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Lecture 7

NATS 1870 Lecture Notes - Lecture 7: Coal Tar, Ionic Bonding, Naphthalene

Natural Science
Course Code
NATS 1870
Robin Kingsburgh

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SC NATS 1870 (lecture) Monday,
November 2, 2009
Late 1660’s, Isaac Newton, prisms break white light into colors
Color depends on wavelength
1856, William experimented with coal tar
Medical test require color for results
Color differentiates acids & how much in there
Covalent Bond
Pair of e- shared equally between 2 atoms
Ionic bond: one atom donates its e- to another atom atoms become a linked positively
charge & negatively charged ion set
Molecules & Color
Configuration of molecules that determines whether a substance appears colored
The energy level structure for the e-
oCertain λ’s get absorbed rest are reflected, give color of substance
By slight changes in structure of molecule or composition
oCreates changes in energy level structure
oCreates changes in λ’s absorbed color appearance change
Common structures in molecules that give rise to color:
1) double covalent bond
Often in large molecules the portion of that molecule responsible for absorbing visible λ’s
has a double bond
2) adding one or more groups of atoms serve as e- donors/acceptors
Change in energy level structure of molecule
Eg. Benzene colorless
Naphthalene colorless
oNO2, OH is added, the molecule absorbs strongly at short λ’s
Martius yellow a deep yellow dye (see PDF for structure)
The groups that are added are sometimes called “auxochromes) can deepen +
intensify color
3) conjugated bonds
Strings of alternating double/single covalent bonds
Eg. In carotene family (over 500 molecules)
One caratenoid absorbs 424nm appears orange
4) porphyrins
Extended ring structures with heavy metal atom (ion in centre)
Chlorophyll green contains mg at centre
Hemoglobin red Fe (iron)
Phthalocyidne blue blue Cu (cyan)
History of chemistry through development of dyes:
1) Perkins violet:
Example of “anoliu” dye from a coal tar derivatives (post-industrial product of natural gas
burning coal tar, lots of molecules central basis in benzene)
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