Alcohols are one of the fundamental classes of organic compounds that every JEE aspirant must master. These compounds contain one or more hydroxyl (-OH) groups attached to a saturated carbon atom. Understanding alcohols' classification, preparation methods, physical and chemical properties, and reaction mechanisms is crucial for both JEE Mains and Advanced exams.
The suffix -ol is added to the parent alkane name by replacing the final e. If multiple hydroxyl groups are present, use prefixes like diol, triol etc. The position of –OH is indicated by the lowest possible number.
Example: \( \mathrm{CH_3CH_2OH} \) is called ethanol.
Hydration of alkenes using dilute \( \mathrm{H_2SO_4} \) results in Markovnikov addition of –OH.
\[ \mathrm{CH_2=CH_2 + H_2O \xrightarrow{H_2SO_4} CH_3CH_2OH} \]
Primary haloalkanes react with aqueous \( \mathrm{KOH} \) or \( \mathrm{NaOH} \) to form primary alcohols via \( S_N2 \) mechanism.
\[ \mathrm{R-X + OH^- \rightarrow R-OH + X^-} \]
Example: \( \mathrm{CH_3CHO + NaBH_4 \rightarrow CH_3CH_2OH} \)
Grignard reagents react with formaldehyde, aldehydes, and ketones to produce primary, secondary, and tertiary alcohols respectively.
\[ \mathrm{R-MgX + H_2CO \rightarrow R-CH_2OH} \]
Alcohols are weak acids (pKa ~16). They can donate \( \mathrm{H^+} \) to strong bases forming alkoxides.
\[ \mathrm{R-OH + Na \rightarrow R-O^-Na^+ + \frac{1}{2}H_2} \]
Alcohols react with active metals (Na, K) releasing hydrogen gas.
Alcohols react with carboxylic acids in presence of acid catalyst to give esters (Fischer esterification).
\[ \mathrm{R-OH + R'-COOH \xrightarrow{H^+} R'-COOR + H_2O} \]
Oxidation of primary alcohols yields aldehydes and further to carboxylic acids depending on oxidizing agent.
\[ \mathrm{R-CH_2OH \xrightarrow{[O]} R-CHO \xrightarrow{[O]} R-COOH} \]
Oxidation of secondary alcohols produces ketones.
\[ \mathrm{R_2CHOH \xrightarrow{[O]} R_2C=O} \]
Generally resistant to oxidation.
Alcohols undergo elimination of water to form alkenes using concentrated \( \mathrm{H_2SO_4} \) or \( \mathrm{Al_2O_3} \).
\[ \mathrm{R-CH_2CH_2OH \xrightarrow{H_2SO_4, \Delta} R-CH=CH_2 + H_2O} \]
Alcohols react with HX acids to form alkyl halides.
\[ \mathrm{R-OH + HX \rightarrow R-X + H_2O} \]
Converts alcohols to alkyl chlorides with retention of carbon skeleton.
\[ \mathrm{R-OH + SOCl_2 \rightarrow R-Cl + SO_2 + HCl} \]
Alcohols can convert into alkyl halides through SN1 or SN2 mechanisms depending on the type of alcohol.
Protonation of –OH makes it a better leaving group, followed by elimination to form an alkene.
Though technically not classified as alcohols in some contexts, phenols contain –OH bonded directly to an aromatic ring. Phenols are acidic and show different chemical behavior.
Contain two hydroxyl groups and show special properties like forming ethers or undergoing oxidation to form aldehydes or ketones.
| Reaction | Reactants | Conditions/Reagents | Products |
|---|---|---|---|
| Hydration of Alkene | Alkene + H2O | Dilute H2SO4 | Alcohol |
| Reduction of Aldehydes/Ketones | Aldehyde/Ketone | NaBH4, LiAlH4 | Primary/Secondary Alcohol |
| Reaction with HX | Alcohol + HX | Heat | Alkyl Halide |
| Dehydration | Alcohol | Concentrated H2SO4, Heat | Alkene + H2O |
| Oxidation | Alcohol | KMnO4, K2Cr2O7 | Aldehyde/Ketone/Carboxylic acid |
Mastering alcohols is vital for JEE success. Focus on understanding reaction mechanisms and the physical and chemical behavior of alcohols. Regular practice and revision of reactions will help secure high marks in organic chemistry sections.