Organic synthesis (A Level only)

Welcome to Organic Synthesis! The Ultimate A-Level Challenge

Hello future chemist! If the previous chapters were about learning the individual steps of organic chemistry, then this chapter—Organic Synthesis (Topic 36)—is where you learn to put it all together. Think of it like this: you've mastered the rules of chess (individual reactions), and now you have to play the full game (multi-step synthesis).

This topic is perhaps the most challenging, but also the most rewarding. It requires you to recall reagents, conditions, and mechanisms from every previous organic chapter and apply them strategically.

Don't worry! We will break down the strategies needed to successfully plan and analyze complex synthetic routes.

Key Takeaway: Synthesis is a Puzzle

Synthesis is the skill of transforming one molecule into another using a sequence of known reactions. The goal is to figure out the path (reagents and conditions) that gets you from the Starting Material to the Target Molecule.

I. Analyzing the Building Blocks: Functional Groups

Before planning any synthesis, the very first step is always to look closely at the molecules involved.

1. Identifying Functional Groups (Syllabus 36.1(a))

When given a complex molecule, you must be able to instantly spot and name all the functional groups present, even if there are several in one molecule.

• Example: A molecule might contain both an alcohol (-OH), an ester (-COO-), and a carbon-carbon double bond (C=C).
• Why this matters: Each functional group determines a specific set of chemical properties and reactions.

2. Predicting Properties and Reactions (Syllabus 36.1(b))

Once identified, you must predict how each group will react under different conditions. This means recalling:

• Reaction Type: Is it Nucleophilic Substitution (SN), Electrophilic Addition (AE), Oxidation, or Hydrolysis?
• Reagents and Conditions: What specifically is needed to make this reaction happen? (e.g., \( \text{H}_2\text{SO}_4 \text{ catalyst} \), heat, or UV light).

II. Strategy for Multi-Step Synthesis (Syllabus 36.1(c))

Devising a synthetic route means planning a sequence of reactions to reach your target. You must think like a detective and often work backwards—a process called Retrosynthesis.

1. The Retrosynthesis Approach: Working Backwards

Instead of starting with the initial molecule, start with the Target Molecule and ask: "What was the immediate precursor (the molecule right before it)?"

Step-by-Step Guide to Planning a Route:

Step 1: Compare. Compare the structure of the starting material and the target molecule.

• What functional groups have been added, removed, or changed?
• Has the carbon chain length changed?

Step 2: Go Backwards. Look at the target molecule's functional group. What is the standard reaction used to make that group? Identify the precursor.

Step 3: Repeat. Does the precursor match the starting material? If not, repeat Step 2, moving further backwards until you reach the starting material.

Step 4: Write Forwards. Once the path is mapped backwards, write the synthesis in the correct, chronological (forwards) order, clearly stating the reagents and conditions for each step.

Analogy: Imagine you want to bake a cake (Target). You realize the last step is icing it, so the precursor is the sponge. To make the sponge, you need ingredients and heat (the starting materials and conditions). Working backwards ensures you don't miss any necessary steps.

2. Common Chain-Length Changes (Crucial Steps)

Changing the number of carbon atoms is often a key marker for a synthesis question. Only a few reactions in the syllabus can achieve this:

• Chain Lengthening (Adding 1 Carbon): The key intermediate is usually a nitrile.
  \( \text{Halogenoalkane} \xrightarrow{\text{KCN in ethanol, heat}} \text{Nitrile} \)
  The nitrile can then be hydrolysed to a carboxylic acid, or reduced to an amine, maintaining the longer chain.
• Chain Shortening (via Decarboxylation): While not a core single reaction in this syllabus, most synthesis focuses on lengthening the chain. If a chain reduction is needed, it might involve oxidation leading to the loss of \( \text{CO}_2 \) or fragmentation, but typically, multi-step routes involve nitrile synthesis to grow the chain.

III. Analyzing Given Synthetic Routes (Syllabus 36.1(d))

Sometimes, the exam provides the route and asks you to analyze it. This involves identifying the specific role of the reagents and predicting unwanted outcomes.

1. Identifying Reaction Type and Reagents

For each arrow in a synthesis diagram, you must define:

1. The Reagent (e.g., \( \text{NaOH(aq)} \), \( \text{KCN(ethanolic)} \)).
2. The Conditions (e.g., room temperature, heat, reflux, UV light).
3. The Type of Reaction (e.g., Nucleophilic Substitution, Reduction, Hydrolysis).

Common Mistake to Avoid: Confusing aqueous (aq) and ethanolic (ethanol) reagents.

Ethanolic reagents (like \( \text{NaOH} \) in ethanol) favor elimination reactions (making alkenes).

Aqueous reagents (like \( \text{NaOH(aq)} \)) favor nucleophilic substitution reactions (making alcohols).

2. Predicting Possible By-Products

In a synthetic route, the main product is what you are aiming for, but often side reactions occur or unwanted products are formed. Recognizing these is key to explaining why certain conditions (like distillation vs. reflux) are used.

• Example: Oxidation of Primary Alcohols:
  • If you want an aldehyde, you use acidified \( \text{K}_2\text{Cr}_2\text{O}_7 \) and distil the product immediately. By-product is the carboxylic acid if the aldehyde is left to react further.
  • If you want a carboxylic acid, you use the same reagent but heat under reflux to ensure complete oxidation.
• Example: Free Radical Substitution: Reaction of an alkane with \( \text{Cl}_2 \) and UV light leads to a mix of mono-substituted, di-substituted, and poly-substituted products (e.g., chloroethane, dichloroethane, etc.), making separation difficult.

IV. The Challenge of Selectivity: Molecules with Multiple Functional Groups

When a molecule has more than one functional group (e.g., an amino acid has both amine and carboxylic acid groups), predicting its reactions requires understanding relative reactivity.

1. Differential Reactivity

A reagent that reacts strongly with one group might leave a weaker group untouched, or vice versa.

• Carbonyls vs. Alkenes:
  • The C=C bond in an alkene undergoes electrophilic addition.
  • The C=O bond in an aldehyde/ketone undergoes nucleophilic addition.
  • A strong reducing agent like \( \text{LiAlH}_4 \) reduces the carbonyl (C=O) but typically leaves the C=C alkene bond intact unless specialized conditions are used (which are outside the scope of 9701).
• Halogenoalkanes (C-X) Reactivity:
  • Halogenoalkanes are highly reactive towards nucleophilic substitution (SN1/SN2 mechanisms).
  • Halogenoarenes (where the X is directly attached to the benzene ring) are very unreactive due to the overlap of the C-X p-orbital with the delocalised ring system, which strengthens the C-X bond (Syllabus 31.1.2). Therefore, a reaction intended for an alkyl halide will likely ignore an aryl halide in the same molecule.

2. Planning Synthesis to Avoid Side Reactions

The key to multi-functional synthesis is choosing reagents that are selective.

Example: Reducing an aldehyde without touching a C=C bond:

If you have an unsaturated aldehyde and want the unsaturated alcohol, you must use a mild reducing agent like \( \text{NaBH}_4 \). This reagent is powerful enough to reduce the \( \text{C=O} \) group but usually too weak to break the \( \text{C=C} \) bond.

Using the very strong reducing agent \( \text{H}_2/\text{Ni} \) or \( \text{LiAlH}_4 \) would likely reduce both the \( \text{C=O} \) and the \( \text{C=C} \) bond, giving an unwanted saturated alcohol product.

V. Synthesis Practice Route Example

Let's plan a two-step synthesis: from ethanol to ethanoic acid.

Target: Ethanoic Acid (Carboxylic Acid)

Starting Material: Ethanol (Primary Alcohol)

Step 1: Check the Difference

Ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)) has two carbons. Ethanoic acid (\( \text{CH}_3\text{COOH} \)) also has two carbons. The chain length is unchanged. We need to perform oxidation.

Step 2: Determine Route

Primary alcohols can be oxidised in two stages:

1. Primary alcohol \( \rightarrow \) Aldehyde
2. Aldehyde \( \rightarrow \) Carboxylic acid

Since we want the final product (the carboxylic acid), we need the vigorous conditions for complete oxidation.

Step 3: Define Reagents & Conditions

Reaction: Complete oxidation of a primary alcohol.

Reagents: Acidified potassium dichromate(VI) or acidified potassium manganate(VII).

Conditions: Heat under reflux (Reflux ensures the intermediate aldehyde does not escape and is fully oxidised).

The Synthetic Route:

\( \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{acidified } \text{K}_2\text{Cr}_2\text{O}_7 \text{ or } \text{KMnO}_4} \text{CH}_3\text{COOH} \)
\( \text{Conditions: Heat under reflux} \)

VI. Comprehensive Review: Organic Synthesis Checklist

To succeed in Organic Synthesis, use the following checklist as a study tool:

• Chain Length: Did the carbon chain change? If yes, look for Nitrile formation (to lengthen) or other key steps.
• Functional Group Change: Can I convert the old group to the new group in one step (Oxidation/Reduction)?
• Location Change: Did the functional group move position? (Often requires a two-step Elimination followed by Addition, e.g., alcohol \( \rightarrow \) alkene \( \rightarrow \) different alcohol).
• Reagent Specificity: Did I choose the right conditions (e.g., \( \text{NaOH(aq)} \) vs. \( \text{NaOH(ethanolic)} \))?
• Stereochemistry: Do I need to worry about cis/trans or optical isomers? (If you form a chiral centre, remember you will likely form a racemic mixture).
• Aromatic vs. Aliphatic: Remember that halogenoarenes are unreactive compared to halogenoalkanes (Topic 31.1.2).

Practice linking reactions together. Every single organic reaction learned previously is a potential step in a synthesis route!