Exp t 31 Synthesis of the Dye Sudan I Addition of Benzenediazonium Ion to 2 Naphthol from K L Williamson Macroscale and Microscale Organic Experiments 2nd Ed 1994 Houghton Mifflin Boston Revised 3 21 01 Introduction Many synthetic dyes often called diazo dyes are based on diazonium chemistry that has been used for over 100 years in synthetic chemistry and this experiment is an example of the synthesis of one such dye 1 phenylazo 2 naphthol or more simply Sudan I 4 The reaction involves two steps the first being the preparation of a solution of the diazonium salt benzenediazonium chloride 2 from aniline 1 N2 Cl NH2 N a N O2 HCl H2O 0 5 C 1 b p 184 C 2 In the second step the solution of 2 is added to 2 naphthol 3 to produce the diazo dye 4 The dye is analyzed by visible light spectrophotometry OH N2 Cl3 NaOH H2O 0 5 C N N OH 4 m p 131 132 C Prelaboratory Exercises Draw the Lewis structures for nitrite ion nitrous acid and a generalized Ar aryl diazonium ion Cautions Aniline 1 is an anticipated carcinogen Wear gloves when handling and handle only in a hood Sodium nitrite and 2 naphthol are toxic Handle with care Be careful not to allow the benzenediazonium chloride solution to warm up or it will decompose leading to undesirable sideproducts The 30 KOH solution used for the UV Vis analysis is very caustic Handle with care Synthesis STEP 1 Preparation of Sodium 2 naphtholate solution In a reaction tube prepare a solution of 78 mg of 2 naphthol 3 m p 123 C in 0 5 mL of 3M NaOH Set aside in an ice bath to cool until ready for use in step 3 STEP 2 Preparation of benzene diazonium chloride 2 In a hood carry out an instant microscale distillation of aniline b p 184 C Refer to the Chem 35 or 36 Lab Guide Appendix A3 section 2d for a description of this method You only need about 0 5 to 1 mL of the impure aniline from the side shelf to do the distillation Use a boiling chip Discard the first two or three cloudy pipet fractions from the instant microscale distillation as these contain mainly water Then collect the next 5 to 10 pipetfuls of condensed vapor in a clean reaction tube The material to be used should be clear and only slightly yellow not cloudy Discard any undistilled aniline into the Nonhalogenated Organics Waste container In a reaction tube weigh 50 mg of the freshly distilled aniline and add 0 25 mL of concentrated 12M HCl and 0 5 mL distilled water Warm and agitate this solution to dissolve the aniline hydrochloride and then cool it in an ice bath to 0 C it is important to keep the reaction cooled to 0 C since above 5 C diazonium salts can form aryl radicals Prepare a solution of 40 mg of sodium nitrite NaNO2 in 0 5 mL water in another reaction tube Add this sodium nitrite solution drop wise to the acidic aniline solution while maintaining the reaction at ice bath temperature Agitate the solution by vigorously flicking the reaction tube with your finger Five min after the addition of the sodium nitrite solution is complete test the reaction solution for excess nitrous acid HONO by touching a small drop of it on the tip the spatula to a piece of KI starch starch iodide paper If the test is positive immediate purple color add urea a few crystals at a time with cooling and agitation Test with KI starch paper again and continue adding urea a few crystals at a time until the KI starch test is negative STEP 3 Preparation of 4 by reaction of 2 with sodium 2 naphtholate Add a small piece of ice to the ice cold solution of sodium 2 naphtholate you prepared at the start of this synthesis Dropwise add the benzene diazonium chloride solution from STEP 2 while keeping both solutions at 0 5 C Agitate the reaction tube by stirring with the bent microspatula to insure complete mixing The reaction initially forms a blood red solid but this changes to a reddish orange precipitate as the pH becomes acidic with the addition of the hydrochloric acid sodium nitrite solution After complete addition the reaction mixture is left at ice bath temperature for 15 min with occasional agitation Isolation and Purification Filter the bright red solid Sudan I on a Hirsch funnel washing it from the reaction tube twice with one or two mL of cold distilled water Air dry this crude product until the next lab Once dry weigh the crude dye product and set aside a small amount for a mp determination Recrystalize the crude product from ethanol Do NOT heat the dye too much because you will turn your nice red dye into a black brown mess Filter dry and weigh the product Typical yields are about 40 to 50 or 30 to 40 mg Determine the m p s of this and the crude product Visible Light Absorption Analysis Prepare a dilute solution for visible light absorption analysis by dissolving about 1 mg of the red product in 2 mL of 30 KOH in methanol 30 methanolic KOH will be found on the hooded shelf in a 125 mL Erlenmeyer flask and adding 100 mL of methanol Use the UV Vis spectrophotometer in the organic lab instrument room to obtain the visible spectrum between 350 nm and 750 nm See Lab Guide and operating instructions near the instrument in 206 Whitmore After running the visible spectrum of this basic solution add a drop of conc hydrochloric acid to the sample cuvette mix check that the solution is acidic with pH paper and run another visible wavelength absorption spectrum You should see a bathochromic shift in the lmax as the pH is changed You may need to dilute your solution with methanol Dilution should be carried out if any peak absorbances are above 1 5 AU Keep diluting the solution with methanol until all peaks are at or below 1 5 AU Cleaning Up Discard undistilled and unused aniline into the Non Halogenated Organics Waste container The filtrates from the crude product isolation and recrystallizaton can also be placed in this container Solutions from the spectrophometric analysis can be flushed down the drain Final Report Attach visible spectra to your report with absorption maxima measured in nm clearly marked on them Answer the following questions 1 Given the hypothesis that nitrous acid HONO is protonated and then loses a molecule of H2O give the likely electrophilic species that attacks the aniline Complete the arrow pushing mechanism for the formation of the diazonium ion Finally complete the mechanism for the subsequent reaction with 2 napthol 2 Why does the reaction between the diazonium ion and 2 naphthol occur primarily on the 1 and not the 3 position as shown in the balanced equation Hint Draw the initially formed