Revise Organic synthesis for Cambridge 9701 (36.1)

Organic synthesis is the art and science of constructing complex organic molecules from simpler, readily available starting materials. It is not a random process of mixing chemicals but a logical, strategic exercise akin to a game of chess or solving a complex puzzle. The primary goal is to devise a reaction sequence, or 'synthetic pathway', that efficiently converts simple precursors into a desired 'target molecule'. An ideal synthesis is characterised by a high overall yield, a small number of steps, the use of inexpensive and safe reagents, and good atom economy. To achieve this, chemists must have a deep understanding of a wide range of reactions and apply a strategic approach to planning the route before any practical work begins.

Instead of guessing a starting point, modern synthesis is planned using retrosynthetic analysis. This involves working backwards from the target molecule. We perform imaginary 'disconnections' of bonds in the target, guided by our knowledge of reliable bond-forming reactions. This process breaks the complex molecule down into simpler precursors. Each disconnection is represented by a special retrosynthetic arrow (=>). The disconnection leads to idealised fragments called 'synthons', which don't usually exist but represent the required reactivity. We then identify the real-world reagents, or 'synthetic equivalents', that correspond to these synthons. For example, disconnecting the C-OH bond of a secondary alcohol suggests it could be made from an aldehyde and a Grignard reagent. This process is repeated until we arrive at simple, commercially available starting materials.

A significant part of any synthetic pathway involves Functional Group Interconversion (FGI). This is any step that converts one functional group into another without altering the carbon skeleton of the molecule. FGI is crucial for manipulating the reactivity of a molecule to set it up for a subsequent, often more complex, reaction. For instance, you might need to form an ester, but your starting material is an alkane. The pathway would involve FGI steps: free-radical substitution of the alkane to a haloalkane, hydrolysis to an alcohol, and then oxidation to a carboxylic acid, which can finally be esterified. Recognising the required FGI sequence is a key skill in planning a synthesis.

Sometimes a reagent required to modify one part of a molecule is incompatible with another functional group present. For example, attempting to reduce an ester to an alcohol using LiAlH₄ in a molecule that also contains a ketone would reduce both groups. To solve this, we use a 'protecting group'. This involves a three-stage strategy: 1) 'Protect' the sensitive group by converting it into an unreactive derivative. 2) Carry out the desired reaction on the other part of the molecule. 3) 'Deprotect' the molecule to regenerate the original functional group. For instance, the acidic -OH of a phenol can be protected by converting it to an ester before nitrating the ring, preventing unwanted side reactions. The ester is then hydrolysed to restore the phenol.

Instead of starting with simple molecules and seeing what you can make, a synthetic chemist starts with the desired product, the 'target molecule'. They then work backwards, step-by-step, breaking it down into simpler precursors. This process is called retrosynthesis. Each backward step is a 'disconnection', often at a key functional group. This continues until you reach simple, readily available starting materials.

Your ability to design a synthesis depends entirely on your knowledge of the organic reaction 'toolkit'. You must be fluent in converting one functional group to another. The map of organic reactions you have built throughout the course is your primary resource here.

Sometimes, a reagent needed for one transformation will also react with another functional group in the same molecule. For example, if you want to reduce an ester to an alcohol but the molecule also contains a ketone, a strong reducing agent like $LiAlH_4$ would reduce both. To solve this, we can temporarily 'protect' the ketone, perform the ester reduction, and then 'deprotect' the ketone to restore it. At A-level, you are not required to know the specific reactions for protection/deprotection, but you must understand the concept and why it is necessary.