Book Cover Organic Chemistry 4e Carey
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Chapter 14: Organometallic Compounds

Summary | Organometallic Compounds | Preparations of Organometallic Reagents | Reactions involving Organometallic Reagents | Retrosynthesis | Self Assessment | Quiz |

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
the disconnection arrow
"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 ? THEY DON'T KNOW THEIR REACTIONS WELL ENOUGH

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 grade 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......

student response to different types of questions Forward thinking 
Fill the gap, forwards or backwards
Think backwards, but lots of help
Think backwards with no help !



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:
+  "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.


How to Plan a Synthesis.....(part 2)

Throughout this page we will collect some simple ideas to keep in mind:

There are two general types of reactions to keep in mind (however the types are not exclusive): Analysis Synthesis Tips Lets look at a generic example.... how could we prepare the following system ?
how can we synthesise this molecule ?
What is the functional group ?
a primary alcohol

Scenario 1
Lets first consider "disconnecting" the R group by breaking the C-C bond at the functional group.

This would correspond to the reaction of an organometallic reagent (which is equivalent to the carbanion, R-) with methanal (which has an electrophilic C)
Scenario 2
Alternatively we could "disconnect" an H atom by breaking a C-H bond at the functional group.
This would correspond to the reaction of a reducing agent such as lithium aluminum hydride with an aldehyde:

Let's think about disconnections in general.
When we break a bond, most likely we will be considering heterolytic reactions involving nucleophiles and electrophiles, so we need to "consider" which fragment is the nucleophile and which is the electrophile.
Consider both options, but typically one will make much more sense than the others provided you know how to recognize nucleophilic and electrophilic centers.

In order to become good at synthesis problems practice is the best tactic. (it also helps you learn your reactions)


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