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

NOT

                              C3H7OH

 

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:

 

Cyclopentanol