Organic synthesis (Topic 18C)

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.

The scheme below highlights the reactions involving alcohols featured in Edexcel A-level chemistry.

The scheme below highlights the reactions involving haloalkanes featured in Edexcel A-level chemistry.

The scheme below highlights the reactions involving alkenes featured in Edexcel A-level chemistry.

The scheme below highlights the reactions of phenol featured in Edexcel A-level chemistry.

The scheme below highlights the reactions involving benzene featured in Edexcel A-level chemistry.

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.

Lithium aluminium hydride in dry ether, is a powerful reducing agent.

This reagent reacts violently with water so must be used under strictly anhydrous conditions with dry ether as a solvent.

Hydrogen gas and a nickel catalyst, , are used for reduction of unsaturated compounds.

Reactions with are associated with creating new carbon-carbon bonds.

1. Haloalkanes react with in a nucleophilic substitution reaction. The ion nucleophile is from an ethanolic solution of .
2. 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.

Reaction schemes with a haloalkane intermediate.

Reaction schemes with a nitrile intermediate.

Reaction schemes with a nitrile intermediate.

Carbon chain length can be increased by the use of Grignard reagents. These are prepared from bromo or iodoalkanes and magnesium.

The haloalkane is refluxed with pieces in a solution with a dry ether solvent. The magnesium inserts into the carbon-halogen bond forming a Grignard reagent, .

The alkyl group of the Grignard reagent holds a 𝛿- charge and acts as a nucleophile. It will attack the carbonyl carbon in aldehydes, ketones or even carbon dioxide.

The products are different classes of alcohols, or from the product is a carboxylic acid.