Physical and Chemical Properties — Revision Notes

⚡ 30-Second Revision

Physical Properties:\n * Higher B.P. than ethers/alkanes (due to H-bonding).\n * Lower alcohols soluble in water (H-bonding with H\(_2\)O).\n * Solubility \(\downarrow\) with increasing alkyl chain.\n- Chemical Properties:\n * Acidity: CH\(_3\)OH > $1^\circ$ > $2^\circ$ > $3^\circ$ (weaker than H\(_2\)O).\n * O-H bond cleavage: Reaction with Na ($2ROH + 2Na \rightarrow 2RONa + H_2$), Esterification ($ROH + R'COOH \xrightarrow{H^+} R'COOR + H_2O$).\n * C-O bond cleavage:\n * With HX: $R-OH + HX \rightarrow R-X + H_2O$. Reactivity: $3^\circ > 2^\circ > 1^\circ$. Lucas test.\n * With PCl\(_5\): $R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl$.\n * With SOCl\(_2\): $R-OH + SOCl_2 \xrightarrow{Pyridine} R-Cl + SO_2 + HCl$ (Darzens process).\n * Dehydration: $R-OH \xrightarrow{H^+} Alkene + H_2O$. Reactivity: $3^\circ > 2^\circ > 1^\circ$. Follows Saytzeff's rule.\n * Oxidation:\n * $1^\circ$ alcohol: $R-CH_2OH \xrightarrow{PCC} R-CHO$ (aldehyde); $R-CH_2OH \xrightarrow{Strong\,oxidant} R-COOH$ (carboxylic acid).\n * $2^\circ$ alcohol: $R_2CH-OH \xrightarrow{Oxidant} R_2C=O$ (ketone).\n * $3^\circ$ alcohol: Resistant to mild oxidation.

2-Minute Revision

Alcohols' properties are dominated by the hydroxyl (-OH) group. Physically, strong intermolecular hydrogen bonding leads to high boiling points and good water solubility for smaller alcohols, decreasing with increasing alkyl chain length.

Chemically, alcohols can act as weak acids (O-H bond cleavage) or nucleophiles/bases (C-O bond cleavage). Their acidity decreases from primary to tertiary alcohols (CH\(_3\)OH > $1^\circ$ > $2^\circ$ > $3^\circ$).

They react with active metals to form alkoxides and with carboxylic acids to form esters. For C-O bond cleavage, alcohols react with hydrogen halides (HX), phosphorus halides (PCl\(_3\), PCl\(_5\)), or thionyl chloride (SOCl\(_2\)) to form alkyl halides.

The reactivity order for these substitutions and for dehydration is $3^\circ > 2^\circ > 1^\circ$. Dehydration, typically acid-catalyzed, forms alkenes, often following Saytzeff's rule. Oxidation products depend on the alcohol type and oxidant strength: primary alcohols yield aldehydes (PCC) or carboxylic acids (strong oxidants); secondary alcohols yield ketones; tertiary alcohols are resistant to mild oxidation.

5-Minute Revision

The unique physical and chemical characteristics of alcohols stem from the polar hydroxyl (-OH) group. \n\nPhysical Properties:\n* Boiling Points: Alcohols have significantly higher boiling points than comparable alkanes or ethers due to extensive intermolecular hydrogen bonding.

This strong attraction requires more energy to overcome during vaporization. Boiling points increase with molecular mass but decrease with branching (due to reduced surface area for van der Waals forces).

\n* Solubility: Lower alcohols (up to 3-4 carbons) are highly soluble in water because their -OH group can form hydrogen bonds with water molecules. As the non-polar alkyl chain lengthens, the hydrophobic character dominates, reducing water solubility.

\n\nChemical Properties (Reactivity):\nAlcohols exhibit reactions involving either the O-H bond or the C-O bond. \n\n1. Reactions Involving O-H Bond Cleavage (Acidic Nature):\n * Acidity: Alcohols are weak acids, weaker than water.

Their acidity order is CH\(_3\)OH > $1^\circ$ > $2^\circ$ > $3^\circ$, due to the electron-donating inductive effect of alkyl groups destabilizing the alkoxide ion. \n * Reaction with Active Metals: $2ROH + 2Na \rightarrow 2RONa + H_2$.

This confirms their acidic nature. \n * Esterification: $ROH + R'COOH \xrightarrow{H^+} R'COOR + H_2O$. Alcohol acts as a nucleophile. \n\n2. Reactions Involving C-O Bond Cleavage (Nucleophilic Substitution/Elimination):\n * Reaction with Hydrogen Halides (HX): $R-OH + HX \rightarrow R-X + H_2O$.

Reactivity order: $3^\circ > 2^\circ > 1^\circ$. Tertiary and secondary alcohols often react via SN1 (carbocation formation), primary via SN2. Lucas test (HCl/ZnCl\(_2\)) distinguishes alcohol types. \n * Reaction with Phosphorus Halides (PCl\(_3\), PCl\(_5\), PBr\(_3\), PI\(_3\)): Converts -OH to -X.

E.g., $R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl$. \n * Reaction with Thionyl Chloride (SOCl\(_2\)): $R-OH + SOCl_2 \xrightarrow{Pyridine} R-Cl + SO_2\uparrow + HCl\uparrow$. Excellent for alkyl chloride preparation due to gaseous byproducts.

\n * Dehydration: $R-OH \xrightarrow{Conc. H_2SO_4/H_3PO_4} Alkene + H_2O$. Reactivity: $3^\circ > 2^\circ > 1^\circ$. Follows Saytzeff's rule (more substituted alkene is major product). Conditions vary with alcohol type (e.

g., $1^\circ$ needs higher temp). \n\n3. Oxidation Reactions:\n * **Primary Alcohols ($1^\circ$):** \n * Mild oxidation (e.g., PCC): $R-CH_2OH \rightarrow R-CHO$ (aldehyde). \n * Strong oxidation (e.

g., K\(_2\)Cr\(_2\)O\(_7\)/H\(_2\)SO\(_4\)): $R-CH_2OH \rightarrow R-COOH$ (carboxylic acid). \n * **Secondary Alcohols ($2^\circ$):** $R_2CH-OH \xrightarrow{Oxidant} R_2C=O$ (ketone). \n * **Tertiary Alcohols ($3^\circ$):** Resistant to mild oxidation due to absence of H on carbinol carbon.

\n\nKey Takeaway: The type of alcohol (primary, secondary, tertiary) dictates its reactivity and the products formed in many reactions. Always consider the specific reagent and reaction conditions.

Prelims Revision Notes

I. Physical Properties:\n* Boiling Point: Alcohols have higher B.P. than alkanes, ethers, haloalkanes of similar M.W. due to intermolecular hydrogen bonding. \n * B.P. \(\uparrow\) with M.W.

(increased van der Waals forces). \n * B.P. \(\downarrow\) with branching (decreased surface area for van der Waals forces). \n* Solubility in Water: Lower alcohols (C1-C3) are highly soluble/miscible due to H-bonding with water.

\n * Solubility \(\downarrow\) with increasing alkyl chain length (hydrophobic part dominates). \n* Density: Generally less dense than water; increases with M.W.\n\nII. Chemical Properties (Reactivity):\n**A.

Reactions Involving O-H Bond Cleavage (Acidic Nature):**\n* Acidity Order: CH\(_3\)OH > $1^\circ$ > $2^\circ$ > $3^\circ$. (Alkyl groups are electron-donating, destabilize alkoxide ion). \n* Comparison: Alcohols are weaker acids than water (pKa ~16-18 vs ~15.

7 for H\(_2\)O). \n* Reaction with Active Metals (Na, K, Al): $2ROH + 2Na \rightarrow 2RONa + H_2$. (Forms alkoxides, liberates H\(_2\) gas). \n* Esterification: $ROH + R'COOH \xrightarrow{H^+} R'COOR + H_2O$.

(Alcohol acts as nucleophile). \n\nB. Reactions Involving C-O Bond Cleavage (Nucleophilic Substitution/Elimination):\n* Reactivity Order: $3^\circ > 2^\circ > 1^\circ$ (due to carbocation stability in SN1/E1).

\n* Reaction with Hydrogen Halides (HX): $R-OH + HX \rightarrow R-X + H_2O$. \n * Reactivity of HX: HI > HBr > HCl. \n * Lucas Test (HCl + Anhydrous ZnCl\(_2\)): \n * $3^\circ$: Immediate turbidity.

\n * $2^\circ$: Turbidity in 5-10 min. \n * $1^\circ$: No turbidity at room temp. \n* Reaction with Phosphorus Halides (PCl\(_3\), PCl\(_5\), PBr\(_3\), PI\(_3\)): Converts -OH to -X. \n * $R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl$.

\n * $3R-OH + PCl_3 \rightarrow 3R-Cl + H_3PO_3$. \n* Reaction with Thionyl Chloride (SOCl\(_2\)): $R-OH + SOCl_2 \xrightarrow{Pyridine} R-Cl + SO_2\uparrow + HCl\uparrow$. (Darzens process, pure alkyl halide).

\n* Dehydration (Elimination): $R-OH \xrightarrow{Conc. H_2SO_4/H_3PO_4, Heat} Alkene + H_2O$. \n * Ease of dehydration: $3^\circ > 2^\circ > 1^\circ$. \n * Follows Saytzeff's Rule: Major product is the more substituted alkene.

\n * Carbocation rearrangements possible for $2^\circ$ and $3^\circ$ alcohols.\n\nC. Oxidation Reactions:\n* **Primary Alcohols ($1^\circ$):** \n * To Aldehyde: $R-CH_2OH \xrightarrow{PCC} R-CHO$.

(PCC is mild, selective). \n * To Carboxylic Acid: $R-CH_2OH \xrightarrow{Strong\,oxidant (K_2Cr_2O_7/H^+, KMnO_4)} R-COOH$. \n* **Secondary Alcohols ($2^\circ$):** $R_2CH-OH \xrightarrow{Oxidant} R_2C=O$ (Ketone).

(Ketones are resistant to further oxidation). \n* **Tertiary Alcohols ($3^\circ$):** Resistant to mild oxidation (no H on carbinol carbon). Under vigorous conditions, C-C bond cleavage occurs.

Vyyuha Quick Recall

All Boys Sing During Organic Reactions: \n* Acidity (CH\(_3\)OH > $1^\circ$ > $2^\circ$ > $3^\circ$) \n* Boiling Point (High due to H-bonding) \n* Solubility (Decreases with chain length) \n* Dehydration ($3^\circ$ > $2^\circ$ > $1^\circ$) \n* Oxidation (PCC for aldehyde, Strong for acid/ketone) \n* Reactions with HX ($3^\circ$ > $2^\circ$ > $1^\circ$) \n\nThis mnemonic helps recall the key properties and reactivity trends of alcohols.