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Organic synthesis - AL only (3.3.14)
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Functional groups react in the same way whether they are the only group present in a small carbon skeleton, or in combination with other functional groups in a larger molecule.
Questions frequently expect learners to recognise functional groups present in complex molecular structures.
Aldehydes and ketones can be easily confused when assigning functional groups.
In an aldehyde, the group carbon atom is always at the end of the chain.
In a ketone, the group carbon atom in a ketone is never at the end of the chain.
The ketone group is recognised by having carbon atoms bonded on either side of the carbonyl carbon, whereas the aldehyde carbonyl is bound to a hydrogen.
The ketone group can not be further oxidised, whereas the aldehyde group can.
Secondary amines and amides are easily confused.
In a secondary amine, two alkyl carbon R groups are bonded to the with a single remaining hydrogen. The lone pair in an amine is basic.
In a secondary amide, a carbonyl group will always bond directly to the nitrogen atom. The amide group is susceptible to hydrolysis, and the nitrogen’s lone pair is not basic.
Question walkthrough
Identifying functional groups
Finding discrete functional groups in complex organic molecules
To successfully develop reaction schemes, or to identify missing details in given reaction schemes, it is useful to commit the reactions covered across the specification to memory.
Fast recall here will save valuable time in the exam.
The scheme below highlights the reactions involving alcohols featured in AQA A-level chemistry.
The scheme below highlights the reactions involving halogenoalkanes featured in AQA A-level chemistry.
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The scheme below highlights the reactions involving alkenes featured in AQA A-level chemistry.
The scheme below highlights the reactions involving benzene featured in AQA A-level chemistry.
Question walkthrough
Identifying reagents and conditions
Looking at the functional group conversion in aliphatic molecules in order to identify the reagents and conditions required
Question walkthrough
Identifying reagents and conditions
Looking at the functional group conversion in aromatic molecules in order to identify the reagents and conditions required
Question walkthrough
Identifying missing information in a reaction scheme
Using knowledge of aliphatic reactions to complete a reaction scheme
Acidified potassium dichromate VI is a commonly used oxidising agent.
It may be shown as
or
Evidence for oxidation is the reduction of the orange dichromate VI ion to the green ion.
Sodium borohydride solution, , and acidified tin, , are both used as reducing agents.
![A table titled 'Reduction [O]' displaying chemical reactions. The first column lists reactants: Aldehyde (CH3CHO + 2 [H]), Ketone (CH3COCH3 + 2 [H]), and Aromatic nitro (C6H5NO2 + 6 [H]). The second column outlines reagents and conditions: NaBH4 (aq) + Reflux for both Aldehyde and Ketone, and Sn / Conc HCl + Reflux for Aromatic nitro. The third column shows the products: Alcohol (1°) (CH3CH2OH), Alcohol (2°) (2CH3COOH), and Aromatic primary amine (C6H5NH2 + 2H2O).](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI4MDU3LCJwdXIiOiJibG9iX2lkIn19--fa10e194897db72528fdf5e28c39d1cb7eafec76/4_OC_15_Organic%20synthesis_019_light.png)
![A table titled 'Reduction [O]' displaying chemical reactions. The first column lists reactants: Aldehyde (CH3CHO + 2 [H]), Ketone (CH3COCH3 + 2 [H]), and Aromatic nitro (C6H5NO2 + 6 [H]). The second column outlines reagents and conditions: NaBH4 (aq) + Reflux for both Aldehyde and Ketone, and Sn / Conc HCl + Reflux for Aromatic nitro. The third column shows the products: Alcohol (1°) (CH3CH2OH), Alcohol (2°) (2CH3COOH), and Aromatic primary amine (C6H5NH2 + 2H2O).](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI4MDU4LCJwdXIiOiJibG9iX2lkIn19--6c8a11c20167deef2daed99f7cf9813febf7c32d/4_OC_15_Organic%20synthesis_019_dark.png)
Hydrogen gas and a nickel catalyst, , are used for reduction of unsaturated compounds.
Reactions with are associated with creating new carbon-carbon bonds.
- Haloalkanes react with in a nucleophilic substitution reaction. The ion nucleophile is from an ethanolic solution of .
- Carbonyls (aldehydes and ketones) react with the nucleophile, in a nucleophilic addition reaction. The ions are made by the reaction of with a dilute acid, to avoid using the highly toxic .
Friedel–Crafts reactions are used to create carbon-carbon bonds in aromatic compounds.
They are identifiable by the use of a Lewis acid catalyst, commonly , anhydrous aluminium chloride.
The catalyst generates the electrophiles necessary to attack, and substitute a carbon skeleton onto, the arene ring.
The use of an acid or base in aqueous solution can either indicate a neutralisation reaction, an electrophilic substitution (base only) or a hydrolysis reaction.
The nature of hydrolysis products is related to the reaction conditions;
- in acidic conditions basic products will be in their protonated form – amines become ammonium salts.
- in basic conditions acidic products will be in their deprotonated form – carboxylic acids become carboxylate salts.
Many synthetic reaction sequences consisting of three or four-step conversions, pass through the same highly versatile functional groups.
Haloalkanes, alcohols, and nitriles are commonly found in the middle of aliphatic reaction pathways.
Reaction schemes with a haloalkane intermediate.
Reaction schemes with an alcohol intermediate.
Reaction schemes with a nitrile intermediate.
Question walkthrough
Planning a synthetic pathway
Linking reactions of aliphatic molecules to produce a four-step synthetic pathway
In organic synthesis, each step is regarded as a single reaction, with its own reactant, reagents, conditions, and product.
Generally, fewer steps give a more efficient process and a better yield as there are fewer transfers required.
The most environmentally friendly synthesis feasible within time, cost, and safety constraints, should be identified for use in the industry.
Catalysts are a good way of reducing energy, increasing reaction rate, and improving product specificity.
Higher percentage yield and percentage atom economy of each reaction step are key considerations in industrial synthesis. Where a co-product is formed, finding a use for it can make a process more viable.
Heat energy from exothermic reactions can be used to generate steam, which can then be used to heat other parts of the synthesis process.
Planning a synthesis to avoid solutions in organic solvents reduces hazards associated with flammability and inhalation.
Where the reaction product forms as a precipitate, rather than in solution, the filtration process is more straightforward and less costly than solvent evaporation.