Organic: Physical Properties

Boiling Points of Alkanes, Alkenes, Alkynes and Haloalkanes

  • The boiling points of molecules are determined by the strength of the intermolecular forces. Therefore, in the example of alkanes, alkenes and alkynes, because they are non-polar and the only intermolecular forces present are weak dispersion forces, as the length of the carbon chain is increased, the overall forces increases and subsequently the boiling point increases as well.
  • Moreover, because straight-chained alkanes are able to fit more closely than branched-chain isomers, therefore they have higher boiling points.
  • It is important to note however, that because Haloalkanes contain bonds that are quite polar resulting from the halo functional group, they not only contain dispersion forces, but also have dipole-dipole attractions which are much stronger and hence have higher boiling points.

Boiling Points of Alcohols, Carboxylic Acids, Amines and Amides

  • Because alcohols, carboxylic acids, amines and amides are all able to form hydrogen bonds, one of the strongest intermolecular forces present, they exhibit higher boiling points than alkanes of an equivalent carbon number.
  • In alcohols, the oxygen is a more electronegative atom than hydrogen, forming a polar bond and allowing for H-Bonds.
  • In amines and amides, the presence of highly polar nitrogen-hydrogen bonds allows for H-Bonds
  • In carboxylic acids, dimers in which two hydrogen bonds occur between two molecules can occur.
  • As the carbon chain is increased, the boiling point of each increases. However, in the case of alcohols, as branching occurs, the boiling point decreases due to the alkyl group’s restriction on the molecules ability to form hydrogen bonds with other molecules

Boiling Points of Aldehydes, Ketones and Esters

  • All these molecules contain a carbon-oxygen double bond. Because oxygen is much more electronegative than carbon, this double bond is polar, resulting in a permanent dipole which can form dipole-dipole attractions with nearby molecules. Hence, as the carbon chain is increased, the boiling point will increase due to the increased dispersion forces present.

Viscosity

  • Remember that the viscosity of a liquid is its resistance to pouring or flowing. The viscosity of a liquid depends on the interactions between molecules. Moreover, as the length of the carbon chain is increased, the viscosity increases as the dispersion forces present increases as well.

Flashpoint

  • The temperature at which a vapour ignites is called the flashpoint. i.e. the lowest temperature at which the liquid forms sufficient vapour to ignite when an ignition source is present. The flashpoint trend corresponds with the trend of boiling points. This is because as the intermolecular forces increase, the vapour pressure i.e. the ability to readily form vapour decreases because molecules cannot break out of their formation as easily thereby corresponding to an increase in flashpoint.