Edexcel International A Level Chemistry

Topic 20: Organic Synthesis

The topic that pulls the whole organic course together. Work out what an unknown compound is from data, build a bigger molecule from a smaller one, plan a route of up to four steps, and prepare and purify a product in the laboratory.

The one idea

Organic chemistry in the exam asks three questions of the same compounds, and this topic is the three of them in turn.

1. What is it? Data about an unknown compound, such as combustion masses, tests, infrared, mass and NMR spectra, are combined to find a formula and then a structure.
2. How do I make it? A route of up to four steps is planned by working backwards from the target, using the reactions you already know. Grignard reagents are the new tool: they build a longer carbon chain.
3. How do I actually do it? Each step needs a procedure: reflux, washing, extraction, drying, distillation, recrystallisation. The purity of the product is checked by a melting or boiling temperature, and the risks are controlled.

Almost nothing in this topic is a new reaction. It is the earlier topics applied to unfamiliar compounds, and written up as exam answers: evidence, deduction, reagent, conditions, product.

How to use these notes

• Pages 1 to 5 are about identifying a compound. Do page 1 first, because every spectrum question rests on a formula. Page 5 puts all the evidence together.
• Pages 6 and 7 are about Grignard reagents. Pages 8 to 10 are about predicting properties and planning routes: the reaction maps on page 9 hold every one-step conversion in the course.
• Pages 11 to 14 are the techniques and risk assessment. Page 15 is Core Practical 16, the preparation of aspirin, which uses them all.
• Mark schemes give a mark for each separate piece of evidence, so show the evidence for every deduction. For every step in a synthesis, give the reagent, the conditions and the named product.
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

Relative atomic masses are the Edexcel IAL periodic table values to one decimal place: H 1.0, C 12.0, N 14.0, O 16.0, Na 23.0, Mg 24.3, Cl 35.5, Br 79.9. Data for infrared, carbon-13 and proton NMR are given in the exam, so these notes quote the ranges for practice only; use the booklet's values.

Pages

1. Formulae from combustion and composition

   Turn the masses of carbon dioxide and water from a combustion, or the percentages by mass of each element, into an empirical formula, then use the molecular ion to reach the molecular formula.

   14 min 20.1

2. Tests for functional groups

   Each test-tube reaction is a question with a yes or no answer. A positive result shows one functional group, a negative result rules it out, and together the answers narrow a formula down to one structure.

   14 min 20.1

3. Infrared and mass spectra

   Infrared absorptions show which bonds are present, and a mass spectrum gives the relative molecular mass and clues to the structure from the way the molecule breaks up. Used together they separate isomers.

   15 min 20.1

4. NMR: environments, shifts and areas

   Carbon-13 and proton NMR both give one peak for each set of equivalent atoms. The number of peaks counts the environments, the chemical shift shows what each atom is next to, and for protons the peak areas give the ratio of hydrogen atoms.

   15 min 20.1

5. NMR splitting and solving structures

   At high resolution each proton peak splits into n + 1 lines, where n is the number of hydrogens on the adjacent carbon atoms. Combine this with every other piece of evidence, and check your answer by predicting its spectrum.

   15 min 20.1

6. Grignard reagents and carbon dioxide

   A Grignard reagent is a halogenoalkane that has been turned inside out. Its carbon atom is now the electron-rich end, so it forms a new carbon-carbon bond with carbon dioxide and gives a carboxylic acid with one more carbon.

   13 min 20.2

7. Grignard reagents and carbonyl compounds

   A Grignard reagent adds its whole R group to the carbon of a C=O group. Methanal gives a primary alcohol, any other aldehyde a secondary alcohol and a ketone a tertiary alcohol, and you can work backwards from the alcohol to the reagents.

   14 min 20.2

8. Unfamiliar compounds

   A compound you have never met is a set of groups you have. Find each functional group, predict what each one does on its own, and then say what the compound will do in terms of those groups.

   14 min 20.3(i)

9. The reaction map

   Every one-step conversion in the course, on two maps. Learn the arrows first and the reagents second, and mark every step that adds a carbon, because those are the steps a synthesis needs when the product has a longer chain.

   15 min 20.3(ii)

10. Planning a route

    Compare the carbon count and the functional groups of the start and the target, work backwards from the target using the reaction maps, and write each step as a reagent, a condition and a named product. Unfamiliar reactions come with the information to use them.

    15 min 20.3(ii)

11. Reflux, washing and solvent extraction

    Reflux lets a slow reaction run hot without losing anything. Washing then removes acid and water-soluble impurities from the crude product, and solvent extraction pulls a product out of water into a liquid that does not mix with it.

    15 min 20.5(i)20.5(ii)20.5(iii)

12. Drying, distillation and steam distillation

    An anhydrous salt removes the last traces of water from a liquid. Distillation then separates liquids by their boiling temperatures, and steam distillation takes a volatile, water-insoluble compound out of a mixture below 100 degrees Celsius.

    15 min 20.5(v)20.5(vi)20.5(vii)

13. Recrystallisation, melting and boiling temperatures

    A solid is purified by recrystallisation from a hot solvent, and its purity is checked by its melting temperature. A liquid is checked by its boiling temperature. A pure compound melts or boils sharply at the value in the data book.

    14 min 20.5(iv)20.5(viii)20.5(ix)

14. Choosing a procedure and controlling risk

    Match the technique to a property of the substances, and match each control measure to a hazard. A good answer names both the procedure and the property that makes it suitable.

    14 min 20.3(iii)20.3(iv)

15. Core Practical 16: preparing aspirin

    Warm 2-hydroxybenzoic acid with ethanoic anhydride and a little acid catalyst, crystallise the crude aspirin, recrystallise it, then check its purity by melting temperature and calculate the percentage yield.

    15 min 20.420.5(iv)20.5(viii)

Also in this topic: Topic 20 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.