But-2-en-1-ol, CH3–CH=CH–CH2OH, is treated with pyridinium chlorochromate (PCC). What is the main product?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → CH3–CH=CH–CHO
Pyridinium chlorochromate (PCC) is a complex of chromium trioxide with pyridine and HCl. It oxidises a primary alcohol to an aldehyde in good yield and does not carry the oxidation on to the acid. Only the –CH2OH group reacts; the C=C double bond is left unchanged.
\( \displaystyle \ce{CH3-CH=CH-CH2OH} \xrightarrow{\text{PCC}} \underset{\text{but-2-enal}}{\ce{CH3-CH=CH-CHO}} \)
Why the other options are incorrect
Option (1) CH3–CH=CH–COOH — Incorrect. A carboxylic acid is obtained directly from a primary alcohol with a strong oxidising agent such as acidified potassium permanganate. PCC stops at the aldehyde.
Option (2) CH3–CH2–CH2–CHO — Incorrect. Butanal would need the C=C bond to be reduced by adding hydrogen. PCC is an oxidising agent and does not touch the double bond.
Option (3) CH3–CH2–CH2–COOH — Incorrect. Both changes are wrong here: PCC neither reduces the C=C bond nor oxidises the alcohol as far as the acid.
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.”
\( \displaystyle \ce{CH3-CH=CH-CH2OH} \xrightarrow{\text{PCC}} \ce{CH3-CH=CH-CHO} \)