Lab 7 Synthesis of E Z Stilbenes by a Wittig Reaction Done by Ramali Gunatilake Lab Partner Kevin Hsu TA Vamsee Handed in 10th April 2014 Abstract The purpose of this lab is to create trans stilbene using the Wittig reaction 50 NaOH is added to Benzyltriphenyl phosphonuim chloride to create the ylide which is then reacted with benzaldehyde to produce a cis stilbene that is isomerized to trans stilbene using I 2 Introduction The Wittig reaction is a reaction between a ylide and the carbonyl aldehyde or ketone group of a molecule In this lab we use Benzyltriphenyl phosphonium to create the ylide The hydroxyl group from the NaOH that is added reacts with one of the hydrogen atoms on the Benzyltriphenyl phosphonium in a nucleophilic SN2 reaction to produce a water molecule and the ylide An ylide is a molecule that has an adjacent positive and negative charge The ylide molecule is then reacted with benzaldehyde to form a cis stilbene The negatively charged C atom on the ylide attacks the carbonyl bond on the benzaldehyde at a 90 angle to form the intermediate Betaine The Betaine then reacts within itself the negatively charged O atom forms a bond with the positively charged P atom creating a 4 membered ring compound called Oxaphosphetane The bonds in this ring then shift around to produce the cis stilbene and a phosphine oxide molecule The cisstilbene is then isomerized using I2 into trans stilbene Mechanism Procedure Refer to lab manual and or lab handout for experimental procedure Results Reactant Product Benzyl triphenylphosphonium chloride Benzaldehye 50 NaOH Diethyl Ether 95 Ethanol Iodine Saturated Sodium Bisulfite E Z Stilbene E Stilbene Theoretical Yield Recovery E and Z stilbene Yield Calculation Mol Wt g mol M Pt C B Pt C Density g ml Amounts Used formed 389 XXX XXX XXX 0 39 106 19 Molar XXX 46 253 8 XXX XXX 114 113 7 XXX XXX 78 37 184 3 XXX XXX 0 789 4 933 0 1 0 5 XXX 1 5 A pinch 104 061 150 XXX 1 48 0 5 180 25 180 25 5 6 125 XXX XXX XXX XXX 0 007 Color Weight g Moles mmol XXX 0 18 0 09 Observed M Pt C XXX 0 007 0 003 100 Yellow White Discussion Regular lab safety procedures are to be observed 50 NaOH can cause damage to skin eyes and mucus membranes Benzyl triphenyphophnium chloride is toxic and a skin irritant Benzaldehyde is a skin irritant Diethyl Ether is extremely flammable and harmful if inhaled Work under the fume hood at all times and wear gloves and goggles The low percent yield of 3 33 is probably due to very little of the cis stilbene converting to transstilbene It can also be due to some of the product being discarded during separation of the aqueous and organic layer Due to the steric hindrance and the mechanism pathway taken to form the betaine molecule the major product that is formed is the cis stilbene According to the experimental data it can be concluded using melting point analysis that the product formed is the E isomer However the low observed melting point of 100 C maybe be due to impurities that weren t removed during the purification step Conclusion The purpose of the experiment was to synthesis trans stilbene using the Wittig reaction A higher yield of the product may have been obtained with a more carful separation of the aqueous and organic layer also the use of alkyl lithium to stabilize the betaine would help favor the trans stilbene