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CH 2270/2290
Melting Point Determination (adapted from Organic Chemistry: A Short Course, H. Hart, L. E. Craine, D. J.
Hart, and T.K. Vinod 13th ed. Houghton-Mifflin, Boston, 2012)
Materials
From the Chemicals Hood:
Nothing from the Stockroom
Unknowns A â F
Urea
Cinnamic acid
Fluorene
Acetanilide
Benzoic acid
Succinimide
Benzamide
Benzilic acid
Adipic acid
N-phenylsuccinimide
Salicylic acid
Melting Range
Background Information: The melting range of a pure solid organic is the temperature range at which the solid is in
equilibrium with its liquid. As heat is added to a solid, the solid eventually changes to a liquid. This occurs as molecules
acquire enough energy to overcome the intermolecular forces previously binding them together in an orderly crystalline
lattice. Melting does not occur instantaneously, because molecules must absorb the energy and then physically break the
binding forces. Typically the outside of a crystal will melt faster than the inside, because it takes time for heat to penetrate.
(Imagine an ice cube melting from the outside in, and not doing so instantlyâŠ)
The melting range of a compound is one of the characteristic properties of a pure solid. The melting range is defined as the
span of temperature from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid.
Most pure organics melt over a narrow temperature range of 1-2 ÂșC, if heated slowly enough. Impure samples will normally
have melting ranges that are both larger (>1 ÂșC) and begin lower.
Taking the melting range of a sample is useful for two reasons:
It allows identification of an unknown sample (compare its observed melting range with that of known compounds).
Assessment of sample purity for a known substance
By comparing observed range for an actual sample to the known range for a pure sample, you can tell whether your actual
sample is pure or contaminated (the range is depressed and broadened) .
The presence of impurity has two effects on a substanceâs melting range
Melting range depression (lower end of the range drops)
Melting range broadening (the range simply increases. Often the low end drops a lot, the high end less so or sometimes not
much at all.) A melting range of 5 ÂșC or more indicates that a compound is impure.
Why? The reason for this depression/broadening is that contaminants disrupt the consistency and organization of the
crystal lattice at the molecular level. Contaminants donât âfitâ correctly into what would be the normal pure lattice.
How does this manifest itself? The disruption weakens the lattice, so that the lattice can be broken down more easily; the
weakened structure melts more easily at reduced temperature (depression) . Disruption of the lattice makes it non-uniform. At
the molecular level, the molecules closest to the impurities melt fastest. Farther away from the impurities, the crystal lattice is
relatively undisturbed and therefore melts at or nearer the normal temperature.
Miscellaneous notes on melting range depression/broadening:
Only âsolubleâ impurities, which are incorporated into the crystal structure at the molecular level, cause depression and
broadening. An insoluble piece of metal or wood ionic salt crystal has negligible effect, because only a few organic
molecules will be in contact and will be affected.
At the chemical level, it is impossible to âraiseâ the melting point of an already pure substance. Its melting point can be
depressed by contamination, but not raised.
Practical: If the melting point for an unknown sample is observed to be in between that of two candidates whose pure melting
points are known, the unknown canât actually be equal to the lower-melting candidate. (Short of the rapid-heating effect, see
later.) Most likely the unknown sample is an impure version of the higher melting candidate. For example: suppose an
