Two Easy Diels-Alder Puzzles

Transcription

1 Two Easy Diels-Alder Puzzles Exp't 2 Adapted by Selembo, Sun, Underwood, Watson, Woycechowsky, Jennifer Woelker & R Minard from Pickering, M, "Three Easy Puzzles based on the Diels-Alder Reaction, Jhem Educ, 67, 52 (1990 ) Revised 3/17/01 Introduction: The Diels-Alder reaction is a pericyclic [+2] cycloaddition, in which a conjugated diene and an isolated alkene add together in a single step without intermediates by a cyclic redistribution of bonding electrons (1) This process is very useful in organic synthesis because it forms two carbon-carbon bonds in one step and is one of the few methods available for forming cyclic molecules (1) (For a more detailed description of the reaction see McMurry, rganic hemistry, 2nd Ed, Brooks & ole, 1988 p and p ) diene dienophile cyclic product ' 2' + sp 2 hybridized carbons sp 3 hybridized carbons Starting diene & "ene" all carbons sp 2 hybridized cyclic transition state cyclic product with two new bonds formed When both the diene and dieneophile are cyclic, it is possible to form two products with different geometries, called exo and endo, as shown below: Is the overlap of p electrons more or less in this transition state than in this one? Exo-isomer minor product Which product, exo or endo, is favored? endo-isomer major product

2 Exp't 2 The objectives of this experiment are: 1) to perform two different Diels-Alder reactions: a) furan and maleic anhydride + endo or exo Diels-Alder product???? furan: a diene maleic anhydride: a dienophile b) 1,3-cyclohexadiene and maleic anhydride + endo or exo Diels-Alder product???? 1,3-cyclohexadiene: a diene maleic anhydride: a dienophile 2) to identify which of the possible structures, the endo or exo form, the products have by melting point determination Adduct exo product endo product furan/maleic anhydride ,3-cyclohexadiene/maleic anhydride Prelaboratory Exercise: 1 omplete the equations above showing both the endo and exo products for each reaction Predict which one is favored, exo product or endo product; refer to your textbook if necessary 2 ow could you synthesize compound 1 (shown below) using a Diels-Alder reaction? 3 aution: 1 Maleic anhydride and the Diels-Alder adducts are intense skin irritants - Wear gloves! 1,3-cyclohexadiene has a noxious odor so work in a hood Furan is very low boiling Synthesis: Prepare 5 ml of a M solution of maleic anhydride in 2 l 2 (Show your calculations in your lab notebook) Filter off the insoluble maleic acid using a irsch funnel and suction filtration Mark two reaction tubes and measure 05 ml furan into one and 025 ml 1,3-cyclohexadiene into the other (be careful with these reagents - they are expensive!) The 1,3-cyclohexadiene reactions is very exothermic, so cool this reaction tube in an ice bath before adding the maleic anhydride (And after the addition, if necessary!) Add the appropriate equimolar amount of maleic anhydride to the reaction tubes (show calculations) The cyclohexadiene reaction turns yellow, the furan reaction does not (The nature of this yellow color is still a topic of research) ork the test tubes, seal the corks with Parafilm, and set them in your locker until the next laboratory period because the reaction takes 2-8 hours to reach completion Isolation and Purification: If crystals haven't formed, insert a clean stirring rod or boiling stick into the reaction tube, remove the wet rod or stick, and allow it to dry, forming seed crystals When reinserted, crystallization should be instantaneous Pipet off

3 the remaining liquid (Pasteur pipet method), wash the crystals with ice-cold 2 l 2 or 1:1 (V/V) 2 l 2 :hexane, let the crystals dry, and determine the yield and melting point Recrystallize the 1,3 cyclohexadiene adduct in about 19 ml of a 5:2 mixture of hexane:ethyl acetate Take the melting point of the furan adduct before recrystallization The furan adduct requires much more "finesse" to recrystallize because it undergoes a reverse or retro Diels-Alder reaction at the melting point Use a hot water bath adjusted to a temperature just below the melting point you obtained to dissolve the adduct Use the same solvent mixture as above, but use only part of the crude product Alternatively, you can try acetone and induce crystallization by dripping hexane into the acetone solution btain melting points of all recrystallized products leanup: Dispose of leftover maleic anhydride/ 2 l 2 solution in the alogenated rganics container and the solids and left-over recrystallizing solvents in Nonhalogenated rganics container Analysis: btain melting points of all recrystallized products Final Report: Determine which of the possible products you have, exo or endo by comparing your melting points with the literature values Answer these questions at the end of your report: 1 Explain why the M solution of maleic anhydride is made in 2l2 and not in water Show the reaction between maleic anhydride and water 2 Give structures for the major Diels-Alder product of the following reactions: 1,3-cyclohexadiene and tetrachloroethene 1,3-cylcohexadiene and fumaric acid, the trans isomer of maleic acid

4 Synthetic Experiment PreLab Grading Sheet Name(s): TA: Date: PreLab For Exp't #:2 Title: Two Easy Diels-Alder Puzzles Date, Name, Desk #, Experiment # & Title(abbreviated after 1 st pg), Section & TA Name Possible Missed Summary 12 Goals 12 Reactions, structures, conditions, diagrams 1 ompleteness of hemical Data Table(s) 6 PreLab Exercise 16 hromatographic Behavior omparison 12 Spectral Features omparison 12 Work-up isolation and recrystallization 12 TTAL FR PRELAB 100 Date anded in: General omments: Total :

5 Synthetic Experiment Final Report Grading Sheet Name: TA: Date: Final Report For Exp't #:2 Title: Two Easy Diels-Alder Puzzles Name, Date, Experiment Title (abbreviated after 1st page) and every page numbered Possible Missed BSERVATIN and DATA - verall organization, readability, completeness 8 Data: Weighing data, molecular weights, moles, d, & v of necessary compounds Yield: Show % yield & wt calculations with limiting reagent Purity: Give pure and impure melting points RESULTS AND DISUSSIN - verall organization, readability, completeness 8 Results; Achievement of goals 16 Product Analysis Data: Quality and Interpretation Structure(s) drawn on each Spectrum or hromatogram Assignment and discussion of any spectral data Mp data and its meaning 2 PSTLAB QUESTINS 16 TTAL PINTS 100 Date anded in: General omments: Total :