QCE Chemistry — Unit 4
Organic Reactions — Flashcards & Quiz
QCE Chemistry Unit 4 covers the key organic reaction pathways: substitution on alkanes, addition to alkenes, oxidation of alcohols, and esterification. You need to predict products, write balanced equations with catalysts, and link each reaction to the functional groups involved. Fluency in IUPAC naming is assumed for reading reaction schemes.
Key Points
- Substitution (alkanes): X₂ + UV light replaces an H with a halogen; product is an alkyl halide plus HX.
- Addition (alkenes): the C=C double bond breaks and two new bonds form. Examples include bromination, hydrogenation, hydration, and HX addition.
- Oxidation of alcohols: primary → aldehyde → carboxylic acid (using KMnO₄ or K₂Cr₂O₇); secondary → ketone (stops there); tertiary resists oxidation.
- Esterification: carboxylic acid + alcohol ⇌ ester + water, catalysed by concentrated H₂SO₄. It is reversible (acidic hydrolysis returns the reactants).
- Saponification: base hydrolysis of an ester gives the sodium carboxylate salt plus the alcohol — the basis of soap making.
- Markovnikov's rule: when HX adds to an unsymmetrical alkene, H bonds to the carbon with more hydrogens (the other gets X).
Common Mistakes to Avoid
- Confusing substitution (alkanes) with addition (alkenes) — they are characteristic of different functional groups.
- Forgetting to balance the equation and include the catalyst (UV, H₂SO₄, heat).
- Claiming tertiary alcohols can be oxidised — they resist oxidation because there is no H on the carbon bearing the –OH.
- Missing Markovnikov's rule when adding HX to an unsymmetrical alkene.
- Mixing up esterification (Fischer, acid-catalysed) with saponification (base hydrolysis).
Exam Strategy
QCAA Unit 4 organic reaction questions ask you to predict products or write equations for named reactions. Method: (1) identify the starting material and functional group, (2) match to the appropriate reaction type, (3) write the product with correct IUPAC name, (4) balance the equation and include conditions (catalyst, temperature, light). Draw structures where helpful — they make mechanism clearer than words.
Sample Flashcards
Q1: What is the formula for calculating atom economy in organic reactions?
Atom economy = (molar mass of desired product / total molar mass of all reactants) × 100%. Green chemistry principle.
Q2: What is regioselectivity in organic reactions?
Preference for reaction to occur at one position rather than another possible position. Markovnikov's rule is example of regioselective addition.
Q3: What is the role of catalysts in organic reactions?
Lower activation energy, increase reaction rate, provide alternative pathway. Don't change equilibrium position. Include acids, bases, metals, enzymes.
Q4: What are the environmental considerations in organic reactions?
Solvent choice, waste production, energy consumption, toxicity of reagents/products. Green chemistry principles guide sustainable synthesis.
Q5: How do you predict the major product of organic reactions?
Consider thermodynamic stability (more stable product favored), kinetic factors (faster pathway), and selectivity rules (Markovnikov, Zaitsev).
Sample Quiz Questions
Q1: Which statement accurately describes the relationship between reaction rate and temperature in most organic reactions?
Answer: Reaction rate generally increases as temperature increases.
Increasing temperature generally increases the kinetic energy of molecules, leading to more frequent and more energetic collisions. This results in a higher proportion of molecules possessing the activation energy needed for reaction, thus increasing the reaction rate.
Q2: Alkanes primarily undergo which types of reactions under typical conditions?
Answer: Substitution and combustion
Alkanes, with their strong C-C and C-H single bonds, are relatively unreactive but undergo substitution reactions (like halogenation) and combustion reactions. Alkenes undergo addition, alcohols undergo oxidation and dehydration, and carboxylic acids/alcohols undergo esterification.
Q3: What is the primary outcome of the halogenation of alkenes using X_2 (e.g., Br_2)?
Answer: Formation of a 1,2-dihalide compound.
Halogenation of alkenes involves the addition of a halogen molecule (X_2) across the double bond, resulting in a vicinal dihalide (or 1,2-dihalide), where each carbon of the original double bond gains a halogen atom. This reaction is also used as a test for unsaturation.
Revision Tip
Organic reaction pathways are pattern recognition — drill a Revizi flashcard deck pairing each functional group with its characteristic reaction and product.
Related Concepts
Last updated: 31 August 2026 · 19 sample flashcards · 20 sample quiz questions