Qualitative Organic Chemistry The science of identifying unknown organic compounds

Qualitative organic chemistry You arrive at work to find a bottle of an unknown chemical on your desk with a note attached – “what is this?” You have no access to the IR, NMR, MS or UV/VIS Can you determine anything about this compound? How? Qualitative organic chemistry

Physical appearance You can note the physical appearance of the sample Solid, liquid? Crystalline form, colour Odour Viscosity Density Etc. Physical appearance

Physical appearance

Simple physical tests If it is a solid, you can obtain a mp A bp can be obtained for a liquid Does a mp tell you very much about the compound? Are all mp’s unique? (there are about 16 million known organic compounds out there) Simple physical tests

The mp may help you determine something about the compound Is it a very low or very high mp (small mw compound, an organic salt?) If you have any suspicions about the compound you can check the mp to see if you are on the right track Simple physical tests

A classic old way of confirming the ID of a compound – the mixed melting point Mix the unknown with a pure known sample Take the mp If the mp stays the same as the pure compound, you have a good chance of being correct (if you add two dissimilar compounds together, the mp will decrease) Mixed mp

solubility You can check to see what the sample dissolves in If it is water soluble it is polar It is also likely to be a low mw compound solubility

If it does not dissolve in water (most organics do not), then see if it will dissolve in dilute acid or base If it dissolves in an acid, it is basic (in organic chemistry, this usually means it is an amine) If it dissolves in a base, it is acidic (a carboxylic acid, a phenol, or a sulfonic acid) Solubility

If it dissolves in both sodium bicarbonate and sodium hydroxide it is probably a carboxylic acid If it dissolves in sodium hydroxide but not in sodium bicarbonate it is probably a phenol solubility

Solubility flow chart

There is a selection of other solvents which may be tried to narrow down the possibilities However, to pin down the absolute identity of a compound on solubility alone is a pretty remote concept solubility

Molecular weight of an acid by titration If you suspect based on solubility tests that you have a carboxylic acid, you can determine its molecular weight by titrating with standard base If know the grams of acid in your sample and the # moles it represents from the titration: Moles = g/mol wt Molecular weight of an acid by titration

If you are a typical organic chemist, you could burn it in the flame of a bunsen burner If it burns with a sooty black flame, it probably contains a benzene ring No sooty flame – no benzene ring Clear blue flame – probably an unsaturated compound (or highly oxygenated compound) What else can we do?

Dip a copper wire into a solution of the compound and then burn it in a flame The Beilstein test – a green flame indicates a halogenated compound More flame tests

Add sample to a test tube complete with a small lump of sodium metal Heat (and take cover) Reaction with hot sodium leads to decomposition of the sample Can now analyze for various elements Eg chloride determined by adding silver nitrate The Sodium Fusion test

Functional group tests There are a large number of chemical spot tests that may be used to further narrow down the possibilities Different functional groups react with certain reagents to produce characteristic colours or precipitates (or the release of a gas) Functional group tests

React with a solution of ferric chloride to produce a purple colour phenols

React with a reagent (2,4- dinitrophenylhydrazine or 2,4-DNP) to form a yellow to orange precipitate Aldehydes react with another reagent (Tollen’s reagent) to form a silver mirror surface on a test tube Methyl ketones react with another reagent (iodoform test) to produce a pale yellow precipitate Aldehydes and ketones

Spot tests

React with chromate oxidizing agents – change colour from red to green (Jones’ reagent) React with acetyl chloride to produce heat Can tell whether the alcohol is primary, secondary or tertiary using Lucas reagent (Zn/HCl) and gauging the speed and ease of reaction alcohols

And so on and so on …

So what info do we have so far? By now, we should know the functional group(s) the compound contains We still do not know the exact compound we have in the bottle however So what info do we have so far?

In the classical tradition, at this point we would do one of two things We would consult an extensive set of tables to match our mp with known precisely determined mp’s and make an educated guess Then check all of the available physical data against our own (appearance, solubility, etc, etc) Where next?

OR… We could create a derivative of our sample and purify it We then would take the mp and check it against a derivative table If both the mp of our original compound and that of our derivative were the same, there was a very good chance we had identified our sample correctly OR…

Derivatization

We could also determine an approximate mw of our compound by: Boiling point elevation Freezing point depression Titration (works very well for carboxylic acids) Molecular weight

Freezing point depression The change in the freezing point is related to the concentration (in molality) of the compound Using equations, you can easily calculate the mw based on the freezing point change and the quantity (in grams) of the unknown used

Time and labour So why do we no longer pursue the classical organic procedures to their ultimate conclusion?