5.4 Organic Functional Groups: Alcohols, Carbonyls, Carboxylic Acids & Amines

5.4 Organic Functional Groups: Alcohols, Carbonyls, Carboxylic Acids & Amines

Organic functional groups are specific arrangements of atoms within molecules that dictate chemical reactivity and physical properties. A comprehensive understanding of oxygen- and nitrogen-containing functional groups—specifically their synthesis, reactivity, acidic/basic behavior, and analytical identification tests—is mandatory for medical cadet candidates.

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Alcohols, Phenols & Ethers

Alcohols ($R-OH$)

Alcohols are organic derivatives of water where one hydrogen atom is replaced by an alkyl group. They are classified into primary ($1^\circ$), secondary ($2^\circ$), and tertiary ($3^\circ$) based on the number of carbon atoms attached to the carbon bearing the $-OH$ group.

The Lucas Test (Distinguishing $1^\circ, 2^\circ, 3^\circ$ Alcohols)

The Lucas reagent consists of an equimolar mixture of anhydrous $ZnCl_2$ in concentrated $HCl$. It converts alcohols to insoluble alkyl chlorides, producing visible cloudiness/turbidity: $R-OH + HCl \xrightarrow{\text{Anhyd. } ZnCl_2} R-Cl \downarrow \text{(Turbidity)} + H_2O$

• $3^\circ$ Alcohols: Form immediate turbidity at room temperature due to rapid formation of a stable $3^\circ$ carbocation.
• $2^\circ$ Alcohols: Form turbidity within 5 to 10 minutes.
• $1^\circ$ Alcohols: Do not produce turbidity at room temperature; require heating.

Oxidation of Alcohols

• $1^\circ$ Alcohols: Oxidized by $K_2Cr_2O_7 / H_2SO_4$ first to aldehydes, then further to carboxylic acids having the same number of carbon atoms.
• $2^\circ$ Alcohols: Oxidized to ketones (resistant to further oxidation under mild conditions).
• $3^\circ$ Alcohols: Resist oxidation in neutral/alkaline media; under harsh acidic conditions, they undergo dehydration to form alkenes.

Phenols ($Ar-OH$) & Acidity Trends

Phenols are compounds containing a hydroxyl group directly bonded to an aromatic benzene ring. Phenols are substantially more acidic than alcohols ($pK_a \approx 10$ for phenol vs $pK_a \approx 16-18$ for ethanol) because the phenoxide ion ($C_6H_5O^-$) generated upon deprotonation is stabilized by resonance delocalization of the negative charge over the aromatic ring.

$\text{Acidity Order: } \text{Carboxylic Acids } (pK_a \sim 4-5) > \text{Phenols } (pK_a \sim 10) > \text{Water } (pK_a = 15.7) > \text{Alcohols } (pK_a \sim 16-18)$

Diagnostic Test for Phenol:

• Ferric Chloride Test: Addition of neutral $FeCl_3$ solution to phenol produces a characteristic violet/purple coloration due to the formation of a ferric phenoxide complex.
• Bromine Water Test: Phenol reacts instantaneously with aqueous $Br_2$ to form a white precipitate of 2,4,6-tribromophenol.

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Aldehydes & Ketones (Carbonyl Compounds)

Aldehydes ($R-CHO$) and ketones ($R-CO-R'$) contain the polar carbonyl group ($>C=\delta^+O^\delta^-$). Carbonyl carbons are $sp^2$-hybridized and undergo nucleophilic addition reactions.

              Nu⁻ (Nucleophile)         H⁺ (Electrophile)
                     │                         │
                     ▼                         ▼
        R ╲                   R ╲  O⁻     R ╲  OH
           C═O  ──►              C ───►      C
        R'╱                   R'╱  Nu     R'╱  Nu
     Trigonal Planar        Tetrahedral Intermediate

Nucleophilic Addition Reactions

1. Addition of $HCN$: Forms cyanohydrins ($R-CH(OH)CN$).
2. Addition of Sodium Bisulfite ($NaHSO_3$): Forms crystalline white bisulfite addition compounds (used for purification of carbonyls).
3. Reaction with Grignard Reagents ($RMgX$):
   • Formaldehyde ($HCHO$) $+ RMgX \xrightarrow{H_3O^+} 1^\circ$ Alcohol.
   • Other Aldehydes ($RCHO$) $+ RMgX \xrightarrow{H_3O^+} 2^\circ$ Alcohol.
   • Ketones ($R_2CO$) $+ RMgX \xrightarrow{H_3O^+} 3^\circ$ Alcohol.
4. Reaction with Ammonia Derivatives: Condensation reactions with hydroxylamine (forms oximes), hydrazine (forms hydrazones), and 2,4-dinitrophenylhydrazine (2,4-DNPH / Brady's Reagent) yielding yellow/orange/red crystalline precipitates.

Diagnostic Distinction Tests: Aldehydes vs. Ketones

Aldehydes are easily oxidized to carboxylic acids, whereas ketones resist mild oxidation. This fundamental difference forms the basis of qualitative distinction tests:

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Carboxylic Acids & Functional Derivatives

Carboxylic acids contain the $-COOH$ group. They are weak organic acids that liberate $CO_2$ gas with sodium bicarbonate ($NaHCO_3$), a key diagnostic test distinguishing them from phenols.

Carboxylic Acid Derivatives

When the $-OH$ group of a carboxylic acid is replaced by other electronegative groups, functional derivatives are formed:

1. Acid Chlorides ($RCOCl$): Formed using $PCl_5$, $PCl_3$, or $SOCl_2$ (thionyl chloride—preferred because byproducts $SO_2$ and $HCl$ are gases).
2. Acid Anhydrides ($(RCO)_2O$): Dehydration of two carboxylic acid molecules using $P_2O_5$.
3. Esters ($RCOOR'$): Prepared via Fischer Esterification (acid-catalyzed reaction between a carboxylic acid and an alcohol): $R-COOH + R'-OH \stackrel{H_2SO_4}{\rightleftharpoons} R-COOR' + H_2O$
   • Esters possess pleasant, fruity odors (e.g., amyl acetate smells like banana; ethyl butyrate smells like pineapple).
   • Saponification is the alkaline hydrolysis of esters yielding soap (salts of fatty acids) and glycerol.
4. Amides ($RCONH_2$): Heating ammonium carboxylate salts ($RCOONH_4 \xrightarrow{\Delta} RCONH_2 + H_2O$).

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Amines: Basicity Trends & Identification

Amines are organic derivatives of ammonia ($NH_3$). They are classified as primary ($RNH_2$), secondary ($R_2NH$), and tertiary ($R_3N$).

Basicity Trends of Amines

Amines behave as Lewis bases due to the unshared lone pair of electrons on the nitrogen atom. In aqueous solution, basicity is governed by inductive effects, steric hindrance, and solvation energy: $\text{Basicity in Aqueous Media: } (CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3 > C_6H_5NH_2$

• Alkylamines are more basic than Ammonia: Electron-donating alkyl groups ($+I$ effect) increase electron density on nitrogen.
• Arylamines (Aniline) are much weaker bases: The lone pair on nitrogen is delocalized into the aromatic $\pi$-system, making it less available for protonation.

Diagnostic Tests for Amines

1. Carbylamine Test (Isocyanide Test): Heating a primary amine ($1^\circ$ aliphatic or aromatic) with chloroform ($CHCl_3$) and alcoholic $KOH$ produces an extremely foul-smelling isocyanide (carbylamine): $R-NH_2 + CHCl_3 + 3 KOH \xrightarrow{\Delta} R-N\equiv C \text{ (Foul Smell)} + 3 KCl + 3 H_2O$ Secondary and tertiary amines DO NOT give this test.
2. Reaction with Nitrous Acid ($HNO_2 = NaNO_2 + HCl$):
   • $1^\circ$ Aliphatic Amines: React with $HNO_2$ at $0-5^\circ\text{C}$ to yield alcohols with brisk evolution of nitrogen gas ($N_2\uparrow$).
   • $1^\circ$ Aromatic Amines (Aniline): Undergo diazotization at $0-5^\circ\text{C}$ to form stable benzenediazonium chloride ($C_6H_5N_2^+ Cl^-$), which couples with $\beta$-naphthol to give bright orange-red azo dyes.