7.LAB 42 – Chemistry – Flashcards
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JOB
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Make 1,4-diphenyl-1,3-butadiene from trans-Cinnamaldehyde Determine stereochem of major product (E,Z)
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RESULT
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(E,E) based on melting point and least steric, with large groups far apart. After synthesizing 1,4-diphenyl-1,3-butadiene from trans-cinnamaldehyde and taking the melting point, the product was the (E,E) isomer (based on melting point and textbook)
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1. Why do you think the product you obtained, rather than another isomer, is the major product of the Wittig reaction?
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a. (E,E) - 1,4-diphenyl-1,3-butadiene is the major product because the transition state leading to the E-isomer has lower energy than the Z-isomer & large groups are far apart
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2. a) Of the three 1,4-diphenyl-1,3-butadiene isomers, (E,E), (E,Z) and (Z,Z) which would be most suitable as a diene in the Diels-Alder reaction? Which would be least suitable?
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??? zz?
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b) Why is (Z,Z)-1,4-diphenyl-1,3-butadiene not a likely product of the reaction you carried out?
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b. The Wittig reaction involves a 4-membered ring intermediate that the Z,Z isomer cannot conform to. Hence, (Z,Z) is not a likely product of the Wittig reaction.
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1. Compare and contract Aldol condensation and the Wittig reaction. Be as specific as possible in the contrasts of the two.
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a. Aldol condensation and the Wittig reaction both consist of an SN2 nucleophilic attack, however Wittig phosphorous ylide (release Ph3P=O) and Aldol enolate (release H2O)
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2. Propose a synthesis of trans-cinnamaldehyde using an aldol condensation. Give the IUPAC name of trans-cinnamaldahyde
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(2E)-3-phenylprop-2-enala. 1-carboxaldehyde-3-phenyl-2-propene. SEE LAB
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3. Explain what a "phase-transfer catalyst" is, and what will be used today that acts as such. Specifically, how is this catalyst involved in today's experiment?
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BenzylTriphenyl Phosphonium iona. A phase transfer catalyst pulls a compound from the aqueous layer of the organic layer. The phase-transfer catalyst in this experiment is [Ph3P-CHPh]Cl-. In the experiment it pulls the ylide from the aqueous to organic layer to react with the cinnamaldehyde in the organic layer.
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4. Draw the 3 possible isomers of the product, 1,4-diphenyl-1,3-butadiene
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a. SEE LAB
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In the Wittig reaction, nuc is
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the phosphorus ylide
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The C=C double bond forms by
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elimination (Ph3P=O)
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In this Experiment, the phosphonium salt will be benzyltriphenylphosphonium chloride
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which is prepared by the reaction of triphenylphospine with benzyl chloride
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This is an Sn2 reaction in which
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triphenylphosphine (nuc) displaces chloride ion (leaving group) from benzyl chloride (substrate)
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he reaction is carried out in high boiling solvent p-cymene
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during a 2 hour reaction period
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Formation of the ylide is facilitated by
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the presence of the phenyl group, which helps stabilize it, so concentrated sodium hydroxide is used rather than a stronger base such as butyllithium
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You will carry out the ylide-forming reaction in
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the presence of cinnamaldehyde, which reacts with the ylide to form the product 1,4-diphenyl-1,3-butadiene
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This reaction takes place at room temp in
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two phase mix, with dichloromethane as the organic phase
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Benzyltriphenylphosphonium chloride itself acts as a phase-transfer catalyst
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apparently escorting the polar ylide from the aqueous to the organic layer, where it can encounter and react with cinnamaldehyde
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Most of the major product will remain in the organic phase
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o The rest is extracted from the aqueous phase with more dichloromethane, which is removed by evaporation
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The crude product is triturated with aqueous 60% ethanol to remove triphenylphosphine oxide and a byproduct of the reaction,
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and then purified further by recrystallization from 95% ethanol
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The flat crystals tend to stick to glass surfaces and are hard to scrape off***
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so you should avoid any unnecessary transfers
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A melting point measurement should tell you whether or not you obtained the desired E,E isomer
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UV spec can also be used to distinguish geometric isomers
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The wittig reaction has a low atom economy ***
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because it produces triphenyphosphine oxide as a by product
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Although the oxide can be recycled to triphenylphosphine
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the process is expensive and produces some waste by product
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Benzyl chloride and triphenylphosphine are somewhat toxic to aquatic organisms
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The EPA classifies dichloromethane as a pollutant
