Reactions of Alkanes and Alkenes

• Reactions of organic compounds involve various transformations of alkanes, alkenes, arenes, and hydroxyl compounds.

Alkanes

• Alkanes undergo free radical substitution, represented by the equation CxHy + X2(g) → CxHy-1X + HX, which requires UV light.
• Alkanes can also undergo reduction in the presence of Ni, heat, or Pt/Pd, rtp, resulting in the addition of hydrogen.

Alkenes

• Alkenes participate in electrophilic addition reactions.
• Halogenation of alkenes involves the addition of X₂ in an organic solvent. The observations show that X2(g) can be Cl2(g) or Br2(g), and the reaction undergoes a heterogeneous catalyst mechanism. A test for this reaction is the disappearance of C=C.
• Alkenes + X2(aq) + H2O produces a mixture of products.
• The addition of HX to alkenes occurs at room temperature and pressure (rtp).
• Alkenes undergo hydration with H2O(g) under specific conditions to form alcohols.
• Mild oxidation of alkenes with KMnO₄(aq) or H₂SO₄(aq) or NaOH(aq) in cold conditions yields diols. [Test: C=C]
• Strong oxidation of alkenes with KMnO₄(aq), H₂SO₄(aq), or NaOH(aq) under heat leads to the cleavage of the double bond. [Test: C=C/C=C]

Arenes (Benzene Derivatives)

• Arenes undergo electrophilic substitution reactions.
• Halogenation using anhydrous FeX3/AIX3 at rtp leads to the substitution of hydrogen by a halogen (X), where X2 = Cl2(g) or Br2(l).
• Alkylation uses anhydrous FeX3/AIX3 at rtp to add an alkyl group (R) to the benzene ring.
• Nitration with concentrated H₂SO₄ and HNO₃ under reflux at 55°C introduces a nitro group (-NO₂) to the benzene ring.
• Oxidation of the side chain (C-containing) of arenes can occur under acidic or alkaline conditions using KMnO₄(aq). Under acidic conditions, H₂SO₄(aq) and heat are used to form a white precipitate of benzoic acid. Under alkaline conditions, NaOH(aq) and heat are used, requiring that the carbon bonded to benzene has at least 1 H.
• Nitro groups on arenes can be reduced to amino groups (-NH₂) using H2(g) with Ni, heat, or H2(g) with Pt/Pd, rtp or Sn in conc. HCl, heat under reflux, then NaOH(aq).

Halogenoalkanes

• Halogenoalkanes undergo nucleophilic substitution reactions.
• Reaction with OH⁻ from NaOH(aq) under heat leads to the formation of alcohols (R-OH).
• Reaction with CN⁻ from NaCN(alc) under heat leads to the formation of nitriles (R-CN).
• Reacting with NH3(alc) in excess under pressure leads to the formation of amines (R-NH2).

Halogen Derivatives

• Elimination reactions of halogen derivatives involve the removal of HX using NaOH(alc) under heat to form alkenes.

Nitriles/Cyanohydrins

• Nitriles undergo hydrolysis under acidic or alkaline conditions.
• Acidic hydrolysis with dilute acid forms carboxylic acids.
• Alkaline hydrolysis with alkali forms carboxylate salts and ammonia.
• Reduction of nitriles with 4[H] yields amines.

Hydroxyl Compounds

• Alcohols can be reduced with Na(s) at rtp to form alkoxides.
• Condensation (Esterification) of alcohols with carboxylic acids using conc. H₂SO₄ under heat forms esters. Phenols do not react with carboxylic acids.
• Alcohols react with aliphatic acyl chloride using pyridine as a solvent. Aromatic acyl chloride uses NaOH as a solvent.
• Oxidation of primary alcohols forms aldehydes or carboxylic acids, while secondary alcohols form ketones.
• Elimination (Dehydration) of alcohols using excess conc. H₂SO₄ at 170°C or Al₂O₃ at 350°C forms alkenes.

Nucleophilic Substitution of Hydroxyl Compounds

• Chlorination with PCI₅ or SOCl₂ forms alkyl chlorides (RCI), with (PCI₅/SOCl₂) useful for testing -OH/-COOH groups. Preferred reaction is with SOCl₂.
• Bromination with PBr₃ or HBr forms alkyl bromides (RBr).
• Iodination with PI or HI forms alkyl iodides (RI).

Tests

• The acid-base reaction of phenols with NaOH(aq) forms phenoxide salts.
• Tri-iodomethane test involves reacting methyl alcohols with I₂(aq) and NaOH(aq) under warm conditions to form a yellow precipitate of CHI₃.
• Ferric chloride test involves reacting phenols with FeCl₃(aq) to observe a purple colouration.

Reactant and Product Observations

• Reactant observations include the color changes of chlorine (Cl2(g)), bromine (Br2(l) and Br2(aq)), and the dissolution of metals.
• OA observations include the decolorization of purple KMnO₄ and the color change of K₂Cr₂O₇.
• Product observations include the formation of steamy white fumes of HX, the effervescence of CO₂ and H₂, along with their respective tests, and the sweet smell of esters.

Directing Effects

• Activating groups such as alkyl groups (-R), -OH, -OR, -NH₂, -NHR, -NHR₂, and -NHCOR are 2,4-directing.
• Halogens and deactivating groups such as -COR, -COOH, -COOR, -NH3+, -NO2, and -CN are 3-directing.

Distinguishing Halides

• Method 1 involves adding NaOH(aq), HNO₃(aq), and AgNO₃(aq), followed by NH₃(aq) to distinguish the halides based on the color and solubility of the precipitates formed.
• Method 2 involves adding equal amounts of AgNO₃(alc) and warming the mixture in a water bath at 50°C to distinguish 1°, 2°, and 3° RX.
• The ethanolic solvent increases the solubility of RX.
• Water acts as a weak nucleophile, so relative rates of substitution can be observed.