Identifying Unknown Alcohol
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  • 1. Identifying an Unknown Alcohol
  • 2. Background Classification Tests are qualitative tests which can indicate the presence of a particular functional group, such as an aldehyde or an alcohol. Two inviolable rules when performing classification tests: 1. Perform the test exactly as described. 2. Always perform tests on known compounds, one that will result in a positive test (known positive) and one that will result in a negative test (known negative) and then perform the tests on the unknown compound.
  • 3. Experimental Procedures Do this experiment before finishing the Grignard purification. For all procedures in this lab, follow the lab manual exactly as written, but change the order. (Do Part 1 after Parts 2 & 3) New experimental order: 1. Lucas Test 2. TCICA Test: use gloves 3. Derivative formation Omit the refractive index and micro-boiling point sections. Do not cross contaminate the contents of your test tubes. There are three Waste Bottles: 1. Recovered Lucas Solutions 2. Recovered TCICA Solutions 3. Recovered Alcohols Your lab report must state which type of alcohol is your unknown.
  • 4. Experimental Procedure Lucas Test The Lucas reagent is a solution of zinc chloride (ZnCl2) in concentrated HCl. A positive test occurs when the alkyl halide formed in the reaction begins to separate from the aqueous solution to form a second layer. This test readily distinguishes between primary, secondary, and tertiary alcohol based on whether a reaction occurs and on the reaction time. (i) 3° alcohol and alcohols that form resonance-stabilized carbocations react to form a cloudy mixture in less than 30 sec at room temperature. R R R R ZnCl - ZnCl2 Cl R C OH R C O+ R C+ R C Cl HCl/H2O H R R R R 3° alcohol: not soluble water soluble Forms Carbocation (SN1 reaction)
  • 5. Experimental – Lucas Test (ii) 2° alcohol can react in 3-10 min, but may need to be warmed. R R R R ZnCl ZnCl2 Cl- R C OH R C O+ R C+ R C Cl HCl/H2O H H H H H 2° alcohol not soluble water soluble (iii) 1° alcohol requires more than an hour to react, if it reacts at all. H ZnCl2 NO REACTION in several hours R C OH HCl/H2O H 1° alcohol The rate of reaction is related to carbocation stability. water soluble 3° > 2° > 1° alcohol
  • 6. Experimental Procedure Lucas Test Prepare 4 test tubes with 2 mL Lucas reagent, 4 drops alcohol, & mix. Record time until cloudy (how long does it take to form cloudy solution?). TCICA Test TCICA (1,3,5-trichloroisocyanuric acid) is a common source of chlorine used to disinfect swimming pools or as an additive in some kitchen cleansers. O O O H H OH Cl Cl N N N N 1 drop HCl(aq) + 3 HCl 3 + 3 + C H 3C N O N O H O N O H Cl alcohol TCICA isocyanuric acid precipitate
  • 7. Experimental - TCICA Test The time it takes for isocyanuric acid precipitate to appear is characteristic of the type of alcohol. O OH O H H N N TCICA 3 R C R 3 R C R + + 3 HCl 15 – 30 sec HCl / CH3CN O N O H ketone H 2° alcohol O not soluble OH O H H TCICA N N 3 R C H 3 R C H + + 3 HCl 3 – 7 min HCl / CH3CN H O N O aldehyde 1° alcohol H OH NO OXIDATION within 1 hr TCICA R C R The rate of isocyanuric acid precipitate HCl / CH3CN appearance(cloudy solution): 3° alcohol R 2° > 1° > 3° alcohol
  • 8. Experimental Procedure TCICA Test Prepare 4 test tubes with 0.5 mL TCICA and 1 drop of 1M HCl, mix together. Add 1 drop of alcohol, each to its own test tube, and mix together. Record time until precipitate forms. Derivative formation (only with unknown) Unknown compounds can be reacted with various reagents to give new compounds called derivatives. Stable, crystalline derivatives can be used to identify unknowns by comparing the derivative melting point with literature values. Several derivatives can be made to confirm an unknown alcohol.
  • 9. Derivative Formation If 1° or 2° alcohol is suspected, follow A. If 3° alcohol (which is slow to react) is suspected, follow B (use catalyst). O O A: NO2 1) heat 10 - 15 min in water bath NO2 R OH + Cl (almost boiling) RO 8 drops 2) cool to RT. if NO precipitate, cool in ice bath 3) neutralized with Na2CO3 NO2 4) filter NO2 5) recrystallize using EtOH 0.23 g Take melting point after recrystallize 3,5-dinitrobenzoyl chloride from EtOH (do not exceed 5 mL ) O O- Cl- + NO2 + NO2 R O R O Cl H H NO2 NO2
  • 10. O Derivative Formation O B: N NO2 1) 1 mL of acetone + NO 2 + Cl 2) 8 drops ROH N 3) heat 10-15 min 4) cool N N NO2 5) neutralized with Na2CO 3 NO2 0.12 g 0.23 g 4-dimethylaminopyridine 6) filter ROH (DMAP) 7) recrystallize using EtOH (tertiary alcohol) O- + NO 2 O- N + NO2 Cl N + N O H R N NO2 O NO 2 NO2 RO N N O + H N N NO2 R O + better leaving group NO2 Ester: take mp after recrystallization NO2
  • 11. Last - Finish Grignard Reaction Triturate residue in distilling flask with cold petroleum ether (not diethyl ether). "…to reduce to fine particles or powder by rubbing, grinding, bruising, or the like." Decant petroleum ether (SAVE small sample of ether for TLC). Recrystallize from isopropyl alcohol to obtain the pure product. Characterization of Your Product: Perform TLC on the crude product, petroleum ether extract, and pure product by using ether to dissolve all solids and using petroleum ether as eluent. Measure mass, mp and calculate %yield. Next week – Comprehensive Final !!! Have Fun!