What a learner can do afterwards
- Orders steps so that each reagent meets only the groups it tolerates
- Inserts protection and deprotection where a reagent would otherwise attack the wrong site
- Multiplies stage yields to get an overall figure and argues for a shorter route on that basis
- Identifies the step most likely to fail on scale and proposes an alternative
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You think in routes, not single reactions. Each reagent you pick must tolerate every other group present at that moment, so order decides success or failure. If you oxidise too early, a later Grignard will attack the carbonyl you just made, so you reorder the steps or mask the group first. You build fragile rings late so they survive the fewest steps.
When a later reagent would attack a group you want to keep, you mask it first, run the step, then reveal it. You hide an aldehyde as an acetal and restore it later with mild acid, or mask an alcohol as a silyl ether. Ethers work well as protection because they are famously unreactive.
You multiply fractional yields to get your overall figure. Three stages at 80 percent give 0.8 times 0.8 times 0.8, about 51 percent. A ten step route at 80 percent per step delivers only about 11 percent overall, while a six step route at the same stage yield delivers 26 percent, so you argue for the shorter route. When a stage needs extreme cold, explosive reagents, or endless chromatography, you redesign around it rather than forcing it through.
You isolate each stage by distillation or recrystallization before the next begins, since impurities carried forward can poison later steps.
You order steps to protect earlier work, hide reactive groups, multiply yields to favour short routes, and redesign risky steps.
More to read
Synthesis, Distillation, & Recrystallization: Crash Course Organic Chemistry #40
A multi step synthesis is a route, not a single reaction. Each reagent you pick must tolerate every other functional group in the molecule at that moment, which is why the order of steps decides success or failure. Oxidise too early and a later Grignard reagent will attack the carbonyl you just made. Nitrate after installing an activator and the new group lands where you want it. Purification matters just as much. Distillation and recrystallization isolate the product of each stage before the next begins, because impurities carried forward can poison later steps.
Medicinal Chemistry and Penicillin Total Synthesis: Crash Course Organic Chemistry #50
Total synthesis of medicines such as penicillin shows every planning principle working at once. Fragile rings must be built late so they survive the fewest possible steps. Reactive side chains are masked with protecting groups and revealed at the end. Above all, the route is kept short, because each extra stage multiplies the losses. A ten step route at 80 percent per step delivers only about 11 percent overall, while a six step route at the same stage yield delivers 26 percent. When a step looks likely to fail on scale, through extreme cold, explosive reagents, or endless chromatography, a good chemist redesigns around it rather than forcing it through.
15: Aldehydes and Ketones
Carbonyl compounds sit at the centre of most synthesis plans because they can be built up or broken down in so many ways. Grignard reagents add to aldehydes and ketones to make larger alcohols. Enolates alkylate at the alpha position. Oxidation climbs from alcohol to carbonyl to acid, and reduction descends the same ladder. Because carbonyls react with so many reagents, they are also the groups most often needing protection. Converting an aldehyde to an acetal hides it while the rest of the molecule is transformed, and mild acid later restores it.
14.1: Alcohols, Phenols, and Ethers
Alcohols appear in countless target molecules, and their acidic proton makes them both useful and troublesome in synthesis. Any strongly basic reagent, from Grignard reagents to enolates, will simply deprotonate a free OH instead of doing the planned chemistry. That is why alcohol protection is one of the first skills a synthesis planner learns. Masking the alcohol as a silyl ether or an acetal lets the rest of the route proceed untouched. Phenols are more acidic still and need the same care. Ethers, by contrast, are famously unreactive, which is exactly why they make such good protecting groups and solvents.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.