Comprehensive University Notes on Organic Chemistry: Alkanes, Alkenes, Alcohols, and Haloalkanes

Fundamental Principles of Organic Chemistry: Nomenclature, Formulas, and Bonding

• Organic Chemistry & Structural Definitions:

  • Homologous Series: A family of organic compounds that possess the same functional group, share similar chemical properties, and exhibit a gradual trend in physical properties. Each successive member of a homologous series differs from the previous one by a $-CH_2-$ repeating unit.

  • Functional Group: An atom or a specific group of atoms within an organic molecule that dictates its characteristic chemical reactivity and properties.

  • Saturated Hydrocarbon: A hydrocarbon containing only single covalent bonds between carbon atoms, holding the maximum possible number of hydrogen atoms per carbon (e.g., alkanes).

  • Unsaturated Hydrocarbon: A hydrocarbon that contains one or more carbon-carbon double ($C=C$) or triple ($C\equiv C$) bonds, allowing it to undergo addition reactions (e.g., alkenes, alkynes).

• Types of Chemical Formulae:

  • Empirical Formula: The simplest whole-number ratio of atoms of each element present in a compound (e.g., the empirical formula of hexane, $C_6 H_{14}$, is $C_3 H_7$; the empirical formula of 3-methylpentan-2,2-diol is $C_3 H_7 O$).

  • Molecular Formula: The actual number of atoms of each element present in a single molecule of a compound (e.g., $C_6 H_{14}$ for hexane).

  • General Formula: An algebraic formula representing the compositional ratio of elements for any member of an entire homologous series:

    • Alkanes: $C_n H_{2n+2}$

    • Alkenes: $C_n H_{2n}$

    • Monohydric Alcohols: $C_n H_{2n+1}OH$ or $C_n H_{2n+2}O$

    • Haloalkanes: $C_n H_{2n+1}X$

  • Structural / Condensed Formula: Shows the detailed arrangement of atoms in a molecule carbon-by-carbon, omitting individual bond lines (e.g., $CH_3 CH_2 CH_2 CH_2 CH_2 CH_3$ or $CH_3(CH_2)_4 CH_3$ for hexane).

  • Displayed Formula: Represents every individual atom and every covalent bond within the molecule as explicit lines.

  • Skeletal Formula: A simplified representation where carbon-bonded hydrogen atoms are omitted, carbon chains are drawn as zig-zag lines, line vertices and ends represent carbon atoms, and functional groups are explicitly shown.

  • Three-Dimensional (3D) Formula: Depicts the spatial spatial orientation of bonds using solid lines (in-plane), wedge bonds (projecting forward), and dashed bonds (projecting backward).

• IUPAC Priority of Functional Groups for Nomenclature:

  • When naming polyfunctional organic compounds, the principal functional group determines the suffix, while lower-priority groups are named as prefixes in alphabetical order:

    1. Carboxylic Acid: Prefix: carboxy-, Suffix: -oic acid (e.g., ethanoic acid)

    2. Sulfonic Acid: Prefix: sulfo-, Suffix: -sulfonic acid (e.g., benzenesulfonic acid)

    3. Ester: Prefix: alkoxycarbonyl-, Suffix: -oate (e.g., methyl ethanoate)

    4. Acid Halide: Prefix: halocarbonyl-, Suffix: -oyl halide (e.g., ethanoyl chloride)

    5. Amide: Prefix: carbamoyl- / aminocarbonyl-, Suffix: -amide (e.g., ethanamide)

    6. Nitrile: Prefix: cyano-, Suffix: -nitrile (e.g., ethanonitrile)

    7. Aldehyde: Prefix: formyl-, Suffix: -al / -carbaldehyde (e.g., ethanal)

    8. Ketone: Prefix: oxo-, Suffix: -one (e.g., 2-propanone)

    9. Alcohol: Prefix: hydroxy-, Suffix: -ol (e.g., methanol)

    10. Thiol: Prefix: mercapto-, Suffix: -thiol (e.g., methanethiol)

    11. Amine: Prefix: amino-, Suffix: -amine (e.g., methylamine)

    12. Alkene: Prefix: -en, Suffix: -ene (e.g., pent-1-ene)

    13. Alkyne: Prefix: -yn, Suffix: -yne (e.g., pent-2-yne)

    14. Alkane / Halocarbon / Nitro: Prefixes: alkyl-, halo- (chloro-, bromo-, iodo-), nitro-

• IUPAC Nomenclature Rules and Examples:

  • Select the longest continuous carbon chain containing the highest priority functional group as the base parent name.

  • Number the carbon chain from the end that gives the principal functional group the lowest locant number.

  • Prefix substituents alphabetically, using numerical locants separated by hyphens, and separate multiple numbers with commas (e.g., di-, tri-, tetra- prefixes are ignored in alphabetization).

  • Nomenclature Examples:
    
    

    • $CH_2=CH-CH_2-CH_2-CH_2OH$: named pent-4-en-1-ol (hydroxyl priority exceeds alkene).

    • $H_3C-C\equiv C-CH_2-NH_2$: named but-2-yn-1-amine.

    • $HC\equiv C-CH=CH-CH_3$: named pent-3-en-1-yne.

    • $H_2C=CH-CH_2-C\equiv CH$: named pent-1-en-4-yne.

    • $CH_3-CO-CH_2-CH_2-COOH$: named 4-oxo-pentanoic acid.

    • 1-bromo-4-chloro-3-nitroheptane**.\n * ethyl\,4-mercaptobutanoate**.

    • 5-chloropent-1-yne**.\n\n* **Isomerism:**\n * **Structural Isomerism:** Compounds possessing the identical molecular formula but different structural arrangements of atoms.\n * *Chain Isomerism:* Differences in the arrangement of the carbon skeleton (e.g., pentane, 2-methylbutane, and 2,2-dimethylpropane for C_5 H_{12}).\n * *Positional Isomerism:* Differences in the position of a functional group or substituent along the same carbon chain (e.g., 1-chlorobutane vs. 2-chlorobutane; propan-1-ol vs. propan-2-ol).\n * *Functional Group Isomerism:* Compounds sharing the same molecular formula but containing different functional groups (e.g., alcohols and ethers such as ethanol CH_3 CH_2 OH$and dimethyl ether$CH_3 O CH_3; aldehydes and ketones).\n * **Stereoisomerism:** Compounds with the same structural formula but different spatial arrangements of their atoms.\n * *Geometric (E/Z or Cis/Trans) Isomerism:* Occurs in alkenes due to restricted rotation around the C=C double bond when each double-bonded carbon atom is attached to two different groups.\n * *Optical Isomerism:* Occurs in molecules containing a chiral center (a tetrahedral carbon bonded to four distinct atoms or groups). Optical isomers (enantiomers) exist as non-superimposable mirror images that lack a plane of symmetry.\n\n* **Atomic Orbitals, Hybridization, and Bonding:**\n * **sp^3 Hybridization:**\n * The mixing of one 2s$orbital and three$2p$orbitals to produce four degenerate$sp^3 hybrid orbitals.\n * Arranged in a **tetrahedral** geometry with bond angles of 109.5^{\circ}.\n * Forms single covalent **\sigma (sigma) bonds** via end-to-end (head-on) orbital overlap along the internuclear axis.\n * Characteristic of saturated alkanes. The high bond enthalpies of C-C$($347\,kJ\,mol^{-1}$) and$C-H$($413\,kJ\,mol^{-1}) bonds along with low polarity make alkanes chemically unreactive.\n * **sp^2 Hybridization:**\n * The mixing of one 2s$orbital and two$2p$orbitals to form three degenerate$sp^2$hybrid orbitals, leaving one unhybridized$p orbital perpendicular to the hybrid plane.\n * Arranged in a **trigonal planar** geometry with bond angles of 120^{\circ}.\n * A C=C$double bond consists of one strong **$\sigma$bond** (head-on$sp^2-sp^2$overlap) and one weaker **$\pi$(pi) bond** (sideways overlap of unhybridized parallel$p orbitals above and below the molecular plane).\n * The electron density of the \pi bond is exposed above and below the plane, making alkenes attractive targets for electrophilic attack.\n\n\n# Alkanes: Radical Substitution, Industrial Processing, and Environmental Chemistry\n\n* **Free Radical Substitution (Photochemical Chlorination of Alkanes):**\n * **Homolytic Fission:** The breaking of a covalent bond where each bonded atom retains one electron from the shared pair, creating highly reactive neutral species with unpaired electrons called **free radicals** (represented by a single dot \cdot).\n * **Heterolytic Fission:** The breaking of a covalent bond where one atom retains both bonding electrons, forming a cation and an anion.\n * **Reaction Mechanism Stages (e.g., Chlorination of Methane or Heptane):**\n 1. *Initiation:* Ultraviolet (UV) light provides photon energy to rupture the non-polar halogen bond homolytically:\n            Cl_2 \xrightarrow{UV} 2Cl\cdot\n 2. *Propagation:* Chain reactions where radicals react with non-radicals to yield new radicals:\n            C_7 H_{16} + Cl\cdot \rightarrow \cdot C_7 H_{15} + HCl\n            \cdot C_7 H_{15} + Cl_2 \rightarrow C_7 H_{15} Cl + Cl\cdot\n 3. *Termination:* Reaction of two free radicals combining to form a stable covalent molecule, ending the chain reaction:\n            Cl\cdot + Cl\cdot \rightarrow Cl_2\n            \cdot C_7 H_{15} + Cl\cdot \rightarrow C_7 H_{15} Cl\n            \cdot C_7 H_{15} + \cdot C_7 H_{15} \rightarrow C_{14} H_{30} (forms dimeric organic by-products containing 14 carbon atoms).\n * *Synthetic Limitations:* Free radical substitution leads to multi-substituted products (e.g., CH_3 Cl, CH_2 Cl_2, CHCl_3, CCl_4) and structural isomers across different carbon positions, producing low yields of specific target halogenoalkanes unless a massive excess of alkane is utilized.\n\n* **Crude Oil Refining and Fractional Distillation:**\n * Crude oil is a complex mixture of unbranched and branched alkanes, cycloalkanes, and aromatic hydrocarbons.\n * **Fractional Distillation (Primary Processing):** Physical separation process based on differences in boiling points.\n * Crude oil is vaporized at approximately 350^{\circ}\text{C} and introduced at the bottom of a fractionating column with a continuous temperature gradient (hotter at the bottom, cooler at the top).\n * Vapors rise; fractions condense at specific height levels where column temperature drops below their boiling points.\n * **Fractional Distillation Products & Boiling Point Trends:**\n 1. *Refinery Gas:* Boiling point < 25^{\circ}\text{C}$, average chain length$C_3 (used for bottled gas, heating).\n 2. *Naphtha:* Boiling point 25 - 60^{\circ}\text{C}$/$60 - 180^{\circ}\text{C}$, average chain length$C_8 - C_{10} (petrochemical feedstock).\n 3. *Petrol / Gasoline:* Boiling point 60 - 180^{\circ}\text{C}$, average chain length$C_8 (motor fuel).\n 4. *Kerosene / Paraffin:* Boiling point 180 - 220^{\circ}\text{C}$, average chain length$C_{12} (jet fuel, domestic heating).\n 5. *Diesel / Gas Oil:* Boiling point 220 - 250^{\circ}\text{C}$/$250 - 300^{\circ}\text{C}$, average chain length$C_{20} (diesel engines, heating).\n 6. *Fuel Oil:* Boiling point 300 - 350^{\circ}\text{C}$, average chain length$C_{40} (ship boilers, power stations).\n 7. *Lubricating Oil, Wax, Bitumen:* Boiling point > 350^{\circ}\text{C}$, average chain length$C_{80} - C_{120} (road surfacing, roofing, lubrication).\n * *Trends down the column (from top to bottom):* Carbon chain length increases, relative molecular mass increases, intermolecular Van der Waals forces increase, boiling points increase, viscosity increases, volatility decreases, and flammability decreases.\n\n* **Cracking and Reforming Processes:**\n * **Economic Need:** Fractional distillation yields an excess of low-demand, long-chain hydrocarbons and insufficient amounts of short-chain fractions (petrol) and alkenes needed for polymer synthesis.\n * **Thermal Cracking:**\n * *Conditions:* High temperatures (750 - 900^{\circ}\text{C}$) and high pressures (up to$70\,atm).\n * *Mechanism:* Free radical mechanism involving carbon-carbon bond cleavage.\n * *Products:* High yield of alkenes (e.g., ethene) and short-chain unbranched alkanes.\n * **Catalytic Cracking:**\n * *Conditions:* Moderate temperatures (450 - 500^{\circ}\text{C}$), slight pressure, in the presence of a **zeolite catalyst** (aluminosilicate,$SiO_2:Al_2O_3).\n * *Mechanism:* Carbocation (ionic) mechanism.\n * *Products:* High yield of branched alkanes, cycloalkanes, and aromatic hydrocarbons (arenes) for high-octane motor fuels.\n * **Reforming:** Chemical rearrangement of straight-chain alkanes into branched or cyclic hydrocarbons and aromatic compounds (e.g., benzene) with the elimination of hydrogen gas (H_2):\n        C_6 H_{14} \rightarrow C_6 H_{12} (\text{hex-1-ene}) + H_2\n        C_6 H_{14} \rightarrow C_6 H_6 (\text{benzene}) + 4H_2\n\n* **Combustion and Pollutants:**\n * **Complete Combustion:** Occurs in excess oxygen, producing carbon dioxide and water:\n        C_n H_{2n+2} + \left(\frac{3n+1}{2}\right) O_2 \rightarrow n CO_2 + (n+1) H_2 O\n        2 C_2 H_5 OH + 6 O_2 \rightarrow 4 CO_2 + 6 H_2 O\n * **Incomplete Combustion:** Occurs under limited oxygen conditions, producing carbon monoxide (CO$), soot/carbon particulates ($C), and water:\n        C_3 H_7 OH + 3 O_2 \rightarrow 3 CO + 4 H_2 O\n        2 C_3 H_7 OH + 5 O_2 \rightarrow 4 CO + 2 C + 8 H_2 O\n * **Environmental Hazards of Combustion Products:**\n * *Carbon Monoxide (CO):* Odorless, toxic gas that binds irreversibly to hemoglobin in red blood cells, blocking oxygen transport; lethal.\n * *Carbon Particulates (C):* Exacerbates respiratory illnesses (asthma), forms smog, acts as a carcinogen.\n * *Sulfur Dioxide (SO_2$):* Produced by combustion of sulfur impurities in fuels ($S + O_2 \rightarrow SO_2$). Oxidizes to$SO_3$and reacts with water vapor to form acid rain ($H_2 SO_4), causing deforestation, aquatic ecosystem death, soil acidification, and building corrosion.\n * *Flue Gas Desulfurization (Removal of SO_2$):* Gases are reacted with basic calcium oxide ($CaO$) or calcium carbonate ($CaCO_3):\n                CaO(s) + SO_2(g) \rightarrow CaSO_3(s)\n                2CaSO_3(s) + O_2(g) \rightarrow 2CaSO_4(s)\n * *Oxides of Nitrogen (NO_x$):* Formed when high combustion temperatures inside car engines cause atmospheric nitrogen and oxygen to react ($N_2 + O_2 \rightarrow 2NO$;$2NO + O_2 \rightarrow 2NO_2). Causes acid rain and photochemical smog.\n * *Carbon Dioxide (CO_2):* Greenhouse gas that absorbs infrared radiation, driving global climate change and sea level rise.\n * **Catalytic Converters:**\n * Installed in automobile exhaust systems using a honeycomb ceramic structure coated with platinum (Pt$), palladium ($Pd$), and rhodium ($Rh) catalysts.\n * Catalyzes reactions converting toxic engine emissions into harmless gases:\n            2CO + 2NO \rightarrow 2CO_2 + N_2\n            C_x H_{2x+2} + (2x + \frac{1}{2}) NO \rightarrow x CO_2 + (x+1) H_2 O + (x + \frac{1}{4}) N_2\n            2CO + O_2 \rightarrow 2CO_2\n\n\n# Alkenes: Reactivity, Electrophilic Addition, and Polymerization\n\n* **Geometric Isomerism (E/Z and Cis/Trans):**\n * *Requirements:* Restricted rotation around the C=C bond and two different groups attached to each carbon of the double bond.\n * **Z (Cis) Isomer:** High-priority groups (determined by Cahn-Ingold-Prelog rules based on higher atomic number) are located on the same side of the double bond.\n * *Properties:* Slightly polar due to non-canceling bond dipoles. Higher boiling point than E isomer due to permanent dipole-dipole attractions. Lower melting point because the asymmetrical "U" shape prevents efficient crystal lattice packing.\n * **E (Trans) Isomer:** High-priority groups are located on opposite sides of the double bond.\n * *Properties:* Non-polar (individual dipoles cancel out). Lower boiling point (only London dispersion forces). Higher melting point due to symmetrical linear packing in the solid state.\n\n* **Laboratory Synthesis of Alkenes:**\n * Synthesized via dehydration of alcohols by heating with a concentrated acid catalyst (H_2 SO_4$or$H_3 PO_4$) at$170^{\circ}\text{C}$, or by passing alcohol vapor over heated aluminium oxide ($Al_2 O_3) catalyst.\n\n* **Test for Unsaturation:**\n * *Reagent:* Bromine water (Br_2(aq), orange/brown solution).\n * *Observation:* Addition of an unsaturated compound (alkene) causes the orange/brown solution to decolorize rapidly to colorless at room temperature.\n\n* **Electrophilic Addition Reactions and Mechanisms:**\n * **Electrophile:** An electron-pair acceptor species carrying a positive charge or partial positive charge (\delta+).\n * **Nucleophile:** An electron-pair donor species possessing a lone pair of electrons or a negative charge.\n * *Mechanism:* The high electron density of the alkene \pi bond attacks an electrophile, forming a carbocation intermediate, which is subsequently attacked by a nucleophile.\n * **Halogenation (+ X_2$, e.g.,$Br_2):**\n * As non-polar Br_2$approaches the$\pi$bond, the electron cloud induces a dipole ($Br^{\delta+} - Br^{\delta-}$). The$\pi$electrons attack$Br^{\delta+}$, displacing$Br^-$and forming a carbocation (or bromonium ion).$Br^- then attacks the carbocation to yield a dihalogenoalkane (e.g., 1,2-dibromoethane).\n * **Hydrohalogenation (+ HX$, e.g.,$HCl, HBr):**\n * Yields a halogenoalkane.\n * **Markovnikov's Rule:** When an asymmetric hydrogen halide (HX) reacts with an unsymmetrical alkene, the hydrogen atom attaches to the carbon atom already bearing the greater number of hydrogen atoms.\n * *Carbocation Stability:* Tertiary (3^{\circ}) carbocation > Secondary (2^{\circ}) carbocation > Primary (1^{\circ}$) carbocation. Alkyl groups are electron-donating via a positive inductive effect ($+I), dispersing the positive charge and stabilizing the intermediate. The reaction proceeds predominantly via the more stable carbocation.\n * **Hydration (+ H_2 O):**\n * *Conditions:* Reaction with steam, concentrated phosphoric(V) acid catalyst (H_3 PO_4$), temperature of$300^{\circ}\text{C}$, pressure of$60 - 70\,atm.\n * *Product:* Alcohol.\n * **Hydrogenation (+ H_2):**\n * *Conditions:* Nickel (Ni$) catalyst at$150^{\circ}\text{C} (or platinum/palladium at room temperature).\n * *Product:* Alkane.\n * *Industrial Application:* Manufacturing of margarine. Polyunsaturated vegetable oils containing multiple C=C$bonds are partially hydrogenated with$H_2 over a nickel catalyst at high temperature and pressure to raise their melting point, converting liquid oils into solid margarine.\n\n* **Addition Polymerization:**\n * Process where unsaturated monomer molecules containing C=C double bonds join together to form long-chain macro-molecules (polymers) with repeating units.\n * *Characteristics:* Involves radical or ionic initiation, propagation, and termination steps. The double bond opens up, leaving only single \sigma bonds in the polymer backbone.\n * *Resin Identification Codes (RIC) & Polymer Properties:*\n 1. **PETE (1):** Polyethylene Terephthalate (soda bottles, jars).\n 2. **HDPE (2):** High-Density Polyethylene. Linear polymer structure with minimal branching, high crystallinity (> 90\%$), density$0.95 - 0.97\,g/cm^3$, melting point$135^{\circ}\text{C}. Strong, rigid, chemically inert, resistant to UV light. Used for milk jugs, detergent bottles, piping.\n 3. **PVC (3):** Polyvinyl Chloride / Poly(chloroethene), monomer CH_2=CHCl. Used for water pipes, window frames, flooring, cable insulation.\n 4. **LDPE (4):** Low-Density Polyethylene. Highly branched structure, low crystallinity (< 50 - 60\%$), density$0.91 - 0.94\,g/cm^3$, melting point$115^{\circ}\text{C}. Flexible, lightweight, degraded by oxygen/UV. Used for squeeze bottles, trash bags, film wrap.\n 5. **PP (5):** Polypropylene / Poly(propene), monomer CH_2=CHCH_3. Used for food containers, straws, auto parts.\n 6. **PS (6):** Polystyrene / Poly(phenylethene), monomer CH_2=CH(C_6 H_5). Used for disposable cutlery, foam packaging, packaging peanuts.\n 7. **OTHER (7):** Polycarbonate, Nylon, ABS, Acrylic, PLA.\n * **Disposal & Environmental Concerns:**\n * Addition polymers are non-biodegradable due to non-polar, strong C-C$$\sigma bonds that resist bacterial enzyme breakdown.\n * *Landfill:* Takes up large volume; persistent pollution.\n * *Incineration:* Releases heat energy, but combustion of PVC releases highly toxic, corrosive hydrogen chloride gas (HCl) and toxic chlorinated dioxins. PVC must be sorted and separated prior to waste incineration.\n * *Recycling Methods:* Sorting plastic by RIC, mechanical melting into granules, chemical feedstock recycling via thermal cracking to regenerate monomers, or developing starch-embedded biodegradable polymers.\n\n\n# Alcohols: Classification, Synthesis, Physical Properties, and Chemical Reactions\n\n* **Classification of Alcohols:**\n * **Primary (1^{\circ}$) Alcohol:** The carbon bearing the$-OH group is bonded to only one other carbon atom (e.g., ethanol, butan-1-ol).\n * **Secondary (2^{\circ}$) Alcohol:** The carbon bearing the$-OH group is bonded to two other carbon atoms (e.g., propan-2-ol, 2-methylpentan-3-ol).\n * **Tertiary (3^{\circ}$) Alcohol:** The carbon bearing the$-OH group is bonded to three other carbon atoms (e.g., 2-methylpropan-2-ol).\n\n* **Physical Properties:**\n * **Boiling Points:** Alcohols have significantly higher boiling points than alkanes of equivalent molar mass due to strong intermolecular hydrogen bonding between polar -OH groups.\n * Boiling point increases with carbon chain length due to increased surface area and stronger London dispersion forces.\n * Boiling point decreases with chain branching because spherical branched molecules have a reduced surface area of contact, weakening dispersion forces.\n * **Solubility in Water:**\n * Small alcohols (methanol, ethanol, propanol) are completely miscible in water because the hydroxyl group readily forms hydrogen bonds with water molecules.\n * Solubility decreases as carbon chain length increases because the non-polar hydrophobic hydrocarbon chain dominates the polar hydrophilic -OH group.\n * Solubility of isomeric alcohols increases with branching because the hydrophobic hydrocarbon surface area is minimized.\n\n* **Industrial Routes for Ethanol Production:**\n * **Route 1: Fermentation of Sugars (Glucose):**\n * *Equation:* C_6 H_{12} O_6(aq) \xrightarrow{\text{yeast}} 2 C_2 H_5 OH(aq) + 2 CO_2(g)\n * *Conditions:* Warm temperature (30 - 35^{\circ}\text{C}$, optimal for yeast enzyme invertase/zymase), atmospheric pressure ($1\,atm), anaerobic environment (airtight container to prevent oxidation to ethanoic acid), batch process lasting several days.\n * *Pros:* Uses renewable plant feedstock (sugarcane, corn, sorghum); low energy input for reaction conditions.\n * *Cons:* Slow batch process; high labor costs; yields dilute aqueous ethanol (< 15\% concentration before yeast dies), requiring energy-intensive fractional distillation; generates large waste volume.\n * *Atom Economy:* \n            \text{Atom Economy} = \frac{2 \times 46.07}{180.16} \times 100\% \approx 51.1\%\n * **Route 2: Direct Hydration of Ethene:**\n * *Equation:* C_2 H_4(g) + H_2 O(g) \xrightarrow{H_3 PO_4} C_2 H_5 OH(g)\n * *Conditions:* High temperature (300^{\circ}\text{C}$), high pressure ($60 - 70\,atm$), concentrated phosphoric(V) acid ($H_3 PO_4) catalyst, continuous process.\n * *Pros:* Rapid continuous process; low labor requirement; produces high purity ethanol with zero by-products (100\% atom economy).\n * *Cons:* Uses non-renewable crude oil feedstock (ethene); high energy consumption to maintain temperature and pressure.\n\n* **Bioethanol and Carbon Neutrality:**\n * **Biofuel:** Any liquid or gaseous fuel derived from biological material (biomass).\n * **Theoretical Carbon Neutrality:** A fuel is defined as carbon neutral if the amount of CO_2 absorbed during biomass growth equals the amount released during production and combustion:\n * *Photosynthesis:* 6 CO_2 + 6 H_2 O \rightarrow C_6 H_{12} O_6 + 6 O_2$(absorbs$6\,mol\,CO_2)\n * *Fermentation:* C_6 H_{12} O_6 \rightarrow 2 C_2 H_5 OH + 2 CO_2$(releases$2\,mol\,CO_2)\n * *Combustion:* 2 C_2 H_5 OH + 6 O_2 \rightarrow 4 CO_2 + 6 H_2 O$(releases$4\,mol\,CO_2)\n * *Net Carbon Balance:* -6 + 2 + 4 = 0\,mol\,CO_2\n * *Invalidity in Practice:* Bioethanol is not strictly carbon neutral because fossil fuels are burned to run agricultural machinery, manufacture fertilizers/pesticides, harvest crops, distill ethanol, and transport fuel.\n * *Socio-Economic Concerns:* Diverting arable land to energy crops causes food price spikes, deforestation, and habitat loss.\n\n* **Chemical Reactions of Alcohols:**\n * **Oxidation Reactions:**\n * *Oxidizing Agents:* Acidified potassium dichromate(VI) (K_2 Cr_2 O_7 / H_2 SO_4$), which changes color from **orange to green** ($Cr^{6+}$reduced to$Cr^{3+}$), or acidified potassium manganate(VII) ($KMnO_4 / H^+$), which changes from **purple to colorless** ($Mn^{7+}$reduced to$Mn^{2+}).\n * *Primary Alcohols (1^{\circ}):*\n * Partial Oxidation (Distillation apparatus): Oxidizes to an **aldehyde**:\n                RCH_2 OH + [O] \rightarrow RCHO + H_2 O\n * Complete Oxidation (Reflux apparatus with excess [O]): Oxidizes to a **carboxylic acid**:\n                RCH_2 OH + 2[O] \rightarrow RCOOH + H_2 O\n * *Secondary Alcohols (2^{\circ}):* Oxidize under reflux to a **ketone**:\n            R-CH(OH)-R' + [O] \rightarrow R-CO-R' + H_2 O\n * *Tertiary Alcohols (3^{\circ}$):* Resistant to oxidation under mild conditions because no hydrogen atom is bonded to the carbon carrying the$-OH group; dichromate solution remains orange.\n * **Qualitative Tests for Alcohols:**\n * *Acidified Potassium Dichromate Test:* Primary and secondary alcohols turn orange dichromate green; tertiary alcohols cause no color change.\n * *Triiodomethane (Iodoform) Reaction:* Specific test for alcohols containing the CH_3 CH(OH)- structure (e.g., ethanol, propan-2-ol, butan-2-ol).\n * *Reagents:* Iodine solution (I_2$) and aqueous sodium hydroxide ($NaOH).\n * *Observation:* Positive test yields a pale **yellow precipitate** of triiodomethane (CHI_3) with an antiseptic smell.\n * **Reaction with Sodium Metal:**\n * Alcohols react weakly as acids with sodium metal, releasing hydrogen gas effervescence and forming a sodium alkoxide solution:\n            2 R-OH + 2 Na \rightarrow 2 R-O^- Na^+ + H_2(g)\n * Effervescence is significantly less vigorous than the reaction of sodium with water.\n * **Substitution / Halogenation Reactions:**\n * *Chlorination:* \n            R-OH + HCl(g) \xrightarrow{ZnCl_2} R-Cl + H_2 O\n            R-OH + SOCl_2(l) \rightarrow R-Cl(l) + SO_2(g) + HCl(g) (Convenient because both by-products are gases).\n            R-OH + PCl_5(s) \rightarrow R-Cl(l) + POCl_3(l) + HCl(g)$(Observed by misty fumes of$HCl).\n            3 R-OH + PCl_3(l) \rightarrow 3 R-Cl(l) + H_3 PO_3(aq)\n * *Bromination:* Generated *in situ* using NaBr$and concentrated$H_2 SO_4 or red phosphorus and bromine:\n            R-OH + HBr \rightarrow R-Br + H_2 O\n            3 R-OH + PBr_3 \rightarrow 3 R-Br + H_3 PO_3\n * *Iodination:* Generated *in situ* using red phosphorus and iodine:\n            3 R-OH + PI_3 \rightarrow 3 R-I + H_3 PO_3\n * **Dehydration to Alkenes:**\n * Heating alcohol with concentrated H_2 SO_4$or$H_3 PO_4$at$170^{\circ}\text{C} yields alkenes via elimination mechanism.\n * Unsymmetric secondary alcohols (e.g., butan-2-ol) produce isomeric mixtures of alkenes (but-1-ene, cis-but-2-ene, trans-but-2-ene).\n * *Side Reaction:* Insufficient heating causes condensation of two alcohol molecules to form an ether (2 R-OH \rightarrow R-O-R + H_2 O).\n * **Esterification:**\n * Reversible reaction between a carboxylic acid and an alcohol heated under reflux with concentrated H_2 SO_4 catalyst:\n            R-COOH + R'-OH \rightleftharpoons R-COO-R' + H_2 O\n * *Using Acyl Chlorides:* Reaction with acyl chlorides (R-COCl$) occurs vigorously at room temperature, is irreversible, gives high yields, and yields$HCl(g) by-product.\n\n\n# Haloalkanes: Mechanisms, Nucleophilic Substitution, Elimination, and Stratospheric Ozone Depletion\n\n* **Classification and Physical Properties:**\n * Haloalkanes contain carbon-halogen bonds (C-X$where$X = F, Cl, Br, I$). Classified as primary ($1^{\circ}$), secondary ($2^{\circ}$), or tertiary ($3^{\circ}).\n * **Polarity & Solubility:** The C-X$bond is polar due to the higher electronegativity of halogens ($C^{\delta+} - X^{\delta-}). Haloalkanes are insoluble in water (cannot form hydrogen bonds) but soluble in organic solvents.\n * **Boiling Point Trends:**\n * Increases down the halogen group (R-F < R-Cl < R-Br < R-I) due to increasing relative atomic mass and polarizability, which strengthen London dispersion forces.\n * Increases with carbon chain length.\n * Decreases with chain branching due to reduced surface contact.\n\n* **Nucleophilic Substitution Mechanisms (S_N1$vs.$S_N2):**\n * **S_N2 Mechanism (Bimolecular Nucleophilic Substitution):**\n * Predominant in **primary halogenoalkanes** (e.g., 1-chlorobutane).\n * *Process:* A single-step concerted process. The nucleophile (Nu^-$) attacks the$\delta+$carbon from the side opposite to the leaving group (backside attack). A 5-coordinate transition state forms simultaneously as the$C-X bond breaks heterolytically.\n * *Kinetics:* Second-order rate law: Rate = k [R-X][Nu^-].\n * *Steric Factors:* Reactivity order: Methyl > Primary > Secondary >> Tertiary. Favored by **aprotic polar solvents** (e.g., acetone) which do not hydrogen-bond to nucleophiles, leaving them "naked" and highly reactive.\n * **S_N1 Mechanism (Unimolecular Nucleophilic Substitution):**\n * Predominant in **tertiary halogenoalkanes** (e.g., 2-chloro-2-methylpropane).\n * *Process:* Two-step mechanism. \n * Step 1 (Slow, rate-determining step): Heterolytic bond fission of C-X to form a stable carbocation intermediate.\n * Step 2 (Fast step): Rapid attack of the nucleophile on the carbocation.\n * *Kinetics:* First-order rate law: Rate = k [R-X].\n * *Stabilization:* Alkyl groups stabilize the tertiary carbocation intermediate via inductive electron donation (+I). Favored by **protic polar solvents** (e.g., water, ethanol) which stabilize carbocations via solvation.\n * **Relative Reactivity of Carbon-Halogen Bonds:**\n * Reactivity is determined by **C-X bond enthalpy (bond strength)**, NOT bond polarity.\n * *Bond Enthalpies:* C-F (467\,kJ\,mol^{-1}) > C-Cl (340\,kJ\,mol^{-1}) > C-Br (280\,kJ\,mol^{-1}) > C-I (228\,kJ\,mol^{-1}).\n * Iodoalkanes react fastest due to the weak C-I bond; fluoroalkanes are chemically inert.\n\n* **Common Nucleophilic Substitution Reactions:**\n * **Hydrolysis with Hydroxide Ions (+ OH^-):**\n * *Reagent:* Aqueous KOH$or$NaOH, reflux.\n * *Product:* Alcohol (R-X + OH^- \rightarrow R-OH + X^-).\n * **Reaction with Cyanide Ions (+ CN^-):**\n * *Reagent:* Ethanolic KCN$or$NaCN, reflux.\n * *Product:* Nitrile (R-X + CN^- \rightarrow R-CN + X^-). Increases the carbon chain length by one carbon.\n * **Reaction with Ammonia (+ NH_3):**\n * *Reagent:* Excess concentrated NH_3 in ethanol, heated under pressure in a sealed tube.\n * *Product:* Primary Amine (R-X + 2NH_3 \rightarrow R-NH_2 + NH_4^+ X^-).\n\n* **Qualitative Test for Halide Ions in Haloalkanes:**\n * *Procedure:* Warm haloalkane with aqueous silver nitrate (AgNO_3$) in ethanol solvent (ethanol acts as a mutual solvent for water and haloalkane), acidified with dilute nitric acid ($HNO_3).\n * *Observations:* \n * *Chloroalkane:* **White precipitate** (AgCl$); precipitate **dissolves in dilute$NH_3(aq)**.\n * *Bromoalkane:* **Cream precipitate** (AgBr$); precipitate is insoluble in dilute$NH_3$, but **dissolves in concentrated$NH_3(aq)**.\n * *Iodoalkane:* **Yellow precipitate** (AgI$); precipitate is **insoluble in concentrated$NH_3(aq)**.\n\n* **Elimination Mechanism in Halogenoalkanes:**\n * *Reagents:* Hot ethanolic potassium hydroxide (KOH$or$NaOH), reflux.\n * *Role of Base:* Hydroxide ion (OH^-$) acts as a base, removing a proton ($H^+$) from a carbon atom adjacent to the$C-X carbon.\n * *Mechanism:* OH^-$attacks adjacent$H$atom$\rightarrow$electron pair moves to form a$C=C$double bond$\rightarrow$halide ion ($X^-) departs.\n * *Products:* Alkene, water, and halide salt.\n * *Un-symmetrical Haloalkanes:* Elimination from 2-bromobutane removes hydrogen from adjacent carbon-1 or carbon-3, yielding a mixture of positional and geometric isomeric alkenes: **but-1-ene**, **cis-but-2-ene (Z-but-2-ene)**, and **trans-but-2-ene (E-but-2-ene)** (major product).\n\n* **Competition between Substitution and Elimination:**\n\n| Variable | Favors Nucleophilic Substitution | Favors Elimination |\n| :--- | :--- | :--- |\n| **Halogenoalkane Type** | Primary (1^{\circ}$) | Tertiary ($3^{\circ}) |\n| **Solvent Medium** | Water (Aqueous) | Pure Ethanol (Ethanolic) |\n| **Temperature** | Lower / Warm | High Temperature / Reflux |\n| **Base Concentration** | Dilute NaOH/KOH$| Concentrated$NaOH/KOH |\n\n* **Stratospheric Ozone Depletion by Chlorofluorocarbons (CFCs):**\n * **Ozone (O_3):** Discovered in 1839 by Christian Friedrich Schönbein. Pale blue gas present in the stratosphere (~25 km altitude, up to 15 ppm) that absorbs harmful solar ultraviolet (UV-B) radiation.\n * **CFC Characteristics:** Chlorofluorocarbons (e.g., CFCl_3$,$CF_2 Cl_2) developed in 1940s/50s as refrigerants, aerosol propellants, foaming agents, and fire retardants. Extremely unreactive, non-toxic, non-flammable, with long atmospheric lifetimes (10 to 100+ years).\n * **Depletion Mechanism (Rowland & Molina, 1974):**\n 1. CFCs diffuse unchanged into the stratosphere, where high-energy UV radiation breaks a weak C-Cl bond homolytically:\n            CF_2 Cl_2 \xrightarrow{UV} \cdot CF_2 Cl + Cl\cdot\n 2. Chlorine free radicals (Cl\cdot) attack ozone molecules in a catalytic cycle:\n            Cl\cdot + O_3 \rightarrow ClO\cdot + O_2\n            ClO\cdot + O\cdot \rightarrow Cl\cdot + O_2\n            *Overall Equation:* O_3 + O\cdot \rightarrow 2 O_2\n 3. Dimerization propagation steps:\n            2 ClO\cdot \rightarrow ClOOCl \xrightarrow{UV} ClOO\cdot + Cl\cdot \rightarrow O_2 + 2 Cl\cdot\n * *Catalytic Power:* A single chlorine radical can destroy up to **100,000 ozone molecules** before being scavenged.\n * **Environmental & Health Consequences:** Increased UV-B exposure leads to skin cancers, eye cataracts, suppressed human immune systems, damaged ocean phytoplankton (disrupting marine food webs), reduced crop yields, degradation of materials, and accelerated global warming.\n * **International Action & Replacements:**\n * *Montreal Protocol (1987, effective 1989):* International treaty mandating the global phase-out of CFC production.\n * *Replacements:* Hydrofluoroalkanes (HFCs, e.g., CH_2 F CF_3$) and hydrochlorofluorocarbons (HCFCs) which lack chlorine radicals or decompose quickly in the troposphere; ammonia and hydrocarbons (propane/butane) as coolants;$CO_2$$ or nitrogen gas as aerosol propellants.