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Experime nt 36B: Benzoic Acid Introduction: For this experiment, a Grignard reagent of phenylmagnesium bromide was prepared from bromobenzene and magnesium turnings in anhydrous ether. This Grignardreagent was then used, with the help of carbon dioxide, to form our final product of benzoic acid. Procedure

Physical Properties Bromobenzene Carbon dioxide Anhydrous ether HCl NaOH Results:Reactions

MW (g/mol) 157.01 44.01 74.12 36.36 39.997

BP (ºC) 156 -78.5 34.6 110 1390

MP (ºC) -30.8 -56.6 -116.3 -27.32 318

Density (g/cm3 ) 1.495 *(1.98 g/L, gas) 0.714 1.18 2.1

PhysicalProperties Phenylmagnesium bromide Benzoic Acid

MW (g/mol) 181.31 122.12

BP (ºC) N.A. 249

MP (ºC) N.A. 122.4

Density (g/cm3 ) 1.14 1.32

Theoretical 2.85g bromobenzene * (1 mol / 157.01g) =0.0182 mol bromobenzene 0.0182 mol * (122.12g / 1 mol) = 2.22g benzoic acid Percent Yield 100% * (2.49g / 2.22g) = 112.2%

Discussion

Our experiment began with preparing our Grignard reagent.One important factor was using the anhydrous ether since water reacts easily with Grignard reagents. Also, all our glassware was cleaned and acetone dried a week in preparation. The drying tubes alsohelped in keeping water out of the reaction. The purpose of later adding hydrochloric acid was to hydrolyze the Grignard adduct and to react with any remaining magnesium. In the separatory funnel, theaqueous layer with the inorganic salts was first removed. Because biphenyl is a byproduct of this reaction, it was then extracted with sodium hydroxide. The remaining aqueous layer was then heated onthe thermowell to evaporate off any ether. The final hydrochloric acid addition was to precipitate out the benzoic acid from the alkaline solution. Our percent yield was too high (2.49g) because ofimpurities. Our experiment was rushed by the substituting T.A., who turned up the heat in order to make it run faster. Also, our product was not allowed proper time to cool before Buchner filtration....
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