Ch 14: How to Plan a Synthesis

Chapter 14: Organometallic Compounds

How to Plan a Synthesis.....(part 1)

The ideas collected here are based on the work of E.J.Corey (Nobel Prize 1990) who was one of the pioneers at trying to design strategies for the synthesis of complex organic molecules.

"Retrosynthesis" means planning a synthesis backwards, by starting at the product, the "target" and taking it back a step at a time to simple, available starting materials or precursors.
In general students dislike these problems because it requires "thinking backwards", good problem solving skills, and a good knowledge of their organic reactions.
In order to "plan" a synthesis, we can break the target down by making a series of "disconnections" - these steps are the reverse of synthetic steps or reactions.

Why do you think most students struggle with synthesis questions ? ANSWER

By now you will probably have written at least one exam that had a lot of reactions on it.... and many of you will have gone into the exam thinking you knew your reactions, but still got a mark you were less than happy with.

Part of the reason for this is shown by the following cartoon that depicts a typical student response to various organic question types......

	Forward thinking
	Fill the gap, forwards or backwards
	Think backwards, but lots of help
	Think backwards with no help !

The MORAL ?

KNOW YOUR REACTIONS !

Using flashcards that YOU make yourself can be a convenient way to learn.
Don't under estimate the power of writing down information yourself as a learning process.
Reading a text book alone is NOT enough.

An instructor will probably expect that you know your reactions inside out and literally back to front. An idea that may help you to prepare for this is treating reactions as a "triangle" of information that connects the starting material, the product and the reagent.

think of starting material, reagent and product all depending on each other

What this is telling you is that certain starting materials can be converted to particular products using the appropriate reagent. Knowing any pair should be enough to give you a good idea of what the third piece of the puzzle is.
This is a lot like a simple math equation relating numbers to each other, for example:

7 + "x" = 11
so "x" = 4

R-Br + "x" = R-I so "x" = I^-

Note that unlike many simple math problems, organic problems often don't have one single solution.