Trends in Boiling Points: Molecular Structure & Organic Compounds

This report investigates the relationship between the boiling points of organic compounds and their molecular structures, focusing on the influence of intermolecular forces. It explains that boiling points are determined by the strength of intermolecular forces such as ionic bonds, hydrogen bonds, dipole-dipole interactions, and Van der Waals forces. The report discusses how the presence of functional groups like OH in carboxylic acids leads to higher boiling points compared to aldehydes with COH groups. It also highlights the impact of molecular structure, chain length, and branching on the surface area and Van der Waals dispersion forces, which consequently affect boiling points. The report presents a table summarizing the boiling points of butanoic acid, pentanal, butane, and 1-butene, and justifies these trends based on the types of intermolecular forces present in each compound, along with the number of carbon atoms and branching. It concludes by emphasizing the significance of hydrogen bonding, dipole-dipole interactions, and molecular shape in determining both boiling and melting points.

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At a certain temperature, vapour pressure of a given liquid reaches a point equivalent to
pressure of its surrounding. This temperature is defined as the boiling point of such a
substance. Organic compounds exhibit a fascinating trend since they tend to have boiling
points based upon certain characteristics particular to every organic compound. According to
Burger, Baibourine, Bruno (2011 p.89), organic compounds comprises of a number of forces
that keep the molecules together. These forces include bonds of either ionic, hydrogen
bonding as well as van der Waal forces of attraction or the dipole to dipole moments or both.
Of all the forces, it has been noted that ionic bonds provides the greatest forces of attraction
hence substances possessing it considerably than other exhibit higher boiling points. After
ionic forces of attraction/bonds, hydrogen bonds ranks second followed by dipole-dipole
forces with the van der Waal forces of attraction being ranked as the weakest of all hence
associated with lowest boiling points in substances in which it occurs compared to the
aforementioned forces respectively. For instance, substances having OH groups including the
carboxylic acids has been identified to have high values of boiling points when compared to
substances having the COH, that is the aldehydes possessing the dipole-dipole bonds.
As per Burger et al (2011 p.121), atomic structure of the particles influences the surface zone
of the atoms which thusly influences the Van der Waals scattering influences and
consequently, the boiling points of the atoms. A long chain natural compound builds the
surface region of the atom and the sub-atomic weight of the particle. Henceforth, the capacity
of the individual particles to draw in one another is additionally expanded. The cooperation
result in expanded scattering effects and along these lines higher boiling points.
Compound structure (Haynes, 2014 p.87).

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Branched molecular structures anyway diminish boiling points of compounds as the surface
zone will be diminished and subsequently, the scattering forces too will be debilitated.
Compound structure (Haynes, 2014 p.132)
The last sub-atomic structure viewpoint influencing boiling point of the substances is the
introduction of the polar group of functionality. As indicated by Haynes (2014 p.192), an
uncovered polar assembly like in the carboxylic acids expands boiling points.
The compound structure of the alcohol (Kerth, 2016 p.78).

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Results (Kerth, 2016 p.108).
Name of the organic
compound.
Molecular Structure. Boiling Point.
Butanoic Acid C4H8O2 164.30C (Royal
Society of
Chemists, 2015)
Pentanal C5H10O 103.70C (Royal
Society of
Chemists, 2015)
Butane C4 H10 -0.5ᵒC
(Cleveland and
Morris, 2013)
1-butene C4 H8 -6.3ᵒC
(Cleveland and
Morris, 2013)

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The above table summarises the trends in boiling points of the listed four compounds. Boiling
points of these organic compounds depict the strength of the intermolecular forces of
attraction that consist each an organic compound. Based on these strength mainly, the trends
in boiling points can be understood. Other factors that contribute towards these trends include
number of carbons as well as branching for a given compound. As aforementioned in the
introduction, number of carbon atoms have a direct bearing on the boiling point of a
compound such that an increase in number of carbon atoms leads to increase in the boiling
point of a given compound. However, branching within the structure has an influence to
decrease in the boiling point hence the higher the branching the lower the boiling point. In
this regard therefore, I seek to discuss the trends in boiling points of the above compounds as
in the section that follows.
In butanoic acid, which is classified as carboxylic acid, there are three notable forces
maintaining the intermolecular bonds. These forces include the hydrogen bonds, dipole-
dipole forces and the van der Waal forces of attraction. A combination of the three bonds
precisely denotes that braking the bonds requires significant energy hence as result the
boiling point of butanoic acid is high. In aldehydes and specifically the compound pentanal,
the molecular structure is sustained by two forces namely; the dipole-dipole forces and the
van der Waal forces of attraction (Vogel, 2013 p.86).
The hydrogen bonding in the butanoic acid molecule outcomes in the practical dissimilarity
in boiling points of the butanoic acid and the pentanal as the bond needs additional energy to
break when paralleled to the dipole-dipole bonds in the pentanal molecule.
While butane is unsaturated, butane is a saturated hydrocarbon with all the carbon atoms
filled. Butene have lower boiling points than butane. Butane have higher molecular weight
and high intermolecular forces of attraction hence has higher boiling point than butane
despite butane having pi bond interactions hence the small difference.

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Alkanes simply have carbon and hydrogen particles with no utilitarian gatherings, so the
main intermolecular power that impacts the breaking point is London scattering powers. The
more the atoms can contact one another, the more London scattering powers there are, and
the higher the boiling point. s a govern, bigger particles have higher boiling (and dissolving)
focuses. Consider the boiling points of progressively bigger hydrocarbons. More carbons and
hydrogens implies a more prominent surface zone feasible for van der Waals association, and
in this way higher boiling points. Beneath zero degrees centigrade (and at environmental
weight) butane is a fluid, in light of the fact that the butane particles are held together by Van
der Waals powers. Over zero degrees, in any case, the atoms increase enough warm vitality to
break separated and enter the gas phase (Aryangat, 2014). The quality of intermolecular
hydrogen holding and dipole-dipole associations is reflected in higher boiling points. Take a
gander at the pattern for hexane (van der Waals collaborations just), 3-hexanone (dipole-
dipole cooperations), and 3-hexanol (hydrogen holding). In every one of the three particles,
van der Waals connections are critical. The polar ketone aggregate permits 3-hexanone to
frame intermolecular dipole-dipole cooperations, notwithstanding the weaker van der Waals
associations. 3-hexanol, as a result of its hydroxyl gathering, can shape intermolecular
hydrogen bonds, which are more grounded yet. Of specific enthusiasm to scientists (and
practically whatever else that is alive on the planet) is the impact of hydrogen holding in
water. Since it can frame tight systems of intermolecular hydrogen bonds, water stays in the
fluid stage at temperatures up to 100 OC notwithstanding its little size. By considering
noncovalent intermolecular connections, we can likewise foresee relative dissolving focuses.
The majority of similar standards apply: more grounded intermolecular collaborations result
in a higher liquefying point. Ionic mixes, not surprisingly, as a rule have high softening points
because of the quality of particle collaborations. Much the same as with boiling points, the
nearness of polar and hydrogen-holding bunches on natural mixes by and large prompts

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higher boiling points (Bettelheim et al., n.d.). The extent of an atom impacts its liquefying
point and in addition its boiling point, again because of expanded van der Waals
communications between particles (Cleveland and Morris, 2013).
What is diverse about melting point slants, that we don't see with boiling point or
dissolvability patterns, is the significance of a particle's shape and its capacity of pack firmly
together. Imagine yourself attempting to make a steady heap of balls in the floor. It simply
doesn't work, since circles don't pack together well - there is almost no region of contact
between each ball. It is simple, however, to make a pile of level articles like books.
A similar idea applies to how well particles pack together in a strong. The level state of
sweet-smelling mixes enables them to pack productively, and hence aromatics have a
tendency to have higher liquefying guides contrasted toward non-planar hydrocarbons and
comparative atomic weights. Looking at the melting points of butane and butene, you can see
that the additional methyl amass on toluene disturbs the particle's capacity to pack firmly, in
this way diminishing the total quality of intermolecular van der Waals powers and bringing
down the boiling point.

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References
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