4.2.3 (b,c) Synthetic Routes

Syllabus

(b) for an organic molecule containing several functional groups:

(i) identification of individual functional groups

(ii) prediction of properties and reactions

{Learners will be expected to identify the functional groups encountered in Module 4.}

Learners will be expected to be able to devise two stage synthetic routes by applying transformations between all functional groups encountered up to this point of the specification.

{Extra information may be provided on exam papers to extend the learner’s toolkit of organic reactions.}

What does this mean?

Two-step synthesis

Any Organic substance that we wish to synthesise requires raw materials.

And it will contain one or more functional group that is unlikely to exist in the raw material.

So we must identify the functional groups we wish our product to have.

And then we can begin to work out how to produce them from the raw material.

To do this well you need to have a firm mental-map of how to inter-convert the functional groups we've seen so far.

And so far we have looked at Alkanes, Alkenes, Alcohols, Ketones, Aldehydes, Haloalkanes and Carboxylic Acids.

Learn this, or something like it.

Examiners are particularly fond of two step syntheses where you are asked to convert one substance to another via a middle substance.

Typically you will have to guess the middle compound and suggest the conditions necessary for both changes.

eg.

Starting with Ethene work out a two-step synthesis of Ethanal, including the necessary conditions and reagents for all reactions.

Starting Molecule: Ethene is an Alkene.

We know how to turn Alkenes into Alcohols, Haloalkanes and Alkanes.

Target Molecule: Ethanal is an Aldehyde

We only know how to make Aldehydes from Primary Alcohols

Reaction Pathway: Ethene →Ethanol →Ethanal

Conditions Reaction 1: Steam and Phosphoric Acid Catalyst.

Conditions Reaction 2: Distill with Acidified Dichromate

Sometimes you're given the pathway and simply have to provide the reagents and conditions.

This is easier but you still need to know your reactions well.

LEARN THEM NOW!

Predicting Properties

This is somewhat simpler than it sounds.

An examiner may give you a molecule that you have never seen and have never studied.

Nevertheless, you should be able to predict its properties and reactions because the examiner can only give you a molecule containing functional groups that you know.

So a molecule with an Alcohol group will behave like an Alcohol no matter how many other functional groups it contains.

So Lactic Acid is acidic- because it contains an acid group.

It can still be Oxidised, however, because the Alcohol group will Oxidise to a Ketone.

It will be very soluble because it is a smallish molecule that can hydrogen bond in two places.

So if you were asked to synthesise Lactic Acid from Prop-2-enoic Acid...

You could still react with Steam in the presence of Phosphoric Acid catalyst although you would make some 3-hydroxypropanoic acid as well.

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