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Diels-Alder Reactions

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Presentation on theme: "Diels-Alder Reactions"— Presentation transcript:

1 Diels-Alder Reactions
1,2,3,4-Tetraphenylnaphthalene Hexaphenylbenzene Dimethyl Tetraphenylphthalate Triptycene Experimental Write-up

2 1,2,3,4-Tetraphenylnaphthalene
500 mg tetraphenylcyclopentadienone (purple solid) 3 mL glyme (1,2-dimethoxyethane) 0.35 mL isoamyl nitrite Large rxn tube & boiling chip Caution: Remove any plastic coatings from reaction tubes. Heat to a gentle reflux and then time for 2-5 min. In a separate small vial dissolve 250 mg anthranilic acid in 2 mL glyme. Add the acid/glyme mixture to the refluxing solution dropwise with a pipette. Bubbling should occur. Look for a color change from dark brown to yellow. If color change does not occur add an additional 0.35 mL isoamyl nitrite. Heat for an additional 2 min. Cap and shake tube and let it stand at room temperature. (Move on to next reaction while compound precipitates out.) Add 10 mL ethanol and 5 mL saturated sodium bicarbonate, shake well and allow solid to form. Vacuum filter: wash with cold water twice followed by cold ethanol. Recrystallize with nitrobenzene/ethanol. Weigh and record MP.

3 Tetraphenylnaphthalene
Benzyne – An Intermediate Solvent? Glyme (1,2-dimethoxyethane) boils at 85ᵒC. This is higher than most other solvents we use in the lab which speeds up the formation of the intermediate. Benzyne will be created in situ from anthranilic acid and isoamyl nitrite. Its instability will cause it to react immediately with tetraphenylcyclopentadienone. In the procedure it talks about bubbling. This gives us an important clue about the nature of this reaction. What is it? It is important to note the structure of your reagents and product. These give important clues such as steric hindrance and stability. Is our product planar or non-planar? Does resonance have a part to play? You will need to answer these questions in your paper. Intermediate structures will need to be shown in your full extended write-up.

4 Hexaphenylbenzene A neat reaction (without solvent)
100 mg tetraphenylcyclopentadienone 500 mg diphenylacetylene Small DRY rxn tube Caution: Remove any plastic coatings from reaction tubes. Turn heat up all the way on sand bath. Reaction requires a temperature of ~300ᵒC. Use a spatula to build up sand around the base of the tube to increase heat. Lightly place a cap on reaction tube in case it bumps. Heat mixture until mixture changes color from purple to brown. Pull out of sand and lightly shake to observe if any white solid forms at the bottom while mixture is cooling. Reflux longer if no solid forms upon cooling. Cool and then add 2 mL diphenyl ether and heat in sand bath until solid is dissolved. Cool tube to RT before placing 2 mL of toluene into the product. Let tube cool or ice bath will make tube shatter if it’s hot. Vacuum filter and wash white solid with toluene.

5 Hexaphenylbenzene A similar intermediate.
Stability is a key factor in all of these reactions. How stable is this product compared to the others? Again, think in terms of sterics and resonance. A neat reaction is performed without a solvent. For a neat reaction to work the melting point of the reactants must be met. Tetraphenylcyclopentadienone has a melting point of ᵒC. Solvents that reach this high of a temperature without boiling off or breaking down are expensive.

6 Dimethyl Tetraphenylphthalate
100 mg tetraphenylcyclopentadienone 0.1 mL dimethyl acetylenedicarboxylate 1 mL nitrobenzene Small rxn tube & boiling stick Caution: Remove any plastic coatings from reaction tubes. Heat rxn tube & contents until reflux. Color will change from purple to a tan. Cool to room temp FIRST then add 3 mL ethanol. Cool in an ice bath. Vacuum filter w/ cold ethanol, dry, and weigh.

7 Dimethyl Tetraphenylphthalate
Higher BP solvent. Why? (Hint: Think in terms of activation energy.) Similar intermediate.

8 Triptycene 400 mg anthracene 0.4 mL isoamyl nitrite
4 mL glyme (1,2-dimethoxyethane) Rxn Tube 1 – w/ boiling chip 520 mg anthranilic acid 2 mL glyme (1,2-dimethoxyethane) Rxn Tube 2 Add contents of tube 2 into tube 1 dropwise over 20-minute period through a septum. After 20 min addition, add 0.4 mL isoamyl nitrite. Reflux for 10 min and then cool to RT. Add 5 mL EtOH followed by 10 mL sodium hydroxide soln (3 M). Vacuum filter and wash with cold ethanol then cold water (brown color should fade). Weigh and record this crude product. Place your crude product in 25 mL round bottom flask and add 200 mg maleic anhydride & 4 mL triglyme (NOT glyme). Fit with a reflux condenser and reflux on sand bath for 5 min. Cool to RT. Add 2 mL EtOH and 6 mL sodium hydroxide soln (3 M). Filter and rinse with cold EtOH followed by cold water. Recrystallize with methanol and filter, dry and weigh.

9 Triptycene cont. Deprotonation of anthranilic acid leads to benzyne precursors through the release of nitrogen and carbon dioxide gas just as in the first reaction. Having the anthrinilic acid and isoamyl nitrite seperated is key since side products are formed if they are added together. This is why the addition must be done dropwise over a 20 min period. Final product mechanism has no intermediate but why did the benzyne add to the anthracene at the middle of the ring and not on the side? The answer lies in resonant structures. Which rings in anthracene have the greatest resonance stability? Solvent is always an important factor. Normally this is done in dichloromethane (BP: 41ᵒC) but we are going to use isoamyl nitrite (BP: 85ᵒC). Why?

10 Experimental Write-Up
Significant figures must be a part of your experimental since they could play a limiting role. Decimals always have zeros in front of them. Notice that all reactants have the same amount of sig figs while solvents only require one. Your product will have the same number of sig figs as your limiting reagent (including your % yeild). Reaction title is bold and there is a period after every title. Title is part of the paragraph. ONLY reactants require mmol amounts, NOT solvents. Use correct notation for mL (not ml or ML). Compound names are not capitalized unless at the beginning of a sentence. Italicized words will never be capitalized. Common abbreviations are acceptable to use. The only exception is % recovery. Solvent volumes should be placed after solvent name, not before. Always include spaces between amounts and units. Example: 2,5-Di-tert-butylacenaphthene. A soln of acenaphthene (15.1 g, 97.7 mmol) and aluminum chloride (2.65 g, 19.9 mmol) in carbon disulfide (100 mL) were added to tert-butyl-chloride (20. mL, 180 mmol) dropwise via syringe over 25 min. The soln was stirred for an additional 3 h, then refluxed for 1 h. Upon cooling, water (100 mL) was cautiously added, followed by concentrated HCl (5 mL) and stirring was continued until all of the dark solid had dissolved. The mixture was extracted with carbon disulfide, dried over Na2SO4, and filtered. The solvent was removed under vacuum and the resulting yellowish wax was recrystallized once in glacial acetic acid and then once in toluene/ethanol to yield a white precipitate (7.42 g, 27.8 mmol, 30.3%): mp ºC. Any color changes and descriptions must be given. Always include a description of your product. No space on % recovery Melting point must be given as a range ONLY if you recorded one.

11 A Write-Up Checklist What you do NOT need to do:
To explain processes such as recrystallization, how to set up an apparatus, or include what you think happened. Check these off after you have completed your write-up. Correct tense – Are there any “I”, “you”, “me”, “they”, “he”, “she” or present tense words (-ing, -es words), etc… in the experimental? Remove them and restructure your sentence(s) to reflect a past tense, 3rd person point of view. All reactants are given mmol amounts while solvents were not. Liquid reactants should have mass included (e.g. “isoamyl nitrite (0.35 mL, 0.30 g, 2.6 mmol)”). Provide information about the product if it was requested in the procedure (mass, MP as a range, BP, % yield). A description of the product should always be included (color, texture, phase, smell, etc…). Crude weights are included in the same experimental paragraph. Shorthand may be used but always define it in its first mention (e.g. “hydrochloric acid (HCl)”). Include conditions such as time under reflux, ice bath, temperatures, crystallization solvents, filtration method, etc… Mass, mmol, and % yield of product reported at end along with MP range if recorded.

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