• Deducing structures: using combustion analysis, % composition, functional group tests and spectra (IR, MS, NMR) to work backwards to a structure.
• Planning reaction schemes: a map of how every functional group converts into every other one — this is the toolkit for synthesis questions.
• Increasing carbon chain length: using Grignard reagents (or nitriles) when the target molecule has more carbons than the starting one.
• Preparation techniques: simple distillation, steam distillation, and heating under reflux — knowing when to use which.
• Purification techniques: separating funnels, drying agents, and recrystallisation.
• Purity techniques: melting point and boiling point determination as evidence of how pure (and what) your product is.

The four-step method

Whenever you're given a mystery compound and a pile of data, always work through the same sequence — don't jump straight to guessing a structure. Examiners reward this order because each step narrows down the possibilities:

1. Find the empirical formula — the simplest whole-number ratio of atoms.
2. Determine the molecular formula — usually using the given relative molecular mass (M_r) to scale up the empirical formula.
3. Identify the functional groups present — from test-tube reactions and/or spectra.
4. Deduce the overall structure — put the pieces together into one consistent skeleton.

Combustion analysis

This is how you get from "burn a known mass of the compound" to "percentage composition of C, H and O." A known mass of the compound is burned completely in excess dry oxygen. All the carbon ends up as CO₂ and all the hydrogen ends up as H₂O — you then work backwards from the masses of those two products to find out how much carbon and hydrogen must have been in the original sample.

The 5-step method

1. Calculate the mass of carbon in the sample (from the mass of CO₂ produced).
2. Calculate the % of carbon in the sample.
3. Calculate the mass of hydrogen in the sample (from the mass of H₂O produced).
4. Calculate the % of hydrogen in the sample.
5. Deduce % oxygen = 100 − %C − %H (oxygen is found "by difference" because it doesn't show up directly in either product).

Characteristic reactions of functional groups

These are the classic "add a reagent, watch for a colour change / precipitate" test-tube reactions. They're often the fastest way to confirm which functional group is present once you have a molecular formula.

Spectral analysis

Why this matters

A huge range of organic products can all be made from a small handful of starting compounds — the trick is knowing which reagents and conditions turn one functional group into another. If you can memorise this map (or reliably reconstruct it), synthesis questions become a matter of "find the shortest path" rather than guesswork.

Aliphatic reactions table

Aromatic reactions table

Designing a reaction pathway

When you're asked to design a synthesis route (sometimes up to 4 steps), always work systematically:

1. Draw the structures of the starting molecule and the target molecule.
2. Check whether they have the same number of carbon atoms. If the target has more carbons, you'll need to lengthen the chain (see Section 4).
3. List everything you could make from the starting molecule, and everything you could make the target molecule from.
4. Find where those two lists overlap, and work out the reagents/conditions that connect them.

Preparing Grignard reagents

If your target molecule needs more carbon atoms than your starting molecule has, a nitrile route (KCN, as above) adds one carbon. But when you need a Grignard reagent, you dissolve a haloalkane in dry ether and react it with magnesium:

Grignard reagents react with carbonyl compounds and CO₂ in a two-step process (mechanism not required at this level): first an addition reaction forms an initial product, then dilute acid is added to hydrolyse it into the final alcohol or carboxylic acid.

Reactions with carbonyl compounds

Simple distillation

Used to separate compounds by boiling point — the substance with the lowest boiling point distils off first. Classic use: making an aldehyde from a primary alcohol using acidified potassium dichromate. The reaction mixture is set up with a still head and condenser (Quickfit apparatus, joints lightly greased for a good seal), heated with an electric heating mantle (safer than a naked flame for flammable organics, and gives controllable heating). The aldehyde has a lower boiling point than the alcohol (it's lost the H-bonding), so it distils off as soon as it forms — before it can be oxidised further to the carboxylic acid.

Steam distillation

Used to separate an insoluble liquid from an aqueous solution. Steam is bubbled through the mixture, mixing the layers so the insoluble compound evaporates along with the water; the resulting distillate can look cloudy if the compound isn't miscible with water.

Two key advantages:

• The insoluble liquid distils at a temperature below its normal boiling point.
• This lowers the risk of thermal decomposition of a delicate organic compound.

Heating under reflux

Many organic reactions are far too slow at room temperature. Refluxing lets you heat a reaction mixture strongly for a sustained time without losing any reactants, products, or solvent — the condenser is clamped vertically so vapours condense and drip straight back down into the flask, rather than escaping (unlike distillation, where you deliberately collect what comes off).

Typical uses: making a carboxylic acid from a primary alcohol (K₂Cr₂O₇/H₂SO₄, reflux), or making an ester from an alcohol and acid with an acid catalyst.

Solvent extraction / separating funnel

When preparing organic liquids, water is often produced alongside (or mixed with) the organic product, forming two immiscible layers. Sodium carbonate solution can be added first to neutralise the mixture and wash out acidic impurities. The mixture is transferred into a separating funnel, inverted and vented repeatedly (15–20 times) to release pressure — done slowly if neutralisation has occurred, to avoid losing product through the stopcock.

Once the layers settle, the bottom layer is run off first through the stopcock (whichever layer that is — aqueous is often, but not always, the denser bottom layer; check by adding a little extra water and seeing which layer grows).

Drying agents

Used to remove traces of water from an organic product — usually anhydrous inorganic salts that readily absorb water and hydrate.

Add a spatula of drying agent, swirl, and check: if it clumps together, there's still water present, so add more. Once some powder remains dispersed as fine, free-flowing particles, the liquid is dry — it should also look clear rather than cloudy. Then decant or filter to recover the dry liquid.

Recrystallisation

Used to purify impure solids. The principle: dissolve both the solid and its impurities in the minimum volume of hot solvent, then let the solution cool slowly. As it cools, the desired solid crystallises back out — but the impurities (present in much smaller amounts) stay dissolved in the solvent, because their solubility limit isn't reached.

1. Dissolve the impure solid in the minimum amount of hot solvent.
2. If solid impurities remain undissolved, do a hot filtration to remove them.
3. Let the solution cool to room temperature — the product crystallises out.
4. Recover the crystals by filtration — faster using Buchner apparatus (filtration under reduced pressure).
5. Wash with a small amount of fresh, cold solvent and allow to dry on filter paper.

Melting point determination

The melting point of a solid indicates both its identity (matched against known/data book values) and its purity.

• Impurities lower the melting point of a solid compared to the pure substance.
• Pure substances melt sharply, over a very narrow (well-defined) temperature range.
• Impure substances melt over a much broader range — a big gap between when melting starts and when it's complete.

Sample must be totally dry and finely powdered (crush against filter paper or a white tile to absorb moisture). Run one tube quickly first to find the approximate range, then repeat with a much slower heating rate for an accurate reading — heating too fast means you overshoot the true melting point.

Boiling point determination

Similarly, the boiling point of a liquid indicates its identity and purity, and is determined by distillation: the sample is gently heated until it boils and the temperature is recorded, then compared with literature/database values.

If the sample contains impurities:

• The boiling point may appear higher than the literature value.
• The sample may boil over a range of temperatures rather than at one single, sharp temperature.

• I can calculate % composition from combustion analysis data (mass of CO₂ and H₂O).
• I can convert % composition into an empirical formula.
• I know which test-tube reagent detects which functional group (bromine, Fehling's, 2,4-DNPH, Na₂CO₃, iodoform, acidified dichromate).
• I understand what IR, MS, ¹³C NMR and ¹H NMR each reveal about a structure.
• I can recall the reagents/conditions for at least 15 of the aliphatic interconversions.
• I know the difference between distilling vs. refluxing an alcohol oxidation, and why it matters.
• I can design a multi-step synthesis route between a starting molecule and a target molecule.
• I know how to make a Grignard reagent and can predict whether it gives a primary, secondary, or tertiary alcohol (or carboxylic acid with CO₂).
• I can describe the apparatus and process for simple distillation, steam distillation, and heating under reflux.
• I can describe how a separating funnel is used to isolate an organic layer.
• I can list common drying agents and describe how to tell when a liquid is dry.
• I can describe the full recrystallisation process, step by step, including Buchner filtration.
• I can explain how melting point range and boiling point are used as evidence of purity.