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Experiment 1: diels-alder and melting point

The purpose of this lab is to introduce the concept of the melting point of an organic compound as a first step in chemical identification of that compound and assessing its purity. In addition, you will synthesize a cyclic compound by employing the famous Diels-Alder Reaction.

Background information

Part 1: melting point

All solids display a characteristic melting point. With a pure substance, a melting point is the quickest and most accessible method for an organic chemist to confirm the identity of a compound (although in all fairness, NMR is the method of choice today because of its’ability to provide evidence of identity with concomitant additional information.).

Melting points can also be used to assess purity. You are all familiar with the concept of freezing point depression from Freshman Chemistry. Impurities in the solution prevent the ordering necessary to form a crystal lattice. This factor explains why salt/water solutions will not freeze until the temperature is as low as -20°C. The concepts involved with melting points are similar. In a perfect crystal, with no impurities, the melting point will occur at one temperature. However when impurities are introduced into the lattice, there is no longer a continuous crystal structure. Instead the solid is made up of different regions, some with more crystal imperfections then others. Not surprisingly the melting point will not be as sharp. This will also cause melting to begin earlier then expected. These impurities can even be water–Houston with high humidity!

It is a combination of these concepts that will be used in this lab. For the first part of the lab you will examine how adding an impurity will affect the melting point of the compound. In the second part of the lab you will determine the identity of an unknown by applying your knowledge of mixing properties that you gained in the first part of this experiment.

It will quickly become apparent that the melting points you measure will not match the melting points found in literature. You are encouraged, and it is to your advantage, to hypothesize why this is the case and to run your hypothesize by your TA to see if you are on the right track.

Find melting point data: For Part I you will be preparing mixtures of two compounds. Before lab you should look up the melting points of these two compounds. Possible sources can be Aldrich (Fondren location: TP202 .A42 2000-2001 REFERENCE-TRADE) or Acros chemical catalogs or online at (External Link) or (External Link) . These compounds are cinnamic acid and urea.

Part 2: diels-alder reaction

The Diels-Alder reaction is essentially a cycloaddition reaction (Cycloaddition reaction is one example of a special class of reactions called pericyclic reaction. Other examples of pericyclic reactions are electrocyclic reactions, sigmatropic reactions) in which an alkene adds to a 1,3-diene to form a 6-membered ring. The reaction is synthetically very useful, due to the formation of cyclic products. This is another Nobel-prize-winning reaction (1950) that provides flexibility to synthetic organic chemists. In this reaction a conjugated diene (this is one with alternating double and single bonds) is heated with a dieneophile - a compound with a multiple bond with an attached electron-withdrawing group (EWG).

Questions & Answers

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At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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Source:  OpenStax, Chem217labsfall07. OpenStax CNX. Oct 16, 2007 Download for free at http://cnx.org/content/col10463/1.4
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