH=CHCH3       


In alcohols, watch out for the position of the hydroxyl group by putting brackets in different isomers:


Butan-1-ol      CH3CH2CH2CH2OH


Butan-2-ol       CH3CH(OH)CH2CH3


In Aldehydes, make sure the carboxyl functional group is written as   –CHO rather than –COH which can be misleading:


Butanal           CH3CH2CH2CHO


In ketones, the same rules apply:


Butanone        CH3COCH2CH3


As for Carboxylic acids, the carboxylic acid functional group can be written in two ways:


Butanoic acid            CH3CH2CH2COOH


Pentanoic Acid          CH3CH2CH2CH2CO2H


Structural formulae for Cycloalkanols include bonds between the carbons to give a distinct shape: