Organic Chemistry Lab 315 Fall, 2014

1. Organic Chemistry Lab 315 Fall, 2014

2. DUE DATES • Today • Notebook Pre-lab should be ready before class. • At end of lab — turn in copy of laboratory notebook pages (pre-lab + exp’t.) for today's experiment (do not turn in any of your pre-lab or in-class notes). • Next Week • Prepare for quiz – assigned reading and video (you can omit sections 11.5, 11.6, 11.9, 11.10) • At beginning of lab — Melting Point and Refractive Index Report (see report instructions given separately). • Please review the policy on late lab reports.

3. In Lab Today • Melting Point – Physical property of a solid • Measure melting points of knowns • Measure melting point of unknown • Identify unknown and confirm identity with mixed melting point • Refractive Index - Physical property of a liquid • Measure nD with known liquid (butyl acetate) • Measure nD for unknown liquid • Pay attention to measurement precision • Identify unknown

4. 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”)

5. 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.

6. 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

7. Mixture Melting Point Mixture Melting Point - used to determine identity of compound: • A compound melts at 133-134. What is it? -Is it aspirin (m.pt. 135) or urea (m.pt. 133) -How could you tell?

8. Mixture Melting Point • Mix unknown compound with a little aspirin and measure melting point -If melting point is still 133-134 degrees, unknown compound was aspirin. -If the melting point is 120-128 deg. 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.

9. Melting Point of a Solid • Obtain a Mel-Temp from Prep Room. Record its number in your notebook. (Always record an instrument number if there are multiple instruments available.) • Inspect the thermometer in your drawer. • 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. -Drop cap. tube through a length of glass tubing to pack solid on bottom.

10. 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: 120) • Record temperature at which all solid is melted (example: 121) • Report the range: 120-121 oC

11. 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 sample. Label the tube with your name and unknown number and give the tube to me.

12. Notes • Take only a small quantity of known samples. • Record Unknown Number! • Record number on Mel-Temp • Do not attempt to re-do a melting point on a previously melted sample. • Put used capillary tubes in broken glass container. • Use air jets (QUIETLY) to rapidly cool Mel-Temp. Temperature needs to be about 10-15 deg. below anticipated m.pt. • Put known compound solid waste in waste jar in hood.

13. Refractive Index • Refractive index is a physical property of a substance, most useful for liquids. • It is an indication of purity • Physical property used for identification • It can be measured very precisely to 4 decimal places.

14. Refractive Index • Defined as the ratio of the velocity of light in air to the velocity of light in the (liquid) medium • Depends on 2 variables: • Temperature (velocity depends on density, which depends on temperature) • Wavelength of light

15. Refractive Index • This symbolism is used to report the refractive index: nDT • The subscript denotes the wavelength of light. In most instruments this is 589 nm, the wavelength of the sodium D-line. • The superscript denotes the temperature of measurement or the correction to a different temperature. • Remember to record the temperature of the measurement. • Then correct to the temperature reported in the literature measurement (0.00045 per degree). It’s not always 20o. • Expect to reproduce a literature value to at least the first two decimal places (three significant digits).

16. Abbé Refractometer • See illustration on p. 847 in Pavia. Keep your text open. • Instructions are posted next to the refractometer.

17. Abbé Refractometer • Use toggle switch (or button) to view numerical scale. What is the refractive index measurement?

18. Abbé Refractometer • Use toggle switch (or button) to view numerical scale. What is the refractive index measurement? nTD = 1.5128 ± .0001 The last digit is estimated.

19. Refractive Index Experiment • The class will be divided into fourths, as will the time remaining in the lab period, in order to measure R.I.’s. • It is your responsibility to take your R.I. during the assigned time, interrupting your melting point experiment, if necessary. • One refractometer will be used only for measuring the R.I. of the known compound, butyl acetate (if two refractometers are available). • After demonstrating that you can measure correctly, take the R.I. of your unknown using the other refractometer. • Record the bar code of the refractometer used in each part. • Record the “official” room temperature written on the board. Do not use any other thermometer for this purpose.

20. Melting Point/Refractive Index Report • See the Handout “Organic Lab Notebook &Reports”. • This information is also on my lab webpage. • The report is due at the beginning of next lab period. • Pay special attention to pages 16-19 in the Manual, regarding finding and citing information for the laboratory.