Organic chemistry
Definition The molecular formula gives the actual number of atoms of each element present in a molecule.
The molecular formula must be a multiple of the empirical formula, i.e. (CH2O)n in this example. In order to discover the value of n in the formula (CH2O)n, we need to know the relative molecular mass of compound X. The mass of the empirical formula CH2O is 30. Thus, if the relative molecular mass of X is found to be 60, then the value of n is 2 and the molecular formula is (CH2O)2, which is more usually written as C2H4O2. This molecular formula, C2H4O2, can represent several compounds in which the two atoms of carbon, four atoms of hydrogen and two atoms of oxygen are arranged differently. These different molecules are called isomers. The molecular formula for X, C2H4O2, can represent several different compounds. The structures of these compounds can be shown by displayed formulae, structural formulae or skeletal formulae.
Essential Notes Isomerism is considered in detail later in this book (section 3.3.1.3) and in Collins Student Support Materials: A-Level year 2 – Organic and Relevant Physical Chemistry, section 3.3.7.
Definition A displayed formula shows all the bonds present in a molecule. Displayed formulae for two isomers of C2H4O2 are shown in Fig 11. H H
C
H
O C
H O
H
H
C
O
O
Fig 11 Two structural isomers of C2H4O2 shown as displayed formulae
C
H
H
However, it is not necessary to show all of the bonds in a molecule to give a true picture of its structure, so it is simpler and more common to use a structural formula.
Definition A structural formula shows the unique arrangement of atoms in a particular molecule in a simplified form, without showing all the bonds. Structural formulae for two compounds with molecular formula C2H4O2 are shown in Fig 12. O H3C
C
O H
OH
C
Fig 12 Two structural isomers of C2H4O2 shown as structural formulae
OCH3
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