Structure, Basic Reactions & Physical Properties of Organic Compounds

Organic Chemistry: Scope & Core Principles

• Study of structure, properties, reactions and synthesis of carbon‐containing molecules.

• Central theme: tetravalency of carbon → vast structural diversity.

• Applications span pharmaceuticals, materials, energy, biochemistry, environmental science.

Organic vs. Inorganic Compounds

• Bonding within molecules

  • Organic → covalent; Inorganic often ionic.

• Intermolecular forces

  • Organic: generally weak (London, dipole, H-bond if heteroatoms).

  • Inorganic: often strong lattice forces.

• Physical state

  • Organic: gases, liquids, low-m.p. solids.

  • Inorganic: high-m.p. solids.

• Flammability – Organics often flammable; inorganics usually non-flammable.

• Water solubility & conductivity

  • Organics: often low solubility, non-conductors.

  • Inorganics: frequently water-soluble, conductive as solutions.

Physical Properties & Intermolecular Forces

• Boiling point (bp) ↔ Vapor pressure (vp): $\text{High bp} \Rightarrow \text{Low vp}$ due to stronger attractions.

• Strength of forces

  • Stronger forces → ↑bp, ↑m.p., ↑viscosity, ↓vp.

• Chain length

  • ↑C chain → ↑surface area → stronger London forces → ↑bp/m.p./viscosity, ↓vp (propane → pentane example).

• Branching

  • ↑Branching → ↓surface area → weaker forces → ↓bp, ↑vp (isomeric C_5H_{12}).

• Functional-group polarity

  • $\text{pentane} < \text{butanal} < \text{butan-1-ol}$ for bp (London < London+dipole < +H-bond).

Bond Types in Carbon

• Single (σ) bond: sharing 2 e⁻.

• Double (σ + π): sharing 4 e⁻.

• Triple (σ + 2π): sharing 6 e⁻.

Isomerism

• Isomers: same molecular formula, different arrangement.

• Structural (constitutional): different connectivity.

• Stereoisomers: same connectivity, different spatial arrangement.

  • Geometric (cis/trans) in rings or C=C.

Functional-Group Identification Tree (excerpt)

• Hydrocarbon only? → $sp^3$ all single bonds = alkane; $sp^2$ = alkene; $sp$ = alkyne.

• Contains O?

  • $\text{C}=O$ & OH on same carbon → carboxylic acid.

  • $\text{C}=O$ without OH → aldehyde (terminal) or ketone (internal).

  • No $\text{C}=O$ but O-C:

  • O bound to one C = alcohol;

  • O bound to two C = ether;

  • O in ring with adjacent C=C = epoxide.

• Contains N?

  • C=O adjacent N → amide.

  • N without adjacent C=O → amine.

Hydrocarbons

Families & Key Features

• Alkanes (saturated): only C–C single; formula $C<em>nH</em>{2n+2}$ (acyclic).

• Alkenes (unsaturated): ≥1 C=C.

• Alkynes (unsaturated): ≥1 C≡C.

• Aromatics: conjugated planar rings (e.g., benzene).

Saturated vs. Unsaturated Illustration

• Saturated: no multiple bonds.

• Unsaturated: contain C=C or C≡C.

Alkanes

• Non-polar; interactions = London dispersion.

• ↑C → ↑dispersion → ↑bp.

• Low density (< water), immiscible with water.

• Low reactivity; key reaction: combustion (highly exothermic).

• Isomer effects: branched n-hexane vs. 2-methylpentane (lower bp).

Alkenes, Alkynes, Aromatics

• Physical properties resemble alkanes (dispersion only).

• Nomenclature: longest chain containing all multiple bonds; locate first unsaturation; endings –ene/–yne.

Cycloalkanes

• Rings $\ge 3$ C; prefix cyclo-.

• Restricted rotation → cis/trans geometric isomers.

• Hydrophobic; smaller rings may be gases/liquids, larger solids.

• Higher bp than acyclic analogues (tighter packing).

• Uses: cyclohexane common lab solvent; industrial syntheses.

Oxygen-Containing Functional Groups

Alcohols (R–OH)

• Classification: 1° (RCH_2OH), 2° (R_2CHOH), 3° (R_3COH).

• Solubility: strong H-bonding; small alcohols miscible; solubility ↓ with chain length.

• Boiling/Melting: higher than hydrocarbons due to H-bonds; 1° > 2° > 3° for bp.

• Flammability: flammable; ↑branching ↑flammability.

• Acidity: weak; react with active metals → alkoxides; acidity ↓ with electron-donating groups.

• Important examples & uses

  • Methanol (wood alcohol) → formaldehyde, MTBE.

  • Ethanol: beverage, solvent, gasohol; denatured forms.

  • Isopropanol: rubbing alcohol.

  • Ethylene glycol (diol): antifreeze, polyesters.

  • Glycerol (triol): sweetener, moisturizer.

• Reactions

  • Dehydration (acid) → alkenes.
    $RCH<em>2CH</em>2OH \xrightarrow{H^+,\;\Delta} RCH=CH<em>2 + H</em>2O$

  • Oxidation

  • 1° $\rightarrow$ aldehyde $\rightarrow$ carboxylic acid.

  • 2° $\rightarrow$ ketone.

  • 3°: generally resistant.

  • Esterification with acids: $RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O$.

Phenols (Ar–OH)

• OH directly on aromatic ring.

• More acidic than alcohols; historical antiseptic use (Lister).

• Medical use limited (<1.5 % solutions) due to toxicity.

Ethers (R–O–R')

• sp³ O, $C–O–C$ angle ≈ $109.5^{\circ}$; net dipole.

• Physical: bp < alcohols, ≈ alkanes; soluble in water (H-bond acceptor); volatility ↑ with smaller R groups.

• Classification: symmetrical vs. asymmetrical.

• Common solvents (e.g., diethyl ether).

Esters (RCOOR')

• Nomenclature: alcohol part → “yl”, acid part → “oate” (e.g., ethyl acetate).

• Formation: Fischer esterification (acid-catalyzed) or acyl chloride/anhydride routes.

• Hydrolysis: acid catalyzed reverse; base (saponification) gives carboxylate.

• Physical: colorless, volatile, flammable; water-soluble decreases with size.

• Flavors & fragrances: isoamyl acetate (banana), ethyl butyrate (pineapple), methyl salicylate (wintergreen), etc.; biofuels.

Carbonyl Compounds

Aldehydes (R–CHO)

• Terminal $C=O$; suffix –al.

• Physical: low-MW often gases (formaldehyde). Short chains pungent; longer pleasant.

• bp < alcohols (no H-bond donor) but > alkanes.

• Soluble; flammable.

• Reactivity: easily oxidized to acids; reduced to 1° alcohols; undergo aldol condensations.

Ketones (R_2C=O)

• Internal $C=O$; suffix –one.

• Physical: distinctive odors (acetone sweet); bp between alkanes & alcohols; water-soluble.

• More resistant to oxidation than aldehydes; reduce to 2° alcohols; participate in aldol.

Carboxylic Acids (RCOOH)

• Nomenclature: drop –e, add –oic acid; carbonyl carbon = C-1.

• Acidity

  • Ka quantified via $pK<em>a = -\log K</em>a$; smaller $pK_a$ → stronger acid.

  • $pK_a$ values: methanoic 3.75 < ethanoic 4.76 < propanoic 4.87 < butanoic 4.82 < benzoic 4.20.

  • Stronger than alcohols due to:

  1. Electron-withdrawing carbonyl weakens O–H.

  2. Conjugate base (carboxylate) resonance-stabilized.

• Substituent effects (inductive)

  • Electron-withdrawing (halogens, NO_2) ↑ acidity; more groups or closer proximity ↑ effect.

  • Order example: $CH<em>3COOH < CH</em>2ClCOOH < CHCl<em>2COOH < CCl</em>3COOH$.

• Physical: often pungent; ↑bp via dimeric H-bonding; water-soluble (small R).

• Reactions

  • Metals → salt + $H_2$.

  • Bases (NaOH) → salts.

  • Carbonates/bicarbonates → salt + $CO_2$ (diagnostic fizz).

  • Reduction (LiAlH_4) → 1° alcohols.

  • Esterification, decarboxylation, oxidative cleavage.

Derivatives

Acyl (Acid) Chlorides (RCOCl)

• Naming: replace “–ic acid” with “–yl chloride”.

• Prepared with $SOCl<em>2, PCl</em>3, PCl_5$.

• Most reactive; sensitive to moisture.

• React with:

  • $H_2O$ → acid + $HCl$.

  • Alcohols → esters.

  • $NH<em>3, RNH</em>2, R_2NH$ → amides.

  • $R'COOH$ → anhydrides.

Acid Anhydrides (RCO)_2O

• Name: replace “acid” with “anhydride”.

• Hydrolysis → 2 acids.

• Alcoholysis (acid catalyzed) → ester + acid.

Amides (RCONH_2, RCONHR', RCONR'R'')

• Naming: replace “–oic acid” with “amide”; N-substituents prefixed N-.

• Formed from acyl chlorides + ammonia/amines or dehydration of ammonium salts.

• Hydrolysis

  • Acidic → acid + $NH_4^+$.

  • Basic → carboxylate + amine.

• Dehydration (P_4O_{10}) → nitriles.

• Stable; weak bases; H-bond donors & acceptors.

Nitrogen Functional Groups

Amines (RNH_2, R_2NH, R_3N)

• Derived from $NH_3$; N is sp³, pyramidal, lone pair basic/nucleophilic.

• Classification: 1°, 2°, 3°.

• Properties: fishy odors (low-MW), H-bonding (1° > 2°), soluble if small.

Imines (C=N), Nitro (RNO_2), Nitriles (RC≡N), Azo (R–N=N–R'), Azides (RN_3) – acknowledged for completeness.

Sulfur Functional Groups

Thiols (R–SH)

• Analogues of alcohols; named “–thiol”.

• Oxidation (mild) → disulfides (R–S–S–R).

• Biological example: cysteine ↔ cystine in hair keratin – disulfide cross-links.

Thioethers (R–S–R'), Disulfides (R–S–S–R), Sulfoxides/Sulfones (not detailed in slides).

Halogenated Organic Compounds

• Alkyl halides (R–X) & acyl halides discussed.

• Uses: anesthetics, solvents, propellants, fire suppressants, pesticides.

• Environmental concerns: CFC-induced ozone depletion (Cl radicals cycle).

• Biological: thyroxine (iodine-containing); iodine deficiency → goiter, mitigated with $KI$ salt.

Fundamental Reaction Types at Carbon

1. Oxidation–Reduction

   • Oxidation: ↑C–O or ↓C–H (e.g., $CH<em>4 \rightarrow CO</em>2$).

   • Reduction: ↑C–H or ↓C–O (e.g., $CO<em>2 \rightarrow CH</em>3OH$).

2. Substitution: atom/group exchange (e.g., $CH<em>4 + Br</em>2 \rightarrow CH_3Br + HBr$).

3. Addition: unsaturated + reagent → single product (e.g., $CH<em>2=CH</em>2 + HBr \rightarrow CH<em>3CH</em>2Br$).

4. Elimination: single reactant → unsaturated product + small molecule (reverse of addition).

Representative Equations (LaTeX format)

• Alkane combustion: $C<em>nH</em>{2n+2} + \frac{3n+1}{2}O<em>2 \rightarrow nCO</em>2 + (n+1)H_2O$.

• Fischer esterification: $RCOOH + R'OH \xrightleftharpoons[\;H_2O\;]{H^+} RCOOR'$.

• Aldehyde oxidation: $RCHO + [O] \rightarrow RCOOH$.

• Ketone reduction: $R<em>2C=O + 2[H] \xrightarrow{NaBH</em>4} R_2CHOH$.

• Carboxylate formation: $RCOOH + NaOH \rightarrow RCOO^-Na^+ + H_2O$.

• Acyl chloride hydrolysis: $RCOCl + H_2O \rightarrow RCOOH + HCl$.

Real-World & Ethical Contexts

• Volatile organics (VOCs) impact air quality & health.

• Halogenated solvents indispensable yet ecologically regulated.

• Bioactive esters and amides underpin flavors, fragrances, and drugs.

• Combustion of hydrocarbons → climate change concerns.

• Synthesis strategies weigh reactivity vs. safety (e.g., handling acyl chlorides, LiAlH_4).

Study Tips & Connections

• Relate functional-group polarity to physical properties.

• Practice IUPAC naming systematically: identify parent, functional priority, locants.

• Map oxidation levels (alkane < alcohol < aldehyde/ketone < acid).

• Use reaction-type framework (add, elim, sub, red/ox) to predict products.

• Compare acidities via resonance, inductive and hybridization effects.

• Visualize stereochemistry with models; cis/trans in cycloalkanes and alkenes.

• Cross-reference with biochemistry: amino acids (amines + carboxylic acids), triglycerides (esters), nucleic acids (phosphate esters), etc.