Chapter 46: Exp. 1 & 4 Synthesis of Methyl Orange
SO3- dye & pH indicatormethyl orange
, 2 HCl
Note: We need to use many chemicals in this experiment. Some of the reagents are the same compound in solutions with different concentrations. Please make sure you check the labels very carefully and use the correct reagent bottles, otherwise your experiment may just fail.
1. Diazotization of sulfanilic acid
Prepare a sand bath of ~100 oC. In a 10 x 100 mm reaction tube add 1.0 mL of 0.40 M sodium carbonate solution. Weigh ~120 mg of sulfanilic acid and add it in small portions to the reaction tube. Each time when a small portion of solid is added use the stainless steel spatula to thoroughly agitate the mixture as gas bubbles (CO2) are formed. After all solid has been added insert the tube to the bottom of the sand bath for ~2 minutes. Use a spatula to break any big particles and constantly agitate the mixture to dissolve most of the solid. (Note: a small amount of black solid might remain insoluble. Just ignore it.) Cool the mixture to room temperature, add ~50 mg of sodium nitrite, and stir the mixture with the spatula for 1 minute. Cool the tube in an ice/water bath. Add one piece of ice to the reaction mixture in the tube followed by 5 drops of concentrated hydrochloric acid. (Concentrated hydrochloric acid has a molarity of 12 M. Make sure you do NOT use the 0.01 M hydrochloric acid.) Agitate the mixture with the spatula for 2 minutes, and some white or yellow precipitates will begin to form. The product is pretty stable and is called a diazonium salt.
If you don’t see significant amount of precipitates after the above process, something must have gone wrong. Please check with Dr. Wang who will watch you repeating the procedure. The repetition should take ~10 minutes or less and can guarantee that you get the desired product in the end.
4. Methyl orange
In a 10-mL Erlenmeyer flask, carefully weigh ~75 mg of N,N-dimethylaniline and ~65 mg of acetic acid. (Caution: These two chemicals are both toxic and smell badly. After weighing keep the flask covered with a watch glass in your hood.) The easiest way to weight these two liquid compounds is to put the Erlenmeyer flaks on the balance, zero it, carefully add N,Ndimethylaniline dropwise from the top opening of balance until you get ~75 mg, zero again, and carefully add acetic acid from the top opening of balance until you get ~65 mg. Shake the diazonium salt mixture in the reaction tube from Experiment 1 into a suspension, and pour the mixture into the Erlenmeyer flask containing N,N-dimethylaniline and acetic acid. Note any color change in your observations. Rinse the residual solid in the reaction tube with a few drops of water and also add the rinse to the Erlenmeyer flask. Stir the mixture with the spatula for 5 min, and add 1 mL of 3 M sodium hydroxide solution (not the 0.01 M NaOH solution) and 2 mL of water. Place the flask in the 100 oC sand bath, and keep constant stirring (to avoid bumping) for 5 minutes. Remove the flask from the sand bath, cool slowly to room temperature, and further cool thoroughly in the ice/water bath for 5 minutes. Set up a filtration apparatus and secure it with clamp. Put a filter paper on the Hirsh funnel and wet it with 2 drops of water. Agitate the mixture in the Erlenmeyer flask with a spatula and pour it out onto the filter paper, perform vacuum filtration with maximum vacuum. Use ~1.0 mL of saturated sodium chloride solution (NaCl) to rinse the residual solid in the flask onto the filter paper. Further wash the solid with another 1 mL of saturated NaCl solution and decant the filtrate down the drain. Without applying vacuum, add ~1 mL of ethanol to the solid on the filter paper. (Note: the product is a salt and thus has minimal solubility in ethanol.) Perform vacuum filtration for 5 min until you get a solid which appears dry. If filtration is smooth, use medium vacuum. If filtration is slow, use maximum vacuum. (The filtration for this experiment is typically very slow. Gentle agitation of the muddy solid with a spatula may help speed up the filtration. Once you see no more liquid dripping from the bottom of the Hirsh funnel, wait for 5 min and then stop the filtration. Make sure there are no significant amount of liquid in the filter flask and that no liquid get into the rubber tube connecting the filter flask to the vacuum outlet.) Determine the weight of your product for later yield calculation.
Dissolve ~ 3 mg (0.003 g, not 0.030 g. This is a very tiny amount. If you use more than this amount your coloring results will be skewed.) of your methyl orange product in 2.0 mL of water in a test tube. If there are still solids remaining add more water until a transparent solution is formed. In two separate test tubes obtain 1 mL of 0.01 M hydrochloric acid and 0.01 M sodium hydroxide, and add 1 drop of the methyl orange solution to each solution. Record the colors and rationalize in your lab report.
Cleaning up. Wash all your glassware and Hirsch funnel clean. Make sure there is no residual colored material left. Dump the methyl orange solid (without weighing paper) into the solid waste container. All liquids from this experiment can be dumped down the drain.
Note on yield calculation: Assuming 100% yields on steps 1 and 2, calculate the mole amounts of sulfanilic acid, sodium carbonate, sodium nitrite, and N,N-dimethylaniline. Based on the stoichiometry of the reactions determine the limiting reactant and calculate the theoretical yield. The statement “The yield should be between 125 mg and 150 mg” on page 596 of the lab manual tells you just a range of the weight of methyl orange that people normally get after performing the reaction. The precise maximal product weight should be based on the exact amounts of reactants that you have used, which are not necessarily the same amounts in the lab manual. Dividing your product weight by the precise maximal product weight then gives you the actual yield.
e.g. If you actually used 1.0 mL of 0.30 mL Na2CO3, 0.1305 g of sulfanilic acid, 0.0524 g of sodium nitrite, and 0.0734 mg of N,N-dimethylaniline, and isolated 0.1354 g of methyl orange. From searching the Internet or calculation we find the molar mass of the reactants and product: Na2CO3 (106.0 g/mol), sulfanilic acid (173.2 g/mol), sodium nitrite (59.0 g/mol), N,Ndimethylaniline (121.2 g/mol), and methyl orange (327.3 g/mol). Now we calculate the mole amounts:
Na2CO3: 1.0 mL x x 0.40 (mol/L) = 4.0 x 10
sulfanilic acid: 0.1305 g ÷ (173.2 g/mol) = 7.54 x 10-4 mol
NaNO2: 0.0524 g ÷ (59.0 g/mol) = 8.88 x 10-4 mol
N,N-dimethylaniline: 0.0734 g ÷ (121.2 g/mol) = 6.06 x 10-4 mol
From the stoichiometry we know that 6.06 x 10-4 mol of N,N-dimethylaniline would require 6.06 x 10-4 mol of diazonium salt, 6.06 x 10-4 mol of NaNO2, 6.06 x 10-4 mol of sulfanilic acid, and 3.03 x 10-4 mol of Na2CO3, and it is apparent that N,N-dimethylaniline is the limiting reactant.
Since the stoichiometry between N,N-dimethylaniline and methyl orange is 1:1, we can now calculate the maximum product mass:
6.06 x 10-4 mol x (327.3 g/mol) = 0.198 g
. the actual percent yield = x 100% = 68% (always keep 2 significant figures
for percent yield)