Chemistry · Alcohols, Phenols And Ethers · NEET
Lucas reagent is concentrated (conc.) hydrochloric acid, HCl, mixed with anhydrous zinc chloride, ZnCl2. Anhydrous means it has no water. This mixture is used at room temperature. In NEET, if a question says conc. HCl + ZnCl2, they are talking about the Lucas test.
The alcohol dissolves in Lucas reagent, so it stays clear at first. When the OH group is replaced by chlorine, the product is an alkyl chloride (R-Cl). Alkyl chlorides do not dissolve in the reagent, so tiny oily droplets appear and make the liquid look cloudy. This cloudiness is the turbidity. The faster the alkyl chloride forms, the faster you see cloudiness.
Tertiary (3 degree) alcohols react fastest and give cloudiness at once. This is because they form a tertiary carbocation, which is very stable. A more stable carbocation forms more easily, so the reaction is quick. Secondary (2 degree) alcohols form a less stable carbocation and take about 5 minutes. Primary (1 degree) alcohols would need a very unstable primary carbocation, so they do not react at room temperature.
Not at room temperature. A primary alcohol stays clear when you add Lucas reagent and wait. NCERT clearly says primary alcohols do not produce turbidity at room temperature. This is a common NEET trap: do not say primary is the most reactive. Primary is the least reactive here.
Add Lucas reagent and watch the clock. Cloudy immediately = tertiary (3 degree). Cloudy after about 5 minutes = secondary (2 degree). Stays clear at room temperature = primary (1 degree). Remember the order of speed: 3 degree > 2 degree > 1 degree.
NEET regularly asks which alcohol reacts fastest with Lucas reagent, or gives statement-based questions about turbidity. If you mix up the reactivity order, you lose an easy 4 marks. The safe rule to memorise is tertiary is fastest and primary does not react at room temperature.
Given below are two statements: Statement I: In the Lucas test, primary, secondary and tertiary alcohols are distinguished on the basis of their reactivity with conc. HCl + ZnCl2, known as Lucas reagent. Statement II: Primary alcohols are most reactive and immediately produce turbidity at room temperature on reaction with Lucas reagent. In the light of the above statements, choose the most appropriate answer:
Which one of the following alcohols reacts instantaneously with Lucas reagent? (The options showed four alcohol structures; option C was the tertiary alcohol.)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
There is no color change. The clear liquid turns cloudy or milky (turbid) because an oily alkyl chloride forms and does not dissolve. You watch for cloudiness, not color.
Anhydrous ZnCl2 acts as a Lewis acid catalyst. It helps the OH group leave so the carbocation can form and the OH is replaced by chlorine faster.
Yes, by timing. Immediate cloudiness means tertiary, cloudiness in about 5 minutes means secondary, and no cloudiness at room temperature means primary.
The Lucas test works well for alcohols that dissolve in the reagent, which are usually the lower (smaller) alcohols with up to about six carbons. Larger alcohols do not dissolve, so you cannot see the clear-to-cloudy change clearly.
It is the same reaction (OH replaced by Cl using conc. HCl and ZnCl2), but the Lucas test uses the difference in speed of that reaction to identify the alcohol class.
