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IDENTIFICATION OF ORGANIC UNKNOWNS ??  Solubility classification ... Classification and...

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  • UNK.1

    IDENTIFICATION OF ORGANIC UNKNOWNS

    SECTIONS :

    Physical examination

    Physical properties

    Solubility classification

    Interpretation of Spectra

    Classification and Chemical tests

    Identification of Organic Unknowns

    Part 1 : Classification of Unknowns I-V

    Part 2 : Individual Unknowns

    Always treat all chemical unknowns with caution.

    Wear gloves and use in a fumehood whenever possible.

    Be aware that many materials are flammable so be aware of nearby flames.

    Sample vials should not be left open.

    Contaminated glassware including pipettes should not be left on the bench.

    All unknowns assigned are pure compounds, and no purification should be necessary.

    PHYSICAL EXAMINATION

    a) State Organic compounds can be solids, liquids or gases at room temperature. Knowing this

    information can be used to narrow the search. Solids can be powders or crystals, and the

    shapes of crystals can be indicative

    b) Colour Common coloured compounds include nitro and nitroso compounds (yellow), -diketones

    (yellow), quinones (yellow to red), azo compounds (yellow to red), and polyconjugated olefins

    and ketones (yellow to red). Phenols and amines are often brown to dark-purple because of

    traces of air oxidation products.

    c) Odour Some liquid and solid amines are recognisable by their fishlike odours; esters are often

    pleasantly fragrant : often described as "fruity". Alcohols, ketones, aromatic hydrocarbons,

    and aliphatic olefins, have characteristic odours. On the unpleasant side, thiols, isonitriles,

    and low MW carboxylic acids are not "nice"!

    PHYSICAL PROPERTIES

    a) Melting Point - This is a characteristic physical property of your unknown and should be determined

    very carefully (see melting point determination) and this value compared with a value previously recorded in

    the literature (see tables of melting points). In comparing your value with melting points in the tables, a

    http://www.chem.ucalgary.ca/courses/351/laboratory/meltingpoint.pdf

  • UNK.2

    leeway of several degrees should be allowed for possible errors in your determination or in the value

    recorded in the table.

    b) Boiling Point - The boiling point of a liquid depends on the molecular weight and the functional groups

    present. So once the functional groups are known it is possible to estimate the likely molecular weight range

    of the compound. For a relatively large amount of sample the boiling point of the unknown is determined by

    carrying out a simple distillation of the liquid. If only small amounts of liquid material are available for boiling

    point determination, the microboiling point apparatus should be used. If the boiling point of your unknown is

    higher than 230 you will be given the approximate uncorrected (i.e. 5) boiling point by your instructor.

    Remember to make an approximate correction for Calgary atmospheric pressure before comparing

    your experimental value with the literature value (see boiling point determination). If you are given a boiling

    point for a compound with b.pts. >230 this will correspond to the value you would measure in Calgary and

    the approximate correction will have to be applied.

    SOLUBILITY CLASSIFICATION

    The solubility of an organic compound in water or aqueous acid or base can provide evidence for the

    presence of several important functional groups, as indicated in the following chart:

    Solvent Some Solubility or Complete Miscibility

    Water alcohols, amines, acids, esters, ketones, aldehydes (with 4 carbons or fewer) 5% NaHCO3 carboxylic acids

    5% NaOH carboxylic acids and phenols

    5% HCl amines

    Solubility in water denotes a rather high ratio of polar groups to carbon chain length, i.e., a low

    molecular weight compound containing an -OH, -NH2 or -CO2H group, or a larger molecule containing more

    than one such group. The presence of an acidic -CO2H or basic -NH2 group in a water-soluble compound

    can be detected by a low or high pH, respectively, of the solution.

    Compounds which are insoluble in water generally dissolve to a significant extent in aqueous acid or

    base if they form an ionic species (review Chem 351 solubility experiment). The solubility of carboxylic acids

    (KA = 10-3 to 10-5) and phenols (KA = 10

    -9 to 10-10) in aqueous hydroxide is due to the formation of the

    carboxylate or phenoxide, since they are much stronger acids than water (KA = 10-14), and the acid-base

    equilibria lie far to the right.

    RCO2H + OH RCO2

    + H2O

    ArOH + OH ArO + H2O

    Carboxylic acids, but not phenols, are also stronger than carbonic acid (KA = 10-7), and are therefore also

    soluble in NaHCO3 solution:

    RCO2H + HCO3 RCO2

    + H2O + CO2

    http://www.chem.ucalgary.ca/courses/351/laboratory/boilingpoint.pdfhttp://www.chem.ucalgary.ca/courses/351/laboratory/boilingpoint.pdfhttp://www.chem.ucalgary.ca/courses/351/laboratory/351expt_01_solubility.pdf

  • UNK.3

    The solubility of amines in dilute aqueous acid similarly reflects the fact that they are stronger bases

    than water, and are converted to an ammonium ion:

    RNH2 + H3O+ RNH3

    + + H2O

    Amines are the only common class of organic compounds which are protonated in dilute aqueous acid.

    EXPERIMENTAL PROCEDURE

    Note Due to the nature of the solubility tests, you don't need to apply all the tests to each compound, the

    results from one test dictate what to do next.

    Place two drops of a liquid unknown, or an equivalent amount of a solid, in a test tube and add about

    0.5 mL of water. If the compound dissolves completely or partially (a swirling appearance in the water, due to

    change in the refractive index, is an indication of some solubility), consider it soluble and proceed no further

    since no information will be gained by further solubility tests.

    If the compound is soluble in water, test the pH of the solution with universal indicator paper. An

    organic acid that is soluble in water will give a solution of pH 2 to 3 (indicator paper red). An amine that is

    water soluble will give a pH of 10 to 11 (indicator paper blue). A blank test should be carried out with pH

    paper on distilled water alone as a control for this test.

    If the sample is not soluble in water, repeat the test using 0.5 mL of 5% hydrochloric acid. If the

    compound dissolves or is significantly more soluble in acid than in water, it can be concluded that it is an

    amine.

    If the compound is insoluble in water and in dilute acid, test for acidic properties by repeating the

    solubility test with 0.5 mL of 5% NaHCO3 solution. If solubility is not observed, test with 5% NaOH to check

    for a phenol.

    INTERPRETATION OF SPECTRA Spectroscopic tables Chem 351 spectroscopy experiment

    Interpret the spectra as fully as possible. Remember infra-red spectroscopy is most useful for

    indicating the presence of functional groups due to the vibrations of these polar covalent bonds. H-NMR

    spectroscopy provides important information on the hydrocarbon portions of the molecules. Remember H-

    NMR can provide four levels of information : number of types of H (based on the number of peak sets), how

    many of each type (from the integration), what chemical types (based on the chemical shifts), and last, but

    not by no means least, how these are connected (from the coupling patterns). The spectral information

    should give indications of functional groups present and will assist greatly when applying the characteristic

    functional group tests.

    http://www.chem.ucalgary.ca/courses/351/laboratory/spec_tables.pdfhttp://www.chem.ucalgary.ca/courses/351/laboratory/351expt_07_spectroscopy.pdf

  • UNK.4

    CLASSIFICATION AND CHEMICAL TESTS

    Note Due to the nature of the functional group tests, you don't need to and should not apply all the tests to

    each compound, the results from one test dictate what to do next. Such an approach can result in

    misleading false positives.

    At this point, since the scope of the experiment is restricted, it should be possible to place the unknown

    in one of the six classes of compounds listed in the tables by reference to the solubility data (and infra-red

    spectrum if provided). A few chemical tests that give a clearly visible indication of a reaction are sometimes

    useful to confirm the presence of functional groups that are indicated by spectra. The reagents for this

    purpose are described below. These tests should be applied only as indicated, and not routinely to all

    unknowns; spurious and confusing results will be obtained. For example, if any of these reagents is applied to

    an amine then false positive results are likely.

  • UNK.5

    ALDEHYDES AND KETONES : 2,4-DINITROPHENYLHYDRAZONES

    All aldehydes and ketones readily form bright yellow to dark red 2,4-dinitrophenylhydrazones. The

    reagent is a solution of 2,4-dinitrophenylhydrazine and sulfuric acid in ethanol, and may be useful in the case

    of neutral compounds that contain a carbonyl group to distinguish between aldehydes or ketones on one

    hand and other carbonyl containing molecules (e.g. esters, amides, carboxylic acids etc.) on the other. Yellow

    derivatives are formed from isolated carbonyl groups and orange-red to red derivatives from aldehydes or

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