Experiment 23: THEORY OF COLOR IN ORGANIC COMPOUNDS; PREPARATION AND APPLICATION OF ORGANIC DYES

Objectives:  To synthesize azo dyes with various amines.  To study the relationship between the extent of conjugation and the color observed.  To study how conjugation and color are affected by pH.

AZO DYE SYNTHESIS  Arene diazonium salts are formed by the reaction between an arylamine and nitrous acid (HNO 2 ).  Since nitrous acid is unstable, it is generated in situ by mixing sodium nitrite (NaNO 2 ) with cold, dilute HCl.  Arene diazonium ions are weak electrophiles in electrophilic aromatic substitution, and will only react with strongly activating rings.

GENERAL CHEMICAL EQUATION Azo linkage

COMPOUNDS USED IN DYE SYNTHESIS amines activator

COLOR REQUIRES THE ABSORPTION OF LIGHT  Molecules absorb Visible or UV light by promoting an electron from the HOMO to the LUMO.  The wavelength of light absorbed is inversely proportional to the energy difference between these orbitals: E = hc/l = h Ground State Excited State smaller energy gap = lower energy of light required = higher light absorbed

ELECTROMAGNETIC SPECTRUM Colored compounds have such extended conjugated systems that their “UV” absorptions extend into the visible region.

REQUIREMENT FOR COLOR  For a substance to have color, it must absorb within the nm region of the spectrum.  Substances which absorb near 350 nm often ‘tail’ into the visible region and appear yellowish. n (nm) Extended Conjugation (>7 double bonds) is required for color in organic compounds

pH AND CONJUGATION  When base is added to a phenol, it is converted to a phenoxide ion.  The phenoxide ion has lone electrons which can contribute to the resonance stabilization of the aromatic ring.  This makes the energy gap between the two orbitals smaller, and a longer wavelength is absorbed, resulting in a different color than is observed for a neutral solution of a dye.

THE COLOR WHEEL An object appears as the complement of the color absorbed (opposite on color wheel) nm nm nm nm nm nm nm

DYES  Dyes are colored compounds which adhere to the material they color.  All of the various forces of intermolecular attraction may be involved in fixing a dye to a fabric: ion-ion, ion-dipole, dipole- dipole, London dispersion (van der Waals), and H-bonding.  The interaction of intermolecular forces between the fabric and the organic dye are responsible for the intensity of the color observed. The stronger the interaction of IMF, the more intense the color will appear.

EXPERIMENTAL PROCEDURE (Part A) Part A involves the preparation of a monoamine dye and the investigation of color based on conjugation of the dye at various pH levels. p-nitroaniline dye m-nitroaniline dye AcidicBasicAcidicBasic pH  use pH Hydrion paper! 112 color OBSERVED  Color that you SEE! YR approximate OBSERVED (nm)  Report entire range of wavelengths! color ABSORBED  Use color wheel, p. 193! VG approximate ABSORBED (nm)  Report entire range of wavelengths!

EXPERIMENTAL PROCEDURE (Part B) Part B involves the preparation of a monoamine dye and a diamine dye, and the comparison of color based on conjugation. o-anisidine dye Dianisidine dye color OBSERVED  Color that you SEE! approximate OBSERVED (nm)  Report entire range of wavelengths! color ABSORBED  Use color wheel, p. 193! approximate ABSORBED (nm)  Report entire range of wavelengths!

SAFETY CONCERNS CAUTION: These dyes may be harmful; avoid contact!

WASTE MANAGEMENT  All dye solutions are to be placed in the container labeled “Organic Waste (Dyes)”.

CLEANING  Be sure to clean all glassware with soap, water, and brush, followed by a rinse with wash acetone, before returning to lab drawer!  Be sure all test tubes are clean and free of dye solution before returning to lab drawer!  DO NOT return any glassware to lab drawer dirty or wet!