Example Lab Report - Nick Johnson Chemistry l 123 3/1...

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Unformatted text preview: Nick Johnson Chemistry l 123 3/1 3f2009 The Wittig Reaction Introduction In this experiment the Wittig reaction was carried out using benzyltriphenylphosphonium chloride and 9-anthraldehyde in DMF. The Wittig reaction was USBfill in this Synthesis because of its ability to create new carbon—carbon double bonds between two separate compounds. This reaction was also used so that the resulting stereochemistry of the product could be examined and hopefully explained as well. Discussion The stereochemistry of the product of the Wittig reaction greatly depends on the type of ylide used to form it. There were three main types of ylides to consider in determining the stereochemistry of the product: non—stabilized (ylides with electron donating groups), semi- stabilized (ylides with phenyl groups). and stabilized ylides (ylides with electron withdrawing groups). Because the group attached to the ylide used in this experiment was a phenyl group. a semi-stabilized ylide was being used. This semi-stabilized ylide went through a late transition state when reacting with the 9—anthraldehyde that resembled the final product more than the starting materials. This indicated that the transition state took place when the molecule had adopted a planar conformation. To minimize steric hindrance. the phenyi groups were pushed away from each other resulting in the trans final product. Another clue the product was trans was gained from examining the lH NMR spectra and noticing the splitting between the Hn and H” were approximately 17 Hz and 16 Hz respectively. This type of splitting was indicative of a trans double bond. This reaction was performed using NaOH instead of something like BuLi because of BuLi’s nucleophilicity as well as basicity. This experiment required a strong base to form the ylide correctly; however, it‘a strong nucleophile such as BuLi were used, it would react readin by adding to the aldehyde and would result in a side product. When running the crude product through TLC. three spots were observed. One of these spots represented the unreacted 9-anthraldehyde because it matched up with the 9—anthraidehyde standard. A Small unknown Spot appeared beneath the Q—anthraldehyde. indicating a more polar compound. which was likely unreacted benzyltriphenylphosphonium chloride. The last of the three spots was the desired product which appeared higher on the TLC plate than the 9- anthraldehyde, indicating it was more non-polar because of the loss of the carbonyl group and addition of a phenyl group attached by a trans-double bond. An important observation noted in the UV analysis was the fluorescence of the final product; this indicated the final product was very conjugated. Benzyltriphenylphosphonium chloride was the limiting reagent in this reaction (.104 g used). Since the reaction has a 1:1 reactant to product molar ratio, a full yield would consist of .075 g of product. This experiment yielded .039 g of product, a 52% yield of the expected desired product. Two major things contributed to a loss of yield in this experiment. The first of these was that not all of the 9—anthraldehyde or benzyltriphenylphosphonium chloride had completely reacted; this was revealed by the. TLC analysis. The second reason product was lost was in the transfer and filtration process followed by recrystallization and another filtration. The loss of yield in this portion was minimized but still significant. Mechanism for the formation of product: f DH ' I T) [Pkgg'flfipha {—-—---} + o \ Ii 2': +§ no i U? PL\________ H‘ Q 93” jest aflp‘“ Mme will +K\:.-\ .1 [2+1] [2+1] pk Pk '— tFo R Conclusion The Wittig reaction proved to be a very usefiil tool in the synthesis of carbon—carbon double bonds in this labOraIOry experiment. A greater understanding of the stereochemistry of this reaction was also gained through this experiment. To obtain a better yield of product, lab technique could be improved, especially in the filtration and recrystallization phases of this experiment to minimize product loss. DW 2 q: .- Questions .2" 2r, 1. Wittig reactions ave two general outcomes based on the ylide used to form the product. A cis d product is obtained through a non-stabilized ylide (electron donating group attached). Because the ylide is not stable due to the group attached to it, the reaction proceeds through an early intermediate (reactant-like transition state, folded or boat). Steric hindrance is created early in the reaction so the transition state adopts a cis conformation to avoid this steric clashing; this results in the cis product. A trans product is obtained through the use of a stabilized or semi-stabilized /ylide (electron withdrawing or phenyl group respectively). Because the ylide is more stable, the reaction can proceed through a later intermediate (product-like transition, planar). This causes steric hindrance to be felt much later so the transition state must change to a trans conformation to avoid steric clashing. This results in the trans product. .In the case of this experiment, a semi- stabilized ylide was used; therefore, for the aforementioned reasons, the major product of this reaction was the trans product. 2.. The protons that were involved in the alkene double bond on the 'H NMR spectra were HD (7.4—7.9 ppm) and H11 (6.5-7.0 ppm) because these two protons had the right chemical shifts for their interactions with the surrounding molecule and because they both spilt into clean doublets. The] values for the splitting of these two protons were approximately 17 Hz and 16 Hz respectively. A cis relationship generally creates J values of about 10 Hz and a trans conformation creates J values around I6 Hz usually. After comparing these values with the indicated protons, it was clear that they participated in a trans double bond indicating that the final product of this reaction was the trans product. . . .. U 5 Hsamfififimfi W W % Emmy? A . . . . . . . . . . . . . . 7.. 6 Rm 101 Hg Hm wwwam QHHHW ww.up thm.mw Hm mmwmm wompx hm.wH wwbo.uw No mmmmm mwomx aw.Hq waw.wm NH mmmom mqum mH.wH mmwm.mo _ MN Nwmwm bHHmfi Hw.mm mmww.mm mm Nmmwm mmwwx Ho.mo mmwm.wq NNHO.©N 4m.wm I erE 2. S 3 Eu .I I...| _ . I . . . mh PM ah P<HWPDIUEHGII u an 333: mama. Emaofimm flNuBVNZ; Emmw- 32 -- U>Hm¥§o= a 2 NNwmubm Mac sot pawn.” r8511 we: 23 oz” 535' 7 - Ewan flu: | 52:, .rr .. Us. a a. .52.. .15” PM man 2.)” m _ ...
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Example Lab Report - Nick Johnson Chemistry l 123 3/1...

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