Determination of melting point of organic compounds
Dotsha J. RaheemCollege of Science- university of salahaddin
Department of Chemistry
Melting Point
1. Is the temperature at which a solid is converted to liquid at normal atmospheric pressure.
2. A more specific definition of m.p. (or f.p.):
Is the temperature at which the solid and the liquid phases are at equilibrium at a specific pressure (normally taken atmospheric unless stated otherwise)
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3. Is the temperature at which the forces which unite the crystals of a solid are ruptured. This results in a change from the crystalline to the amorphous state. This change involves the absorption of a characteristic amount of heat called the heat of melting or fusion.
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Importance of m.p.
1. It is a physical property used for identification.
2. It is an important indicator for purity.
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Factors affecting m.p.
M.wt. or size of the molecule
Branching
Intermolecular forces
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CompoundMolecular
formula Molecular
weightMelting
point
methane CH4 16 -182
ethane C2H6 30 -183
propane C3H8 44 -188
butane C4H10 58 -138
pentane C5H12 72 -130
M.wt. or size of the molecule
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Branching
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1. Ionic, Dipole - Dipole attractions
Intermolecular forces
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2. Hydrogen Bonding
• a special kind of dipole-dipole
force that occurs when a H atom
is bonded to one of the very
electronegative atoms, F, O, or N.
• Electronegative atom must have
at least a lone pair of electrons
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- 82 oC0 oC
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The H-F, H-O, and H-N bonds are very polar, because the electronegative atom draws the bonding electron pair strongly to itself. This leaves the hydrogen nucleus exposed
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Intermolecular forces
Non-polar molecules do not have dipoles like polar molecules, but they can solidify. This is due mainly to the very important van derwaals forces that exist in these molecules
3. Van der Waals forces
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dispersion forces < dipole-dipole interactions < hydrogen bonds 13
Types of m.p. and effect of impurities
In the presence of impurities
m.p. will be lower
needs less energy to destroy the
crystal lattice
Melting range will be broader
Due to the presence of different
types of interactions
Pure compound,melting range 1-2 oC
impure compound,melting range >2 oC 14
Ionic vs. covalent compounds
NaCl m.p. 801 oC
In ionic compounds the structural units are ions, and the strong intermolecular interactions are overcome only at very high temperatures
Na+
Na+
Na+
Na+ Na+
Na+
Na+
Na+ Na+
Na+
Na+
Na+
Na+Na+
Cl-
Cl-Cl- Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl- Cl-
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Methane m.p. -182 oC
In covalent compounds, the intermolecular forces are very week if compared to the forces in ionic compounds (and m.p. represents only the amount if heat needed to overcome the intermolecular interactions but not the covalent bonds)
CH4CH4
CH4 CH4
CH4
CH4
CH4 CH4
CH4
CH4
CH4
CH4CH4CH4CH4
CH4 CH4CH4
CH4 CH4 CH4CH4
CH4CH4
CH4
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