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Formulae, Functional Groups, and Naming of Organic Compounds

• Hydrocarbons: Compounds consisting exclusively of carbon and hydrogen atoms.

• Alkanes: A family of simple hydrocarbons containing no functional groups.

• Carbon Chain Prefixes: Naming organic molecules requires identifying the length of the unbranched carbon backbone (up to 6 carbons):

  • 1 Carbon: meth-
  • 2 Carbons: eth-
  • 3 Carbons: prop-
  • 4 Carbons: but-
  • 5 Carbons: pent-
  • 6 Carbons: hex-

• Representations of Organic Molecules:

  • Molecular Formula: Expresses the actual count of each atom of every element present in a single molecule.
  • Structural Formula: Shows the carbon-by-carbon sequence and structural arrangement, indicating attached hydrogens and functional groups explicitly (e.g., $CH_3CH_2OH$).
  • Skeletal Formula: Illustrates only the carbon skeleton bonds as lines. Carbon and hydrogen atoms attached to carbon are omitted, while functional groups and non-carbon atoms are shown. Simplifies large, complex structures.
  • Displayed Formula: Shows every individual atom and every single covalent bond present within the molecule.

• Homologous Series: A family of organic compounds sharing the same functional group, where each successive member differs by a $-\text{CH}_2-$ unit. The functional group dictates the characteristic chemical reactivity of the series.

• Overview of Aliphatic Homologous Series:

  • Alkanes: Suffix -ane. Example: Propane, Molecular formula: $C_3H_8$, Structural formula: $CH_3CH_2CH_3$.
  • Alkenes: Suffix -ene. Example: Ethene, Molecular formula: $C_2H_4$, Structural formula: $CH_2CH_2$ ($CH_2=CH_2$).
  • Halogenoalkanes: Prefixes Fluoro-, Chloro-, Bromo-, Iodo-. Example: Chloroethane, Molecular formula: $C_2H_5Cl$, Structural formula: $CH_3CH_2Cl$.
  • Alcohols: Suffix -ol. Example: Ethanol, Molecular formula: $C_2H_6O$, Structural formula: $CH_3CH_2OH$.
  • Aldehydes: Suffix -al. Example: Ethanal, Molecular formula: $C_2H_4O$, Structural formula: $CH_3CHO$.
  • Ketones: Suffix -one. Example: Propanone, Molecular formula: $C_3H_6O$, Structural formula: $CH_3COCH_3$.
  • Carboxylic Acids: Suffix -oic acid. Example: Ethanoic acid, Molecular formula: $C_2H_4O_2$, Structural formula: $CH_3COOH$.
  • Esters: Prefix Alkyl-, Suffix -oate. Example: Methyl ethanoate, Molecular formula: $C_3H_6O_2$, Structural formula: $CH_3COOCH_3$.
  • Amines: Suffix -amine. Example: Methylamine, Molecular formula: $CH_5N$, Structural formula: $CH_3NH_2$.
  • Nitriles: Suffix -nitrile. Example: Ethanenitrile, Molecular formula: $C_2H_3N$, Structural formula: $CH_3CN$.

• Systematic IUPAC Rules for Naming Organic Compounds:

  1. Find and name the longest continuous carbon chain containing the principal functional group.
  2. Identify the principal functional group to determine the primary suffix or prefix.
  3. Number the continuous carbon chain from the end that gives the principal functional group the lowest possible locator number.
  4. Identify side chains or secondary functional groups and place them as prefixes at the start of the name in alphabetical order (e.g., ethyl- before methyl-).
  5. Use multiplicative prefixes (di-, tri-, tetra-) when multiple identical functional groups or side chains are present.

• Nomenclature Examples:

  • Example 1: A 6-carbon chain (hex-) containing an alcohol functional group ($-OH$) on the second carbon (-2-ol), an ethyl group on carbon 3 (3-ethyl), and a methyl group on carbon 5 (5-methyl) is named 3-ethyl-5-methylhexan-2-ol (or 3-ethyl-5-methylhexanol).
  • Example 2: A symmetrical 4-carbon chain (but-) containing two alcohol groups on carbons 2 and 3 (-2,3-diol) is named butan-2,3-diol.

Characteristic Organic Reactions and Terminology

• Fundamental Terms:

  • Functional Group: An atom or group of atoms responsible for the characteristic chemical reactions of a given organic compound.
  • Saturated Compound: A molecule containing only single bonds between carbon atoms, holding the maximum possible number of hydrogen atoms.
  • Unsaturated Compound: A molecule containing at least one carbon-carbon double ($C=C$) or triple ($C \text{≡} C$) bond, thus not holding the maximum possible hydrogen atoms.
  • Homolytic Fission: The cleavage of a covalent bond where each bonded atom retains one electron from the shared pair, forming two free radicals.
  • Heterolytic Fission: The cleavage of a covalent bond where one of the bonded atoms retains both electrons from the shared pair, forming positive and negative ions.
  • Free Radical: An uncharged species possessing an unpaired electron, denoted by a single dot (e.g., $Cl^\bullet$).
  • Initiation Step: The primary chemical reaction step that produces free radicals from a neutral molecule, triggering subsequent radical chain reactions.
  • Propagation Step: A secondary reaction step in a radical chain mechanism where a free radical reacts with a molecule to form a new molecule and a new free radical, resulting in no net gain or loss of radicals.
  • Termination Step: The final step in a radical chain mechanism in which two free radicals combine to form a stable covalent bond, ending the chain reaction.
  • Nucleophile: An electron-rich species (molecule or ion) that donates an electron pair to an electron-deficient carbon atom to form a covalent bond.
  • Electrophile: An electron-deficient species (molecule or ion) that accepts an electron pair from an electron-dense region to form a covalent bond.
  • Addition Reaction: A reaction in which two or more reactant molecules combine to form a single larger product molecule.
  • Substitution Reaction: A reaction in which an atom or functional group in a molecule is replaced by another atom or functional group.
  • Elimination Reaction: A reaction in which a simple molecule (such as $H_2O$ or $HCl$) is removed from a larger molecule, creating a double or triple bond.
  • Hydrolysis: The breakdown of a chemical compound or bond through reaction with water.
  • Condensation: A reaction where two molecules join together to form a larger compound, accompanied by the elimination of a small molecule such as water.
  • Oxidation: The loss of electrons, loss of hydrogen atoms, or gain of oxygen atoms by a molecule.
  • Reduction: The gain of electrons, gain of hydrogen atoms, or loss of oxygen atoms by a molecule.

• Structural Shapes: Organic molecules can exist as straight-chained, branched, or cyclic structures depending on carbon skeleton connectivity.

Hybridisation and Bonding in Organic Molecules

• Concept of Hybridisation: The mathematical linear combination of atomic orbitals of different energy levels to form new equivalent hybrid orbitals.

• Electron Promotion in Carbon:

  • Ground state carbon electron configuration: $1s^2 2s^2 2p^2$
  • Upon receiving excitation energy, an electron is promoted from the $2s$ orbital to the empty $2p_z$ orbital, yielding the promoted electronic configuration: $1s^2 2s^1 2p^3$
  • This promotion is energetically favourable because having four unpaired electrons reduces inter-electronic repulsion and permits the formation of four covalent bonds, producing overall greater stability.

• Types of Hybridisation in Carbon:

  • $sp$ Hybridisation: One $2s$ orbital combines with one $2p$ orbital to produce two equivalent $sp$ hybrid orbitals. The two $sp$ orbitals orient at $180^\bullet$ to minimize repulsion, yielding a linear shape.
  • $sp^2$ Hybridisation: One $2s$ orbital combines with two $2p$ orbitals to generate three equivalent $sp^2$ hybrid orbitals, leaving one unhybridised $2p$ orbital. The three $sp^2$ orbitals lie in a plane oriented at $120^\bullet$ to each other (trigonal planar shape). The unhybridised $2p$ orbital is oriented perpendicular ($90^\bullet$) to this plane.
  • $sp^3$ Hybridisation: One $2s$ orbital combines with all three $2p$ orbitals to form four equivalent $sp^3$ hybrid orbitals directed toward the corners of a regular tetrahedron with bond angles of $109.5^\bullet$ (tetrahedral shape).

• Bonding in Ethane ($C_2H_6$):

  • Each carbon atom undergoes $sp^3$ hybridisation.
  • A single carbon-carbon $\text{σ}$ (sigma) bond is formed by the end-to-end direct overlap of one $sp^3$ orbital from each carbon atom.
  • The remaining three $sp^3$ orbitals on each carbon overlap end-to-end with the $1s$ atomic orbital of hydrogen atoms to form six $C-H$ $\text{σ}$ bonds.
  • The molecular geometry around each carbon atom is tetrahedral with bond angles of $109.5^\bullet$.
  • Free rotation occurs readily around carbon-carbon single $\text{σ}$ bonds because the end-to-end orbital overlap maintains cylindrical symmetry during rotation.

• Bonding in Ethene ($C_2H_4$):

  • Each carbon atom undergoes $sp^2$ hybridisation.
  • One $sp^2$ orbital from each carbon atom overlaps end-to-end to form a central $C-C$ $\text{σ}$ bond.
  • The remaining two $sp^2$ orbitals on each carbon overlap end-to-end with hydrogen $1s$ orbitals to form four $C-H$ $\text{σ}$ bonds in a single plane.
  • The unhybridised $2p$ orbitals situated at $90^\bullet$ to the planar $\text{σ}$-framework overlap sideways above and below the plane of the carbon atoms to form a second bond, called a $\text{π}$ (pi) bond.
  • Ethene exhibits a trigonal planar arrangement around each carbon with bond angles of $120^\bullet$.
  • Rotation around the $C=C$ bond is restricted because rotation would break the sideways overlap of the parallel $2p$ orbitals forming the $\text{π}$ bond.

Isomerism in Organic Compounds

• Structural Isomerism: Compounds possessing the same molecular formula but different structural formulae. Categorized into three types:

  • Chain Isomerism: Differences occur in the arrangement or branching of the main carbon backbone.
    • Example: Pentane ($CH_3CH_2CH_2CH_2CH_3$) versus 2,2-dimethylpropane ($C(CH_3)_4$).
  • Functional Group Isomerism: Contains entirely different functional groups.
    • Example: Propanone ($CH_3COCH_3$, a ketone) versus Propanal ($CH_3CH_2CHO$, an aldehyde).
  • Positional Isomerism: The carbon backbone and functional groups remain identical, but the position of the functional group changes along the chain.
    • Example: But-2-ene ($CH_3CH=CHCH_3$) versus But-1-ene ($CH_2=CHCH_2CH_3$).

• Stereoisomerism: Compounds possessing the same molecular formula and structural formula, but differing in the three-dimensional spatial arrangement of their atoms.

• Geometrical (Cis-Trans or E/Z) Isomerism:

  • Occurs due to restricted rotation around a $C=C$ double bond.
  • Requires two different groups attached to each carbon atom of the double bond.
  • Cis Isomer: The highest priority groups attached to each double-bonded carbon reside on the same side of the double bond (both above or both below).
  • Trans Isomer: The highest priority groups reside on opposite sides diagonally across the double bond.
  • Group priority is determined by atomic number.
  • Example: In an alkene with a methyl group ($-CH_3$) and ethyl group ($-CH_2CH_3$) attached to opposite carbons of the double bond, priority is established per carbon by comparing atomic numbers of attached atoms.

• Optical Isomerism:

  • Occurs when a compound contains a chiral centre (chiral carbon).
  • Chiral Centre: A carbon atom bonded to four completely different atoms or groups of atoms.
  • Optical isomers (enantiomers) are non-superimposable mirror images of each other.
  • Example: 2-hydroxybutane / butan-2-ol ($CH_3CH(OH)CH_2CH_3$) contains a central chiral carbon bonded to: $-H$, $-CH_3$, $-OH$, and $-CH_2CH_3$.
  • Represented by drawing tetrahedral 3D structures around the chiral carbon atom, with one enantiomer as a exact reflected mirror image of the other.

Alkanes: Reactivity, Production, Reactions, and Environmental Impact

• Reactivity of Alkanes:

  • Alkanes are chemically unreactive toward most reagents.
  • Reason 1: High bond enthalpies of non-polar $C-C$ and $C-H$ $\text{σ}$ bonds require high energy to cleave.
  • Reason 2: Electronegativities of carbon ($2.5$) and hydrogen ($2.1$) are almost identical, rendering alkane molecules non-polar and unreactive toward nucleophiles or electrophiles.

• Production of Alkanes:

  • Hydrogenation of Alkenes: Addition of hydrogen gas ($H_2$) across the $C=C$ double bond of an alkene.
    • Conditions: Platinum ($Pt$) or Nickel ($Ni$) catalyst, elevated temperature (heat).
    • Equation: $C_2H_4 + H_2 \rightarrow C_2H_6$
  • Cracking of Long-Chain Alkanes: Thermal breakdown of long-chain alkanes into smaller, economically useful alkanes and alkenes.
    • Conditions: Aluminum oxide ($Al_2O_3$) catalyst, high temperature.
    • Equation: $C_9H_{20} \rightarrow C_6H_{14} + C_3H_6$

• Combustion of Alkanes:

  • Alkanes act as effective fuels due to highly exothermic oxidation reactions.
  • Complete Combustion: Occurs in excess oxygen gas, yielding carbon dioxide ($CO_2$) and water ($H_2O$).
    • Equation 1: $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$
    • Equation 2: $2C_6H_{14} + 19O_2 \rightarrow 12CO_2 + 14H_2O$
  • Incomplete Combustion: Occurs in limited oxygen supply, yielding carbon monoxide ($CO$), solid carbon particulates (soot, $C$), water ($H_2O$), and small amounts of $CO_2$
    • Equation 1: $2C_3H_8 + 7O_2 \rightarrow 6CO + 8H_2O$
    • Equation 2: $2C_2H_6 + 3O_2 \rightarrow 4C + 6H_2O$
    • Equation 3: $4CH_4 + 5O_2 \rightarrow 2CO + 8H_2O + 2C$

• Toxicity and Hazards of Combustion Products:

  • Carbon Monoxide ($CO$): Colourless, odourless, toxic gas. Binds irreversibly to oxygen-binding sites on hemoglobin in red blood cells, inhibiting systemic oxygen transport.
  • Carbon Particulates ($C$): Fine unburned soot particles that pollute the air and cause severe respiratory diseases upon inhalation.
  • Oxides of Nitrogen ($NO_x$): Formed inside internal combustion car engines due to high temperatures and pressures causing atmospheric nitrogen ($N_2$) and oxygen ($O_2$) to react.
    • React with unburnt hydrocarbons in sunlight to produce harmful ground-level ozone.
    • Dissolve in cloud water vapour to form acid rain, leading to: lake/river acidification (toxic to aquatic life), vegetation and tree destruction, and structural erosion of limestone buildings.

• Catalytic Converters:

  • Fitted to vehicle exhaust systems to remove toxic engine emissions.
  • Structure: Ceramic honeycomb structure coated with a thin layer of precious heterogeneous metal catalysts (rhodium and platinum) to maximize surface area.
  • Redox Equation: $2NO + 2CO \rightarrow N_2 + 2CO_2$

• Free Radical Substitution of Alkanes:

  • Alkanes react with halogens ($Cl_2$ or $Br_2$) under ultraviolet (UV) light exposure.
  • Overall reaction: $CH_4 + Br_2 \rightarrow CH_3Br + HBr$ or $CH_3CH_3 + Cl_2 \rightarrow CH_3CH_2Cl + HCl$
  • Mechanism Steps (Reaction of Methane with Chlorine):
    1. Initiation: Homolytic fission of the halogen molecule induced by UV light.
       • $Cl_2 \rightarrow 2Cl^\bullet$
    2. Propagation: Radical chain reactions that consume and regenerate halogen radicals.
       • $Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl$
       • $CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$
    3. Termination: Combination of any two free radicals to form a stable molecule.
       • $Cl^\bullet + CH_3^\bullet \rightarrow CH_3Cl$
       • $Cl^\bullet + Cl^\bullet \rightarrow Cl_2$
       • $CH_3^\bullet + CH_3^\bullet \rightarrow C_2H_6$

• Crude Oil Refining and Industrial Cracking:

  • Crude oil is unrefined petroleum composed of complex aliphatic and aromatic hydrocarbon mixtures.
  • Fractional Distillation Procedure:
    1. Crude oil is vaporized in a furnace.
    2. Vapours enter a fractionating column with a temperature gradient (hot at the bottom, cooler at the top).
    3. Vapours rise; hydrocarbons condense at distinct levels corresponding to their specific boiling points.
    4. Longest-chain hydrocarbons with highest boiling points fail to vaporize and collect at the bottom as a viscous residue.
  • Thermal Cracking:
    • Conditions: High temperature ($\text{∼ } 1000^\bullet\text{C}$), high pressure ($\text{∼ } 70\text{ atm}$).
    • Products: High yields of alkenes.
  • Catalytic Cracking:
    • Conditions: Zeolite catalyst, slight pressure, high temperature ($\text{∼ } 450^\bullet\text{C}$).
    • Products: Mostly aromatic hydrocarbons and motor fuels.
  • Examples: $C_{10}H_{22} \rightarrow C_5H_{12} + C_5H_{10}$ or $C_{15}H_{32} \rightarrow 2C_2H_4 + C_3H_6 + C_8H_{18}$ (Decane $\rightarrow$ ethene + octane).

Alkenes: Structure, Synthesis, Addition Reactions, and Oxidation

• Nature and Identification:

  • Unsaturated hydrocarbons containing at least one $C=C$ bond.
  • Test for Alkenes: Shake with aqueous bromine ($Br_2(aq)$). Solution changes rapidly from brown/orange to colourless.

• Formation of Alkenes:

  • Elimination of Halogenoalkanes: Heating halogenoalkanes under reflux with ethanolic sodium hydroxide ($NaOH$).
  • Dehydration of Alcohols:
    • Vapour Phase: Pass ethanol vapour over heated aluminum oxide ($Al_2O_3$) catalyst: $CH_3CH_2OH \rightarrow CH_2CH_2 + H_2O$
    • Acid Catalysed: Heat alcohol with concentrated sulfuric acid ($H_2SO_4$) or concentrated phosphoric acid ($H_3PO_4$). Phosphoric acid is preferred because excess concentrated $H_2SO_4$ acts as a strong oxidising agent, converting alcohol to $CO_2$ while reducing to hazardous $SO_2$ gas.
    • Unsymmetrical Alcohols: Dehydration yields structural and geometrical isomer mixtures (e.g., dehydration of butan-2-ol yields but-2-ene [cis and trans] and but-1-ene).
  • Cracking: Thermal/catalytic cracking of long-chain alkanes using an $Al_2O_3$ catalyst: $C_9H_{20} \rightarrow C_6H_{14} + C_3H_6$

• Electrophilic Addition Reactions of Alkenes:

  • Halogenation: Reaction with $Cl_2$ or $Br_2$ at room temperature yields dihalogenoalkanes (e.g., ethene + $Br_2 \rightarrow$ 1,2-dibromoethane).
  • Hydrohalogenation: Reaction with gaseous hydrogen halides ($HCl$, $HBr$, $HI$) at room temperature yields halogenoalkanes.
  • Hydrogenation: Reaction with hydrogen gas ($H_2$) in the presence of a Nickel ($Ni$) or Platinum ($Pt$) catalyst at $150^\bullet\text{C}$ yields alkanes: $CH_2CH_2 + H_2 \rightarrow CH_3CH_3$
  • Hydration: Reaction with steam ($H_2O(g)$) in the presence of a phosphoric acid ($H_3PO_4$) catalyst yields alcohols: $CH_2CH_2 + H_2O \rightarrow CH_3CH_2OH$

• Markovnikov’s Rule and Inductive Effects:

  • When an unsymmetrical reagent ($HX$ or $H_2O$) reacts with an unsymmetrical alkene, the hydrogen atom attaches to the double-bonded carbon atom that is already bonded to the greater number of hydrogen atoms.
  • Reaction of Propene with Steam: $CH_3CHCH_2 + H_2O \rightarrow CH_3C(OH)CH_3$ (Propan-2-ol is the major product; propan-1-ol is the minor product).
  • Theoretical Basis: Electrophilic addition proceeds via the most stable carbocation intermediate. Alkyl groups exhibit a positive inductive effect ($+\text{I}$), donating electron density toward the positively charged carbon and stabilizing the intermediate. Carbocation stability order: Tertiary ($3^\bullet$) > Secondary ($2^\bullet$) > Primary ($1^\bullet$).

• Detailed Reaction Mechanisms:

  • Electrophilic Addition of $Br_2$ to Ethene:
    1. The electron-dense $C=C$ double bond approaches $Br_2$, inducing a dipole ($Br^{\text{δ}+}-Br^{\text{δ}-}$).
    2. Electron pair from the double bond attacks $Br^{\text{δ}+}$, forming a $C-Br$ bond and breaking the $Br-Br$ bond heterolytically to yield a bromide ion ($Br^-$) and a carbocation intermediate.
    3. The lone pair on $Br^-$ attacks the carbocation to form 1,2-dibromoethane.
  • Electrophilic Addition of $HBr$ to Propene:
    1. Polar $H^{\text{δ}+}-Br^{\text{δ}-}$ approaches propene. Double bond electrons attack $H^{\text{δ}+}$.
    2. Hydrogen adds preferentially to carbon 1 to generate the secondary carbocation $CH_3-\text{C}^+H-CH_3$, which is stabilized by two electron-donating methyl groups (versus the less stable primary carbocation $CH_3-CH_2-\text{C}^+H_2$).
    3. $Br^-$ attacks the secondary carbocation, forming 2-bromopropane as the major product.

• Oxidation Reactions of Alkenes:

  • Cold, Dilute Acidified Manganate(VII) Ions ($KMnO_4/H^+$):
    • Mild oxidation breaks the $\text{π}$-bond to form a 1,2-diol (alkane-diol).
    • Equation: $CH_2CH_2 + H_2O + [O] \rightarrow CH_2(OH)CH_2(OH)$
    • Observation: Purple solution decolourises to colourless.
  • Hot, Concentrated Acidified Manganate(VII) Ions ($KMnO_4/H^+$):
    • Severe oxidative cleavage completely breaks both the $\text{σ}$ and $\text{π}$ bonds of $C=C$, forming carbonyl compounds based on substitution:
      • Carbon attached to two alkyl groups ($R_2C=$) oxidation yields a ketone (resists further oxidation).
      • Carbon attached to one alkyl group and one hydrogen ($RCH=$) oxidation yields an aldehyde, which rapidly oxidises further to a carboxylic acid.
      • Carbon attached to two hydrogen atoms ($CH_2=$) oxidises to methanal, which oxidises further to methanoic acid, and ultimately to carbon dioxide ($CO_2$) and water ($H_2O$).

Halogenoalkanes: Synthesis, Reactions, Mechanisms, and Reactivity Trends

• Classification:

  • Primary ($1^\bullet$): Halogen bonded to a carbon attached to 1 alkyl group (or zero, e.g., $CH_3X$).
  • Secondary ($2^\bullet$): Halogen bonded to a carbon attached to 2 alkyl groups.
  • Tertiary ($3^\bullet$): Halogen bonded to a carbon attached to 3 alkyl groups.

• Synthetic Routes to Halogenoalkanes:

  1. Free Radical Substitution of Alkanes: $CH_3CH_3 + Cl_2 \rightarrow CH_3CH_2Cl + HCl$ (Requires UV light).
     • Further substitution can occur sequentially up to hexachloroethane ($C_2Cl_6$):
       • $Cl^\bullet + CH_3CH_2Cl \rightarrow ^\bullet CH_2CH_2Cl + HCl$
       • $^\bullet CH_2CH_2Cl + Cl_2 \rightarrow ClCH_2CH_2Cl + Cl^\bullet$
  2. Electrophilic Addition to Alkenes: Addition of $X_2$ or $HX$ at room temperature.
  3. Substitution of Alcohols:
     • Using Hydrogen Chloride: $1^\bullet$ and $2^\bullet$ alcohols react extremely slowly. Tertiary alcohols react rapidly with concentrated $HCl$ at room temperature: $(CH_3)_3COH + HCl \rightarrow (CH_3)_3CCl + H_2O$
     • Using Hydrogen Bromide: React alcohol with $KBr$ and concentrated $H_2SO_4$ (generates $HBr$ in situ): $CH_3CH_2OH + HBr \rightarrow CH_3CH_2Br + H_2O$
     • Using Hydrogen Iodide: React alcohol with $KI$ and concentrated phosphoric(V) acid ($H_3PO_4$). Concentrated $H_2SO_4$ cannot be used because it oxidises $I^-$ to $I_2$: $CH_3CH_2OH + HI \rightarrow CH_3CH_2I + H_2O$
     • Using Phosphorus Halides:
       • Heat with $PCl_3$: $3CH_3CH_2OH + PCl_3 \rightarrow 3CH_3CH_2Cl + H_3PO_3$
       • Room temperature with $PCl_5$ (produces steamy white fumes of $HCl$): $CH_3CH_2OH + PCl_5 \rightarrow CH_3CH_2Cl + HCl + POCl_3$
     • Using Thionyl Chloride / Sulfur Dichloride Oxide ($SOCl_2$): Room temperature reaction: $CH_3CH_2OH + SOCl_2 \rightarrow CH_3CH_2Cl + HCl + SO_2$

• Nucleophilic Substitution Reactions of Halogenoalkanes:

  • The polar $C^{\text{δ}+}-X^{\text{δ}-}$ bond renders carbon susceptible to attack by nucleophiles ($OH^-$
  • Hydrolysis: Heat halogenoalkane with warm aqueous $NaOH$.
    • Equation: $CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-$
  • Nitrile Formation: Heat halogenoalkane under reflux with warm ethanolic potassium cyanide ($KCN$). Extends the carbon chain length by one unit.
    • Equation: $CH_3CH_2Br + CN^- \rightarrow CH_3CH_2CN + Br^-$
  • Primary Amine Formation: Heat halogenoalkane in a sealed tube under pressure with excess ethanolic ammonia ($NH_3$).
    • Equation: $CH_3CH_2Cl + 2NH_3 \rightarrow CH_3CH_2NH_2 + NH_4^+Cl^-$

• $S_N1$ vs $S_N2$ Nucleophilic Substitution Mechanisms:

  • $S_N2$ Mechanism (Bimolecular Nucleophilic Substitution):
    • One-step concerted reaction mechanism.
    • Nucleophile attacks the $C^{\text{δ}+}$ from the backside opposite to the leaving halogen group, forming a 5-coordinate transition state.
    • Causes complete spatial inversion of configuration (Walden inversion).
    • Dominates in primary halogenoalkanes due to minimal steric hindrance around the primary carbon.
  • $S_N1$ Mechanism (Unimolecular Nucleophilic Substitution):
    • Two-step reaction mechanism.
    • Step 1 (Slow, rate-determining step): Spontaneous heterolytic cleavage of $C-X$ bond generates a planar carbocation intermediate.
    • Step 2 (Fast step): Nucleophile rapidly attacks the carbocation from either side.
    • Dominates in tertiary halogenoalkanes because bulky alkyl groups prevent backside $S_N2$ attack (steric hindrance) and stabilize the tertiary carbocation intermediate via positive inductive effects.
  • Secondary Halogenoalkanes: Proceed via a mixture of both $S_N1$ and $S_N2$ mechanism routes.

• Bond Enthalpy vs Reactivity Trends:

  • Reactivity of halogenoalkanes is determined by carbon-halogen bond strength (bond enthalpy), not bond polarity.
  • Bond enthalpy sequence: $C-F > C-Cl > C-Br > C-I$.
  • $C-F$ bond is the strongest (highest bond enthalpy), rendering fluoroalkanes unreactive.
  • $C-I$ bond is the weakest (lowest bond enthalpy), rendering iodoalkanes the most reactive.

• Qualitative Silver Nitrate Analytical Test:

  1. Warm halogenoalkane sample with aqueous $NaOH$ in an ethanol/water solvent mixture to hydrolyze halogenoalkane.
  2. Acidify with dilute nitric acid ($HNO_3$) to neutralize remaining $OH^-$ ions (prevents false positive $Ag_2O$ precipitate).
  3. Add aqueous silver nitrate ($AgNO_3$):
     • Chloroalkane: Forms a white precipitate ($AgCl$). Precipitate dissolves in dilute $NH_3(aq)$ and dissolves in concentrated $NH_3(aq)$.
     • Bromoalkane: Forms a cream precipitate ($AgBr$). Precipitate remains insoluble in dilute $NH_3(aq)$, but dissolves in concentrated $NH_3(aq)$.
     • Iodoalkane: Forms a yellow precipitate ($AgI$). Precipitate remains insoluble in both dilute and concentrated $NH_3(aq)$.
  • Precipitation Rate: Silver iodide ($AgI$) forms fastest; silver chloride ($AgCl$) forms slowest.

• Elimination Reaction of Halogenoalkanes:

  • Heat halogenoalkane under reflux with ethanolic sodium hydroxide ($NaOH$).
  • Hydroxide ion acts as a base, abstracting a proton ($H^+$) from an adjacent carbon atom while the halogen atom leaves, forming an alkene, water, and sodium halide salt.

Hydroxy Compounds (Alcohols): Classification, Preparation, and Chemical Properties

• Classification:

  • Primary ($1^\bullet$): $-OH$ attached to carbon bonded to 1 alkyl group.
  • Secondary ($2^\bullet$): $-OH$ attached to carbon bonded to 2 alkyl groups.
  • Tertiary ($3^\bullet$): $-OH$ attached to carbon bonded to 3 alkyl groups.

• Methods of Preparing Alcohols:

  1. Electrophilic addition of steam to alkenes ($H_3PO_4$ catalyst, heat).
  2. Cold, dilute acidified $KMnO_4$ addition to alkenes (forms diols).
  3. Nucleophilic substitution of halogenoalkanes with warm $NaOH(aq)$.
  4. Reduction of Aldehydes: $RCHO + 2[H] \rightarrow RCH_2OH$ (Using $LiAlH_4$ reducing agent).
  5. Reduction of Ketones: $R_1COR_2 + 2[H] \rightarrow R_1CH(OH)R_2$ (Using $LiAlH_4$).
  6. Reduction of Carboxylic Acids: $RCOOH + 4[H] \rightarrow RCH_2OH + H_2O$ (Using $LiAlH_4$ via aldehyde intermediate).
  7. Acid Hydrolysis of Esters: Reversible reaction heating ester with dilute acid and excess water: $CH_3CH_2COOCH_3 + H_2O \rightleftharpoons CH_3CH_2COOH + CH_3OH$

• Combustion of Alcohols:

  • Complete: $CH_3CH_2OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$
  • Incomplete: $2CH_3CH_2OH + 3O_2 \rightarrow 2CO + 2C + 6H_2O$

• Chemical Reactions of Alcohols:

  • Reaction with Metallic Sodium ($Na$):
    • Sodium reacts steadily with alcohols producing a soluble ionic alkoxide salt and bubbling hydrogen gas ($H_2$).
    • Equation: $2CH_3CH_2OH + 2Na \rightarrow 2CH_3CH_2ONa + H_2$
    • Sodium ethoxide ($CH_3CH_2O^- Na^+$) is an ionic salt.
    • Used to safely dispose of small sodium residues or test for hydroxyl groups.
  • Substitution to Halogenoalkanes: Reaction with $HX$, phosphorus halides ($PCl_3, PBr_3, PI_3, PCl_5$), or $SOCl_2$.
  • Oxidation using Acidified Potassium Dichromate(VI) ($K_2Cr_2O_7 / H^+$):
    • Orange dichromate ions ($Cr_2O_7^{2-}$) are reduced to green chromium ions ($Cr^{3+}$).
    • Primary Alcohols: Partially oxidised to aldehydes when distilled off immediately: $CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O$. Fully oxidised under reflux to carboxylic acids: $CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O$.
    • Secondary Alcohols: Oxidised under reflux to ketones: $CH_3CH(OH)CH_3 + [O] \rightarrow CH_3COCH_3 + H_2O$. Resists further oxidation.
    • Tertiary Alcohols: Resistant to oxidation because no hydrogen atom is attached to the hydroxyl-bearing carbon atom.
  • Dehydration to Alkenes: Heated over $Al_2O_3$ catalyst or with concentrated $H_2SO_4$ / $H_3PO_4$.
  • Esterification: Heating alcohol with carboxylic acid in the presence of concentrated $H_2SO_4$ catalyst yields an ester and water.

• Tri-iodomethane (Iodoform) Analytical Test for $CH_3CH(OH)-$ Group:

  • Alcohols containing the specific structural fragment $CH_3CH(OH)-$ (such as ethanol and secondary methyl alcohols like propan-2-ol) react with alkaline aqueous iodine ($I_2(aq) + NaOH(aq)$).
  • Reaction forms a characteristic pale yellow precipitate of tri-iodomethane ($CHI_3$) accompanied by a carboxylate ion ($RCO_2^-Na^+$).

• Acidity of Alcohols vs Water:

  • Dissociation equilibria:
    • Alcohol: $R-OH(aq) \rightleftharpoons R-O^-(aq) + H^+(aq)$
    • Water: $H_2O(aq) \rightleftharpoons OH^-(aq) + H^+(aq)$
  • Both water and alcohols act as weak acids, but alcohols are weaker acids than water.
  • Explanation: The alkyl group ($-R$) is electron-donating ($+\text{I}$ effect), which increases electron density on the oxygen atom of the alkoxide ion ($R-O^-$). This stabilizes the positive charge attraction for $H^+$ ions, shifting the equilibrium further to the left compared to water.

Carbonyl Compounds: Aldehydes and Ketones

• Structure and Nomenclature:

  • Both contain the polar carbonyl group ($C=O$).
  • Aldehydes: Carbonyl group situated at the terminal end of the carbon chain ($-CHO$). Suffix -al.
  • Ketones: Carbonyl group situated internally within the carbon chain ($-CO-$). Suffix -one.

• Preparation:

  • Primary alcohol oxidation via distillation yields Aldehydes: $CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O$
  • Secondary alcohol oxidation under reflux yields Ketones: $CH_3CH(OH)CH_3 + [O] \rightarrow CH_3COCH_3 + H_2O$

• Reduction Reactions:

  • Reducing agents: Sodium borohydride ($NaBH_4$) in aqueous methanol or lithium aluminum hydride ($LiAlH_4$) in dry ether.
  • Aldehyde reduction yields Primary Alcohols: $RCHO + 2[H] \rightarrow RCH_2OH$
  • Ketone reduction yields Secondary Alcohols: $R_1COR_2 + 2[H] \rightarrow R_1CH(OH)R_2$

• Nucleophilic Addition with Hydrogen Cyanide ($HCN$):

  • Aldehydes and ketones react with $HCN$ to form hydroxynitriles.
  • $HCN$ is extremely toxic, so it is generated in situ using potassium cyanide ($KCN$) or sodium cyanide ($NaCN$) acidified with dilute acid.
  • Nomenclature: The nitrile carbon atom ($-C \text{≡} N$) is assigned position Carbon-1.
    • Example 1: Ethanal + $HCN \rightarrow$ 2-hydroxypropanenitrile
    • Example 2: Propanone + $HCN \rightarrow$ 2-hydroxy-2-methylpropanenitrile
  • Mechanism of Nucleophilic Addition:
    1. The carbonyl bond is strongly polar ($C^{\text{δ}+}=O^{\text{δ}-}$).
    2. The nucleophilic cyanide ion ($:CN^-$) attacks the electron-deficient $C^{\text{δ}+}$ carbon atom, breaking the $C=O$ $\text{π}$-bond to yield an alkoxide intermediate possessing a negatively charged oxygen atom ($:O^-$).
    3. The oxygen atom abstracts a proton ($H^+$) from an $HCN$ molecule or acid, forming the hydroxynitrile product and regenerating the $CN^-$ catalyst.

• Analytical Tests for Carbonyl Compounds:

  • 2,4-Dinitrophenylhydrazine (2,4-DNPH / Brady’s Reagent):
    • Addition of 2,4-DNPH to an aldehyde or ketone forms a distinct yellow or orange precipitate.
    • Confirms presence of a carbonyl ($C=O$) group.
  • Distinguishing Aldehydes from Ketones (Aldehydes easily oxidise; ketones resist oxidation):
    • Acidified Potassium Dichromate(VI): Aldehyde turns orange solution to green ($Cr^{3+}$). Ketones show no visible change (remains orange).
    • Tollen’s Reagent (Ammoniacal Silver Nitrate): Warm with aldehyde to form a silver mirror on test tube walls ($Ag^+ + e^- \rightarrow Ag(s)$). Ketones show no visible change.
    • *Fehling’s Reagent