Introduction to Selected Homologous Series and Organic Chemistry Fundamentals

Overview and General Principles of Organic Chemistry

• Importance of Organic Chemistry: Organic chemistry is a distinct and crucial branch of chemistry, defined by the uniqueness of carbon and the vast array of carbon compounds it forms.
• Conceptual Foundation: Building upon junior secondary knowledge and specific curriculum topics (V and VI), these notes cover structural characteristics, systematic (IUPAC) naming, and common trivial names of carbon compounds used in academic and daily contexts.
• Scope of Study: Students are expected to understand:
  • Isomerism: Including structural isomerism, cis-trans (geometric) isomerism, and enantiomerism (optical isomerism).
  • Functional Groups: Systematic naming for alkanes, alkenes, haloalkanes, alcohols, aldehydes, ketones, carboxylic acids, esters, unsubstituted amides, and primary amines containing up to eight carbon atoms in their chains.
  • Chemical Identification: Using reagents, reaction conditions, and observations (such as Markovnikov’s rule for alkene reactions with hydrogen halides) to distinguish functional groups and identify unknown compounds.
  • Synthesis and Application: The inter-conversion between functional groups to synthesize substances like aspirin, detergents, nylon, and polyesters.
• Polymers and Detergents:
  • Nylon and Polyesters: Identified as condensation polymers; students must recognize their structures and write formation equations.
  • Detergents: Characterized by hydrophobic and hydrophilic parts that enable emulsifying and wetting properties. Understanding the cleansing action of soaps vs. soapless detergents based on structural differences is required.

Homologous Series and Functional Groups

• Definition of Homologous Series: A family of compounds that share:
  1. The same general formula.
  2. The same functional group, resulting in similar chemical properties.
  3. Physical properties (e.g., boiling point) that exhibit a gradual change as the series progresses.
  4. Adjacent members that differ by a single $-\text{CH}_2-$ unit.
• Definition of Functional Group: An atom or a specific group of atoms within a molecule that is responsible for the majority of the compound's chemical reactions and properties.
• Table of Common Functional Groups and Series:
  • Alcohols: Contains the hydroxyl group ($\text{-OH}$). General formula: $R\text{-OH}$. Example: Ethanol ($CH_3CH_2OH$).
  • Aldehydes: Contains a terminal carbonyl group ($-CHO$). General formula: $(H \text{ or } R)CHO$. The carbon in the CHO group is always counted as position 1. Example: Ethanal ($CH_3CHO$).
  • Ketones: Contains a non-terminal carbonyl group ($-CO-$). General formula: $R\text{-CO-}R'$. The carbonyl carbon must not be the first or last carbon in the chain. Example: Propanone ($CH_3COCH_3$).
  • Carboxylic Acids: Contains the carboxyl group ($-COOH$). General formula: $(H \text{ or } R)COOH$. The carbon in COOH is strictly counted as carbon number 1. Example: Ethanoic acid ($CH_3COOH$).
  • Esters: Contains the ester group ($-COO-$). General formula: $(H \text{ or } R)COOR_1$. Formed from an acid and an alcohol. Example: Methyl ethanoate ($CH_3COOCH_3$).
  • Amides (Unsubstituted): Contains the amide group ($-CONH_2$). General formula: $(H \text{ or } R)CONH_2$. Example: Ethanamide ($CH_3CONH_2$).
  • Amines (Primary): Contains the amine group ($-NH_2$). General formula: $R\text{-}NH_2$. Example: Ethanamine ($CH_3CH_2NH_2$).

Systematic Naming of Organic Compounds

• Classification of Hydrocarbons:
  • Aliphatic: Can be acyclic (open chain) or cyclic.
  • Aromatic: Includes benzene and compounds exhibiting chemical behavior similar to benzene.
• Naming Conventions:
  • Carboxylic Acids: Suffix "-oic acid". If two carboxyl groups are present, it is a dibasic acid with the suffix "-dioic acid".
  • Alcohols: Suffix "-ol". When the hydroxyl group is directly attached to a benzene ring, the compound is called a phenol.
  • Aldehydes: Replace the ending "-e" of the alkane with "-al". For aromatic aldehydes where CHO is on a benzene ring, the name is benzaldehyde.
  • Ketones: Replace the ending "-e" of the alkane with "-one".
  • Esters: Named as two words. The first word is the alkyl group from the alcohol (suffix "-yl"); the second word is the carboxylate from the acid (suffix "-oate").
  • Amides: Replace "-oic acid" with "-amide".
  • Amines: Replace the ending "-e" of the alkane with "-amine".
• Trivial Names and Uses:
  • Methanal: Formaldehyde. Used as a preservative for biological specimens, disinfectant, and in polymer production.
  • Trichloromethane: Chloroform. Used as an anaesthetic and solvent.
  • Propanone: Acetone. Used as a solvent.
  • Propan-2-ol: Isopropyl alcohol. Used as a disinfectant, fuel additive, and solvent.
  • Ethanoic acid: Acetic acid. Used in vinegar, making esters, and polymer production.

Physical Properties of Organic Compounds

• General Principles:
  • Solubility: Depends on the balance of attraction between solute-solute/solvent-solvent particles versus solute-solvent particles. Soluble if solute-solvent attraction is strong enough to overcome internal attractions.
  • Melting/Boiling Points: Influenced by molecular size (affecting van der Waals’ forces) and the presence of specific intermolecular forces (e.g., hydrogen bonding).
  • Density: Related to the efficiency of molecular packing.
• Alkanes and Alkenes:
  • Solubility: Non-polar; insoluble in water but soluble in non-polar solvents like $CCl_4$. This is because the energy released forming hexane-water van der Waals' forces is less than the energy needed to break water's hydrogen bonds.
  • Density: Liquid alkanes are less dense than water because they pack less efficiently and have weaker intermolecular forces than water's hydrogen bonds.
  • Boiling Points: Increases with molecular size due to stronger van der Waals’ forces. Branched-chain alkanes have lower boiling points than straight-chain isomers because they have a smaller surface area, resulting in weaker intermolecular forces.
• Haloalkanes:
  • Boiling Point Trends: BP increases with chain length and as the halogen group is descended ($Cl < Br < I$) because larger atoms/molecules have stronger van der Waals' forces.
  • Comparison: Haloalkane BPs are higher than alkanes of similar mass because the carbon-halogen bond is polar (due to electronegativity differences), leading to stronger van der Waals’ forces.
  • Solubility: Generally insoluble in water. Despite the polar C-X bond, the energy released forming haloalkane-water attractions cannot overcome the strong hydrogen bonds between water molecules.
• Alcohols:
  • H-Bonding: Alcohols have much higher BPs than alkanes of similar mass due to strong intermolecular hydrogen bonding.
  • Solubility: Methanol, ethanol, and propan-1-ol are miscible with water in all proportions (small alkyl groups). As the hydrocarbon chain length increases, the hydrophobic alkyl group predominates, and solubility decreases.
  • Density: Most are less dense than water. However, diols (e.g., ethane-1,2-diol, density $1.109 \, g \, cm^{-3}$) and triols (e.g., propane-1,2,3-triol, density $1.216 \, g \, cm^{-3}$) are denser than water.
• Aldehydes and Ketones:
  • Intermolecular Forces: Higher BPs than alkanes (polar carbonyl group) but lower than alcohols/carboxylic acids (cannot form H-bonds with themselves).
  • Solubility: Only lower members (e.g., ethanal, propanone) are water-soluble via H-bonding with water.
• Carboxylic Acids:
  • Boiling Points: Highest among previously mentioned groups due to extensive H-bonding and the formation of stable dimers (two molecules joined by two H-bonds), which effectively increases molecular size and van der Waals' forces.
  • Solubility: First four members are miscible with water. Solubility decreases as chain length increases.
• Esters, Amides, and Amines:
  • Esters: Lack H-atoms bonded to Oxygen, so they cannot H-bond with each other. BPs are lower than equivalent acids/alcohols but similar to aldehydes/ketones. Lower members are soluble in water.
  • Amides: Have very high BPs due to extensive H-bonding ($-NH_2$ to $C=O$). Lower members like methanamide are highly water-soluble.
  • Amines: Primary amines form H-bonds with each other, but these are weaker than in alcohols because Nitrogen is less electronegative than Oxygen. Lower members ($C_1-C_4$) are very soluble in water.

Questions & Discussion

• Calculation of Molecular Formula (Compound W):
  • Data: Relative molecular mass ($M_r$) of $W = 88.0$. Combustion of $1.32 \, g$ of W produces $2.64 \, g \, CO_2$ and $1.08 \, g \, H_2O$.
  • Moles of C: $\frac{2.64 \, g}{44.0 \, g \, mol^{-1}} = 0.06 \, mol$. Mass of C = $0.06 \times 12.0 = 0.72 \, g$.
  • Moles of H: $2 \times \frac{1.08 \, g}{18.0 \, g \, mol^{-1}} = 0.12 \, mol$. Mass of H = $0.12 \times 1.0 = 0.12 \, g$.
  • Mass of O: Total mass $1.32 \, g - (0.72 \, g + 0.12 \, g) = 0.48 \, g$. Moles of O = $\frac{0.48 \, g}{16.0 \, g \, mol^{-1}} = 0.03 \, mol$.
  • Empirical Formula: Ratio $C:H:O = 0.06 : 0.12 : 0.03 = 2 : 4 : 1$. Empirical formula is $C_2H_4O$.
  • Molecular Formula: $n \times (12.0 \times 2 + 1.0 \times 4 + 16.0) = 88.0 \implies 44n = 88 \implies n = 2$. Molecular formula is $C_4H_8O_2$.
• Structure Identification (Compound X):
  • Structure: $CH_2=CHCH_2OH$.
  • Naming Analysis: The hydroxyl group ($-OH$) takes priority over the double bond. The carbon chain is numbered starting from the carbon attached to the hydroxyl group. Thus, the correct systematic name is prop-2-en-1-ol.
• Methanol Properties:
  • Solubility: Miscible with water.
  • Reactivity/Observation: Neutral to litmus; burns with a non-luminous (blue) flame.
• Ethane-1,2-diol:
  • Solubility: Highly soluble in water because it is a small molecule with two hydroxyl groups capable of extensive hydrogen bonding with water molecules.