The reagents used for dehydration of alcohols
(A) Conc. \( H_2SO_4 \)
(B) Phosphoric acid
(C) Alumina
(D) Anhydrous \( ZnCl_2 \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B), (C) and (D)
Dehydration of an alcohol is the removal of a molecule of water (OH from one carbon and H from the next) to form an alkene. It needs a protic acid or a dehydrating catalyst. The acid protonates the –OH group so that it can leave as water, a carbocation is formed, and loss of a proton gives the alkene. Tertiary alcohols are dehydrated most easily, so they need milder conditions:
\( \displaystyle \ce{C2H5OH} \xrightarrow[\text{443 K}]{\text{conc. }\ce{H2SO4}} \ce{CH2=CH2 + H2O} \)
\( \displaystyle \ce{CH3CH(OH)CH3} \xrightarrow[\text{440 K}]{85\%\ \ce{H3PO4}} \ce{CH3CH=CH2 + H2O} \)
\( \displaystyle \ce{(CH3)3C-OH} \xrightarrow[\text{358 K}]{20\%\ \ce{H3PO4}} \ce{(CH3)2C=CH2 + H2O} \)
(A) Conc. \(H_2SO_4\) — Correct. A strong protic acid and a strong dehydrating agent; ethanol gives ethene at 443 K.
(B) Phosphoric acid — Correct. A protic acid used for secondary and tertiary alcohols (85% and 20% \( \ce{H3PO4} \)).
(C) Alumina — Correct. Alcohol vapour passed over heated \( \ce{Al2O3} \) loses water; alumina acts as a dehydrating catalyst.
(D) Anhydrous \(ZnCl_2\) — Correct. A Lewis acid catalyst that helps the –OH group to leave.
NCERT (2026-27 edition):
Dehydration
