Melting Point
Determination
BIO 105L (Organic Molecules 1) Laboratory
Ms. EMELIE A. ONA
- Molecules in a crystal are
arranged in a regular
pattern
MELTING - Melting occurs when the
fixed array of molecules in
POINT the crystalline solid
rearrange to the more
random, freely moving
liquid state
- Transition from solid to
liquid requires energy in
the form of heat to break
MELTING down the crystal lattice
POINT
- Temperature at which
this transition occurs is
the solid’s melting point.
- an important physical
property of any solid
MELTING compound
POINT - useful in establishing its
identity and as a
criterion of its purity
- until the advent of modern
chromatography and
MELTING spectroscopy, the melting
point was the primary
POINT index of purity for an
organic solid
- the melting point, or
correctly the melting range,
of a crystalline organic
MELTING compound is determined
THEO
POINT by the strength of the
intermolecular forces
RY between its molecule-
hydrogen bonds, dipole-
dipole interactions, and van
der Waals interactions
- these forces hold the
MELTING molecules together in an
THEO
POINT orderly crystalline array
and must be overcome for
RY the molecules to enter the
less orderly liquid phase
- large molecular surface
MELTING area and high molecular
THEO
POINT symmetry are associated
with greater intermolecular
RY forces and higher melting
points
- melting point range is
generally reproducible for a
pure compound
MELTING
relatively pure compounds
THEO
POINT -
normally melt over a narrow
RY temperature range of 0.5°C -
1.5°C whereas impure
substances often melt over
a much larger range
- adding greater amount
MELTING of an impurity generally
THEO
POINT causes a greater
decrease in the melting
RY point
Impurities lower melting
point:
- takes less energy to disrupt
MELTING crystal lattice when
THEO
POINT impurities are present
RY - melting point will be lower
- melting point will be
broader
What is the
importance of
knowing the melting
point of organic
compounds?
- important physical property of
a compound
- can be used to identify a
MELTING substance and as an indication
of its purity
POINT
- defined as a temperature at
which the solid exists in
equilibrium with its liquid
under an external pressure of
one atmosphere
- Pure substances usually
have melting point ranges
of a degree or two (1-2°C)
MELTING POINT
- Impure substance (which
are mixtures of two or
more substances) often
have wider ranges while a
wide range suggests that it
is not
- It is often, but not always
MELTING true, that an impure
POINT substance will have a
lower melting point than
the same substance when
it is pure.
- melting point range of >5°C
indicates that the substance
is impure
MELTING
- for a material whose
POINT identity is known, an
estimate of degree of purity
can be made by comparing
its melting point with that of
a pure sample
What are the factors
that affect the
melting point?
Size of the
molecules
http://amrita.olabs.edu.in/?sub=73&brch=7&sim=33&cnt=1
- if the melting point of two pure sample shows a
clear difference in melting points, it indicates that
the two compounds must have different structural
arrangement, or they must have different
arrangements of atoms or configurations
Force of
attraction
between the
molecules http://amrita.olabs.edu.in/?sub=73&brch=7&sim=33&cnt=1
Stronger intermolecular interactions result in
higher melting points. ionic compounds usually
have high melting points because the electrostatic
forces holding the ions (ion-ion interaction) are
much stronger.
Force of
attraction
between the
molecules http://amrita.olabs.edu.in/?sub=73&brch=7&sim=33&cnt=1
In organic compounds the presence of polarity, or
especially hydrogen bonding generally leads to
higher melting point.
THANK
YOU!
Techniques in Organic Chemistry by Jerry R. Mohrig, Christina Noring Hammond, Paul F. Schatz (z-lib.org)
https://www.slideserve.com/shae/theory-of-melting-point
http://amrita.olabs.edu.in/?sub=73&brch=7&sim=33&cnt=1
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Organic_Chemistry_Lab_Techniques_(Nichols)/06%3A_Miscel
laneous_Techniques/6.01%3A_Melting_Point/6.1C%3A Melting_Point_Theory
