CH 242 EXPERIMENT #1

A DIELS –ALDER REACTION (Synthesis of cis—1,2,3,6-tetrahydro-4,5-dimethylphthalic anhydride)

Background The Diels-Alder reaction is an important, synthetically useful reaction in organic chemistry. It is named after Otto Diels and his student Kurt Alder who were awarded the Nobel Prize in chemistry in 1950 in recognition of the importance of their discovery. The Diels-Alder reaction is the cycloaddition of a 1,3-π system (a diene) with a π bond that usually bears an electron-withdrawing group (a dienophile) to produce a six-membered ring. The reaction is concerted (bond breaking and forming occur simultaneously) and therefore results in high stereoselectivity (products with predictable stereochemistry).

diene head-to-head overlap of p-orbitals to form two new sigma bonds dienophile

In this experiment, you will perform a Diels-Alder reaction using 2,3-dimethyl- 1,3-butadiene as the diene and maleic anhydride as the dienophile. These substrates react to form the Diels-Alder product, cis—1,2,3,6-tetrahydro-4,5-dimethylphthalic anhydride. (The dark dots in the structure of the product, below, imply that the hydrogen atoms at these positions are both “up” as drawn and therefore are cis to each other.) After synthesizing your anhydride, you will need to purify it by recrystallization, and characterize it by obtaining a melting point (the literature value is 78-79 °C) and an IR spectrum.

O O

+ O O

O O 2,3-dimethyl- maleic anhydride cis—1,2,3,6-tetrahydro-4,5- 1,3-butadiene dimethylphthalic anhydride

1 Week 1: Develop Your Experimental Procedure:

Unlike past laboratory experiments, you will spend the first laboratory period developing the experimental procedure that will be used in Week 2 to run your Diels-Alder reaction. While your laboratory instructor can help to point you in the right direction, it is up to you to determine how you will conduct the experiment, and how you will isolate, purify, and analyze your product (there is a lot that goes into conducting a chemical experiment!). By the end of your first laboratory period, you should have enough time to develop and submit your typed experimental procedure to be used in Week 2 (see below for directions regarding the format).

Experiment Guidelines:

You should use 1.0 g of maleic anhydride, and have that compound be your limiting reagent.

You will need to determine how to:

• Choose a proper reaction vessel

• Successfully react the two components (What amounts? What solvent? What temperature? For how long?)

• Analyze if/when the reaction is complete

• Remove any unreacted starting materials (if necessary) and efficiently isolate your product (Is an aqueous workup needed? Under what conditions?)

• Remove any unwanted solvent(s) from your product

• Determine the melting point of your unpurified product

• Purify your product using recrystallization (What solvent should be used?)

• Determine the melting point of your recrystallized product

• Calculate your percent yield for the reaction based on your purified product

• Characterize your product by obtaining an IR spectrum (Note: you do not need to include how to run the IR in your procedure.)

Week 1 Prelab:

1. Read section 12.12 of your text, The Diels-Alder Reaction of Dienes. 2. In CH241 laboratory, you were introduced to several techniques that you will need to use throughout the semester. From your laboratory notes and the CH241 laboratory

2 experiments, review the following concepts and procedures: flow diagrams, reflux apparatus assembly, heating at reflux, TLC, flash chromatography, extraction, recrystallization, melting point determination, use of the rotary evaporator, etc.

Week 1 Experimental Procedure Format:

The procedure that you submit should be in outline format, to make it easy to read and follow, with sufficient detail to clearly define the experimental steps that will be taken. This should allow you to organize your time efficiently. A flow diagram is optional, but may be very useful in identifying how the product will be isolated from the by-products of the reaction or from unreacted starting material. Be sure to write your procedural outline with sufficient detail as this will be the only procedure that you will have to follow. Remember to include the melting points of all reactants and products, compound solubilities, safety and disposal information, and any other relevant information that you will use throughout the experiment.

Week 2: Perform the Experiment:

Using your Week 1 procedure – go to it!

Report

The report for this experiment should be typed and include the following information:

1) A introduction (1-2 sentences) stating the purpose of this experiment 2) A discussion that includes an analysis of your chemical yield, melting points and IR spectrum. Make sure to clearly label all relevant peaks on your IR spectrum (include the spectrum with your report) and also clearly explain how you know your spectrum correlates to the Diels-Alder product and not one (or more) of the starting materials. 3) A conclusion (2-3 sentences) that includes a graphical depiction of the reaction mechanism for the Diels-Alder reaction that you performed. All chemicals structures should be drawn using ChemDraw. Be sure to use curved arrows to show the movement of electrons and show clearly the stereochemistry of the product. 4) An analysis of the 1H and 13C NMR spectra (supplied below) of your Diels-Alder product. Clearly label all relevant peaks directly on the spectra and include labeled spectra with your report. In your report, briefly discuss how you assigned the peaks for each spectrum. Note: the spectra below are not “perfect” like you might find in a textbook. Instead, they are good representations of spectra that you might obtain experimentally. This means that some of the observed signals (and integrations) must be carefully interpreted as to their relevance!

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