At what step in the experimental process were benzene and biphenyl impurities re
ID: 1020304 • Letter: A
Question
At what step in the experimental process were benzene and biphenyl impurities removed?
Nucleophilc R- (or, in this case, Ar-) group to the electrophilic carbonyl group (O=C=O) also requires anhydrous conditions. The importance of a protic-free (anhydrous) environment cannot be overemphasized, for failure in generating the Grignard reagent is often associated with the presence of protic material (Figure 3). As in this experiment, a common impurity, biphenyl (R-R - C_6H_6-C_6H_6) may also result from a coupling reaction to form a different hydrocarbon (Figure 4). H_2O + R-MgBr R-H + HO-MgBr R-Br + R-MgBr R-R + MgBr_2 EXPERIMENTAL: Caution: All ethers are extremely flammable. Observe caution in handling chemicals. Until the Grignard reagent is reacted with the electrophile, all apparatus and chemicals must be absolutely driedll Anhydrous diethyl ether must be used m preparation of the Grignard reagent. After the addition reaction, . t-butyl methyl ether is employed in extraction. Preparation of phenylmagnesiumbromide: Provisions for cooling the reaction flask are necessary In this experiment. Set up your apparatus according to the mo CM provided in the laboratory The 100 ml RBF must be secured by a clamp to the ring stand. Place the addition funnel in one of the Claisen side arms. A condenser set up for refluxing conditions (circulating water) is fitted to the other side arm Allow Sufficient space below the reaction flask to permit positioning of the heating well or water bath while controlling the reaction progress. Weigh 0.6 g magnesium turnings. If the metal is dull, use a spatula to break the magnesium turnings into pieces to provide a 'fresh' (unoxidixed) surface before placing them Into the RBF Dispense 25 mL of bromobenzene via the directly into the addition funnel. Using the dry 25 mL graduated cylinder, measure and add 20 mL anbydrous diethyl ether to the addition funnel Using caution. the ring stand may be 'rocked' to cause mixing of the bromobenzene/ether mixture. Through the stopcock, release about half of the mixture into the flask. Allow the mixture to stand and observe over a ten-minute period whether the reaction is occurring, Alter an induction period, signs of a cloudy or darkening solution (brownish color) with bubbling Indicate an exothermic reaction When the reaction is proceeding smoothly and bubbling without external heating add the remaining bromobenzene/ether solution dropwise. In some cases Where the reaction may be 'sluggish', the Claisen side-arm stopper may be removed and a dry stirring rod inserted to 'nudge' several Of the magnesium turning. Consult a TA or Instructor and exercise care to present breakage of the flask (a common mistake which requires repeating the painful and time-consuming procedural). Heating the reaction flask initially may help, but prolonged beating is unnecessary (See Helpful Mints tor more information). At times, the reaction flask may have to lx cooled with ice water (briefly) to moderate the exothermic reaction. As the reaction subsides (some Mg residue may remaining heat the nurture for an additional 15 minutes at reflux in a hot water bath. Nucleophilic addition of the Grignard reagent to CO_2: Weigh out to. 10 g dry ice on several paper towels (tared). Fold the towels, crush, and place the dry ice In a 100 or 200 mL beaker. Under the fume hood, pour with stirring (caution!!) the Grignard reagent solution (contents of the reaction flask) over the dry ice (a CO_2 "fog" may be observed). Cover the beaker with a watch glass, and allow the reaction to proceed (sublimation of excess CO_2 will occur) until warmed to room temperature An additional 5-10 mL anhydrous diethyl ether may be required since the resulting addition silt becomes viscous. After completion of the Grignard reaction, hydrolysis with arid is required to convert the carboxylate salt to the free carboxylic a ad, which will be extracted into the ether layer. Add co, 10 g crushed ice followed by 4 mL of concentrated MCL Stir the mixture until two layers are evident. Additional acid may be used to dissolve residual magnesium and sate. If more ether is required, add co. 10 mL t-butyl methyl ether. Transfer the contents into a separatory funnel Remove and discard the lower layer. Wash the remaining ether layer In the separatory funnel with at. 10 mL Up water. The benzoic acid product is now extracted (3 Times 10 mL 5 % NaOH). After each extraction, remove the aqueous layer to a separate container prior to the next addition. Determine which layer contains your product and place that layer In a 125mL Erlenmeyer flask. Retain the other layer ur.nl the lab is complete and discard it into the aqueous waste container. On a hotplate, warm to about 60 degree C the Erienmeyer flask containing the base extracts to remove residual dissolved ether. Allow the flask to cool to room temperature (an ice bath may be used) and slowly add with string 4 mL of concentrated HCl solution (Use pH paper to ensure solution is acidic). Collect the crystals under suction using a funnel. By pressing the crystals with a cork and washing with cold water, the relatively pure benzoic acid Is air dried by continuing suction. Remove the product and allow the crystals to dry in your drawer until next lab period. Record the melting point and yield The IR Spectrum may be recorded or provided by your Instructor.Explanation / Answer
In the following step in the experimental process were benzene and biphenyl impurities removed
When benzoic acid product is extracted (3 * 10 mL 5% NaOH), benzoic acid dissolves in aqueous phase in the form of sodium salt. The organic phase (ether layer) contains benzene and biphenyl impurities. The organic layer containing benzene and biphenyl impurities is then discarded.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.