Lab 1 lecture

Lab 1 lecture - Experiment#1: Grignard Synthesis of 2...

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Unformatted text preview: Experiment #1: Grignard Synthesis of 2 ­Methyl ­2 ­Hexanol ObjecAves: •  To synthesize a 3o alcohol from an alkyl halide and a ketone using a Grignard reacAon. •  To purify product using a liquid extracAon method and a disAllaAon method Before coming to lab… •  Review the following techniques: –  ExtracAon –  Drying organic solvents with K2CO3 –  DisAllaAon •  You will be expected to perform these will liPle review, as you learned these techniques in the first semester lab. CHEMICAL EQUATION MAKING THE GRIGNARD REAGENT •  Organic halides react with magnesium metal in ether to yield an organomagnesium halide: RMgX ETHER R ­X + Mg  ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­> R ­Mg ­X Where R= 1o, 2o, or 3o alkyl, aryl or alkenyl X= Cl, Br, I •  The C ­Mg bond is a highly polar covalent bond. The carbon atom is both nucleophilic and basic making it very reacAve with a wide variety of E+. WHEN THE GRIGNARD REAGENT MEETS WATER… •  Since the carbon atom of a Grignard reagent is so nucleophilic and basic, it reacts with proton donors (Brönsted acids) such as H2O, ROH, RCOOH, RNH2 to yield hydrocarbons. •  This makes it extremely important to keep the reacAon flask and solvent completely dry of water. MECHANISM EXPERIMENTAL PROCEDURE (Synthesis of Grignard reagent…) •  Place Mg turnings and iodine in 100mL flask •  Clamp flask to ring stand and set up remainder of reflux with addiAon apparatus. •  Start reacAon with a small amount of n ­ butylbromide Once the reacAon starts, add ether to dilute •  •  Add remaining n ­butylbromide and ether to sep funnel. •  Add to reacAon mixture dropwise once reacAon starts EXPERIMENTAL PROCEDURE (Synthesis of alcohol product…) •  Add acetone/ether to sep funnel. •  Add to reacAon mixture dropwise. •  Cool reacAon flask in ice water bath. •  Add ice and NH4Cl 1:1 to a beaker Transfer reacAon mixture, warmed to room temperature to beaker. •  EXPERIMENTAL PROCEDURE (PurificaRon…) Set up an extracAon apparatus. Transfer the liquid from the beaker to the funnel. Extract aqueous layer with ether Wash with water, NaHCO3, and then water. Transfer organic layer to a 50 mL flask and dry over K2CO3. •  Transfer dried organic liquid to a beaker. •  Set ­up disAllaAon apparatus and collect product at 132˚C ­145˚C •  •  •  •  •  Notes on ExtracAon Which phase is H2O? It depends on the organic solvent. If you know the organic solvent, how do you know which phase is H2O? The denser solvent is on the bottom. DistribuAon Coefficient (K) •  When a compound is distributed between an organic solvent and water, the raAo of the solute concentraAon in both phases can be expressed as: •  Any compound with K > 1.5 can be separated from water by extracAon with an insoluble organic solvent. •  If the compound is impure: •  inorganic impuriAes remain in the water layer; •  organic impurites will also migrate to the organic layer. Phase Interface Emulsions •  Occasionally an emulsion forms between the two layers. •  It appears as a 3rd milky layer between organic and aqueous phases. •  Emulsions don’t appear during diethyl ether extracAons but are common for chlorinated solvents. VenAng •  Work in a hood while performing an extracAon •  VenAng releases gases that may otherwise build up and pop out the stopper. •  Shake the two phases, invert the separatory funnel, point the funnel toward the back of the hood and open the stopcock. Aqueous SoluAons •  These aqueous soluAons are someAmes used instead of pure H2O during extracAon 1.  Aqueous acid (HCl, NH4+) removes organic bases; 2.  Aqueous base (NaOH, NaHCO3, Na2CO3) removes organic acids; 3.  Saturated NaCl (aq) (or brine) is used when the solute is slightly water ­soluble. K generally increases going from water to brine. Removing Bases •  SeparaAng these two compounds is simple because one is neutral and the other basic. •  Ionic compounds always go to the aqueous layer. Removing Acids •  SeparaAng these two compounds is simple because one is neutral and the other acidic. Removing Acids •  Be very careful venAng extracAons in which NaHCO3 (aq) or Na2CO3 (aq) are used to remove acids. •  Both form CO2 (g) with acids that can pop off the stopper. •  Amer adding NaHCO3 (aq), swirl once, then vent. SalAng Out •  •  •  •  For extracAons using brine and an organic solvent: Organic solutes become less soluble in the aqueous phase. Water becomes less soluble in the organic layer. Both of these are good from a chemist’s perspecAve. Some Advice •  Clearly label all beakers or flasks that hold your extracAon layers. •  Never throw any extracAon layer away unAl you are certain it doesn’t have your product! •  Amer you open the stopcock, liquid won’t drain from the separatory funnel unless… the stopper is removed. DisAllaAon •  process of heaAng a liquid unAl it boils, capturing and cooling the resultant hot vapors, and collecAng the condensed vapors •  idenAficaAon and the purificaAon of organic compounds •  The amount of each liquid in the disAllaAon flask is unimportant. DisAllaAon Set ­up Thermometer Set ­up Too high Too low ...
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This note was uploaded on 01/20/2012 for the course CHEM 325BL at USC.

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