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Distillation and Gas Chromatography of Alkanes: Separation and Analysis Techniques

Learn the techniques of simple distillation and Gas Chromatography (GC) to separate and identify compounds in a mixture of alkanes. Use GC retention times and area percent values to determine purity and evaluate the effectiveness of the separation method.

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Distillation and Gas Chromatography of Alkanes: Separation and Analysis Techniques

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  1. Experiment 2 DISTILLATION AND GAS CHROMATOGRAPHY OF ALKANES

  2. Objectives • To learn the techniques of simple distillation and Gas Chromatography (GC). • To use the simple distillation technique to separate a mixture of compounds. • To identify and quantify compounds in the mixture using GC retention times and area percent values. • To determine the purity of the compounds and evaluate effectiveness of separation method based on GC data.

  3. Before coming to lab… • It is *highly recommended* that you watch the following video prior to coming to lab. • http://www.youtube.com/watch?v=3JlIPnyrZMw

  4. Structure, IMF and Boiling Point • Alkanes = consist of ONLY carbon and hydrogen. • Straight chain alkanes = greater surface area contact = greater LDF between molecules = HIGHER BP! • Branched alkanes = less surface area contact = less LDF b/n molecules = LOWER BP! Notice more contact between molecules here!

  5. Distillation • Distillation = separates/purifies liquids. • Heat mixture of two or more liquids in a flask. • More volatile liquid vaporizes 1st, vapor passes into a water-cooled condensing column, it can reverts back into a liquid (condenses) on the cool glass, then trickles into a collection flask. • With further heating the less volatile liquids vaporize and distill at higher temperatures.

  6. Distillation • Involves vaporization, condensation, and collection of the condensate (distillate). • The temperature of the distillate is measured by a thermometer placed in the path of the vapor. • Collect four 5 mL fractions in small test tubes! • Leave the last 5 mL fraction behind in the round bottom flask!

  7. Simple Distillation • Simple distillation is most effective when the mixture contains only onevolatile component. • More than one….most effective if the boiling points of the components differ by at least 70 ºC.

  8. Table 2.1: Distillation Results

  9. Chromatography • Chromatography = collective term for a family of laboratory techniques for the separation of mixtures. • Chromatography involves passing a mixture dissolved in a mobile phase (which moves) through a stationary phase (doesn’t move). • Different levels of interaction b/n compounds and mobile vs. stationary phases separates the analyte to be measured from other molecules in the mixture and allows it to be isolated.

  10. Gas Chromatography (GC) • In GAS chromatography, the MOBILE PHASE = He(g). • Gas chromatography is only used to separate volatile compounds in lab.

  11. STATIONARY PHASE = a nonpolar packing material. When analyzing compounds of similar polarity, the compounds will elute in order of volatility. Lower BP = low retention time (Rt) Gas Chromatography Higher boiling component Lower boiling component

  12. EXAMPLE GC RESULTS Original Sample Mixture Original Sample • The individual fractions collected are analyzed by GC during the course of the distillation. Based on peak size, it is evident that as the distillation proceeds, the later fractions contain mainly the higher boiling component of the mixture. Pentane ( GC Sample Solvent) pentane Alkane #1 hexane Alkane #2 Unknown X Fraction 2 Notice Fraction 1 contains more of Alkane #1 (lo BP) than Alkane #2 (hi BP) Fraction 1 Notice Fraction 5 contains ONLY of Alkane #2 (hi BP), and NO Alkane #1 (lo BP) Fraction 5 Fraction 5

  13. Product Analysis(Quantification of Components…Adjusted Area Percent) • Most often you will dissolve your compound or mixture in a low boiling solvent for GC analysis. • The relative areas of the components of interest must therefore be adjusted, to exclude the large % area of the solvent peak. • Refer to page 21 for further explanation. Adjusted area % = area % of peak of interest X 100 sum of area % of ALL peaks of interest* *omit area % of solvent peak

  14. OVERVIEW • Select and obtain 25 mL of an unknown. • Set up and perform a simple distillation. • Collect (4)-5 mL fractions in small test tubes. • Prepare GC samples of Fraction #1 and Fraction #4. Submit for analysis. • Pick up GC results when ready and record standard retention times from STANDARD CHROMATOGRAM. • Identify alkanes in unknown by comparing sample retention times to standard retention times. • Quantify alkanes at beginning and end of distillation by calculating Adjusted Area Percent.

  15. Experimental Procedure(Simple Distillation) • Add the unknown mixture and 3 boiling chips to a round bottom flask. • Set up a simple distillation apparatus. • Begin water flow through condenser and apply heat to the flask. • Collect liquid distillate in small test tubes as it leaves the condenser. • Keep track of the temperature range for each fraction as it is collected. • Collect (4) 5 mL fractions, leaving the final 5 mL in the boiling flask. • Prepare GC sample for analysis.

  16. Product Analysis(GC Analysis) • Analyze your FIRSTand FOURTH fractions by GC. • SAMPLE PREP: • Transfer 5 drops of your sample into an auto sampler vial using a plastic pipet. • Add 1 mL of GC solvent (pentane) from the solvent dispenser in balance room. • Be sure to record your GC vial slot # in your laboratory notebook. • You will receive an email to your UNCW account when GC results are ready for pick-up!

  17. Table 2.2: GC Results • You cannot complete this table until you obtain your GC results from the board outside of the labs. • The standard chromatogram will be provided on the front of the envelope containing the chromatograms. • Use only the standard run for your lab section!

  18. SAFETY CONCERNS • All compounds used in today’s experiment are extremely flammable. Wear safety goggles at all times and use extreme caution!

  19. WASTE MANAGEMENT • Pour all liquid waste into the bottle labeled “LIQUID ORGANIC WASTE”. • Do NOT pour any waste down the drain!

  20. CLEANING • Disassemble distillation apparatus in your lab hood. • In your lab hood, rinse all glassware with wash acetone ONLY directly into a beaker from your drawer (DO NOT REMOVE DISTILLATION APPARATUS FROM YOUR HOOD AT ANY TIME.) • Return distillation glassware to case in your hood. • Return all other glassware to your lab drawer.

  21. IN LAB QUESTIONS(The following questions should be answered in laboratory notebook.) • How does the shape of an alkane affect the surface area and intermolecular forces between alkane molecules? What affect does this have on the boiling point of an alkane?

  22. IN LAB QUESTIONS(The following questions should be answered in laboratory notebook.) • All of the structures shown below have the molecular formula C6H14. Copy the structures into your laboratory notebook. Look up the boiling point of each compound and record the value under the name of each structure. Circle the structure which you would expect to have the highest boiling point. Draw an “X” through the structure which you would expect to have the lowest GC retention time.

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