Here\'s a nice green chemistry procedure for you. One of the biggest issues in O
ID: 992065 • Letter: H
Question
Here's a nice green chemistry procedure for you. One of the biggest issues in Organic Chemistry involves the use of solvents, many of which have toxicity and environmental concerns. Reaction methods that don't involve solvents are an increasing area of focus. In this lab we will prepare 1,4-diphenyl-1.3-butadiene via a Wittig synthesis where no solvent is used. Place 300 mg of benzyltriphenylphosphonium chloride and 650 mg of potassium phosphate into a small mortar. Add 100 uL of cinnamaldehyde. Grind this stuff together for quite a while (20 minutes or so). Make sure you get it all ground up well. Vacuum filter, using water to transfer the material into the Hirsch funnel. Recrystallize with ethanol. Dry the crystals in the Hirsch funnel. Save the filtrate, and the solids of course, for TLC analysis. Determine percent yield and mp. Analyze the solid (dissolve some of it in acetone) and the filtrate via TLC on fluorescent plates, using hexane as the developing solvent (you must do this the same lab period in which you run the reaction). This Wittig reaction produces both the trans.trans product (which is a solid) and the cis, trans product (which is a liquid). The cis, trans product typically has a higher Rf than the trans, trans product. You may also see triphenylphosphine oxide (the "other" Wittig product), which will probably not move hardly at all on the TLC plates. (It's also possible to identify the trans, trans product because it fluoresces more brilliantly than the others.)
Explanation / Answer
to find the theoretical yield find the limiting reagent, i.e. the reactant which is present in lowest no. of moles.
benzyltriphenylphosphonium chloride molar mass = 389, therefore no. of moles = mass / molar mass = 0.3 g/ 389 g = 7.7 x 10-4 moles
cinnamaldehyde density = 1.05 g/mL
therefore mass = density x volume = 1.05 g/mL x 0.1 mL = 0.105 g
cinnamaldehyde molar mass = 132
therefore moles = 0.105/132 = 7.95 x 10-4
therefore benzyltriphenylphosphonium chloride is limiting reagent, therefore to get theoretical yield of 100% we should get product = 7.7 x 10-4 moles of product =
product 1, 4 diphenyl 1, 3 butadiene molar mass = 206
therefore theoretical yield = 206 x 7.7 x 10-4 = 0.158 g.
your experimental yield = (0.18/0.158) X 100 = 113%
i.e. your product was not dry and had solvent so the percent yield is more than 100%