Organic Chemistry Lab 315 Fall, 2014

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.

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 n D with known liquid (butyl acetate) –Measure n D for unknown liquid –Pay attention to measurement precision –Identify unknown

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”)

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.

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

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

Mixture Melting Point Mix unknown compound with a little aspirin and measure melting point -If melting point is still degrees, unknown compound was aspirin. -If the melting point is deg. The unknown compound is not aspirin. Mix the unknown compound with urea. -The melting point should still be deg. since urea is the only other possibility.

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.

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 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: o C

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.

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 deg. below anticipated m.pt. Put known compound solid waste in waste jar in hood.

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.

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

Refractive Index This symbolism is used to report the refractive index: n D T –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 ( per degree). It’s not always 20 o. Expect to reproduce a literature value to at least the first two decimal places (three significant digits).

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

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

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

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.

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 in the Manual, regarding finding and citing information for the laboratory.

Operation of the Abbé Refractometer See p. 847 in Pavia for illustration 1.Insert sample between prisms. 2.Switch on and position light. 3.Rotate adjustment knob until two fields are visible in the eyepiece; reposition light for best contrast. 4.Rotate compensating drum until borderline is sharp and achromatic. 5.Rotate adjustment knob until borderline is centered on crosshairs. 6.Press display switch; read refractive index; record temperature. 7.Clean and dry prisms; close prism assembly.

Abbé Refractometer Adjustments: –eyepiece to focus; –adjustment knob to move light field dividing line between cross-hairs; –drum to remove colored horizontal band between light and dark field