Reaction Pathways

Syllabus Statement Deduce reaction pathways given the starting material and the product. (This is the same for HL and SL – HL just have to know MORE pathways!)

Sometimes a product can’t be made directly from the given starting material. E.g. convert 1-iodopropane into propanal This can’t be done directly – we have no mechanism for converting a haloalkane into an aldehyde.

It is usually easier to approach the problem backwards! What substance CAN be made into an aldehyde? We only know 1. A primary alcohol

So we know our pathway ends . . . Alcohol  aldehyde Or more specifically . . . Propan-1-ol + [O] propanal + H2O What conditions would we use? Distil with acidified pot dichromate(VI)

So can we make a primary alcohol from 1-iodopropane? Yes! Hydrolyse with aqueous OH-

So the full reaction mechanism is . . . 1-iodopropane + OH-  1-propanol +I- 1-propanol +[O]  propanal + H2O

Another example . . . Give a pathway for producing butanone from but-2-ene. The only way of making a ketone is from a secondary alcohol . . . So we must make butan-2-ol

We can make butan-2-ol from but-2-ene by reacting the alkene with water. (addition reaction) So the full mechanism But-2-ene + H2O  butan-2-ol (this should have H+ catalyst written above the arrow, but I can’t do that on PowerPoint) Butan-2-ol + [O]  butanone (reflux with hot acidified dichromate(VI)

This can be summarised for SL

Diagram of common organic reactions . 11

And the additional bits for HL . . .

Diagram of common organic reactions 13

I find these summaries confusing. And they don’t have the conditions needed for the reactions. So I would like you to produce your own summary in the form of a table.

Starting material Finishing material Reagent and conditions Mechanism required Alkane halogenoalkane Alkane + UV light Yes dihalogenoalkane