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?

Physical appearance

You can note the physical appearance of the sample

Solid, liquid?Crystalline form, colourOdourViscosityDensityEtc.

Physical appearance

Simple physical tests

If it is a solid, you can obtain a mpA 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

Mixed mp

A classic old way of confirming the ID of a compound – the mixed melting point

Mix the unknown with a pure known sampleTake the mpIf 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)

solubility

You can check to see what the sample dissolves in

If it is water soluble it is polarIt 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 flow chart

solubility

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

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

What else can we do?

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 ringClear blue flame – probably an

unsaturated compound (or highly oxygenated compound)

More flame tests

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

The Sodium Fusion test

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 sampleCan now analyze for various elementsEg chloride determined by adding silver

nitrate

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)

phenols

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

Aldehydes and ketones

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

Spot tests

alcohols

React with chromate oxidizing agents – change colour from red to green (Jones’ reagent)

React with acetyl chloride to produce heatCan tell whether the alcohol is primary,

secondary or tertiary using Lucas reagent (Zn/HCl) and gauging the speed and ease of reaction

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

Where next?

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)

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

Derivatization

Molecular weight

We could also determine an approximate mw of our compound by:

Boiling point elevationFreezing point depressionTitration (works very well for carboxylic

acids)

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

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

Time and labour