Organic synthesis

Organic Synthesis: Building Molecular Puzzles (9701 A Level Chemistry)

Welcome to the exciting world of Organic Synthesis! This chapter isn't about learning new reactions, but about applying all the reactions you've learned in the AS and A Level organic syllabus (Alkanes, Alkenes, Alcohols, Carbonyls, Carboxylic Acids, Nitrogen Compounds) to create a target molecule.

Think of organic synthesis as playing with LEGO bricks. You know what each brick (reaction) does, and now you are the architect, deciding which bricks to use and in what order to build your desired structure.

Don't worry if this seems tricky at first—it requires practice and systematic thinking. We will break down the strategies needed to master multi-step conversions.

1. The Synthesis Mindset: Retrosynthesis

When faced with a complex synthesis problem, most students try to work forward: "What happens if I react A with B?" This often leads to dead ends.

Professional chemists use retrosynthesis: working backwards from the target molecule.

1.1 The Backward Strategy (The P.A.C. Method)

Imagine you want to go from Compound A to Target Compound Z.

P.A.C. Mnemonic for Synthesis Planning:

• P: Product (Target): What is the final functional group?
• A: Anchor (Precursor): What is the immediate precursor molecule that can form this product in a single, known reaction?
• C: Connection: How can I make that precursor (the Anchor) from my Starting Material (A)?

Example: Making a Primary Amine from an Alcohol.
The Primary Amine (Z) is usually made by reducing a Nitrile (Anchor). How is the Nitrile made? From a Halogenoalkane. How is the Halogenoalkane made? From the original Alcohol (A).

Alcohol \(\rightarrow\) Halogenoalkane \(\rightarrow\) Nitrile \(\rightarrow\) Amine

2. Devising Multi-Step Synthetic Routes (Syllabus 36.2)

Devising a route involves linking known reactions together. Routes are often categorized by whether they involve changing the functional group or changing the carbon chain length.

2.1 Synthesis Strategy 1: Changing Functional Groups

This is often straightforward, using oxidation or reduction reactions.

The Oxidation/Reduction Ladder

This is the most fundamental concept for converting between oxygen-containing functional groups.

1. Primary Alcohols (\(1^{\circ}\))

\(1^{\circ}\text{ Alcohol } \overset{[\text{O}]}{\underset{\text{Distil}}{\longrightarrow}} \text{ Aldehyde } \overset{[\text{O}]}{\underset{\text{Reflux}}{\longrightarrow}} \text{ Carboxylic Acid}\)

Reagents ([O]): Acidified Potassium Dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7\)) or Potassium Manganate(VII) (\(\text{KMnO}_4\)).

2. Secondary Alcohols (\(2^{\circ}\))

\(2^{\circ}\text{ Alcohol } \overset{[\text{O}]}{\underset{\text{Reflux}}{\longrightarrow}} \text{ Ketone}\)

Key Point: \(\text{K}_2\text{Cr}_2\text{O}_7\) turns from orange to green when an alcohol is successfully oxidised.

3. Reductions
To go down the ladder (e.g., Carboxylic Acid \(\rightarrow\) Alcohol), you use strong reducing agents.

• Carboxylic Acid \(\overset{\text{LiAlH}_4}{\longrightarrow}\) Primary Alcohol
• Aldehyde/Ketone \(\overset{\text{NaBH}_4 \text{ or } \text{LiAlH}_4}{\longrightarrow}\) Alcohol (Primary/Secondary, respectively)

Analogy: Think of the reduction steps as taking the elevator down from the 3rd floor (Carboxylic Acid) to the 1st floor (Alcohol). \(\text{LiAlH}_4\) is the super-fast elevator!

2.2 Synthesis Strategy 2: Changing Carbon Chain Length

In many synthetic problems, you need to add or subtract a carbon atom.

A. Adding a Carbon Atom (Nitrile & Cyanohydrin Synthesis)

Extending the carbon chain by one carbon is achieved mainly through two key pathways:

Pathway 1: Halogenoalkane to Nitrile (Nucleophilic Substitution)
Convert the starting material into a halogenoalkane, then react with cyanide:

\(\text{Halogenoalkane } \overset{\text{KCN in ethanol, heat}}{\longrightarrow} \text{ Nitrile}\)

Pathway 2: Carbonyl to Hydroxynitrile (Nucleophilic Addition)
Aldehydes and ketones react with \(\text{HCN}\) (in the presence of \(\text{KCN}\) / alkali catalyst) to form 2-hydroxynitriles (cyanohydrins):

\(\text{RCHO} + \text{HCN} \overset{\text{KCN/OH}^-}{\longrightarrow} \text{RCH(OH)CN}\)

Converting the Nitrile Group:
The nitrile group (\(-\text{C}\equiv\text{N}\)) can subsequently be transformed into two crucial products:

• Carboxylic Acid: Nitrile \(\overset{\text{Dilute acid (e.g., } \text{H}_2\text{SO}_4\text{), reflux}}{\longrightarrow}\) Carboxylic Acid (\(\text{R}-\text{COOH}\))
• Primary Amine: Nitrile \(\overset{\text{LiAlH}_4 \text{ or } \text{H}_2/\text{Ni}}{\longrightarrow}\) Primary Amine (\(\text{R}-\text{CH}_2\text{NH}_2\))

B. Removing Carbon Atoms (Carboxylic Acid Decarboxylation & Cleavage)

While less common in standard A Level synthesis questions, synthesis routes can shorten chains via oxidative cleavage:

• Oxidation of Alkenes (Hot, concentrated, acidified \(\text{KMnO}_4\)): This breaks the \(\text{C}=\text{C}\) double bond, producing ketones, carboxylic acids, or \(\text{CO}_2\) (if terminal).

3. Analyzing Synthetic Routes (Syllabus 36.3)

Examiners often present a multi-step route and ask you to analyze it. You must systematically explain three things for each step:

3.1 Identifying Reaction Type and Reagents

For every arrow in a synthetic route, ask:

• Reagents: What chemicals are added? (e.g., \(\text{Br}_2\), \(\text{NaOH(aq)}\), \(\text{KMnO}_4\)).
• Conditions: Is heat required? Is the mixture distilled or refluxed? Is a catalyst needed (e.g., concentrated \(\text{H}_2\text{SO}_4\), \(\text{AlCl}_3\), \(\text{UV}\) light, \(\text{Pt}/\text{Ni}\))?
• Reaction Type: What category of reaction is it? (e.g., Nucleophilic Substitution, Electrophilic Addition, Hydrolysis, Oxidation, Condensation, Addition-Elimination).

Common Pitfall to Avoid: Confusing aqueous and ethanolic \(\text{NaOH}\).

• \(\text{NaOH(aq)}\) + heat: Favours Nucleophilic Substitution (\(\text{Halogenoalkane} \rightarrow \text{Alcohol}\)).
• \(\text{NaOH}\) in ethanol + heat: Favours Elimination (\(\text{Halogenoalkane} \rightarrow \text{Alkene}\)).

3.2 The Importance of By-products

Although not always required in the final answer, understanding the by-products confirms you know the stoichiometry and mechanism.

• In Electrophilic Addition of \(\text{HBr}\) to an alkene, there are no significant organic by-products, only the haloalkane product.
• In Hydrolysis of an ester using dilute acid, the by-products are the original alcohol and carboxylic acid.
• In reactions involving Acyl Chlorides (Addition-Elimination), the primary by-product is usually a mineral acid like \(\text{HCl}\).
  Example: \(\text{RCOCl} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{HCl}\)

4. Advanced Connections: Common 'Hub' Molecules

In synthesis, certain functional groups act as critical "hubs" because they are easy to produce and can be easily converted into many other groups. Master the conversion paths in and out of these three hubs:

4.1 Hub 1: The Halogenoalkane

Halogenoalkanes (\(\text{R}-\text{X}\)) are synthesis gold because the polarised \(\text{C}-\text{X}\) bond is highly reactive to nucleophiles.

• In: Made from an \(\text{Alcohol}\) (e.g., using \(\text{PCl}_5\)) or an \(\text{Alkene}\) (Electrophilic addition of \(\text{HX}\)).
• Out (Substitution Pathways - using \(\text{NaOH(aq)}\)):
  • \(\text{R}-\text{X} \rightarrow \text{Alcohol}\) (\(\text{NaOH(aq)}\), heat)
  • \(\text{R}-\text{X} \rightarrow \text{Nitrile}\) (\(\text{KCN}\) in ethanol, heat) \(\rightarrow\) (further steps to \(\text{COOH}\) or \(\text{NH}_2\))
  • \(\text{R}-\text{X} \rightarrow \text{Amine}\) (\(\text{NH}_3\) in ethanol, heated under pressure)
• Out (Elimination Pathway - using \(\text{NaOH}\) in ethanol):
  • \(\text{R}-\text{X} \rightarrow \text{Alkene}\) (\(\text{NaOH}\) in ethanol, heat)

4.2 Hub 2: The Alkene

Alkenes (\(\text{C}=\text{C}\)) are reactive due to the \(\pi\) bond and undergo electrophilic addition.

• In: Made from an \(\text{Alcohol}\) (Dehydration, conc. \(\text{H}_2\text{SO}_4\)/heat or \(\text{Al}_2\text{O}_3\)/heat) or a \(\text{Halogenoalkane}\) (Elimination, \(\text{NaOH}\) in ethanol/heat).
• Out (Addition Pathways):
  • \(\text{Alkene} \rightarrow \text{Alkane}\) (\(\text{H}_2, \text{Pt}/\text{Ni}\), heat)
  • \(\text{Alkene} \rightarrow \text{Halogenoalkane}\) (\(\text{HX(g)}\), room temp.)
  • \(\text{Alkene} \rightarrow \text{Alcohol}\) (\(\text{Steam}, \text{H}_3\text{PO}_4\) catalyst)
  • \(\text{Alkene} \rightarrow \text{Diol}\) (Cold, dilute, acidified \(\text{KMnO}_4\))

4.3 Hub 3: The Carboxylic Acid / Acyl Chloride

Carboxylic acids (\(\text{RCOOH}\)) and their derivatives (like \(\text{RCOCl}\)) allow for important condensations and changes in the oxygen-containing functional group.

• Carboxylic Acid Reactions:
  • \(\text{RCOOH} \rightarrow \text{Ester}\) (Esterification with alcohol, conc. \(\text{H}_2\text{SO}_4\))
  • \(\text{RCOOH} \rightarrow \text{Acyl Chloride}\) (using \(\text{PCl}_5, \text{PCl}_3\) or \(\text{SOCl}_2\))
  • \(\text{RCOOH} \rightarrow \text{Primary Alcohol}\) (\(\text{LiAlH}_4\))
• Acyl Chloride Reactions (Very fast and useful):
  • \(\text{RCOCl} \rightarrow \text{Carboxylic Acid}\) (\(\text{H}_2\text{O}\), room temp.)
  • \(\text{RCOCl} \rightarrow \text{Ester}\) (\(\text{Alcohol}\) or \(\text{Phenol}\))
  • \(\text{RCOCl} \rightarrow \text{Amide}\) (\(\text{NH}_3\) or \(\text{Amine}\))

5. Working with Aromatic Synthesis (Benzene)

Aromatic synthesis (starting with benzene or methylbenzene) requires an additional layer of complexity: directing effects.

Benzene undergoes Electrophilic Substitution. When a substituent is already on the ring, it dictates where the next group goes:

• 2, 4, 6-directors (ortho/para directors): These activate the ring, making substitution easier. Examples: \(-\text{NH}_2, -\text{OH}, -\text{R}\) (alkyl groups).
• 3, 5-directors (meta directors): These deactivate the ring, making substitution harder. Examples: \(-\text{NO}_2, -\text{COOH}, -\text{COR}\).

Synthesis Tip for Benzene:
If you need a meta product (e.g., 3-nitrobenzoic acid), you must introduce the meta-directing group first.
If you need an ortho/para product (e.g., 4-bromomethylbenzene), you must introduce the ortho/para-directing group first.

Example: Making 3-Nitrobenzoic Acid from Benzene

Route:
1. Benzene \(\overset{\text{Friedel-Crafts Alkylation (e.g., } \text{CH}_3\text{Cl}/\text{AlCl}_3)}{\longrightarrow}\) Methylbenzene (\(-\text{R}\) is a 2,4,6-director).
2. Methylbenzene \(\overset{\text{Hot alkaline }\text{KMnO}_4\text{/dilute acid}}{\longrightarrow}\) Benzoic Acid (\(-\text{COOH}\) group is a 3,5-director).
3. Benzoic Acid \(\overset{\text{Conc. }\text{HNO}_3/\text{Conc. }\text{H}_2\text{SO}_4, 50^{\circ}\text{C}}{\longrightarrow}\) 3-Nitrobenzoic Acid.

6. Key Takeaway Strategy

When preparing for synthesis questions, always have a mental map (or flow chart) covering three things:

6.1 Functional Group Interconversion Map

Ask yourself: "What are the ways to get TO this group?" and "What are the ways to get FROM this group?"

Organic synthesis is a test of comprehensive knowledge, not just rote memorization. Practice drawing out the conversions until the reagents and conditions become second nature. You've got this!