A Greener Wittig Reaction
Purpose
In this experiment, we performed a Wittig reaction using benzyltriphenylphosphonium and 4-methylbenzaldehyde in order to determine the stereochemistry of the maor al!ene product using gas chromatography"
Results #he mass of the the puri$ed product product was appro approximately ximately %"&' grams" grams" #he expected yield can be calculated using the mass of the triphenylphosphonium triphenylphosphonium because that is the limiting reagent" Approximately (") grams of the starting material was used which translates to %"%%4* moles of starting material" +ince all of the reactions are in a one-to-one ratio, the resulting yield should also be %"%%4* moles" #he molar mass of the product should be '(%"'& grams which means that the mass of the expected yield is %"& grams" sing a simple yield calculation,
Actual Yield Expected Yield
=
0.72 g 0.97 g
=
0.74 =74 yield
" #he melting point of the puri$ed product
was measured to be about ('(". to ('*"( degrees /elsius" In the crude gas chromatography, it is e0ident that there are multiple pea!s with each pea! representing a di1erent compound" #he ones towards % minutes are the sol0ents such as water and dicholormethane" #he next fastest compound is at '"& minutes" #his is the aldehyde that was added in excess to ensure the reaction was only limited by the other starting material" At the other end of the spectrum at around ) minutes is the lefto0er byproduct of the reaction, the triphenylphospine oxide" 2oth of these match with their respecti0e reference chromatograms as shown on the course webpage" #his lea0es only ' other signi$cant pea!s one at 4"4 and one at ."( minutes" It is !nown that the 3 isomer elutes faster than the isomer because of 0an der Waals forces" #herefore, the pea! at 4"4 min is the 3 isomer and the pea! at ."( min is the isomer" In the puri$ed gas chromatography, it can be obser0ed that the pea!s in general are much smaller, and that pea!s of the extraneous material i"e" extra starting material and reaction byproduct is signi$cantly reduced" In addition, the height of the 3-isomer pea! is also reduced well below the pea! of the -isomer" #he 53 ratio is calculated by di0iding the area under the cur0e of the isomer and di0iding it by the area under the cur0e of the 3 isomer" #he 53 ratio of the crude
product is calculated as
calculated as
65564 10847
128034 153148
6.04
=
"
0.84
=
, and the ratio of the puri$ed product is
Discussion (" 2ased on your puri$ed melting point as well as the chromatogram of your puri$ed product, how successful was the recrystallization6 #he recrystallization was 0ery successful at remo0ing the impurities of the product" While in the crude product chromatography, the height of the pea!s of the byproduct and reagent ri0aled that of the desired product, they were negligible compared to the product pea!s in the puri$ed sample" #he melting point of the product was not as accurate" #he melting point of methoxystilbene is approximately (7. o-(7)o/" #he melting point obtained in this experiment is about ('* o to (7%o / so they are slightly di1erent" #his may be due to an error in ta!ing the melting point either heating too fast or not ha0ing the temperature set correctly" '" What was the maor al!ene isomer in your product mixture6 8o you thin! that sterics plays a large part in this outcome6 In the crude product, the al!ene isomers seemed to be approximately e9ui0alent, but after recrystallization, the trans or con$guration became the maority" An interesting obser0ation is that the ratio of to 3 changes drastically after recrystallization" It goes from being approximately e9ual to being hea0ily in fa0or of the isomer this is because during the puri$cation process, the cis isomer also got remo0ed from the system" 2enzene is an electron donating group so it will push the reaction to form a non-stabilized ylide which leads to 3-al!enes" In the crude product it can be seen that there is a slightly higher amount of 3-al!ene than -al!ene" :owe0er, benzene is a 0ery wea! electron donating group so it will not signi$cantly a1ect the 53 ratio, only push it slightly towards the 3-isomer"
7" /ompare the changes between this ;greener< Wittig reaction and the ;normal< Wittig reaction described in the bac!ground section" /omment of the change in en0ironmental impact that each change made" In the greener reactions, the strong base changes from n2u=i to >a?:" n2u=i is 0ery dangerous and is highly @ammable and reacti0e" It will catch $re spontaneously if exposed to air speci$cally the water in the air" It may also cause reproducti0e toxicity" #he sol0ent that is used for this compound, >,>dimethylformamide is 0ery toxic to humans that can cause damage to organs as well as to the health of an unborn child" While using these compounds may impro0e yield, they are 0ery harmful to humans and if mixed with the water supply, cause drastic en0ironmental damage" 4" What principles of green chemistry were followed in this experiment6 Which were not6 #here are twel0e principles of green chemistry pre0ent waste, maximize atom economy, design less hazardous chemical syntheses, design safer chemicals and products use safer sol0ents and reaction conditions, increase energy eBciency, use renewable feedstoc!s, a0oid chemical deri0ati0es, use catalysts, not stoichiometric reagents, design chemicals and products to degrade after use, analyze in real time to pre0ent pollution, and minimize the potential for accidents" In this experiment, most of these principles were followed" Atom economy is eBcient, 0ery little excess chemicals was used" #he much more dangerous chemicals were replaced by >a?: and water" While >a?: is somewhat hazardous, it is much less so than the other bases used in Wittig reactions" nergy eBciency was de$nitely obser0ed, in a normal Wittig reaction, the compound is heated, but in this experiment the reaction occurred under room temperature" In this experiment, no catalysts were used and the reactant was calculated in stoichiometric amounts which
goes against one of the chemicals" Also, the compounds used in the experiment are not designed to degrade after use as a yield calculation is needed and ha0ing the chemicals degrade could a1ect the experiment" +ome principles didnCt ha0e too much rele0ance to this experiment such as the analysis of pollution and a0oiding temporary modi$cations of chemicals"
