UNKNOWN UNKNOWNS Requirements: classify your functional group

Transcription

1 UNKNOWN UNKNOWNS Candidates: ALCOHOLS Table 70.2, p. 766, 767 CARBOXYLIC ACIDS Table 70.1, p 764,765 ALDEHYDES Table 70.3, p. 767, 768 AMINES Table 70.5, p. 769, 770 KETONES Tabl e 70.14, p. 776 Requirements: You will be given three unknowns. Identify your unknowns. Make derivatives of two out of the three (time permitting). Get melting point/boiling point on both starting material and Titrate and get molecular weight if your sample is an acid. Include NMR for starting material. (If solubility permits). Do whatever solubility/chemical tests you find useful. General Strategy: The strategy simplifies into two major levels. The first level is to classify your functional group, as quickly as possible. Chemical tests (including solubility tests) and NMR are both useful for this purpose. Until you know your functional group, a melting point or boiling point alone is of limited use. And until you know your functional group, you can t know how to make the right The second level is to make derivatives and specifically identify your materials. NMR's, mp/bp on starting material, mp on derivative, and titration information for acids should provide adequate information! Review of Chemical Tests 1. Solubility Tests, especially for Amines or Carboxylic Acids -Add 2 drops of sample if it is a liquid or a little spatula quantity if you have a solid to a small test tube. Then add 15 drops of water solution. Add more water solution if you think partial solubility is occurring. a. in water b. in HCl/water c. in NaOH/water d. in NaHCO 3 /water (carboxylic acids cause bubbling and dissolve slowly) Note: The NaOH and NaHCO 3 test are really the only tests for acids. Interpretation: If the sample is insoluble in neutral water, then solubility in HCl is proof of an amine, and solubility in NaOH and NaHCO 3 is proof of a carboxylic acid In general, solubility in neutral water is indicative of whether a sample has few carbons (soluble) or many carbons (insoluble). In general, whether a sample sinks or floats relative to neutral water is indicative of whether or not aromaticity is present 2: Chromic Acid Test ("Jones' Test") for Alcohols or Amines. Cr +6 oxidizes 1 alcohols (to carboxylic acids) and 2 alcohols (to ketones), but does not oxidize 3 alcohols. Cr +6 (red-orange) gets reduced to Cr +3 (blue-green) in the process. Amines are oxidized even more easily. Aldehydes also are oxidized, but more slowly. -Put 20 drops of acetone (solvent) and 1 drop of unknown into a test tube. Add 1 drop of chromic acid solution and mix. Note: Amines are oxidized as well, so they too give a gross response. Interpretation: a positive test involves both a color change, from orange green/brown/black, and formation of a precipitate. Amines give a more rapid and intense response than. 3. CuSO 4 test for Amines, not Alcohols. -Add 10 drops of CuSO 4 reagent, then 2 drops of unknown. If the solubility is poor, add an additional 10 drops of ethanol to try to improve the solubility. Note: Alcohols don t respond, so it is useful for distinguishing amines from alcohols. Interpretation: Alcohols should not react, but amines should change the color, and may cause a precipitate 4: "DNP" Test for Aldehydes and Ketones. (2,4-Dinitrophenylhydrazine test.) -Put 10 drops of DNP solution into test tube. Add 2 drops of unknown and mix. Interpretation: a positive test involves formation of a precipitate.

2 5: Schiff's Test for Aldehydes, not Ketones. -Add 15 drops of Schiff's reagent, then 2 drops of unknown, and shake. If your sample is water insoluble, add an additional 10 drops of ethanol Interpretation: Water soluble aldehydes turn pink rapidly. Aldehydes that have low water solubility usually turn pink fairly quickly, after the ethanol is used to improve cosolubility. DERIVATIVES A. Carboxylic acids: Anilide Derivative ( See p 764, 765) If your unknown is a carboxylic acid, ask the instructor for a sample of the associated acid chloride. Place 10 drops of the acid chloride into a large test tube. Add a stir bar, and add 2 pipets of ether. To this solution add 20 drops of aniline, dropwise (may spatter if you add it all at once) and stir for 5 minutes. The primary precipitate that forms is the aniline hydrochloride salt. Then add 2 pipets of acqueous NaOH, and continue stirring for an additional five minutes. This will dissolve the aniline hydrochloride salt; if some precipitate remains it is the derivative itself. Use a long pipet to remove the acqueous layer from the bottom of the test tube. (Any unreacted acid chloride should be removed by the basic water.) Then add 2 pipets of aqueous HCl, and stir vigorously. Use a long pipet to remove the acqueous layer. (The aniline should be removed in the process.) If you have a significant amount of precipitate at this point, it is the desired Filter directly over a Hirsch funnel. Rinse with some HCl/water and then some cold ethanol to get your crude If following the acid wash you do not have a precipitate, transfer the ether to a 25-mL Erlenmeyer and concentrate it on the hot plate to remove ether. Remove the residue and let cool; should be crystalline Rrecrystallize from ethanol. Ideal volumes will vary depending on your unknown, but normally a good starting guess will be about 4 ml of ethanol. The addition of some drops of water may help to produce the crystals. B. Alcohols: See p for Derivativ e Table: Use the 3,5- Dinitrobenzoate Column To a large test tube add about 0.3 g of 3,5-dinitrobenzoyl chloride and then 10 drops of your alcohol. Heat the mixture so that the solid material melts, using a Bunsen burner, and keep it warm enough over a 5-minute period so that it stays in the liquid form without solidifying. Do not overheat! Warm it up enough to melt, but otherwise try to keep it as cool as possible. Allow the melt to cool and solidify. Cut up the crystalline mass with a spatula, add 2-3 ml of sodium hydroxide solution, and stir/grind the mixture vigorously with a glass rod, then add a stir bar and vigorously stir for 5 minutes. Collect it by filtration on a Hirsch funnel, and wash it with 3 x 3 ml of water. The solid/powder is your Recrystallize/digest the derivative in a small Erlenmeyer or beaker by adding 3 ml of ethanol, and heat the solution at ~80 C for 5 minutes. Let cool to room temp, further cool on ice, and filter and dry as usual. If you have no crystals, boiling off some ethanol and/or adding drops of water may help. You may wish to recrystallize again to get a more perfect product, but this may not be necessary. Disposal: Filtrates down the drain. Note: Some hindered alcohols have trouble making this Phenylurethanes: See p for Derivative Table Flame dry a large test tube, and add a stir bar. Add 10-drops of your alcohol, then add 10 drops of phenyl isocyanate. Heat this solution in a hot-water bath for 5 minutes, then dilute with 3 ml of hexane or ligroin while stirring, cool, and let crystallize while stirring. (The stirring helps to get oils to solidify). The crude crystals may be satisfactory. To recrystallize, either use hexane (or ligroin), or else ethanol/water.

3 C. Ketone/Aldehyde Unknowns: 2,4-DNP Derivatives (p. 776, 767) -Put 2 pipets of 2,4-DNP solution into a large test tube, add a magnetic stirbar, and to a wellstirred solution add 15 drops of your unknown. After 15 minutes, add 4 pipets of cold water, cool on ice, filter on a Hirsch funnel, wash with cold water, and wash with a small amount (not more than two pipets) of cold ethanol. Aspirate thoroughly, and hopefully get a crude mp. If you have enough material, recrystallize from ethanol/water. May want to further scale up if recovery is poor. NOTE: Saturated carbonyl derivatives usually are yellow, unsaturated are usually orange/red. Disposal: Into DNP waste container. D. Amines See p for Derivative Table: Use the Benzamide Column -Add either 15 drops or 0.15 g of your amine to a large test tube. With vigorous stirring, add about 15 drops of benzoyl chloride (dropwise, may react vigorously). Then add 2 ml of aqueous sodium hydroxide solution (it should be somewhere between M). After about 10 minutes of shaking, acidify with aqueous HCl. (Use litmus or ph paper to confirm that the ph is on the acidic side of 7.) Cool on ice, filter the lumpy product through the Hirsch funnel, and wash with 3x3 ml of cold water. Recrystallize using a minimum of ethanol-water.

4 Unknown Report Sheet Unknown No. Name Final Answer: Structure and Compound Name Data Summary measured literature value bp or mp of starting material measured literature value mp of derivative 1. Physical Examination of Starting Material a) Physical State b) Color c) Odor 2. Solubility Tests on Starting Material Solvent: Water? Aq NaOH Aq NaHCO Aq HCl Solubility: If Insoluble in Water, Does it Float or Sink? Conclusions: 3. Chemical Tests Result Conclusi on Jones Reagent (Chromic Acid) CuSO 4 Test 2,4-Dinitrophenylhydrazone Schiff's Reagent Conclusions: 4. If I have an acid, what is the approximate molecular weight (mw) of my sample, based on my titration? Approximately g/mol. (Show your calculations, or attach a sheet with calculations!)

5 5. Attach H-NMR and C-NMR if you ran one. (Any obvious key structural information that would help your thinking? Needn t write anything for Jasperse s sake, but might help your own thinking.) 6. Preliminary Candidates (Needn t fill this out, but often helpful) Possible literature literature What else can I do, or think about, Compounds mp or bp mw mp of derivatives that could shorten this list? 7. Derivative Prepared observed mp literature mp Crude Recrystallized 8. What is My Actual Unknown? (Structure and Name) 9. Comments, difficulties, etc..