Doc Brown's Chemistry  Theoretical Chemistry - Equilibria - Chemical Equilibrium 8.3

 8.3 Comparative boiling point plots-graphs for selected homologous series of organic molecules

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses) GCSE Notes reversible reactions-equilibrium * EMAIL query?comment

8.3 Comparative Boiling Point Plots for six Organic Homologous Series

but only comparing linear molecules of the six selected aliphatic homologous series

and comments on the intermolecular forces (intermolecular bonding) involved

Read first 8.2.1 A summary of Van der Waals forces and an introduction to intermolecular forces

8.3 Plot (a) Boiling Point versus the number of electrons in the molecule

8.3 Plot (a) Boiling Point versus the number of electrons in the molecule

This is the best comparison that you can, because, this gives a baseline of the effect of instantaneous dipole - induced dipole interactions i.e. intermolecular forces common to all molecules with the same number of electrons in the molecule and they are all approximately linear molecules.

Three general trends are immediately discernable.

• With increased molecular mass or electrons, the boiling point increases i.e. what you would expect from increasing instantaneous dipole - induced dipole attractive forces.

• An important 'sub-trend' influence of a polar bond giving rise to an extra contribution to the intermolecular forces considerably decreases with increase in carbon chain length.

• Generally speaking the more polar the molecule the higher the boiling point for those molecules of similar molecular mass or number of electrons, but this is only significant for small very polar molecules.

• BUT the predominant intermolecular force for most molecules is the instantaneous dipole - induced dipole EXCEPT for a few small highly polar molecules exhibiting hydrogen bonding e.g. methanol and ethanol, methanoic and ethanoic acid, methanamide and ethanamide where the permanent dipole - permanent dipole interactions are very significant.

• In order of increasing boiling point ....

  • Alkanes have the relatively lowest boiling point because only instantaneous dipole - induced dipole interactions can contribute to the intermolecular forces.

  • Primary halogenoalkanes (n-alkyl halides) have slightly higher boiling points than alkanes because of the carbon-chlorine polar bond (^δ+C-Cl^δ-) giving a permanent dipole - permanent dipole an extra small contribution to the intermolecular forces.

  • Primary aliphatic amines are the next highest boiling because the N-H bond (^δ-N-H^δ+) is more polar than the C-Cl bond and these permanent dipole - permanent dipole interactions will give rise to hydrogen bonding but not as strong as for alcohols.

  • Aldehydes (similar for isomeric ketones) are very similar to the linear primary aliphatic amines. It would appear here that the permanent dipole - permanent dipole intermolecular attractive forces due to the polarised carbonyl bond (^δ+C=O^δ-) have about the same effect as the hydrogen bonding in the amines.

  • Alcohols (alkanols) show significantly higher boiling points due to hydrogen bonding BUT only for the lower members.

  • Carboxylic acids (alkanoic acids) are even higher than alcohols because there are extra >C=O ... >C=0 attractions as well as hydrogen bonding or you could argue there are two sites on the molecule for hydrogen bonding, again only for the lower members, and dimers are readily formed for the lower members of the series.

    • Note: For ethanoic acid, if you do the plot point with double the electrons i.e. as in the dimer, the point is then close to the alkane curve!

    • This is what you expect if the liquid ethanoic acid is a cyclic hydrogen bonded dimer and you assume the only attractive intermolecular force is due to the instantaneous dipole - induced dipole interactions!

• Although each interaction is minute, collectively, even the weakest of intermolecular forces can add up to give an impressive effect.

  • Non-polar alkanes are only attracted to each other via the weakest of Van der Waals forces (instantaneous dipole - induced dipole interactions) but once the carbon number gets high, so does the boiling point! and enthalpy of vaporisation which eventually equals bond energies, so very high molecular mass hydrocarbons can thermally decompose before they boil!

  • Some examples of high boiling alkanes are

  • C₃₂H₆₆ M_r = 451, 258 electrons, bpt 467^oC (740K), ΔH_sub(s=>g)  = 271 kJ mol^-1

  • C₃₅H₇₂ M_r = 493, 282 electrons, bpt 490^oC (763K)

8.3 Plot (b) Boiling Point versus the molecular mass

8.3 Plot (b) Boiling Point versus the molecular mass

These plots give a very similar pattern to the electron number plots, but its still best to think of electron number (plot (a)).

8.3 Plot (c) Boiling Point versus the number of carbon atoms in the molecule

8.3 Plot (c) Boiling Point versus the number of carbon atoms in the molecule

• This plot disregards the number of electrons or molecular mass and is not the most useful set of graphs for comparing the boiling points of organic homologous series.

• It is however, a way of comparing the boiling points of various homologous series for the same number of carbon atoms in the molecule.

• All the plot lines show the steady increase of boiling point with carbon number as an extra -CH₂- group is successively added and the relatively diminishing effects of a permanent dipole as the instantaneous dipole - induced dipole interactions almost totally dominate with longer carbon chain lengths.

• It should be remembered that although each interaction is minute, collectively, even the weakest of intermolecular forces can add up to give an impressive effect.

• The ultimate molecular structure held together solely by the weakest of the Van der Waals forces, i.e. the instantaneous dipole - induced dipole  intermolecular forces, is carbon in the form of graphite (trillions of atoms per molecule!) which only sublimes at over 3500^oC! The total intermolecular attractive force is so great that graphite won't melt at normal pressure and above 3500^oC the thermal vibration is the molecular lattice is then great enough to disrupt it and the solid vaporises directly in molecular chunks of the lattice (1-4 carbon atoms?).

• -

Copyright © Dr W P Brown 2000-2010 All rights reserved on the revision notes pages, quizzes, worksheets, x-words etc.