Organic Chemistry Lab 315 Fall, 2016

DUE DATES Today Next Week At beginning of lab – TLC Report At end of lab -- copy of laboratory notebook pages for today's experiment Next Week Gas Chromatography Report (see instructions on my lab web page.)

Gas Chromatography Uses The first two also apply to TLC Separation of compounds in a mixture Determination of purity of a compound Quantitative determination of relative amounts of components of a mixture Isolation of pure compounds The first two also apply to TLC The second two pertain mainly to GC

Gas Chromatography Components of a Gas Chromatograph Carrier gas Injector port (heated) Column (containing stationary phase) in oven Detector Recorder

Gas Chromatography Practice Inject liquid sample with a microliter syringe through a rubber septum. Heated injection port vaporizes liquid sample Carrier gas (He) mixes with sample and both move through the column. Column packed with the stationary liquid phase - a non-volatile liquid, wax, or low melting solid. Liquid phase is coated onto a support material, such as crushed firebrick.

Gas Chromatography The sample dissolves in and interacts with the molecules in the liquid phase as it is pushed through the column by the carrier gas (elutes). Different components of the sample will interact to different extents with the liquid phase. Some components of the mixture will interact less and move faster; other will interact more and move more slowly through the column. The more volatile (low boiling point) compounds travel faster through the column with the liquid phase we are using.

Gas Chromatography The components of the sample, now separated, pass over the detector (thermal conductivity detector, TCD). The output is recorded as a chromatogram.

Gas Chromatography The components of a sample mixture are separated over time and are recorded as peaks on the chromatogram. The time each component spends on the column is the retention time (t). In this example, the component peaks are well resolved (separated). The peaks are symmetrical and the leading and trailing ends of each peak are on the baseline.

Gas Chromatography Factors Affecting Separation Flow Rate of Carrier Gas Temperature of Column Choice of Liquid Phase Molecular weights, functional groups, and polarities of component molecules are factors in selecting liquid phase. Length of Column Similar compounds require longer columns than dissimilar compounds. Isomeric mixtures often require quite long columns.

Gas Chromatography Quantitative Analysis The area under a peak is proportional to the amount (moles) of compound eluted. See Pavia for method of triangulation to calculate peak areas. Do your calculations exactly as shown there. The mole percent composition of a mixture can be approximated by comparing the relative areas of the peaks in the chromatogram. This method assumes that the detector is equally sensitive to all compounds and its response is linear.

Gas Chromatography - Experiment Gas Chromatography of Alkyl Acetates Inject standard mixture of a known equimolar mixture of four esters: (ethyl, propyl, butyl, and pentyl acetate). Inject Unknown (two or more of the esters). KEEP THE VIALS CAPPED AT ALL TIMES Ethyl acetate is volatile and will evaporate

Gas Chromatography - Experiment The retention times of the peaks will be printed on the chromatogram. The esters will elute in order of increasing boiling point. Match to retention times of known esters in order to identify ester components in unknown. Calculate peak areas and approximate mole percentages of components of unknown ester mixture.

Experiment Notes Clean the syringe before injecting a new mixture. Draw up acetone into the syringe. Expel the acetone into the waste beaker. Repeat twice more. Pull out and depress the syringe plunger several times to air-dry the syringe. You may see a small acetone peak in your next injection if the syringe is not dry. Ignore the peak.

Experiment Notes There are 2 GC’s. Work in groups of 2-3 (alphabetically by 1st name). Each group injects standard equimolar ester mixture. Each student injects an unknown ester mixture. No student may leave the lab until the last group of students finishes. Alexandra Briana Djanabou Elisabeth Eman Emily G. Emily E. Erin Gregory T. Gregory L. Hemani Kelsey Kevin Lisa Maritza Marlon Nelia Rabia Samuel Sara Sumaiyah Teresa Young

Notebook Record all the chromatographic conditions Temperature of injection port Temperature of detector Flow rate of carrier gas Identity of liquid phase column

Notes Review significant figures. All calculations of peak area to be done exactly as shown in the Manual on the chromatogram itself. Read the guidelines on the class web page for writing a GC Report.

Refractive Index While waiting to use the GC Choose one of the liquids below and take its refractive index. Be sure to record the known identity in your notebook. Record room temperature in your notebook. Pay attention to measurement precision. BuOAc Hexane Toluene

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 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 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).

Abbé Refractometer See illustration of the refractomer. 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? nTD = 1.5128 ± .0001 The last digit is estimated.

Lab Clean-up