Primary, secondary and tertiary alcohols can be ...

CBSE Class 12 Chemistry 2026 All Sets Solved Paper

Question : 9 of 16

Marks: +1, -0

Primary, secondary and tertiary alcohols can be distinguished by:

Lucas test
Fehling’s test
Tollens’ test
Hinsberg’s test

Solution:

The Lucas test is the standard laboratory method used to differentiate between primary (

1^∘

), secondary (

2^∘

), and tertiary (

3^∘

) alcohols based on their reactivity with Lucas Reagent (a mixture of concentrated HCl and anhydrous

ZnCl_2

).
The reaction involves the formation of an alkyl chloride, which is insoluble in the reagent and appears as turbidity (cloudiness).
The speed of this reaction depends on the stability of the carbocation formed:

Alcohol Type	Reaction Observation	Reason
Tertiary ( $3^∘$ )	Turbidity appears immediately.	Forms a very stable $3^∘$ carbocation.
Secondary ( $2^∘$ )	Turbidity appears within 5–10 minutes.	Forms a moderately stable $2^∘$ carbocation.
Primary ( $1^∘$ )	No turbidity at room temperature (may appear on heating).	Forms an unstable $1^∘$ carbocation.

(B) Fehling’s test: Used to distinguish between aldehydes (positive) and ketones (negative). It does not react with alcohols.
(C) Tollens’ test: Known as the "Silver Mirror Test," it is used to identify aldehydes. Like Fehling's, it is used for carbonyl compounds, not alcohols.
(D) Hinsberg’s test: As we discussed earlier, this is used to distinguish between amines, not alcohols.

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