•Hydrocarbons are non-polar, so there is only weak dispersion forces between molecules. This means that the boiling points are relatively low.
•Longer chains of hydrocarbons have a greater boiling points. This is due to more dispersion force between the molecules.
•Straight chained molecules are closer together compared to their branched isomers. This means that branching will reduce a molecules boiling point.
•The addition of functional groups –double bonds, triple bonds, halogens, carbonyl groups etc, increase the boiling point. This is due to stronger intermolecular bonding such as dipole-dipole and hydrogen bonding.
•Boiling points generally increase in the following order:
alkane-alkene-alkyne (dispersion forces) --> halogen organic molecules, aldehydes, ketones, esters (dipole-dipole only since hydrogen is only bonded to carbon)---> alcohols, carboxylic acids, amines, amides.(hydrogen bonding).
•The solubility in water of a molecule will depend on its ability to hydrogen bond with the water molecules.
•Longer chains of hydrocarbons become less soluble- this is because non-polar part of the molecule increases disrupting hydrogen bonding.
Solubility in organic solvents
•Organic solvents are non-polar- so as a hydrocarbons chain length increases so does it solubility in organic solvents.
•Smaller molecules are kept together due to their polarity and they do not disperse in a non-polar solvent.
•Alkanes, alkenes, alkynes, aldehydes, ketones, esters, alcohols, amines and amides solubility in non-polar solvents all increase with increasing chain length.