OCR A-Level Chemistry A: Module 4 (Core organic chemistry) overview

What this module demands

Module 4 of OCR A-Level Chemistry A is the foundation of organic chemistry: the systematic language for naming compounds, the structural and stereo isomerism that follows from how atoms are arranged, and the reactions and mechanisms of the four core homologous series (alkanes, alkenes, alcohols and haloalkanes). It rewards confident mechanism drawing, precise recall of reagents and conditions, and the ability to read structure from infrared and mass spectra.

This guide walks through Module 4 in specification order and sets out the exam patterns OCR repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Basic concepts and hydrocarbons (4.1)

Basic concepts of organic chemistry (4.1.1) sets out the formulae (empirical, molecular, general, structural, displayed and skeletal), the idea of a homologous series and functional group, IUPAC nomenclature, and the two families of isomerism: structural (chain, position and functional group) and stereoisomerism (E/Z, decided by the Cahn-Ingold-Prelog priority rules).

Alkanes (4.1.2) are saturated hydrocarbons with only sigma bonds and tetrahedral carbon. They are fairly unreactive, undergoing complete and incomplete combustion and free-radical substitution with halogens in ultraviolet light, by the initiation, propagation and termination mechanism.

Alkenes (4.1.3) have a reactive, electron-rich $\text{C=C}$ double bond (one sigma and one pi bond, trigonal planar carbon). They undergo electrophilic addition with hydrogen, halogens, hydrogen halides and steam, follow Markownikoff's rule via the more stable carbocation, and form addition polymers.

Oxygen and halogen chemistry, synthesis and analysis (4.2)

Alcohols (4.2.1) are classified as primary, secondary or tertiary. They burn, are oxidised by acidified potassium dichromate(VI) (primary to aldehyde then carboxylic acid, secondary to ketone, tertiary not at all), are dehydrated to alkenes, and are converted to haloalkanes.

Haloalkanes (4.2.2) have a polar carbon-halogen bond and undergo nucleophilic substitution (with hydroxide, cyanide and ammonia) and elimination (with hot ethanolic hydroxide). Carbon-halogen bond enthalpy controls the rate, and the chlorofluorocarbon (CFC) story links the chemistry to ozone depletion.

Organic synthesis (4.2.3) is about planning multi-step routes between functional groups and using practical techniques (reflux, distillation, purification).

Analytical techniques (4.2.4) introduce infrared spectroscopy (identifying functional groups from characteristic absorptions) and mass spectrometry (the molecular ion peak gives the relative molecular mass and fragment ions reveal structure).

How this module is examined

A typical OCR profile for Module 4:

• Mechanism questions. Free-radical substitution, electrophilic addition (with Markownikoff reasoning) and nucleophilic substitution, all with correct curly arrows.
• Reagents and conditions. What reagent and conditions convert one functional group into another, a recall-heavy but high-yield area.
• Synthesis routes. Choosing a sequence of steps to reach a target molecule from a given starting material.
• Structure determination. Reading functional groups off an infrared spectrum and a relative molecular mass off the molecular ion peak in a mass spectrum.

Check your knowledge

A mix of mechanism, recall and analysis across Module 4. Attempt them under timed conditions, then check against the solutions.

1. Name the three stages of the free-radical substitution mechanism. (3 marks)
2. State the reagent and conditions for the electrophilic addition of bromine across an alkene, and the observation. (2 marks)
3. Give the reagent, conditions and product when a primary alcohol is heated under reflux with excess acidified potassium dichromate(VI). (2 marks)
4. State the reagent and conditions for the elimination of a haloalkane to form an alkene. (2 marks)
5. State the approximate infrared absorption (in $\text{cm}^{-1}$) of a $\text{C=O}$ bond. (1 mark)
6. Explain what the molecular ion peak in a mass spectrum tells you. (2 marks)