Naphthol Red (Pigment red 170 or PR170) is an organic
pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
extensively used in
automotive coating
A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Powde ...
s and painting.
It is produced synthetically by converting
p-aminobenzamide into the corresponding
diazonium compound
Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide.
General properti ...
followed by coupling with 3-hydroxy-2-naphththoic acid (2-ethoxy)anilide ("Naphtol AS-PH" dye precursor).
:
In the solid state the
hydrazo tautomer
Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert.
The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydr ...
forms and several
crystal structures exist. In the initial α
polymorph the molecules are arranged in a herringbone pattern with extensive hydrogen bonding. The φ polymorph is more
dense
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
and more stable and produced industrially by thermal treatment in water at 130
°C under pressure. In this phase the molecules are planar and arranged in layers. Extensive
hydrogen bonding
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
exists within the layer but between layers the only interactions are
Van der Waals force
In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...
s. Dense crystal structures are preferred for pigments used in coatings because in the event of
photochemical
Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400–7 ...
decomposition
Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is ...
the fragments are locked in place and are able to recombine. Research shows that by replacing the
ethoxy group in this compound by a
methoxy
In organic chemistry, a methoxy group is the functional group consisting of a methyl group bound to oxygen. This alkoxy group has the formula .
On a benzene ring, the Hammett equation classifies a methoxy substituent at the ''para'' position as ...
group the crystal structure is less stable and in the final application and the color fades more easily. By careful selection of
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''sid ...
s it is possible to optimize crystal structure and improve optical properties .
References
# {{Note, Schmidt ''Crystal Structures of Pigment Red 170 and Derivatives, as Determined by X-ray Powder Diffraction '' Martin U. Schmidt, Detlef W. M. Hofmann, Christian Buchsbaum, Hans Joachim Metz,
Angewandte Chemie International Edition
''Angewandte Chemie'' (, meaning "Applied Chemistry") is a weekly peer-reviewed scientific journal that is published by Wiley-VCH on behalf of the German Chemical Society (Gesellschaft Deutscher Chemiker). Publishing formats include feature-length ...
Volume 45, Issue 8, Pages 1313 - 1317 2006''
Abstract
Pigments
Organic pigments