QCE Chemistry — Unit 4 Topic 1
Organic Reactions — Flashcards & Quiz
Organic reactions and reaction pathways sit in Unit 4 Topic 1 (Properties and structure of organic materials) of the QCE Chemistry 2025 syllabus, and this subject matter can be assessed in the external examination. You need to write equations for reactions of alkanes, haloalkanes, alkenes, alcohols and carboxylic acids, classify each as addition, elimination, substitution or redox, apply Markovnikov's rule, and use chemical tests to distinguish alkanes from alkenes and primary, secondary and tertiary alcohols. Mechanisms for substitution and elimination reactions are not required.
Key Points
- Substitution (alkanes): alkane + X₂ in UV light → haloalkane + HX.
- Haloalkanes: substitution with aqueous NaOH gives an alcohol and with NH₃ gives an amine; elimination (hot NaOH in ethanol) removes HX to give an alkene.
- Addition (alkenes): the C=C bond breaks; H₂ (Ni or Pt catalyst) gives an alkane, X₂ a dihaloalkane, HX a haloalkane and H₂O (acid catalyst) an alcohol.
- Markovnikov's rule: when HX or H₂O adds to an unsymmetrical alkene, H bonds to the carbon that already has more hydrogens.
- Oxidation with acidified K₂Cr₂O₇ or KMnO₄: primary alcohol → aldehyde → carboxylic acid; secondary → ketone; tertiary does not react.
- Esterification: carboxylic acid + alcohol ⇌ ester + water (concentrated H₂SO₄ catalyst); it is reversible. A carboxylic acid and an amine form an amide.
- Tests: alkenes rapidly decolourise bromine water; dichromate turns orange → green and manganate(VII) purple → colourless with primary and secondary alcohols.
Common Mistakes to Avoid
- Confusing substitution (alkanes, haloalkanes) with addition (alkenes). They are characteristic of different functional groups.
- Leaving out conditions or catalysts (UV light, H₂SO₄, heat, Ni or Pt) when writing equations.
- Claiming tertiary alcohols are oxidised. They resist oxidation because the carbon bearing the –OH has no hydrogen atom.
- Ignoring Markovnikov's rule when adding HX or H₂O to an unsymmetrical alkene.
- Writing esterification with a single arrow. It is reversible and reaches equilibrium.
Exam Strategy
QCE Chemistry has an external examination (50%) covering Units 3 and 4, and organic reactions can be assessed in it, often as a reaction pathway question. Method: (1) identify the functional groups in the starting material and the product, (2) choose the reaction type that converts one to the other, (3) state the reagents and conditions (catalyst, heat, UV light), (4) write the balanced equation with structural formulas and IUPAC names, (5) for multi-step pathways, work backwards from the product.
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: 9 October 2026 · 19 sample flashcards · 20 sample quiz questions