Which of the following can be used to distinguish between the two compounds (A) and (B) given below?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → All of them
(A) \( \ce{CH3CH2CH2CH2OH} \) is butan-1-ol, a primary alcohol; (B) \( \ce{CH3CH2CH(OH)CH3} \) is butan-2-ol, a secondary alcohol. A test distinguishes them when one gives a visible result and the other does not. Each of the three tests does this.
Oxidation with PCC followed by Tollens’ test — PCC oxidises (A) to the aldehyde butanal and (B) to the ketone butan-2-one:
\( \displaystyle \ce{CH3CH2CH2CH2OH} \xrightarrow{\text{PCC}} \ce{CH3CH2CH2CHO} \)
\( \displaystyle \ce{CH3CH2CH(OH)CH3} \xrightarrow{\text{PCC}} \ce{CH3CH2COCH3} \)
On warming with Tollens’ reagent, butanal is oxidised and gives a silver mirror; butan-2-one, a ketone, gives none.
\( \displaystyle \ce{CH3CH2CH2CHO + 2[Ag(NH3)2]^+ + 3OH^- -> CH3CH2CH2COO^- + 2Ag + 2H2O + 4NH3} \)
Lucas reagent (conc. \( \ce{HCl} \) + \( \ce{ZnCl2} \)) — the alkyl chloride formed is insoluble and makes the solution turbid. (B), a secondary alcohol, reacts faster and turns turbid on standing; (A), a primary alcohol, gives no turbidity at room temperature.
\( \displaystyle \ce{CH3CH2CH(OH)CH3 + HCl} \xrightarrow{\ce{ZnCl2}} \ce{CH3CH2CHClCH3 + H2O} \)
Iodoform test (I2/NaOH) — (B) has the \( \ce{CH3CH(OH)-} \) group, which the reagent oxidises to \( \ce{CH3CO-} \); this then gives a yellow precipitate of iodoform. (A) has no such group and gives no precipitate.
\( \displaystyle \ce{CH3CH2CH(OH)CH3 + 4I2 + 6NaOH -> CHI3 v + CH3CH2COONa + 5NaI + 5H2O} \)
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “… A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl.”
8.2.1 Preparation of Aldehydes and Ketones – “Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols, respectively (Unit 7, Class XII).”
8.4 Chemical Reactions – “The mild oxidising agents given below are used to distinguish aldehydes from ketones: (i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal.”
7.4.4 Chemical Reactions – “The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another (Lucas test). Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl2) while their halides are immiscible and produce turbidity in solution. … Primary alcohols do not produce turbidity at room temperature.”
8.4 Chemical Reactions – “Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.”