It’s all about maps.
Maps are easy to understand – they make distances and relationships concrete.
Take a map of Texas. Imagine you’re in Laredo, and you want to go to Dallas. How can you get there? You have choices. A map helps you see the possibilities.
At one level of organic synthesis you can think of functional groups as being like cities on a map, and reactions that link them are like roads.
Let’s look at a reaction map for ketones (not comprehensive)
If you look at all these reactions – forward and backward – you can link functional groups to each other through these types of reactions.
You can build reaction maps for other functional groups. Here’s one for secondary alcohols.
The more reactions you learn, you’ll see that some types of functional groups (like ketones) are very “busy” – like central hubs, there’s lots of reactions that link to them (and link from them). On the other hand, other functional groups are a little like Laredo: dusty, one-horse towns in the middle of nowhere. (Of all the functional groups you learn about, ethers fall into this category the best).
Let’s go back to our problem. If you identify the functional groups involved, it can help you to identify what types of reactions are possible for getting you from your starting point to the destination.
We’re asked to go from an aldehyde to a tertiary alcohol.
Go back to the reaction map. Look for the tertiary alcohol as a product. Then work backwards. How do you get there? One way is from a ketone, in a Grignard reaction. Then think backwards from there – how to get there? One way is to oxidize a secondary alcohol. And if you trace back secondary alcohols, you can get there from the aldehyde and a Grignard reaction.
Once you know what reactions to use, it’s much easier to design your synthesis. Here, the problem is identifying what alkyl groups to use in each of the two Grignard reactions.
Here’s one key way in which roadmaps and reaction maps are different, however. In real life, you want to pick the shortest route from, say, Laredo to Dallas, especially if your car’s A/C is on the fritz in the Texas heat. And yes, in the lab, chemists will choose the shortest, most efficient route from one starting material to the final product. Thankfully, for our purposes – which is ultimately just an intellectual exercise just done “on paper” – the key lesson is just to get there. If your synthesis is longer than it needs to be – even if you end up driving through Moose Jaw, Saskatchewan on your way to Dallas – don’t worry about it too much at this point. Efficiency is a goal for later.
P.S. Here is an an excellent reaction map by Adam of the Chemistry Blog