Comprehensive Study Notes on Pre-Medical and Pre-Pharmacy Organic Chemistry

Origin and Definition of Organic Chemistry

• Conceptual Learning Objectives: By the conclusion of this curriculum, students should be conversant with the chemistry of compounds integral to medical and pharmacy disciplines.
• Historical Origins:
  • Inorganic vs. Organic: Traditionally, a sharp distinction was made between these two classes of compounds.
  • Vital Force Theory: It was originally believed that a "vital force" was required to create organic substances, which could only be produced by living organisms.
  • Friederich Wöhler (1828): The German chemist Friederich Wöhler synthesized Urea, an organic compound, from an inorganic salt, ammonium cyanate ($NH_4CNO$), via simple heating, thereby challenging the Vital Force Theory.
• Modern Definition: Organic chemistry is the study of carbon-containing compounds, with the notable exception of simple compounds such as:
  • Carbonates ($CO_3^{2-}$)
  • Hydrogen carbonates ($HCO_3^{-}$)
  • Carbon dioxide ($CO_2$)
  • Carbon monoxide ($CO$)
• Statistical Context: There are currently over 6 million known organic compounds.

Uniqueness of the Carbon Atom

• Octet Stability: Carbon achieves a fully shared octet of electrons in its compounds.
• Bonding Versatility: Carbon can form strong single, double, and triple bonds to itself.
• Catenation: This is the unique ability of carbon to form long chains or rings by bonding with other carbon atoms. Other elements may exhibit catenation, but not to the extent of carbon.
• Tetravalency: Carbon forms four covalent bonds.
• Key Heteroatoms: Carbon frequently bonds with Hydrogen ($H$), Nitrogen ($N$), Oxygen ($O$), Phosphorus ($P$), and Sulphur ($S$).
• Importance and Applications:
  • Living Systems: Foundational to biochemistry, pharmacy, and medicine.
  • Products: Present in synthetic fibers, plastics, artificial sweeteners, and drugs.
  • Energy: The primary constituent of coal and petroleum.

Analysis of Organic Compounds: Qualitative Analysis

• Definition: Qualitative analysis involves the detection of various elements present in an organic compound.
• Detection of Carbon ($C$) and Hydrogen ($H$):
  • The sample is heated strongly with cupric oxide ($CuO$).
  • Carbon is oxidized to carbon dioxide: $C + 2CuO \rightarrow 2Cu + CO_2$.
  • Hydrogen is oxidized to water: $2H + CuO \rightarrow Cu + H_2O$.
  • Test for $CO_2$: The gas is passed through lime water; a milky appearance confirms its presence.
  • Test for water: The presence of water is confirmed via an anhydrous copper sulphate test (turns from white to blue).
• Detection of Nitrogen ($N$), Sulphur ($S$), and Halogens ($X$):
  • Lassaigne’s Test: Named after French chemist J.L. Lassaigne. Elements are converted into their ionic forms by fusing the organic compound with sodium metal ($Na$).
  • Sodium Extract (Lassaigne's extract): The fused mass is extracted in aqueous solution.
• Detection Procedures:
  • Nitrogen ($N$): Extracts sodium cyanide ($NaCN$). Adding ferrous sulphate ($FeSO_4$) forms sodium ferrocyanide: $6NaCN + FeSO_4 \rightarrow Na_4[Fe(CN)_6] + Na_2SO_4$. Treatment with ferric chloride produces a "Prussian Blue" complex (ferric ferrocyanide): $3Na_4[Fe(CN)_6] + 4FeCl_3 \rightarrow Fe_4[Fe(CN)_6]_3 + 12NaCl$.
  • Sulphur ($S$): Extracts sodium sulphide ($Na_2S$) via $2Na + S \rightarrow Na_2S$.
    • Nitroprusside Test: Adding sodium nitroprusside results in a purple/violet color due to sodium thionitroprusside formation.
    • Lead Acetate Test: Another method for identifying sulphide ions.
  • Nitrogen and Sulphur Together: Sodium fusion produces sodium thiocyanate ($NaSCN$), which reacts with $Fe^{3+}$ to form a blood-red complex: $[Fe(SCN)]^{2+}$.
  • Halogens ($Cl, Br, I$): Forms sodium halide ($NaX$). The extract is acidified with dilute $HNO_3$ and treated with silver nitrate ($AgNO_3$).
    • Chlorine ($Cl$): White curdy precipitate ($AgCl$), soluble in ammonium hydroxide ($NH_4OH$).
    • Bromine ($Br$): Yellowish-white precipitate ($AgBr$), sparingly soluble in $NH_4OH$.
    • Iodine ($I$): Yellow precipitate ($AgI$), insoluble in $NH_4OH$.

Analysis of Organic Compounds: Quantitative Analysis

• Types:
  • Gravimetric Analysis: Determining the mass of a constituent isolated in elementary form or as a compound of definite composition.
  • Volumetric (Titrimetric) Analysis: Measuring the volume of a substance or the volume of a reagent required to combine with a substance in known proportions.
  • Colorimetric Analysis: Determining concentration with the aid of a color reagent; widely used in medical and industrial laboratories.
• Combustion Analysis (Carbon and Hydrogen):
  • Burning an unknown sample in excess oxygen ($O_2$).
  • Logic: All Carbon becomes $CO_2$ (ratio of C in $CO_2$ is $12/44$). All Hydrogen becomes $H_2O$ (ratio of H in $H_2O$ is $2/18$).
  • Oxygen content is usually determined by subtraction.
• Quantitative Halogen Estimation (Carius Method):
  • The compound is heated with fuming nitric acid and silver nitrate in a sealed hard glass tube (Carius tube).
  • Calculations: $\text{Percentage of Halogen} = \frac{\text{atomic mass of X} \times m_1}{\text{molecular mass of AgX} \times m} \times 100$, where $m$ is the mass of the organic compound and $m_1$ is the mass of $AgX$ formed.
• Quantitative Nitrogen Estimation:
  1. Dumas Method: The compound is heated with copper oxide in a $CO_2$ atmosphere. Free Nitrogen gas ($N_2$) is collected over potassium hydroxide ($KOH$).
     • Calculation of volume at STP: $V_{STP} = \frac{P_1 V_1 \times 273}{760 \times T_1}$, where $P_1 = \text{Atmospheric Pressure} - \text{Aqueous Tension}$.
     • $\text{Percentage of N} = \frac{28 \times V_{STP} \times 100}{22400 \times m}$.
  2. Kjeldahl’s Method:
     • Digestion: Heating the compound with concentrated $H_2SO_4$ to convert nitrogen to ammonium sulphate.
     • Distillation: Adding excess $NaOH$ to liberate ammonia ($NH_3$), which is absorbed into a standard acid solution.
     • Titration: Estimating unreacted acid by titrating with a standard alkali.
     • Exceptions: Not applicable to compounds with nitrogen in nitro or azo groups, or nitrogen within a ring (e.g., Pyridine).

Classification of Organic Compounds

• Acyclic or Open Chain Compounds: Also called Aliphatic compounds; consists of straight or branched chains.
• Cyclic or Closed Chain Compounds:
  • Alicyclic: Aliphatic cyclic compounds, either homocyclic (carbon-only rings) or heterocyclic (rings containing other atoms).
  • Aromatic Compounds: Special types including benzene and related rings.
    • Benzenoid: Contain benzene rings.
    • Non-benzenoid: Aromatic without benzene rings.
    • Heterocyclic Aromatic: Aromatic rings containing atoms like $S$, $O$, or $N$.

Homologous Series and Functional Groups

• Functional Group: An atom or group of atoms responsible for the characteristic chemical properties of a compound (e.g., $-OH$ hydroxyl, $-CHO$ aldehyde, $-COOH$ carboxylic acid).
• Homologous Series: A family of compounds with the same functional group and general molecular formula.
  • Successive Members: Differ by a $-CH_2$ unit.
  • Examples: Alkanes, alkenes, alkynes, alkanols, alkanoic acids, etc.

Nomenclature of Organic Compounds

• IUPAC System: Developed by the International Union of Pure and Applied Chemistry.
• Basic Rules:
  1. Identify the longest carbon chain (Parent chain): 1C=meth, 2C=eth, 3C=prop, 4C=but, 5C=pent, etc.
  2. Identify the primary functional group ($-ane, -ene, -yne$).
  3. Number the chain to give substituents the lowest possible numbers.
  4. Use hyphens to separate numbers from words (e.g., 2-methyl) and commas to separate numbers (e.g., 2,2-dimethyl).
  5. List substituents alphabetically.
• Trivial/Common Names: Historically based on origin (e.g., Formic acid from red ants - Latin formica; Citric acid from citrus fruits).

Isomerism

• Definition: Two or more compounds with the same molecular formula but different properties.
• Structural Isomerism:
  • Chain: Different carbon skeletons.
  • Position: Different position of functional groups or substituents.
  • Functional Group: Same formula but different functional groups.
  • Metamerism: Different alkyl chains on either side of a functional group.
• Stereoisomerism:
  • Geometric (Cis-Trans or E-Z): Occurs due to restricted rotation around C=C double bonds. Cis (Z) identifies groups on the same side; Trans (E) identifies groups on opposite sides.
  • Optical: Involves a chiral center (carbon attached to four different groups).
    • Enantiomers: Non-superimposable mirror images.
    • Dextro (d): Rotates plane-polarized light to the right.
    • Laevo (l): Rotates plane-polarized light to the left.
    • Racemic Mixture: Equal mixture of d- and l-enantiomers; does not rotate light.
    • Resolution: The process of separating a racemic mixture.

Hydrocarbons: Alkanes (Saturated)

• Definition: Contain only hydrogen and carbon with single C-C bonds. Historically called Paraffins (Latin: little affinity).
• General Formula: $C_nH_{2n+2}$.
• Hybridization: $sp^3$ hybridization results in a Tetrahedral geometry with bond angles of $109.5^o$.
• Physical Properties:
  • Nonpolar bonds; exhibit Van der Waals forces.
  • Boiling point increases with chain length (surface area) and decreases with branching.
  • States: $C_1-C_4$ (gases), $C_5-C_{17}$ (liquids), $C_{18}+$ (solids).
• Chemical Properties: Generally inert.
  • Combustion: Complete ($CO_2 + H_2O$) or Incomplete ($CO + H_2O$).
  • Cracking (Pyrolysis): Cleavage of long chains into smaller molecules using heat.
  • Substitution (Halogenation): Replacement of $H$ by halogens ($F_2 > Cl_2 > Br_2 > I_2$) in the presence of UV light.

Hydrocarbons: Alkenes (Unsaturated)

• Definition: Contain at least one C=C double bond. Also known as Olefins (oil-forming).
• Structure: Consists of one strong sigma ($\sigma$) bond (head-on overlap, enthalpy $397\,kJ/mol$) and one weak pi ($\pi$) bond (lateral overlap, enthalpy $284\,kJ/mol$). Bond length is $134\,pm$ (shorter than alkane single bonds at $154\,pm$).
• Hybridization: $sp^2$ hybridization resulting in a planar molecule with $120^o$ angles.
• Chemical Properties (Addition Reactions):
  • Hydrogenation: Adding $H_2$ with Pd, Pt, or Raney Ni catalyst.
  • Halogenation: Adding $Cl_2$ or $Br_2$ to form vicinal dihalides. Bromine water test (loss of reddish-orange color) used for unsaturation.
  • Hydrohalogenation ($HX$):
    • Markovnikov Rule: The halide attaches to the carbon with fewer hydrogens.
    • Anti-Markovnikov (Peroxide/Kharash Effect): In the presence of peroxides, $HBr$ adds in the opposite orientation.
  • Ozonolysis: Addition of $O_3$ to form an ozonide, followed by cleavage with $Zn/H_2O$ to determine double-bond position.
  • Polymerization: Small molecules (monomers like ethene) combine to form large molecules (polymers like polythene).