Detecting Elements and Groups in Organic Compounds: Quick Revision

A brief note on Detecting Elements and Groups in Organic Compounds. Read it before a test. Try each question at the end before you open its answer.

Carbon and hydrogen

• Heat the dry compound with dry copper(II) oxide.
• Carbon dioxide turns lime water milky. Water turns white copper(II) sulphate blue.
• Too much carbon dioxide clears the lime water again.

The sodium fusion extract

• Fuse the compound with sodium, plunge it into water, boil and filter.
• Sodium makes $\text{NaCN}$, $\text{Na}_{2}\text{S}$ and $\text{NaX}$. With both N and S and little sodium it makes $\text{NaSCN}$.
• Nitrogen: iron(II) sulphate, warm, then acid. Prussian blue, $\text{Fe}_{4}[\text{Fe}(\text{CN})_{6}]_{3}$.
• Sulphur: sodium nitroprusside gives violet. Acetic acid and lead acetate give black $\text{PbS}$.
• Nitrogen and sulphur together: blood red with iron(III) chloride.
• Halogens: boil with nitric acid, then silver nitrate. White (dissolves in dilute ammonia solution), pale yellow (sparingly), yellow (does not).
• Hydrazine and diazonium salts give no Prussian blue.

Functional groups

• Double bond: bromine water and Baeyer's reagent lose their colour.
• Phenol: violet with neutral iron(III) chloride. White tribromophenol with bromine water.
• Carbonyl: orange 2,4-DNP precipitate. Aldehyde: silver mirror, pink Schiff's reagent. Aliphatic aldehyde: red Fehling's precipitate.
• Iodoform: yellow with $\text{CH}_{3}\text{CO}\text{-}$ or $\text{CH}_{3}\text{CH}(\text{OH})\text{-}$.
• Acid: effervescence with sodium hydrogencarbonate.
• Primary amine: carbylamine smell. Primary aromatic amine: orange-red azo dye.

A worked example

A chlorine compound of mass $0.850\ \text{g}$ gives $0.861\ \text{g}$ of silver chloride. What is the percentage of chlorine?

1. Moles. $\dfrac{0.861}{143.5} = 6.00\ \text{mmol}$.
2. Mass of chlorine. $6.00 \times 35.5 = 213\ \text{mg}$.
3. Percentage. $\dfrac{0.213}{0.850} \times 100 = 25.1\ \%$.

Practice with new numbers

A program to try

The program works out the percentage of a halogen from the mass of its silver halide.

Mistakes to avoid

• Skipping the nitric acid before silver nitrate.
• Using Fehling's solution for benzaldehyde.
• Reading a blood red colour as nitrogen alone. It shows nitrogen and sulphur together.
• Calling every alcohol an iodoform positive.

Check yourself

1. What does a violet colour with sodium nitroprusside show?

2. Which reagent separates an aldehyde from a ketone?

3. How can sodium hydrogencarbonate tell benzoic acid from phenol?

4. A precipitate is white and dissolves in dilute ammonia solution. Which halogen is it?