Organic Synthesis (OCR A-Level Chemistry A): Revision Notes

Synthetic routes

What is organic synthesis?

Organic synthesis is the process of building complex molecules from simpler starting materials. This technique is fundamental to modern chemistry and has enormous practical applications, particularly in the pharmaceutical industry. When you take medicine for a headache or illness, there's a good chance that drug was created through organic synthesis.

The key idea is that chemists can transform simple, readily available chemicals into sophisticated molecules with specific properties. This might involve just one step, but more often requires multiple steps where functional groups are systematically changed or added to build up the desired structure.

Recognising functional groups

Before you can plan any synthesis, you must be able to identify the different functional groups present in both your starting materials and target molecules. A functional group is the reactive part of a molecule that determines its chemical properties and reactions.

Here are the key functional groups you need to know for A-Level chemistry:

Identifying functional groups in complex molecules

When you look at a molecule, scan through it systematically to find each functional group. Let's look at an example:

This molecule contains three functional groups:

• An alcohol group ($\ce{-OH}$) - highlighted in yellow
• An alkene group ($\ce{C=C}$) - the carbon-carbon double bond
• A carboxylic acid group ($\ce{-COOH}$) - highlighted in peach/orange

Another example:

This molecule contains:

• An acyl chloride group (the $\ce{C=O}$ bonded to $\ce{Cl}$) - highlighted in coral
• Additional reactive sites shown in the colour coding

Being able to spot all functional groups present is the first critical step in planning any synthesis route.

Understanding synthetic routes

The flowchart below shows all the main interconversions between functional groups that you've studied. This is one of the most important diagrams you'll use in organic chemistry:

Key features to notice:

• Alcohols sit at the centre because they can be converted to many other functional groups
• Primary alcohols can be oxidised to aldehydes (with distillation) or carboxylic acids (with reflux)
• Secondary alcohols can be oxidised to ketones
• Alkenes are versatile starting points that can form alkanes, haloalkanes, or alcohols
• Haloalkanes can be converted to alcohols through nucleophilic substitution

Predicting properties and reactions

Understanding what reactions each functional group undergoes helps you predict synthetic routes. For example, consider the compound prenol, which contains both an alkene and a primary alcohol:

Because prenol contains multiple functional groups, it can undergo different types of reactions:

Planning two-stage syntheses

What is a target molecule?

A target molecule is the compound you're trying to make through organic synthesis. In a simple synthesis, you might be able to make the target molecule directly from your starting material in just one step. However, most syntheses require multiple steps because you need to change functional groups or add carbon atoms to build up the structure you want.

The two-stage synthesis method

For A-Level, you'll typically work with two-stage syntheses. To plan these successfully, you need to:

1. Identify the functional groups in both your starting molecule and target molecule
2. Identify the intermediate compound that links the starting and target molecules together
3. State the reagents and conditions needed for each step of the synthesis

The intermediate is the "stepping stone" molecule - it's what you make in the first step so that you can then convert it to your target in the second step.

Worked examples of two-stage synthesis

Important points about oxidation conditions

This equation shows the further oxidation of an aldehyde to a carboxylic acid, which is why distillation is used when you want to stop at the aldehyde stage.

Advanced technique: ozonolysis

This technique can be incorporated into multi-step syntheses. For example, to make propanoic acid from pent-3-ene:

Step 1: Ozonolysis $\ce{C2H5CH=CHC2H5 + O3 + H2O → 2C2H5CHO + H2O2}$

This breaks the double bond to form propanal (an aldehyde).

Step 2: Oxidation
$\ce{C2H5CHO + [O] → C2H5COOH}$

Reagents and conditions: Reflux with acidified potassium dichromate(VI) ($\ce{K2Cr2O7/H2SO4}$)

This oxidises the aldehyde to propanoic acid (a carboxylic acid).

Remember!