16.2 Major Functional Groups & IUPAC Nomenclature

16.2 Major Functional Groups & IUPAC Nomenclature

Quick Summary: Functional groups dictate organic chemical reactivity and physical properties. Systematic IUPAC nomenclature identifies the principal functional group to establish the parent suffix, selects the longest carbon chain containing it, and assigns lowest locants. Alcohols form hydrogen bonds and undergo substitution-dependent oxidation (1°, 2°, 3°). Carbonyls (C=O) define aldehydes, ketones, carboxylic acids, esters, and amides. Aldehydes reduce Tollens' reagent to silver metal, distinguishing them from ketones. Carboxylic acids condense with alcohols to yield esters (Fischer esterification), while amines act as bases and condense with acyl donors to form resonance-stabilized, planar amides.

---

1. IUPAC Nomenclature Rules

Systematic IUPAC nomenclature derives names using four components: Locant(s) - Prefix(es) + Parent Chain + Infix (Saturation) + Suffix (Principal Group)

Priority & Procedure

1. Principal Group: Select highest-priority group for parent suffix: acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene = alkyne > ether > haloalkane.
2. Parent Chain: Choose longest chain containing the principal functional group and maximum multiple bonds.
3. Numbering: Number from the end nearest the principal group to yield lowest locants.
4. Substituents: Assign locants and prefixes to branches, halogens, or secondary groups.
5. Alphabetization: Assemble prefixes alphabetically (ignoring di-, tri-, tetra-), using hyphens between numbers and letters.

---

2. Oxygen-Containing Groups: Alcohols & Ethers

Alcohols (R-OH, Suffix -ol)

Alcohols contain a hydroxyl group on an sp3 carbon, classified as primary (1°), secondary (2°), or tertiary (3°).

• Properties: Polar O-H bonds enable intermolecular hydrogen bonding, elevating boiling points (ethanol boils at 78°C vs. dimethyl ether at -24°C) and granting water solubility for C1–C3 alcohols.
• Oxidation: 1° Alcohols oxidize to aldehydes (with PCC) or carboxylic acids (with aqueous K2Cr2O7/KMnO4); 2° alcohols oxidize to ketones; 3° alcohols resist oxidation due to absence of a carbinol C-H bond.

Ethers (R-O-R', Prefix alkoxy- or Suffix -ether)

Ethers bridge two carbon groups with oxygen in a bent geometry (~110°). Lacking O-H bonds, ethers cannot self-associate via hydrogen bonding, producing low boiling points similar to alkanes of comparable mass. Oxygen lone pairs accept hydrogen bonds from water, granting diethyl ether moderate solubility. Ethers are unreactive, serving as inert solvents.

---

3. Carbonyl Compounds: Aldehydes & Ketones

The polar carbonyl group (C=O) contains an sp2 carbon (120°) strongly polarized toward oxygen (Cδ+=Oδ-).

• Aldehydes (R-CHO, Suffix -al): Terminal carbonyl with at least one hydrogen.
• Ketones (R-CO-R', Suffix -one): Internal carbonyl bonded to two carbons.
• Distinguishing Tests: Because aldehydes possess a carbonyl C-H bond, they oxidize readily:
  • Tollens' Test: Aldehydes reduce [Ag(NH3)2]+ to a reflective silver mirror: R-CHO + 2 [Ag(NH3)2]+ + 3 OH- → R-COO- + 2 Ag(s) + 4 NH3 + 2 H2O
  • Fehling's / Benedict's Test: Aldehydes reduce blue Cu2+ to brick-red Cu2O; ketones give negative tests.

---

4. Carboxylic Acids & Esters

Carboxylic Acids (R-COOH, Suffix -oic acid)

Carboxylic acids feature a carboxyl group (-C(=O)OH), forming stable hydrogen-bonded dimers with high boiling points (ethanoic acid: 118°C). Resonating carboxylate ions stabilize their weak Brønsted acidity (pKa ≈ 4–5), reacting with bases to form water-soluble carboxylate salts.

Esters (R-COOR', Suffix -oate)

Esters are carboxylic acid derivatives where acyl carbon binds to an alkoxy group, known for sweet, fruity aromas.

• Fischer Esterification: Acid-catalyzed reversible condensation of a carboxylic acid with an alcohol: R-COOH + R'-OH ⇌ R-COOR' + H2O
• Saponification: Base hydrolysis using hydroxide yields a carboxylate salt and an alcohol: R-COOR' + NaOH → R-COO- Na+ + R'-OH

---

5. Nitrogen Functional Groups: Amines & Amides

Amines (R-NH2, R2NH, R3N, Suffix -amine)

Amines are organic ammonia derivatives with a pyramidal sp3 nitrogen (107°) bearing a lone pair. 1° and 2° amines form hydrogen bonds. Amines act as weak bases (pKb ≈ 3–4) and nucleophiles, neutralizing acids to form water-soluble alkylammonium salts (R-NH3+ Cl-).

Amides (R-CO-NR'2, Suffix -amide)

Amides form via condensation of acyl derivatives with amines. Delocalization of the nitrogen lone pair into the carbonyl π*-orbital imparts ~40% double-bond character to the C-N bond, enforcing planarity, preventing rotation, and rendering amides neutral (pKb ≈ 15). This forms the protein peptide bond.

---

6. Functional Groups Reference Table

---

7. Worked Examples: IUPAC Naming

Example 1: Polyfunctional Alcohol

Problem: Name CH3-CH(OH)-CH(CH3)-CH2-CH3 systematically.

• Step 1: Hydroxyl (-OH) has priority over alkyl branches, establishing suffix -ol.
• Step 2: Longest chain has five carbons (pentane). Numbering from the left gives locants 2 for -OH and 3 for methyl.
• Conclusion: Systematic name is 3-methylpentan-2-ol.

Example 2: Ester Nomenclature

Problem: Name the ester formed from propanoic acid and methanol: CH3CH2COOCH3.

• Step 1: Oxygen-attached alkyl group is methyl.
• Step 2: Three-carbon acyl group from propanoic acid is propanoate.
• Conclusion: Systematic name is methyl propanoate.