Testing, Testing, 1, 2, 3... (CH4/CH2)

One of the hardest aspects of organic chemistry is remembering all the wet tests that you need to carry out to differentiate between different functional groups that you are likely to come across. 

Not only do you have to remember the compounds that give a positive (and the ones that don't), you need to  remember the observations, the chemicals you need to add to get this thing to work and some seemingly random facts that someone deemed important. 

To help you through this, here is a list of all the tests you are likely to come across and an attempt at the relevant detail. I have also indicated whether these are relevant for CH2 or CH4

Tollen's Reagent (CH4), Fehling's Reagent (CH4), Acidified Dichromate (CH4 and CH2)
These three are collectively known (by me at least) as the oxidation tests, each depends on a molecule being oxidised. So molecules that are easily oxidised give a postive, namely aldehydes, primary alcohols and secondary alcohols. 

The only thing that changes is the observation.

So for Tollen's Reagent you see a silver mirror (or more likely if your test tubes could do with a wash - a grey ppt)

For Fehling's Reagent you see a orangey brown ppt

For Acidified Dichromate you see the orange solution go green

Other facts worth remembering are...

• Tollen's reagent is fairly rubbish as an oxiding agent so it is not strong enough to work with primary alcohols (such as ethanol)
• Fehling's is almost the same Benedicts solution (remember from biology, the test for reducing sugars)

Iodoform Test (CH4)

Iodoform (or triiodomethane or CHI3) is the name of the test but in this case it is not the name of the reagent you add. In fact, Iodoform is the yellow stuff you see at the end that tells you the test has worked. 

The reagent you add is aqueous alkaline iodine (or iodine, mixed with aqueous sodium hydroxide) it also works with Potassium Iodide and Sodium Chlorate (I), You generally don't need to warm it but if its being a bit slow a bit of heat helps.

The observation is a yellow ppt (of Iodoform) that smells of ...erm...Iodoform which is basically an antiseptic smell, which makes sense as iodoform used to be used as antiseptic.

People often come unstuck in what gives a positive. Positives are given by methyl ketones or methyl hydroxyls. That is -OH or C=O groups being on the carbon next to a CH3. For example -  ethanal, ethanol, propan-1-ol, Propanone  all give positives but methanol, propanal and pentan-3-one all give negatives. Think this one through, draw the molecules and try and work it out, if you are still stuck have a look here. 

2,4 DNPH (CH4)

Nice and easy one this one. Positives are given by aldehydes and ketones and it looks like a bright orange ppt. The only other factoid that they want you to know here is that this is an example of an addition-elimination reaction. Don't bother learning the mechanism just remember that fact.

Lucas Test (CH4)

This is technically not in the syllabus so they can't ask a question where this is the only answer and it won't be in the mark scheme as a possible answer but it does work and could get you out of jail if you don't know the actual answer but if you can don't use this as your answer of choice as it won't be in the mark scheme and a half asleep examiner may well mark it wrong if they aren't concentrating.

Anyway (briefly), what is it? Reagent is Conc Hydrochloric Acid and Anhydrous Zinc Chloride. Tertiary Alcohols give a white ppt v quickly, secondary alcohols give a white ppt v slowly. Primary Alcohols don't give a white ppt but neither does anything else so that fact doesn't help you much.

Sodium Carbonate Test (CH2 and CH4)

Simple idea this, acids bubble with metal carbonates. So if you add sodium carbonate solution to any acid (including carboxylic acids) you will get bubbles that turn lime water milky. 

Tests for Phenol (CH4)

Two of these, either... 

•  add Iron (III) Chloride and the solution goes purple

            or

• add bromine water and it decolourises and you get a white ppt

Enough said, lets move on.

Test for Alkenes (CH2 and CH4)

This is an easy one you should all know from GCSE. Alkenes decolourise bromine water. In other words it goes from being a browny-orange colour to being colourless (NOT CLEAR!!!)

Test for Amines (CH4)

This is a spin off from all that di-azo stuff that you do. When you add nitrous acid to an amine it forms a di-azo compound that is really unstable so it immediately breaks down to give off bubbles of nitrogen gas

Test for Amides (CH4)

Again this is a consequence of a conversion reaction. When you sodium hydroxide to an amide and heat it a bit it gives off ammonia. You will know because it stinks but if your nose isn't working then test it with damp red litmus paper and it will go blue. The only possible confusion here is with ammonium salts which also give off ammonia with sodium hydroxide but that will happen in the cold, so it is usually possible to tell the difference.

Test for Halogenoalkanes (CH4 and CH2)

Halogenoalkanes are known for being unreactive so the only way to test for them is to chop a bit off and test that. So, add some sodium hydroxide, this will then release the halide ion. 

The halide ion can then be tested for by adding a mixture of silver nitrate and nitric acid (the acid is just there to neutralise any excess sodium hydroxide from earlier).

The observation will depend on the halide ion - so white ppt means chloride (so it was a chloroalkane) cream ppt means bromide (so it was a bromooalkane) and yellow ppt means iodide (so it was an iodoalkane).

There we have it all the test in one place. If I have missed anything please leave a comment and I will add it. 

Y12 if you are getting stressed by the 4 you have to learn for CH2 spare a thought for Y13 who had to learn all of these others for CH4, as will all of you next year...