Introduction to Organic Synthesis
Welcome to Topic 18C: Organic Synthesis! This is the chapter where everything you have learned in organic chemistry comes together. Think of it like being given a box of LEGO bricks (functional groups) and a manual. Your job is to figure out how to build a complex structure from simple starting materials.
In this topic, we focus on deducing structures using analytical data, designing reaction schemes to build molecules, and mastering the practical techniques used in a real lab to make and purify these compounds. Don't worry if it feels like a lot to remember—we will break it down step-by-step!
1. Deducing Organic Structures
Before you can build a molecule, you often need to identify an unknown one. In the exam, you will be like a detective using several "clues" to solve the mystery of a molecule's identity.
A. Combustion Analysis
By burning a sample and measuring the masses of \(CO_{2}\) and \(H_{2}O\) produced, we can calculate the empirical formula. Combined with the molar mass from a mass spectrum, we find the molecular formula.
B. Functional Group Reactions
Chemical tests tell us what "parts" the molecule has. For example:
• If it fizzes with \(NaHCO_{3}\), it’s a carboxylic acid.
• If it forms a silver mirror with Tollens' reagent, it’s an aldehyde.
• If it decolourises bromine water, it has a \(C=C\) bond.
C. Spectroscopy (The Big Three)
1. Mass Spectrometry: Look for the molecular ion peak (\(M^{+}\)) to find the relative molecular mass. High-resolution mass spec (Topic 19A) can give masses to four decimal places, helping distinguish between molecules with similar masses.
2. Infrared (IR) Spectroscopy: Identifies specific bonds. For example, a broad "cup" shape around \(3200-3600\text{ cm}^{-1}\) indicates an \(O-H\) group.
3. NMR Spectroscopy: \(^{13}C\) NMR tells us how many different carbon environments there are. \(^{1}H\) NMR (High Resolution) tells us about the hydrogen atoms, their environments, and their neighbors using the \((n+1)\) splitting rule.
Quick Tip: Always start with the IR to find functional groups, then use NMR to piece the carbon skeleton together!
2. Designing Reaction Schemes
A reaction scheme is a sequence of reactions used to convert a starting material into a target molecule. For Edexcel 9CH0, you need to be able to follow or devise schemes of up to four steps.
When designing a scheme, ask yourself two questions:
1. Is the carbon chain the same length, or does it need to get longer?
2. What functional groups need to change?
Common Bridges between topics:
• Alkanes to Halogenoalkanes: Radical substitution (\(Cl_{2}\), UV light).
• Halogenoalkanes to Alcohols/Amines/Nitriles: Nucleophilic substitution.
• Alcohols to Carbonyls: Oxidation with acidified \(K_{2}Cr_{2}O_{7}\).
• Nitriles to Carboxylic Acids: Acid hydrolysis.
3. Chain Lengthening: Grignard Reagents
One of the most important tools for an organic chemist is the Grignard reagent. These are used specifically for chain lengthening (adding more carbons to your molecule).
Formation:
A Grignard reagent (general formula \(RMgX\), where \(X\) is a halogen) is made by reacting a halogenoalkane with magnesium ribbon in dry ether.
Example: \(CH_{3}CH_{2}Br + Mg \rightarrow CH_{3}CH_{2}MgBr\) (Ethylmagnesium bromide).
Key Reactions:
Grignard reagents act as nucleophiles because the carbon bonded to the magnesium is slightly negative (\(\delta -\)).
• With Carbon Dioxide (\(CO_{2}\)): Produces a carboxylic acid after dilute acid is added. This adds one carbon atom to the chain.
• With Methanal: Produces a primary alcohol (adds 1 carbon).
• With other Aldehydes: Produces a secondary alcohol.
• With Ketones: Produces a tertiary alcohol.
Hazard Note: Grignard reagents are highly reactive with water, which is why they must be prepared in dry ether (anhydrous conditions).
4. Practical Techniques in Synthesis
Making the molecule is only half the battle; you also have to isolate it and make sure it is pure. These techniques are often tested in Paper 3.
Refluxing
This allows you to heat a reaction mixture for a long time without losing volatile organic solvents or reactants. The vapors rise, hit the condenser, turn back into liquid, and drip back into the flask.
Distillation
Separates liquids based on their boiling temperatures.
• Steam Distillation: Used for compounds that have high boiling points or decompose when heated directly. The steam lowers the temperature at which the organic compound distills, preventing decomposition.
Washing and Solvent Extraction
Using a separating funnel, we "wash" the product to remove impurities. For example, adding \(NaHCO_{3}\) to remove acid impurities. We then separate the organic layer from the aqueous (water) layer.
Recrystallisation (For Purifying Solids)
1. Dissolve the impure solid in the minimum volume of hot solvent.
2. Filter while hot to remove insoluble impurities.
3. Let the solution cool slowly so crystals of the pure product form.
4. Filter under reduced pressure (using a Buchner funnel) and wash with a little ice-cold solvent.
Drying
After extraction, the organic liquid often contains traces of water. We add an anhydrous salt (like \(MgSO_{4}\) or \(CaCl_{2}\)) which acts as a drying agent by soaking up the water. The liquid is clear when dry.
Determining Purity
• Melting Temperature: Use a melting point apparatus. A pure solid will have a sharp melting point that matches the data book value. If it is impure, it will melt over a wide range and at a lower temperature.
• Boiling Temperature: Measured during distillation; pure liquids have a constant, specific boiling point.
5. Safety and Hazards
When planning a synthesis, you must identify hazards and implement control measures. Hazards are the "dangers" (e.g., toxicity, flammability), and control measures are how we stay safe.
Common Control Measures:
• Flammable liquids: Use a water bath or electric heating mantle instead of a Bunsen burner.
• Toxic fumes: Carry out the reaction in a fume cupboard.
• Corrosive chemicals: Wear gloves and safety goggles.
• Pressure build-up: Never heat a "closed system" (ensure the apparatus is open to the air at one point).
Key Takeaway Summary:
Organic synthesis is a logical puzzle. You use spectroscopy and chemical tests to identify your starting point, Grignard reagents to grow the carbon chain, reaction schemes to change functional groups, and purification techniques like recrystallisation and distillation to get a clean final product.