Named reactions and tests

Hand an ISC examiner three unlabelled bottles of alcohol and they expect you to sort them with one reagent and a stopwatch — that is Lucas' test. Hand them phenol and they expect salicylic acid out of Kolbe's reaction, salicylaldehyde out of Reimer–Tiemann, and a violet flash out of neutral FeCl₃. Unit 7, "Alcohols, Phenols and Ethers," bundles five such signature transformations: Lucas' test, Kolbe's (Kolbe–Schmitt) reaction, the Reimer–Tiemann reaction, the ferric chloride test, and Williamson's synthesis. None needs a mechanism written out in full — the syllabus asks for reagent, condition and observable outcome, not orbital pictures — but each hinges on one clean piece of chemical logic worth actually understanding rather than memorising. This article builds that logic once, so the five names stop being a list to cram and become a working toolkit for identifying and making these compounds.

The core insight

Five named reactions and tests let you distinguish and synthesise alcohols, phenols and ethers by watching for a specific rate, colour or product: Lucas' test uses carbocation-stability-controlled turbidity, Kolbe's and Reimer–Tiemann exploit the phenoxide ring's nucleophilic ortho position, and FeCl₃/azo coupling exploit the phenolic ring's unique reactivity that plain alcohols lack.

The core idea

Lucas' test sorts 1°, 2°, 3° alcohols by how fast they turn cloudy with Lucas reagent (conc. HCl plus anhydrous ZnCl₂), which converts the alcohol to an insoluble alkyl chloride:

R\text{–OH} + HCl \xrightarrow{ZnCl_2} R\text{–Cl} + H_2O

where: $R$ –OH is the alcohol, ZnCl₂ is a Lewis-acid catalyst that activates the –OH as a leaving group, and R–Cl is the water-insoluble product whose appearance is the turbidity you time. Tertiary alcohols turn turbid instantly, secondary within about 5 minutes, primary not at all at room temperature.

Kolbe's (Kolbe–Schmitt) reaction carboxylates sodium phenoxide with CO₂ under heat and pressure, then acidification gives salicylic acid — the direct precursor of aspirin.

Reimer–Tiemann reaction treats phenol with chloroform and NaOH; a dichlorocarbene intermediate ( $:CCl_2$ ) attacks the phenoxide ring to give, after hydrolysis, salicylaldehyde (o-hydroxybenzaldehyde).

FeCl₃ test and the azo dye test are both phenol-specific colour tests: neutral FeCl₃ gives a violet iron–phenolate complex, while cold alkaline coupling with a diazonium salt (e.g. benzenediazonium chloride) gives a bright azo dye — neither works on a plain alcohol. All five share one exam habit worth noticing: each is examined at the level of reagent, condition, product/observation, not full mechanism. The syllabus reserves "with mechanism" for esterification and dehydration of alcohols elsewhere in this unit; here you are scored on whether you can name the correct reagent, get the conditions right (heat, pressure, acid or alkali), and state what you would actually see or isolate.

Figure

Lucas test turbidity timeline for primary secondary and tertiary alcohols showing tertiary reacting instantly secondary in about five minutes and primary not reacting at room temperature

Why it works

Lucas' test is not testing solubility or acidity — it is timing a carbocation. ZnCl₂ coordinates to the –OH oxygen, converting a poor leaving group (OH⁻) into a good one (a Zn-coordinated water molecule), and the alcohol ionises through an $S_N1$ pathway:

R\text{–OH} \xrightarrow{ZnCl_2} R\text{–}\overset{+}{O}H_2 \xrightarrow{-H_2O} R^+ \xrightarrow{Cl^-} R\text{–Cl}

where: $R^+$ is the carbocation intermediate and its rate of formation, not its final concentration, is what you are watching. Tertiary carbocations are the most stabilised by hyperconjugation and the inductive push of three alkyl groups, so they form fastest — turbidity is instant. Secondary carbocations are stabilised less, so the alkyl chloride forms slowly enough that you see turbidity only after several minutes. Primary carbocations are so unstable that the $S_N1$ pathway barely proceeds at room temperature; a primary alcohol needs heating to react at all, which is why it stays clear under the test's normal conditions. The same "which position is most nucleophilic or most stabilised" logic explains the phenol reactions. In sodium phenoxide the negative charge is delocalised onto the ring, most heavily at the ortho and para carbons; CO₂ in Kolbe's reaction and dichlorocarbene in Reimer–Tiemann are both electrophiles, so they attack there. Kolbe's reaction additionally favours ortho attack because the incoming carboxylate can chelate the adjacent phenoxide oxygen in the transition state, which is why salicylic acid (the ortho isomer) dominates over the para product. FeCl₃ and the azo dye test work only on phenols because it is the ring-stabilised phenoxide-like electron density of the aromatic –OH that forms the coloured complex or performs the coupling; a saturated alcohol's –OH has no such delocalisation to offer.

Worked example

Problem. Three unlabelled bottles hold 1-propanol, 2-propanol and 2-methyl-2-propanol (tert-butanol). Identify each using Lucas' test, and explain the reasoning.

Step 1. Add Lucas reagent (conc. HCl/ZnCl₂) to equal small samples of each alcohol at room temperature and note the time to turbidity — the exact procedure the syllabus expects you to describe.

Step 2. Predict the order from carbocation stability: 3° > 2° > 1°. tert-Butanol should react fastest, 1-propanol slowest (effectively not at all without heating).

Step 3. Read the observations against that prediction. Bottle showing immediate cloudiness within seconds is 2-methyl-2-propanol (3°). Bottle turning turbid only after 5–10 minutes is 2-propanol (2°). Bottle remaining clear at room temperature is 1-propanol (1°), since primary carbocations do not form readily enough without extra heat.

Answer: the bottle turbid instantly is tert-butanol, the one turbid after ~5 minutes is 2-propanol, and the one that stays clear is 1-propanol — the identification rests entirely on timing, not on whether turbidity appears at all.

Competitive stretch

A mixture-identification question may add phenol to the set. Phenol does not undergo Lucas' test cleanly (its –OH is not simply alkyl), so examiners instead pair Lucas' test for the alcohols with the FeCl₃ test to pick out phenol: add neutral FeCl₃ to the fourth bottle — a violet colour confirms phenol, distinguishing it from all three alcohols in one extra step without touching Lucas reagent at all.

Common traps

Describing Lucas' test as "precipitate forms for 3° and 2°, none for 1°" — it is the timing of turbidity across all three classes that distinguishes them, not a strict presence/absence split; primary alcohols simply never turn turbid at room temperature.
Confusing Kolbe's reaction (phenoxide + CO₂ → salicylic acid, an addition/carboxylation) with Kolbe's electrolytic decarboxylation of carboxylic acids — same surname, unrelated reaction from a different chapter.
Forgetting the Reimer–Tiemann reaction needs alkaline conditions throughout and gives the ortho aldehyde as the major product via dichlorocarbene attack on the phenoxide ring, not on neutral phenol.
Assuming FeCl₃ tests all –OH compounds — it is specific to phenols (and enols); simple alcohols such as ethanol or benzyl alcohol give no colour at all.
Treating Lucas' test as a way to distinguish phenol from alcohols — phenol's aromatic –OH does not undergo the same clean $S_N1$ substitution, so examiners pair the FeCl₃ test with Lucas' test rather than relying on Lucas' test alone whenever phenol is in the mix.

Ask the tutor: ask the AI tutor for a one-page side-by-side table of all five named reactions/tests with reagents, conditions and expected observations for last-minute revision.

Why does tert-butanol react so much faster than 1-propanol in Lucas' test if both just have an -OH group?

Because Lucas' test does not react directly with the –OH group — it reacts with the carbocation that forms after –OH leaves. That step goes through an $S_N1$ pathway, and tertiary carbocations are stabilised by hyperconjugation from three alkyl groups, so they form almost instantly. A primary carbocation has almost no such stabilisation, so the ionisation step is far too slow at room temperature, and you see no turbidity without heating. The –OH is identical in both cases; the carbon skeleton around it is what changes the rate.

Is Kolbe's reaction here the same as the Kolbe reaction I saw with carboxylic acids?

No, and this is a favourite examiner trap. This Kolbe's (Kolbe–Schmitt) reaction carboxylates sodium phenoxide with CO₂ to build salicylic acid — it is a synthesis, adding a new –COOH group to the ring. The other "Kolbe reaction" is Kolbe's electrolytic decarboxylation of carboxylic acid salts, which removes CO₂ and couples two alkyl radicals; it belongs to the carboxylic acids chapter and has nothing to do with phenols. Same chemist, two unrelated reactions — always check which chapter you're in.

Do I need to write the full mechanism for Reimer-Tiemann or FeCl3 for ISC marks?

No. The syllabus flags esterification and dehydration of alcohols as the reactions expected "with mechanism"; the five named reactions and tests in this article are expected at the level of reagents, conditions and the product or observation — for example, "phenol + CHCl₃ + NaOH, then hydrolysis, gives salicylaldehyde" is a full-marks answer without drawing the dichlorocarbene step. Understanding the carbocation/electrophile logic (as in this article) still helps you avoid mixing up which position reacts, even though you won't be asked to draw every arrow.