Organic Chemistry Lab 315 Fall, 2014

DUE DATES Today –At beginning of lab – Isolation of Caffeine Report –At end of lab -- copy of laboratory notebook pages for today's experiment Next Week –No lab. Monday classes meet Two Weeks –Gas Chromatography Report (see instructions on my lab web page.) –Spectroscopy Problem II. #3

Report Notes – Last Week Reports You need to follow instructions. –Comprehensive instructions for the Unknown lab report were on the web site. –Such things as page order; drawing the structure on the IR and marking the absorptions; how to approach the written analysis, etc. –These kinds of details count and will count more in future.

Report Notes – Last Week –And most importantly, making sure your experimental and literature IR match. Not an approximate match, but a “fingerprint” match. –The output from the IR spectrometer is a spectrum (plural: spectra). –In succeeding weeks, there will be guidelines for writing different kinds of reports. Please make sure you read the appropriate guideline, as well as the “general” reports guideline.

Report Notes – Last Week It is not allowable to cite “experimental error” to explain away discrepancies. –You’ve been told many times to check your thermometer for gaps and to confirm it is reading a reasonable room temperature. Do not say “thermometer error” to explain a b.pt. that doesn’t correspond to literature. B.pts. are not as accurate indicators of purity as m.pts. are. –Never again should you say there is something wrong with the refractometer. If you think there is, tell me during lab. They are highly precise and accurate instruments. It is (almost) better to say you still have not learned how to use the instrument.

Spectroscopy Problem Notes Each problem in Part II will count 20 points. Half of the points are for consistency and confirming identity of unknown by looking it up on a spectra database. –If you say it is a particular compound, then it is easily confirmed with an exact match to the literature. –I know the answers are “out there”, so if you are actually doing these problems by yourself, then just check the answer in the database. –There is very little excuse for giving a structural formula that does not have the same M.W. as the mass spectrum M+ peak! If you made a major mistake on #1 (they will be returned during lab) that you repeated for #2, then you can turn #2 in by Thursday, 12:00 noon, with no penalty.

Gas Chromatography Uses -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

Gas Chromatography Components Carrier gas Injector Column 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 phases 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 Determination of Retention Time

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. –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. –See Pavia for method of triangulation to calculate peak areas. Do your calculations exactly as shown.

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). Determine retention times of the known compounds.

Gas Chromatography - Experiment Inject Unknown mixture (two or more of the compounds in the known mixture). Determine retention times of unknown components. Match to retention times of known compounds to identify components. Calculate peak areas and approximate mole percentages of components of unknown mixture.

Notes There are 2 GC’s. Work in groups of 2-3 (alphabetically by 1 st name – see last slide). –Each group injects standard sample mixture. –Each student injects an unknown sample. –No student may leave the lab until the last group of students finishes. Abbas Ashely Ben Khoa Ly Megan Muhammad Nafisa Ngoc Olivia Richard Shepket Stephan Stephanie Tatiana Tyler Veronika

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

Notebook Keeping Record all the chromatographic conditions –Temperature of injection port –Temperature of detector –Flow rate of carrier gas –Identity of liquid phase column –Recorder chart speed

Lab Clean-up Idris