Chemistry A: Predicting and Identifying Reactions and Products

Key Organic Reactions

Alkanes

• Main reactions:

  • Combustion:

    • Complete: Alkane + O₂ → CO₂ + H₂O

    • Incomplete: Alkane + O₂ → CO + H₂O (or C + H₂O in extreme cases)

  • Halogenation:

    • Requires UV light

    • CH₄ + Cl₂ → CH₃Cl + HCl

    • Mechanism: Free radical substitution (initiation, propagation, termination)

Alkenes

• Addition reactions:

  • Hydrohalogenation: HX (HCl, HBr) → haloalkane (follows Markovnikov's rule: H attaches to the carbon with more Hs)

  • Halogenation: X₂ (Br₂, Cl₂) → dihalogenoalkane (anti addition, forms a cyclic halonium ion intermediate)

  • Hydration: H₂O/H₂SO₄ → alcohol (follows Markovnikov's rule)

  • Hydrogenation: H₂/Ni catalyst → alkane

• Oxidation:

  • KMnO₄/H⁺ (cold) → diol (syn addition)

  • KMnO₄/H⁺ (hot) → carboxylic acids or ketones depending on alkene structure

• Key mechanism: Electrophilic addition

Alcohols

• Oxidation reactions:

  • Primary alcohols: → Aldehyde (mild oxidant) → Carboxylic acid (strong oxidant like K₂Cr₂O₇/H⁺)

  • Secondary alcohols: → Ketone only (K₂Cr₂O₇/H⁺)

  • Tertiary alcohols: Resistant to oxidation

• Other reactions:

  • Dehydration: H₂SO₄/heat → alkene

  • Esterification: Alcohol + carboxylic acid → ester + H₂O (acid catalyst required)

  • Reaction with metals: Na + alcohol → sodium alkoxide + H₂

  • Substitution with HX: Alcohol + HX → haloalkane + H₂O

Haloalkanes

• Nucleophilic substitution reactions:

  • OH⁻ (aq) → alcohol

  • NH₃ (ethanolic) → amine

  • CN⁻ (ethanolic) → nitrile

• Elimination reaction:

  • OH⁻ in alcohol → alkene + H₂O

• Mechanisms:

  • SN1 (favored by tertiary haloalkanes, forms carbocation intermediate)

  • SN2 (favored by primary haloalkanes, involves a concerted mechanism)

Aldehydes & Ketones

• Oxidation:

  • Aldehydes → Carboxylic acids (using Tollens' or Fehling's reagent)

  • Ketones resist oxidation under normal conditions

• Reduction:

  • Aldehyde → Primary alcohol (NaBH₄ or LiAlH₄)

  • Ketone → Secondary alcohol (NaBH₄ or LiAlH₄)

• Addition reactions:

  • HCN → Hydroxynitrile (nucleophilic addition)

Carboxylic Acids

• Main reactions:

  • Neutralization: Acid + base → salt + H₂O

  • Esterification: Acid + alcohol → ester + H₂O (acid catalyst required)

  • Reduction: LiAlH₄ → primary alcohol

Aromatics (Benzene & Derivatives)

• Electrophilic substitution reactions:

  • Nitration: HNO₃/H₂SO₄ → Nitrobenzene (NO₂⁺ is the electrophile)

  • Halogenation: X₂/AlX₃ → Halobenzene (forms an arenium ion intermediate)

  • Alkylation/Acylation (Friedel-Crafts):

    • R-Cl/AlCl₃ → Alkylbenzene

    • RCOCl/AlCl₃ → Acylbenzene

Key Inorganic Reactions

Alkali Metals (Group 1)

• Reactions with water:

  • Metal + H₂O → Metal hydroxide + H₂

  • Example: 2Na + 2H₂O → 2NaOH + H₂

• Reactions with oxygen:

  • Na₂O, K₂O (oxides), Na₂O₂ (peroxide), KO₂ (superoxide)

• Reactivity trend: Increases down the group

Halogens (Group 17)

• Displacement reactions:

  • More reactive halogen displaces a less reactive one in a halide solution

  • Example: Cl₂ + 2KBr → 2KCl + Br₂

• Disproportionation reactions with alkalis:

  • Cold: X₂ + OH⁻ → XO⁻ + X⁻ + H₂O

  • Hot: X₂ + OH⁻ → XO₃⁻ + X⁻ + H₂O

• Reactivity trend: Decreases down the group

Transition Metals

• Formation of colored complex ions with ligands

  • Example: [Cu(H₂O)₆]²⁺ (blue) vs. [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue)

• Variable oxidation states

  • Example: Fe²⁺ (green) vs. Fe³⁺ (yellow/brown)

• Formation of colored precipitates with OH⁻

  • Example: Fe²⁺ + 2OH⁻ → Fe(OH)₂ (green ppt)

  • Fe³⁺ + 3OH⁻ → Fe(OH)₃ (brown ppt)

Period 3 Oxides

• Basic oxides (react with acids to form salt + water):

  • Na₂O, MgO

• Amphoteric oxide (reacts with both acids & bases):

  • Al₂O₃

• Acidic oxides (react with bases to form salt + water):

  • SiO₂, P₄O₁₀, SO₃

• Trend: Acidity increases across the period