Hard Organic Questions from AQA A-Level Chemistry Paper 2

• Organic chemistry questions make up by far the biggest proportion of difficult questions on the organic and physical paper
• The harder physical chemistry topics are on kinetics, Arrhenius in particular
• Organic mechanisms account for the highest proportion (roughly 20%) of difficult paper 2 questions
• Questions on practical chemistry (especially organic laboratory techniques) are the next highest proportion
• Organic analysis (especially NMR) and organic synthesis account for a high proportion of the most demanding questions
• As for paper 1, multistep calculations also frequently discriminate between top grades
• A large proportion of difficult paper 2 questions are based on isomers – in fact, AQA seem to have a bit of an obsession with them!

Contents

Ah, organic chemistry, with its mysterious symbolic language, complicated naming system, curly arrows, reagents and conditions.

It seems to strongly divide my students. They either love it, or hate it (I’m in the former category, but I’m biased because my PhD was in organic chemistry. Still rubbish at drawing hexagons though!).

Organic chemistry is challenging because there is a lot of content involving some difficult and abstract ideas, which must first be understood before it can be applied, and then there is a lot of application.

It’s no surprise to see that most of the hard questions are on organic mechanisms. Nor is it a surprise that practical chemistry accounts for a higher proportion of hard questions on paper 2 than paper 2. Those organic practicals have a lot more apparatus and techniques to get your head around.

NMR has a reputation for being a difficult topic, so you’d expect to see it in this guide. One of the biggest challenges with NMR is solving the questions within the time limit. AQA also specialise in these odd ‘puzzle’ questions that can be fiendishly difficult to solve quickly, for example:

(OK, I may have made that question up. But the real ones aren’t much better!)

Also, make sure you know your isomers inside out. AQA love isomers. They particularly love the joyful combination of isomers and NMR.

Let’s look at the topics that examiners say are the ones that most frequently separate “top students from less able ones” (AQA’s wording, not mine).

Amount of substance

• Combustion analysis, involving the ideal gas equation, to deduce the identity of organic compounds.
• Extended calculations using experimental data, particularly involving unfamiliar practicals.

Biological chemistry

• Drawing DNA bases correctly to show hydrogen bonding.
• Drawing nucleotides.
• Recognising and drawing disulfide bridges between peptide chains.
• Types of interactions between amino acids and peptide chains and their relative strengths.
• Drawing zwitterions.

Equilibrium

• Predicting the effect of changing reactant or product concentrations on the position of equilibrium and Kc.
• Calculating amounts/concentrations from K_c using algebra.

Kinetics

• Deducing initial concentrations based on comparative rate data.
• Understanding the sample and quench method.
• Using excess reactants to give pseudo-zero order conditions.
• Observations that signal consumption of reagents in kinetic studies.
• Calculations based on rearranging the Arrhenius equation.
• Plotting Arrhenius graphs.
• Calculating activation energy from the gradient of Arrhenius graphs

Organic analysis

• Using IR and NMR data alongside chemical information to deduce structures, particularly of isomers.
• Using ¹³C NMR to distinguish or identify isomers.
• Using ¹H NMR to distinguish or identify isomers.
• Predicting the number of NMR peaks for given structures.
• Explaining the appearance of NMR multiplets in terms of the n+1 rule.
• Explaining the chemical shifts of structural units within molecules.
• Explaining the use of solvents and the TMS standard in NMR.
• Polarity of organic molecules and impact on Rf values in chromatography.
• How solvent affects Rf values in chromatography.
• Use of HRMS to distinguish compounds.

Organic chemistry (isomers, functional groups, reactions, nomenclature)

• Identifying higher amine by-products formed from multiple substitutions in nucleophilic substitution reactions.
• Identifying functional groups in polyfunctional molecules.
• Identifying by-products of reactions, particularly those involving polyfunctional reactants.
• Identifying the products of hydrolysis (esters and amides).
• Identifying and drawing isomers from chemical and analytical data.
• Identifying and drawing stereoisomers from nucleophilic addition reactions.
• Identifying the isomeric products of elimination reactions.
• Identifying reagents and intermediate compounds in organic synthesis.
• Identifying monomers for condensation polymers
• Drawing the requested structure type correctly (displayed, skeletal etc.).
• Using functional groups tests to distinguish compounds.
• Understanding the specificity of reagents towards functional groups (e.g. NaBH₄ reduces carbonyls but not alkenes).
• Deducing the structure of cyclic molecules based on chemical and analytical data.
• Explaining the relative nucleophilicity and basicity of amines.
• Explaining the rationale behind organic mechanisms
• Writing novel mechanisms.
• Drawing correct mechanisms and intermediates for
  • Friedel-Crafts acylation
  • Electrophilic addition
  • Nucleophilic addition-elimination
• Writing equations for hydrocarbon cracking.
• Writing equations to describe radical substitution reactions.
• Writing equations for formation of biodiesel from esters.
• Writing balanced equations for organic reactions involving unfamiliar reactants (especially oxidations and reductions).
• The chemistry of biofuel production and their advantages.
• Influence of the inductive effect on rates and regioselectivity.
• The addition of electrophiles to unsymmetrical alkenes and carbocation stability.
• Explaining the stability of benzene in terms of its structure (delocalisation and resonance)

Organic nomenclature

• Nomenclature of nitriles.
• Nomenclature of alkene stereoisomers (E-Z system).
• Nomenclature of molecules with multiple functional groups.
• Nomenclature of more structurally complex cyclic molecules.

Practical chemistry

• Relating intermolecular forces to boiling points and distillation
• Strategies to increase yield when distilling liquids
• Understanding the stages of recrystallisation
• Understanding the work-up stages in organic synthesis, including use of quenching reagents and drying agents
• The apparatus and techniques involved with preparing a pure organic liquid
• Understanding the immiscibility of solvents and water
• How to accurately weigh samples
• Drawing apparatus diagrams

Organic chemistry involves concepts that you need to understand, and skills that you need to acquire and practise to achieve fluency. I talk about the important of practising skills in my blog about getting an A* in A-Level chemistry.

Skills-based topics are about knowing how to do something. This means things like laboratory skills, such as how to conduct experiments, use laboratory equipment, and carry out specific techniques. It also includes the ability to perform calculations, name organic compounds, write mechanisms, analyse NMR data, and solve problems using set routines and methods.

The concepts you learn about in organic chemistry teach you the underlying principles and theories – the why behind the procedures and phenomena.

For many topics in organic chemistry, you need to understand the principles first before you can successfully acquire the skill – for instance, you can’t mindlessly push curly arrows around and hope to get a mechanism right. You need to understand electron movement, bond formation and breaking, and the role of reaction intermediates.

It’s this blend of concepts and skills, together with all the content to learn, which makes organic chemistry a nightmare for so many students.

Here are some strategies that I hope will help you.

• One of the most important things to understand is that curly arrows are describing electron flow – how electrons move during a chemical reaction when bonds are broken and formed.
• Understanding how electrons move is so important in organic chemistry. Reactions usually involve the movement of electrons from a region of higher electron density (like a lone pair or a π-bond) to a region of lower electron density, like an atom with a complete or partial positive charge.
• You absolutely must write out organic mechanisms to master them. It’s the only way to learn them properly. As you write them, think about where the curly arrows are going and why.
• Most organic reactions can be understood in terms of interactions between nucleophiles (electron pair donors) and electrophiles (electron-pair acceptors). Recognising and predicting the behaviour of these species is key to understanding organic mechanisms.
• The reactivity of organic molecules is largely determined by their functional groups. Knowing the typical reactions of these functional groups is essential for predicting and understanding organic mechanisms, and for figuring out an unfamiliar mechanism.
• The basic types of organic mechanisms (addition, elimination, and substitution) provide you with the means to understand more complex mechanisms. Each mechanism follows certain general patterns that can be applied across specific reactions and extended to unfamiliar mechanisms.
• A lot of the why? behind organic mechanisms – the full explanation – is missing from A-Level chemistry. You are not told why the chloride ion gets kicked out in a nucleophilic addition-elimination, or why bonds appear to sometimes just break by themselves, causing H⁺ ions to fall off.  This understanding is not required – you just have to accept that’s how it is.

• NMR is mostly about application – yes, there are some concepts you need to understand (like the n+1 rule) but after that, it’s mostly skills and procedures that you must repeatedly use so you can solve NMR problems quickly in a timed exam.
• As you practise NMR questions, pay attention to the patterns that you see in ¹H NMR.
• Certain groups are common in organic chemistry and so often appear in NMR spectra as recognisable patterns.
• For example, a 3H triplet and a 2H quartet indicates the presence of a CH₃CH₂– group. Two triplets close together is often telling you there is a -CH₂CH₂– group.
• Learning to recognise these distinctive groups and their patterns in NMR can greatly reduce the time taken to deduce the structure. This is incredibly valuable because of the time pressure of organic analysis questions.

• Understanding the theory behind the practicals is essential to score highly on practical questions.
• Ensure you have detailed notes for each of the required practicals that include:
  • A labelled diagram of the apparatus.
  • An explanation of apparatus choice (e.g. why a conical flask not a beaker in a titration, why a pipette not a measuring cylinder?)
  • A step-by-step description of the technique.
  • An explanation of what each step is for.
  • The most common mistakes.
  • The impact of common mistakes (for example, what happens if you do leave the funnel in the top of the burette?)
  • Strategies for reducing percentage uncertainty (usually, this means modifications to the procedure so that larger measurements are taken – how this is done is different for each practical).
• For example, with a technique like recrystallisation, it’s not good enough to just memorise the steps.
• You’re more likely to be asked questions like, why should a minimum volume of hot solvent be used, or why the filtrate is washed with cold solvent?

• Most of the difficult questions on kinetics involve the Arrhenius equation, so make sure you know it in both its exponential and logarithmic forms.
• It’s especially important to practise plotting Arrhenius graphs and working out the activation energy from the gradient of the straight line.
• Plotting graphs by hand, and calculating gradients, isn’t something that you probably do that often, so you can easily take far too long on these questions because you’re out of practise.
• Work through practise past paper questions and make sure your calculated gradient and activation energy is within the range accepted on the mark scheme.

For strategies on tackling multistep calculations found on paper 2, see my other guides here, here and here.

Reviewing the examiners reports for paper 2, it’s clear that organic chemistry should be your priority if you’re aiming for a top grade.

Mechanisms, NMR, and organic synthesis are important focus areas for your revision, requiring dedicated practise after you’re learned the important principles.

And let’s not forget those isomers 🙂

Good understanding of the apparatus and techniques used in each of the practicals is also very important. Here, understanding is key. You can’t just rely on memorisation of procedures because they usually ask for what’s behind them.

If you’ve found this guide useful, or have anything you want to add, let me know in the comments section!