Organic Chemistry Lab 315 Spring, 2017 (Dr. Pant’s section)

1. Organic Chemistry Lab 315 Spring, 2017 (Dr. Pant’s section) Please see the next slide concerning your assigned bench in the laboratory.

3. DUE DATES • Today • Notebook Pre-lab should be ready before class. • The Pre-lab includes the entries in the Manual: Title, References, Purpose, Apparatus, Chemical Property Table (where applicable). All of these are done before lab. • Note: You do not need to prepare a Chemical Property Table for this lab. The table of m.pts. in the Manual are sufficient until you identify your unknown and write your report. • During lab, you will write the Procedure as you perform the experiment. You will also record your Observations and write a Results, and Conclusion.

4. DUE DATES • Today, cont’d. • Please see the Manual for details regarding what each of these includes. • At end of lab — turn in copy of all laboratory notebook pages (pre-lab + exp’t.) for today’s experiment (except your pre-lab or in-class notes). • The Quiz will cover the assigned reading in the Manual, the textbook, and the video. • You may take notes on this assignment in your notebook and you may use the notebook during the Quiz. • You may NOT use any other notes or materials, except your notebook, during the Quiz.

5. DUE DATES • Next Week – Recrystallization Experiment • Prepare for quiz – assigned reading and video • Due at beginning of lab — Melting Point Report (see Report instructions given separately on the class web page http://mason.gmu.edu/~sslayden/Lab/sws/315/chem315.htm). • Please review the policy on late lab reports.

6. Clarificaton of “melting” in MIT video • The illustration in the video looks like a molecular model with atoms (balls) and bonds (sticks). But this picture (as you can tell from the accompanying narration) is meant to show the intermolecular interactions (sticks) between molecules (balls). It is a very poor choice for an organic lab video! (See the next page for a better picture.) • The narrator said: “At a certain point, the molecular kinetic energy is high enough to overcome the attractive forces that are holding the molecules together. As a result, the crystal lattice breaks down and the molecules enter a liquid state.” • The next video illustration looks like the “bonds” are breaking and the “atoms” are moving far away from each other (gas-like). • It is not surprising that you might have the impression that melting was a bond-breaking process. Of course, it is “only” a reversible phase change, solid → liquid. • continued on next page

7. Here is a stick-rendered picture of one of the polymorphic crystal structures of aspirin(see structure below). You can see the discrete molecules of aspirin arranged in the crystal lattice space. (These large structures are symbolized by the “balls” in the MIT video.) The faint blue lines are the important intermolecularattractivehydrogen-bond dipoles (not a covalent bond!) between aspirin molecules. (These are symbolized by the “sticks” in the MIT video.) The hydrogen-bond dipoles are disrupted with heat as the molecules vibrate and move away from each other in the ordered lattice and eventually become more disordered in the liquid state. Aspirin has a melting point of ~135°.

8. Theory of Melting Point • Indication of purity of a solid • Physical property used for identification • Pure compounds melt within a 1-3 degree range (it is always a range, not a “point”)

9. Melting Point Range Behavior • Melting begins when the first drop of liquid appears (record the temperature) • Melting ends when the entire mass turns to a clear liquid (record the temperature) • Non-melting behavior may be observed: sweating, shrinkage, softening, discoloration, decomposition (if you’re not sure, record what you see – it’s never incorrect!) • Different people may obtain slightly different results (+ 1-2 degrees). These are all “correct”. • Note: a literature “melting point” is the high end of the range.

10. Melting Point of an Impure Solid • When a compound contains an impurity: • melting point range will be lower than for the pure compound • melting point range will be broader than for the pure compound

11. Mixture Melting Point Mixture Melting Point - used to determine identity of compound: • A compound melts at 133-134. Assume there are two possibilities: Whichis it? -Is it aspirin (m.pt. 135) or urea (m.pt. 133)? -How could you tell?

12. Mixture Melting Point • Mix unknown compound with a little aspirin and determine the melting point of the mixture. -If melting point is still 133-134 degrees, unknown compound was aspirin. -If the melting point is 120-128 deg. (for example, it is lower and broader than for pure aspirin), the unknown compound is not aspirin. • Mix the unknown compound with urea. -The melting point should still be 133-134 deg. since urea is the only other possibility.

13. In Lab Today - Overview • Melting Point – Physical property of a solid • Measure melting points of knowns. • Measure melting point of unknown. • Identify unknown by its m.pt. and confirm its identity by the mixed melting point method (the unknown is one of the known compounds).

14. Melting Point of a Solid • Obtain a Mel-Temp or DigiMelt in the lab. Record its number in your notebook. (Always record an instrument number if there are multiple instruments available.) • Inspect the thermometer before using it. • Load the Capillary Tube -Crush a small sample using a spatula and a watch glass. -Lightly press open end of cap. tube into sample 1-2 mm (the Mohrig text says 2-3 mm – this is too much). -Drop cap. tube through a length of glass tubing to pack solid on bottom.

15. Melting Point of a Solid • Do one fast determination to get rough approximation of melting point (for an unknown) • Then, with a new sample, increase temperature rapidly to within 10-15 degrees from anticipated melting point, then lower heat rate setting and slowly approach m.pt. 1-2 deg./min. • Record temperature at which solid first starts to melt (example: 120o) • Record temperature at which all solid is melted (example: 124o) • Report the range: 120-124o C

16. Melting Point of a Solid • Compare your experimental range with literature value(s) for your report. • Before leaving today, pack a capillary tube with the proper amount of unknown sample. Label the tube with your name and unknown number and give the tube to me. • You can make a label with a small rectangle of paper and affix it like a “flag” at the top of the capillary tube.

17. Notes • Take only a small quantity of known samples on a watch glass. • Record Unknown Number! • Do not attempt to re-do a melting point on a previously melted sample. • You can put 3 capillary tubes in the apparatus at one time. It is useful especially for redoing an unknown and a side-by-side comparison with 2 mixtures of the unknown + known. • Put used capillary tubes in broken glass container. • Use air jets (QUIETLY) to rapidly cool apparatus. Temperature needs to be about 10-15 deg. below anticipated m.pt. • Put known compound solid waste in waste jar in hood.

18. Melting Point Report • See the lab webpage information “Organic Lab Notebook &Reports” (http://mason.gmu.edu/~sslayden/Lab/sws/315/chem315.htm). • The Melting Point report will have a special format that is also posted on the webpage. • The report is due at the beginning of the next lab period. • Pay special attention to pages in the Manual regarding finding and citing information for the laboratory.

19. End of lab clean-up