Experiment23: 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 (HNO2). • Since nitrous acid is unstable, it is generated in situ by mixing sodium nitrite (NaNO2) 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 = hn Excited State Ground State smaller energy gap = lower energy of light required = higher l 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 400-700 nm region of the spectrum. • Substances which absorb near 350 nm often ‘tail’ into the visible region and appear yellowish . n l(nm) 1 180 3 250 5 345 7 400 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 400-420 nm 650-750 nm 420-430 nm 430-490 nm 580-650 nm An object appears as the complement of the color absorbed (opposite on color wheel) 560-580 nm 490-560 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.
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.
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!
LABORATORY NOTEBOOK(Pre-lab) • OBJECTIVE(Must clearly state…) • What compounds will be made and how • What will be investigated with the compounds • CHEMICAL EQUATION • Include the general chemical equation from the top of page 193. • TABLE OF PHYSICAL DATA (Complete the following table using a site listed on WWW Links ONLY. Wikipedia is unacceptable!) • REFERENCE TO PROCEDURE (Must include…) • full title including edition and author names • numbers where actual procedure can be found
LABORATORY NOTEBOOK(In-lab) • DATA/CALCULATIONS • Dye colors observed and approximate wavelengths • Dye colors observed and approximate wavelengths • EXPERIMENTAL PROCEDURE • In paragraph form, BRIEFLY describe the procedure that you actually followed during the lab. • Paragraph must be written in PAST TENSE, PASSIVE VOICE. • Include any volumes or weights of chemicals used during the experiment. • Include any mistakes, accidents or observations if applicable.