Revise Organic synthesis for Cambridge 9701 (21.1)

At AS Level, you have learned a variety of reactions that transform one functional group into another. Success in synthesis problems relies on your fluent recall of these transformations. The main reaction types include substitution (nucleophilic), addition (electrophilic and nucleophilic), elimination, oxidation, reduction, and hydrolysis. Each reaction has specific reagents and conditions that you must know.

For any synthesis involving more than one step, a plan is essential. While you can try to work forwards from the starting material, this can lead to dead ends. A more powerful and professional approach is retrosynthesis – working backwards from the target molecule. You ask the question: 'What reaction could I do to make this target molecule?' This identifies a precursor molecule. You then repeat the process for the precursor until you arrive at the given starting material.

Target Molecule $\Rightarrow Precursor\ Molecule$

This special arrow (a retrosynthetic arrow) means 'is made from'. Once you have a complete backwards plan, you can write the synthesis in the forward direction, specifying all reagents and conditions for each step. This method provides a logical and structured way to solve complex synthesis problems.

Often, there can be more than one possible reaction pathway. The best route is usually the one with the fewest steps and the highest yield. A key aspect of control is manipulating reaction conditions. For example, the oxidation of a primary alcohol like propan-1-ol can yield either propanal (an aldehyde) or propanoic acid (a carboxylic acid). To obtain the aldehyde, we use distillation to remove it as it forms, preventing further oxidation. To obtain the carboxylic acid, we use reflux to ensure the reaction goes to completion. This demonstrates how conditions are just as important as the reagents themselves.