A molecular formula counts atoms but does not show their connections. Condensed and skeletal formulae show connectivity more efficiently. In skeletal structures, each unlabelled line end or corner is a carbon, and enough carbon-bound hydrogens are implied to give carbon four bonds. Heteroatoms and their attached hydrogens are shown. Isomers have the same molecular formula but different structures.
Choose a parent chain containing the principal functional group, number it to locate that group and then name substituents. Compare properties using molecular size, polarity and hydrogen-bonding ability. Larger similar molecules generally have stronger dispersion forces. Hydrogen bonding often raises boiling point, while a long non-polar region can reduce water solubility. Condensation joins molecules while releasing a small molecule such as water.
Example 1
Find the molecular formula of CH3CH2CH2OH.
Count three C atoms, eight H atoms including the hydroxyl H, and one O:C3H8O. Its name is propan−1−ol.
Example 2
Compare ethanol and ethane boiling points using structure.
Ethanol can form intermolecular hydrogen bonds through O–H; ethane has only dispersion forces. Ethanol therefore has the higher boiling point despite their similar small carbon skeletons.
Watch out:Isomers can have different atom connections and properties despite the same atom counts.
Organic formulae, naming and properties practice | Year 12 Chemistry | Ascent Arcade