Organic synthesis, NMR and chromatography

1. What is a synthetic route?

1. A single reaction only
2. A sequence of reactions converting a starting material into a target molecule
3. A type of catalyst
4. A purification method

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B: A sequence of reactions converting a starting material into a target molecule. Multi-step syntheses are common in organic chemistry.

2. Why might a protecting group be used?

1. To make a molecule more reactive everywhere
2. To temporarily block a functional group so another reaction can occur elsewhere
3. To permanently change the molecule
4. To increase yield only

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B: To temporarily block a functional group so another reaction can occur elsewhere. It's removed once the other part of the synthesis is complete.

3. What does retrosynthesis involve?

1. Working forwards from starting materials
2. Working backwards from the target molecule to suggest starting materials
3. Measuring reaction rate
4. Testing for functional groups only

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B: Working backwards from the target molecule to suggest starting materials. This is a useful planning tool for multi-step synthesis.

4. What piece of equipment separates an organic product from an aqueous layer?

1. A burette
2. A separating funnel
3. A condenser
4. A thermometer

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B: A separating funnel. Immiscible liquids can be separated by density in a separating funnel.

5. How can the purity of a solid organic product be checked?

1. By its colour
2. By comparing its melting point to the known value; a sharp melting point indicates purity
3. By its smell
4. By its mass alone

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B: By comparing its melting point to the known value; a sharp melting point indicates purity. Impurities usually lower and broaden the melting point range.

6. Why might a chemist choose an acid anhydride rather than an acyl chloride in synthesis?

1. It reacts faster and more violently
2. It is generally safer and produces a less corrosive by-product
3. It never reacts with alcohols
4. It always gives a lower yield

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B: It is generally safer and produces a less corrosive by-product. Acid anhydrides are often preferred for practical and safety reasons.

7. Which sequence converts bromoethane into propanoic acid?

1. Mg in dry ether; then H₂O
2. KCN in ethanol, heat; then dilute HCl, reflux
3. Excess NH₃; then HCl
4. NaOH(aq), reflux; then K₂Cr₂O₇

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B: KCN in ethanol, heat; then dilute HCl, reflux. KCN gives propanenitrile, adding one carbon. Hydrolysis of the nitrile with dilute acid under reflux gives propanoic acid.

8. Why do multi-step syntheses often give a low overall yield?

1. Reagents are used in less than 1:1 ratios
2. Losses at every step accumulate
3. Intermediates always decompose
4. Each step needs a higher temperature

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B: Losses at every step accumulate. Each step has losses from incomplete reaction, side reactions and purification, and the fractional yields multiply.

9. When purifying a liquid by redistillation, which fraction is collected?

1. The residue left in the flask
2. The first few drops
3. All liquid collected below 100 °C
4. The fraction distilling at the product's known boiling point

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D: The fraction distilling at the product's known boiling point. Collecting only the fraction that distils at the product's known boiling point leaves lower and higher boiling impurities behind.

10. Anhydrous magnesium sulfate is added to a crude liquid organic product. What is its purpose?

1. To neutralise acidic impurities
2. To raise the boiling point
3. To separate the layers
4. To remove traces of water

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D: To remove traces of water. Anhydrous magnesium sulfate absorbs water by forming a hydrate. It dries the liquid and is then removed by filtration or decanting.