Experiment Six - Pre-Lab

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Experiment Six – Identification of an Unknown Aldehyde or Ketone XXXXX University of South Carolina CHEM 332L-016 TA: March 14, 2023

Purpose/Objective The purpose of this lab is to prepare a semicarbazone derivative and a 2,4- dinitrophenylhydrazone (2,4-DNP) derivative of unknown aldehydes or ketones, recrystallize them, and determine their melting point. Through this process, the identity of the unknown substances can be determined. Introduction Carbonyl compounds are sp 2 hybridized and trigonal planar with bond angles of 120°. A carbonyl group is relatively uncrowded, but as the alkyl groups around the carbonyl carbon increases, the reactivity of the carbonyl compound decreases. Since oxygen is more electronegative than carbon, oxygen has a greater share of the double bond’s electrons making the carbonyl group polar. Meaning the carbon is an electrophile, electron-deficient, which is subject to a nucleophilic attack. Reactions of carbonyl compounds are mostly acid catalyzed. Through this reaction, the acid will protonate the electronegative oxygen to form a carbocation, which is more susceptible to nucleophilic addition. All reactions are reversible because either of the two atoms with a lone pair attached to the tetrahedral carbon can be protonated and therefore eliminated. The reactions can be acid catalyzed with a controlled pH of 4.5. Derivatives are able to provide a solution by creating a solid for analysis and the solid derivative should be safe, easy to make, has a sharp melting point, and should be stable to hydrolysis. Imines, where the nitrogen atom carries an electronegative group that has a lone pair of electrons, are usually stable. This is due to the electronegative substituent that can participate in the delocalization of the imine double bond. The delocalization decreases the small positive charge on the carbon atom of the imine bond makes it less susceptible to nucleophilic attack. 2,4-

dinitrophenylhydrazine dissolved in EtOH/H 3 PO 4 will form a solid 2,4-dinitrophenylhydrazone when an aldehyde or ketone is reacted with it. Reaction Schemes/Structures 2,4-DNP Semicarbazide hydrochloride Reagents Name, Structure, MW (g/mol) Melting °C Boiling °C Density g/mL Properties 2,4-DNP Derivative (MP) Semicarb Derivative (MP) Benzaldehyde C 7 H 6 O 106.12 -26 178- 179 1.045 Flammable liquid; Cherry- like scent 237 °C 216 °C 3-Heptanone C 7 H 14 O 114.18 -39 147 0.818 Colorless liquid with a mild fruity odor; slightly soluble 81 °C 101 °C Acetophenone C 8 H 8 O 120.15 19.6 202 1.03 Colorless liquid but solid at lower temperatures 238 °C 198 °C Diethyl ketone C 5 H 10 O 86.1334 -42 101 0.814 Flammable liquid 156 °C 138 °C Cyclohexanon e -47 155 0.947 Flammable liquid oily, white to slightly yellow liquid with a peppermint-like 162 °C 166 °C

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C 6 H 10 O 98.1444 odor Name, Structure, MW (g/mol) Melting °C Boiling °C Densit y g/mL Properties Safety Phosphoric acid H 3 PO 4 97.99506 21 158 1.685 Viscous, colorless, odorless, liquid; solidifies at 70 °F hygroscopic Corrosive to tissue and irritating to the skin, mucous membranes, upper respiratory tract and eyes; fatal if swallowed Ethanol C 2 H 5 OH 32.042 -114.1 78.3 0.789 Colorless liquid; pleasant alcoholic odor Harmful by ingestion, inhalation, or skin absorption; irritant or the eyes, nose, throat, and skin (flashback along the vapor trail may occur) Methanol CH 3 OH 32.042 -98 64.6 0.791 Colorless liquid with a characteristic pungent odor May cause severe skin and eye irritation; it can be absorbed through the skin; it may cause narcosis Pyridine C 5 H 5 N 79.1012 -41.6 115.2 0.9819 Colorless or yellow liquid with a penetrating sickening stench Toxic by ingestion, inhalation and contact of the skin, eyes, and respiratory tract; NH 3 may cause sterility in males 2,4-DNP C 6 H 6 N 4 O 4 198.1378 198 - - Flammable liquid; orange crystals with ammonia-like odor May cause eye irritation; Flammable; Possible mutagen Semicarbazide hydrochloride CH 6 ClN 3 O 175-177 ~185 - Very soluble in water; white crystals Toxic; local irritant

111.53309 Procedure 1. Obtain 1 mL of the unknown from the hood and record the number. 0.5 mL will be used for the semicarbazide test and 0.5 mL will be used for the 2,4-DNP test. Preparation of 2,4-DNP Derivative (Brady’s Reagent): 1. From the fume hood acquire 2 mL of the 2,4-DNP stock solution and place it in a medium test tube 2. Add 0.5 mL of the unknown compound to the test tube 3. Place a beaker 2/3 full of water with a thermometer on the hot plate. Make sure that the thermometer is not touching the bottom of the beaker. Make sure that the temperature maintains 60-65 °C 4. Heat the mixture 5-10 minutes in the warm water bath 5. Remove the test tube from the water bath and cool the mixture to room temperature (10- 15 minutes) to begin crystallization. Place the test tube in an ice bath until crystallization is complete. If needed scratch the inside of the test tube with a glass stirring rod to induce crystallization 6. Use suction filtration to isolate the solid product. Wash the solid product with 1 mL ice- cold water dropwise 7. Transfer the product to a small test tube and rescrystallize as follows: a. Cover the crude 2,4-DNP derivative in the test tube with a small amount of warm ethanol b. Gently heat while agitating on the water bath c. Slowly add ethanol dropwise until all the solid has dissolved

d. NOTE: some soluble and/or insoluble impurities may not dissolve regardless proceed to Step 9 8. Slowly cool the hot solution to room temperature, then place it in an ice bath to complete the crystallization 9. Use suction filtration to isolate the solid. Use 1 mL of ice-cold ethanol to wash the product 10. Collect the pure product, dry, and determine the melting point Preparation of Semicarbazone Derivative: 1. In a medium test tube, place 0.5 mL of stock semicarbazide hydrochloride solution 2. Add 0.5 mL of the unknown compound and 1-2 mL of methanol 3. Add 10 drops of pyridine. Warm the solution gently in the warm water bath for 8-10 minutes 4. Slowly cool the solution in the test tube to room temperature. Place the test tube in an ice bath to complete the crystallization. Scratch the inside of the rube with a glass rod to induce crystallization if needed 5. Use suction filtration to isolate the cool mixture and collect the crude solid product 6. Use 10 drops of ice-cold water followed by 5 drops of ice-cold methanol to wash the crude product 7. Recrystallize the crude product from methanol using steps 8-9 of the 2,4-DNP derivative 8. Use suction filtration to collect the pure product, allow the product to air dry and determine the melting point Waste Reagent Disposal

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Excess aqueous solutions should be washed down the drain with excess water. Dispose of the 2,4-DNP filtrate liquid into the organic liquid waste container. Aldehyde/ketone derivatives should be disposed of in the solid organic waste container. Soiled gloves and paper towels must be disposed of in proper containers. Broken glass is to be disposed of in the broken glass container.