Properties & Physical Behaviour of Organic Compounds

Hydrocarbons (Alkanes, Alkenes, Alkynes, Cycloalkanes)

• Overall description
  • Covalent, non-polar molecules present as solids, liquids, or gases.
  • Insoluble or only sparingly soluble in water; density lower than water.
  • Readily soluble in non-polar solvents, especially organic (e.g. benzene, toluene).
  • Do not conduct electricity in any physical state.
  • Combust easily → exothermic.
    • Complete combustion: $\text{CO}<em>2$ + $\text{H}</em>2\text{O}$ (no soot).
    • Incomplete combustion: soot formation.
  • Reactivity trends
    • Saturated (alkanes): undergo substitution.
    • Unsaturated (alkenes, alkynes): undergo addition.
• Intermolecular force governing physical properties = London (dispersion) force only.
  • Magnitude ↑ with molar mass → members of a homologous series show ↑ boiling/melting point with added C.

Tabulated data – straight-chain alkanes (representative)

Alkanes vs. Cycloalkanes (C₃–C₈)

• $T<em>b$ and $T</em>m$ for cycloalkanes are systematically higher than the straight-chain counterparts because of stronger surface contact (↑ London force) and restricted rotation.
• Graphs supplied: linear upward trend with carbon number; parallel but offset lines for each class.

Alkenes vs. Alkynes (straight chain)

• Comparable molar masses: alkynes boil a few degrees higher than the corresponding alkenes (e.g. $\text{but−1−yne}$ 8.1 °C vs. $\text{but−1−ene}$ −6.3 °C) due to slightly greater π-electron polarizability.

Structural influence on boiling point (isomerism)

• For a given formula, more branching → lower $T_b$ (surface area ↓ ⇒ London force ↓).
  • $C<em>6H</em>{14}$ isomers:
    • $n$-hexane 69 °C > 2-methylpentane 63 °C > 3-methylpentane 59 °C > 2,2-dimethylbutane 50 °C.
• Cyclization raises $T_b$ dramatically (heptane 98 °C vs. methyl-substituted hexanes 79–92 °C).
• Practice question (formula $C<em>5H</em>{12}$): three isomers; predicted order $\text{n}$-pentane > isopentane > neopentane.

Aromatic hydrocarbons

• Exhibit conjugated cyclic $\pi$-systems → exceptional stability.
• Physical data
  • Benzene $C<em>6H</em>6$: $T<em>m$ 5.5 °C, $T</em>b$ 80.1 °C.
  • Naphthalene $C<em>{10}H</em>8$: $T<em>m$ 80.3 °C, $T</em>b$ 217.9 °C.
  • Anthracene & phenanthrene $C<em>{14}H</em>{10}$: $T<em>m$ 216/99 °C, $T</em>b$ 340 °C.
• Common uses (moth balls, pharmaceuticals, etc.).

Fundamental intermolecular forces (recap)

• London (dispersion) → all molecules; dominant in non-polar systems.
• Dipole–dipole → polar molecules (aldehydes, ketones, esters, etc.).
• Hydrogen bonding (H-bond) → molecules containing $\text{N–H}$, $\text{O–H}$, or $\text{F–H}$ capable of interacting with lone pairs.

Oxygen-containing organic compounds

Alcohols ($R{-}OH$)

• Comprise polar $\text{–OH}$ + non-polar hydrocarbon tail.
• Polarity decreases as carbon chain increases → water solubility drops (propan-1-ol miscible, butan-1-ol partially, pentan-1-ol poorly soluble).
• Form extensive hydrogen bonds with water and with themselves → high $T<em>b$ relative to molar mass (compare $\text{CH}</em>3OH$ 64.5 °C vs. ethane −88.6 °C).
• Trends: $T<em>b$ & $T</em>m$ ↑ with molar mass; branching lowers $T_b$ (butan-1-ol 117–118 °C > butan-2-ol 98–100 °C > 2-methyl-2-propanol 82–83 °C).
• Chemical notes: weakly acidic, react with $2\,\text{Na}$ to liberate $\text{H}_2$.

Ethers ($R{-}O{-}R'$)

• Have the same formula $C<em>nH</em>{2n+2}O$ as alcohol isomers; moderately polar but cannot hydrogen-bond to each other → low $T_b$ close to alkanes.
• Slight water solubility (hydrogen-bond acceptor only).
• Highly flammable; historical anesthetics (diethyl ether). Polymer example: polyethylene glycol (PEG).

Aldehydes ($R{-}CHO$)

• Contain a carbonyl group at chain end → polar, sharp odors (formalin, cinnamaldehyde).
• Small aldehydes miscible with water (H-bond acceptor). $T_b$ higher than alkanes but below alcohols (propanal 48 °C vs. butane −0.5 °C vs. propan-1-ol 97 °C).

Ketones ($R{-}CO{-}R'$)

• Internal carbonyl; less reactive than aldehydes but similar polarity.
• Small ketones water-miscible (acetone $T<em>b$ 56 °C). $T</em>b$ order for similar molar mass: alkane < aldehyde ≈ ketone < alcohol.
• Experimental comparison: propan-2-one 56 °C vs. propan-2-ol 82.5 °C.

Carboxylic acids ($R{-}COOH$)

• Weak Bronsted acids (partial ionization $RCOOH \rightleftharpoons RCOO^- + H^+$).
• Strongly polar; dimerize via two hydrogen bonds → very high $T_b$ (acetic acid 118 °C > ethanol 78 °C).
• Solubility: C₁–C₄ acids highly miscible; beyond C₅ solubility drops (hexanoic acid 1.1 g/100 g H₂O).

Esters ($R{-}COOR'$)

• Pleasant fragrances (pineapple, oil of wintergreen, nail-polish removers).
• Polar but cannot hydrogen-bond to themselves → $T_b$ lower than isomeric acids and alcohols, higher than alkanes.
• Small esters slightly soluble in water; solubility falls rapidly with chain length.

Nitrogen-containing organic compounds

Amines ($R{-}NH<em>2$, $R</em>2NH$, $R_3N$)

• Basic due to lone pair on N; react with water to form $RNH_3^+ + OH^-$ and with acids to give ammonium salts.
• Low C-count (≤3) = gases; higher = liquids/solids.
• Hydrogen-bond donor/acceptor → $T_b$ higher than alkanes but lower than alcohols (ethanamine 16.5 °C vs. propane −42 °C vs. ethanol 78 °C).
• Solubility decreases with chain length.

Amides ($R{-}CONH<em>2$, $R{-}CONHR'$, $R{-}CONR'</em>2$)

• Strongly polar; capable of extensive hydrogen bonding.
• High $T_b$ (always higher than corresponding acids, esters, or amines of same molar mass).
• Water solubility good for small amides; decreases with chain length.
• Neutral in aqueous solution because electron withdrawal by carbonyl lowers basicity of nitrogen.
• Important in biology (peptide bond) and materials (nylon-6,6).

Combined trends & comparative hierarchy of boiling points (≈ same molar mass)

\text{Alkane} < \text{Ether} < \text{Aldehyde} \approx \text{Ketone} < \text{Amine (1°,2°)} < \text{Alcohol} < \text{Carboxylic acid} < \text{Amide}

Experimental / numerical highlights

• Solubility lab (alcohols): propan-1-ol miscible; butan-1-ol ~7 drops miscible; pentan-1-ol only 2 drops (incipient phase separation).
• Example acid data at $20\,^{\circ}!\text{C}$: acetic, propionic, butyric fully miscible; pentanoic 2.4 g/100 g water; hexanoic 1.1 g/100 g.
• Amines: butan-1-amine $T_b$ 77 °C; pentan-1-amine 104 °C; hexan-1-amine 132 °C.
• Amides: propanamide 213 °C; butanamide 216 °C; pentanamide 225 °C.

Practice problem (isomer $C<em>5H</em>{10}$, three structures)

• Drawings: 1-pentene, 2-methyl-1-butene, 2-pentene (trans/cis counted as same constitutional isomer).
• Order $T_b$ high → low: most linear > monosubstituted (methyl branch) > more branching/cis isomer.

Key take-home principles

• London force strength ∝ molar mass & contact surface; branching reduces contact.
• Ability to hydrogen-bond (donor/acceptor count) overwhelmingly raises boiling point & water solubility.
• Presence of permanent dipole raises $T_b$ relative to non-polar analogs but less than H-bonding.
• Functional group ranking for polarity/H-bond capability underlies nearly every trend in solubility, $T_b$, and chemical reactivity across organic families.