Organic Synthesis: Designing a Reaction Pathway (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about organic synthesis and how to design a reaction pathway.

The reaction maps show which functional groups can be turned into which, and designing a synthesis means finding a route through those maps from a starting molecule to a target.

A wide range of organic products can be made from just a few starting compounds, given the right reagents and conditions. Understanding how the different functional groups interconvert is what makes it possible to design a synthetic route. The reaction maps set out those conversions, the design method finds a route through them, and green chemistry decides which of the possible routes is the one to use.

We'll look at the aliphatic reaction map first, then the aromatic one. Then the steps involved in designing a pathway from a starting molecule to a target, and finally what makes one route better than another.

The functional groups you need are alkanes, alkenes, halogenoalkanes, alcohols, carbonyls, carboxylic acids and their derivatives, amines and nitriles. This map summarises the key transformations between them in aliphatic chemistry. Each arrow is a reaction with its own reagent, conditions and reaction type, covering substitution, addition, elimination, oxidation and reduction. Every aliphatic synthesis you design is a route traced across this map.

Aromatic compounds have their own map. It starts from benzene, runs through intermediates including benzaldehyde and nitrobenzene, and reaches products such as an azo dye. Most of the steps are electrophilic substitutions. The rest are a free radical substitution, a reduction, a diazotisation and a coupling. You read it the same way you read the aliphatic map by finding the functional group you want, then working back to something you can make.

Designing a reaction pathway means planning a sequence of reactions that converts a starting molecule into a target molecule as efficiently as possible. Begin by drawing the structures of both. Compare the number of carbon atoms in each. If the target contains more carbon atoms, the carbon chain needs extending, and one method is to introduce a nitrile group by nucleophilic substitution, which increases the chain length by one carbon.

Next, identify the functional groups present in the starting compound and determine which reactions it can undergo. Then consider which functional groups are present in the target molecule and how they can be formed. Look for intermediate compounds that could link the starting material to the target. Match the functional groups carefully, and specify the appropriate reagents and conditions for each step in the pathway.

Chemists often have several possible ways to make a target molecule, and the best route balances efficiency, safety and sustainability. Choosing a route with fewer steps reduces waste and energy use, which is better for the environment and more cost-effective. Reactions with a high atom economy are preferred, because they maximise the use of the starting materials. Safer solvents and less hazardous reagents are also key considerations. These are the twelve principles of green chemistry, and they decide which of the possible routes through the maps is the one worth using.

You could be asked to design a synthesis with up to four steps. Being able to combine these reactions in multi-step syntheses is a key exam skill, so practise working backwards from target molecules.

The reaction maps show which functional groups convert into which, in both aliphatic and aromatic chemistry. To design a pathway, draw the starting and the target molecule, compare their carbon chains, and extend the chain with a nitrile group if the target is longer. Match the functional groups, then give the reagents and conditions for each step. Where more than one route exists, the one with fewer steps, higher atom economy and safer reagents is the one to choose. The maps tell you what is possible, the method finds a route, and green chemistry decides which route to take.

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