The red line graph shows the boiling point of
alkanes from methane CH4 (boiling point -164oC/109 K)
to tetradecane C14H30 (boiling point 254oC/527 K).
[Remember K = oC + 273]
The red line represents linear alkanes in all
the graphs 1-3 and is a useful baseline to compare the intermolecular
bonding present in other homologous series of non-cyclic aliphatic compounds.
'yellow line' of linear primary alcohols,
the graph goes from methanol CH3OH (bpt 65oC/338 K) to
decan-1-ol CH3(CH2)9OH (bpt 230oC/503 K).
So, in this discussion we are comparing the red line
(linear alkanes) with the line (linear primary alcohols, '1-ols') AND
comparing molecules with the same number of electrons.
A plot of number of electrons in
any molecule of a homologous series versus its boiling point (K) shows a steady rise
with a gradually decreasing gradient.
I consider this the best for
comparison of the effects of intermolecular bonding between different
REMINDER: Intermolecular forces are all about
partially positive (δ+)
sites and partially negative
sites on molecules causing the attraction between neighbouring molecules -
though their origin can differ.
I think Graph 1 is the best graph to look at
the relative effects on intermolecular forces (intermolecular bonding) on
boiling point because it is the distortion of the electron clouds (e.g. in
non-polar alkanes), that gives rise to these, weak, but not insignificant forces,
known as instantaneous dipole - induced dipole forces.
From Graph 1 you can see
the effect of the
permanently polar oxygen - hydrogen bond (Hδ+-Oδ-)
increases the intermolecular forces of attraction, and raising the boiling point compared to non-polar molecules
of similar size in terms of numbers of electrons (clouds).
The hydrogen bonding is in addition to
the intermolecular attractive force compared to non-polar molecules.
Even so, for most polar molecules, the
majority of the intermolecular force is still due to the instantaneous
dipole - induced dipole attractions.
This is an added effect in attracting the alcohol molecules
together much more strongly than just the instantaneous dipole - induced dipole
forces - but it isn't necessarily the largest contributor to the total
intermolecular force of attraction between molecules.
The total intermolecular force is hydrogen
bonding (via the OH group) plus the instantaneous dipole - induced dipole
attraction forces (from the whole molecule).
A minor contribution is from
permanent dipole - induced dipole attraction.
Alcohols are permanently polarised
molecule due to the highly polar bond
δOHδ+ caused by the difference in
electronegativities between oxygen and hydrogen i.e. O (3.5) >
H (2.1). This causes the extra permanent dipole permanent
dipole interaction between neighbouring polar molecules via
Note that the lone pairs on
the most electronegative atom are important to show on a
fully detailed diagram (though I haven't always done so on
The hydrogen bond is directional i.e. the proton
lines up with the lone pair on the oxygen which is effectively the delta
minus and this should come out in a full diagram showing the
hydrogen bonding between molecules.
Total intermolecular force =
(instantaneous dipole induced dipole) + (permanent dipole permanent dipole
including hydrogen bonding) +
(permanent dipole induced dipole)
The effect of hydrogen bonding on the
boiling point is very significant for the lower alcohols, but its effect
decreases as the carbon chain length increases.
For methanol: Total intermolecular force =
(61.3% instantaneous dipole induced dipole) + (30.3% perm. dipole
permanent dipole including hydrogen bonding) + (8.4% permanent dipole
For ethanol: (42.6% instantaneous dipole induced dipole) + (47.6%
permanent dipole permanent dipole including H
bonding) + (9.8% permanent dipole induced dipole)
2methylpropan2ol: Total intermolecular force was: (67.2% instantaneous dipole
induced dipole) + (23.1% permanent dipole permanent dipole including H
bonding) + (9.7% permanent dipole induced dipole)
BUT, the % intermolecular attractive
force from hydrogen bonding will tend to decrease, and the % contribution
of the instantaneous dipole - induced dipole forces increases, as the alkyl
chain gets longer and the boiling points of higher members of the linear
alcohols converges towards the boiling point curve of alkanes!
The increase in
intermolecular attractive forces, means the molecules need a
higher kinetic energy to overcome the intermolecular forces
and escape from the liquid surface, so they
have a higher boiling point and increased enthalpy of
vapourisation compared to alkanes.
For a broader discussion see
on boiling points and intermolecular forces see:
Introduction to Intermolecular Forces
Detailed comparative discussion of boiling points of 8 organic molecules
Boiling point plots for six
and for wider reading on
intermolecular bonding forces
Other case studies of
boiling points related to intermolecular forces
Evidence and theory
for hydrogen bonding in simple covalent hydrides