The Lucas test is a chemical test used to differentiate between primary, secondary, and tertiary alcohols based on their reactivity towards Lucas reagent. The Lucas reagent is a mixture of concentrated hydrochloric acid (HCl) and anhydrous zinc chloride (ZnCl2).
When Lucas reagent is added, the alcohols transform into corresponding alkyl chlorides.
We distinguish between these alcohols by observing the different rates of alkyl chloride formation.
Tertiary alcohols quickly form chlorides,............ while secondary alcohols take 2 - 3 minutes,............In the case of primary alcohols formation of the alkyl chloride requries heating.
Keep in mind that, in primary alcohols, the alcohol group is attached to the primary carbon, which in turn is linked to only one carbon.
Secondary alcohols have the alcohol group attached to a secondary carbon, which is connected to two carbon atoms.
In case of tertiary alcohols, there is tertiary carbon that is connected to three other carbons.
Here is how the Lucas test is done.
1. A sample of the alcohol is mixed with Lucas reagent.
2. The mixture is then heated gently.
3. Depending on the type of alcohol, different reactions occur, which can be observed by changes in the appearance of the mixture.
The reactions typically proceed as follows:
Primary alcohols: React slowly with Lucas reagent, often requiring heating for an extended period of time (sometimes hours) to produce a cloudy solution due to the formation of the corresponding alkyl chloride.
Secondary alcohols: React more quickly than primary alcohols, usually within a few minutes at room temperature, forming a cloudy solution due to the formation of the alkyl chloride.
Tertiary alcohols: React rapidly with Lucas reagent, often producing an immediate, milky solution or an immediate separation of an organic layer due to the formation of the alkyl chloride.
This test is valuable in organic chemistry for quickly determining the type of alcohol present in a sample based on its reactivity. It's important to note that the Lucas test is qualitative and not quantitative, meaning it provides information about the presence or absence of certain functional groups rather than precise measurements.
Now, let's discuss the underlying principle. When alcohols react with Lucas reagent, they undergo nucleophilic substitution, replacing the OH group with a chloride ion. This reaction involves two key steps:
The first step involves, departure of OH group to form a carbocation. This step is the rate-determining one.
Then The carbocation then combines with a chloride ion to form an alkyl chloride.
The rate of the reaction, depends on the rate of formation of carbocation. Which in turn depends on the stability of carbocation. Since the stability of carbocation decreases from tertiary to secondary to primary, the rate of the reaction also reflects the same order.
| Test | Observation | Inference |
| Alcohol + Lucas reagent | Solution becomes turbid immediately | Tertiary alcohol. |
| Solution becomes turbid after 2-3 minutes | Secondary alcohol |
|
| Solution becomes turbid only after heating | Primary alcohol |
Question-1) The reagent used in lucas test to distiguish alcohols is:
a) Na / EtOH
b) ZnCl2
c) [Fe(CN)5NO]2-
d) HCl / ZnCl2
Answer: D
Question-2) Which of the following hydroxyl compound is expected to give the turbidity immediately in lucas test?
a) ethyl alcohol
b) 2-propanol
c) 2-methyl-2-propanol
d) 2-tertiary butyl phenol
Hint: tertiary alcohols give turbidity immediately. 2-methyl-2-propanol is otherwise known as tertiary butly alcohol.
Answer: c
Question-3) How does a primary alcohol typically react in the Lucas test?
a) It produces an immediate milky solution.
b) It forms a cloudy solution after prolonged heating.
c) It shows no reaction.
d) It generates a color change in the solution.
Answer: b
Question-4) What is the primary purpose of the Lucas test in organic chemistry?
a) To determine the molecular weight of alcohols
b) To differentiate between alkanes and alcohols
c) To identify the presence of hydroxyl groups in compounds
d) To distinguish between primary, secondary, and tertiary alcohols
Answer: d
Question-5) Which of the following alcohols would produce a cloudy solution after a prolonged heating period in the Lucas test?
a) Ethanol
b) 1-butanol
c) 2-butanol
d) 2-methyl-2-propanol
Hint: Ethanol is a primary alcohol
Answer: a
Author: Aditya vardhan Vutturi