Alcohols are the compounds containing hydroxyl group attached to the alkyl group, whereas phenols are hydroxyl derivatives of aromatic hydrocarbons in which hydroxyl group is directly attached to carbon atom of aromatic ring. Alcohols are versatile compounds and can behave both as nucleophiles as well as electrophiles.
Phenols, too are reactive compounds and are known to undergo reactions on : phenolic group, benzene ring as well as various other name reactions like Reimer-Tiemann reaction, Kolbe's reaction etc.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1-methylcyclohexene
Alcohols lose a molecule of water on heating with concentrated H2SO4 and give alkenes. The reaction goes through a carbocation, and tertiary alcohols react most easily because they give the most stable (tertiary) carbocation. When a proton can be lost from more than one neighbouring carbon, the major product is the more substituted alkene. 1-Methylcyclohexan-1-ol is a tertiary alcohol. First the –OH is protonated, then water leaves:
\( \displaystyle \ce{C6H10(CH3)OH + H+ <=> C6H10(CH3)OH2+} \)
\( \displaystyle \ce{C6H10(CH3)OH2+ -> C6H10(CH3)+ + H2O} \)
Loss of a proton from a ring \( \ce{CH2} \) next to C-1 gives a ring double bond carrying the methyl group, a trisubstituted alkene, the major product 1-methylcyclohexene:
\( \displaystyle \ce{C6H10(CH3)+ -> C6H9CH3 + H+} \)
Option (1) methylenecyclohexane — Incorrect. It forms when the proton is lost from the \( \ce{CH3} \) group; the exocyclic double bond is only disubstituted, so it is the minor product:
\( \displaystyle \ce{C6H10(CH3)+ -> C6H10=CH2 + H+} \)
Option (2) 3-methylenecyclohexene (a diene) — Incorrect. Two double bonds would need the loss of two molecules of water; one dehydration gives one double bond.
Option (4) cyclohexylmethanol — Incorrect. This is a primary alcohol, an isomer of the starting alcohol; acid does not move the –OH group to a new carbon.
NCERT (2026-27 edition):
Dehydration – “Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with a protic acid e.g., concentrated H2SO4 or H3PO4, or catalysts such as anhydrous zinc chloride or alumina. … Secondary and tertiary alcohols are dehydrated under milder conditions.”