6.2. Nitrogen Compounds, Polymers and Synthesis: Module 6: Organic Chemistry & Analysis Chemistry OCR A A Level

Basicity

Amines react with water to form an alkaline solution. The lone pair of electrons on the amine’s nitrogen atom can accept a hydrogen from a water molecule, therefore acting as a base. This releases OH- ions into the solution.

To Produce Salts

Amines react with acids to form an ammonium salt. Again, the amine acts as a base and accepts a proton to form a quaternary ammonium salt.

Nucleophilic Substitution

Aliphatic amines can be produced from the nucleophilic substitution reaction between a halogenoalkane and ammonia in a sealed tube. One mole of halogenoalkane reacts with two moles of ammonia, producing a primary amine and an ammonium salt (ammonium ion bonded to a halide ion).

Reduction of Nitriles

Aliphatic amines can also be produced by the reduction of nitriles by hydrogenation. This reduction requires a combination of hydrogen with a nickel catalyst (catalytic hydrogenation).

Preparation of Aromatic Amines

Aromatic amines can be produced from the reduction of nitrobenzene using concentrated hydrochloric acid (HCl) and a tin catalyst. Aromatic amines consist of an amine group and a benzene ring.

Zwitterions

Zwitterions are dipolar ions with a positive charge in one part of the molecule and a negative charge in another part of the molecule. The zwitterionic form of an amino acid is the state in which the amine group has a positive charge (+NH3) and the carboxyl group has a negative charge (COO-).

Zwitterions form at the isoelectric point, which is the pH at which the overall charge of the molecule is zero.

Amides

Amines can also undergo nucleophilic addition-elimination reactions with acyl chlorides to produce amides and N substituted amides.

N-substituted Amides

Secondary amides are also known as N-substituted amides. They have an additional carbon chain bonded to the nitrogen atom.

Chiral Centres

A chiral centre is a carbon atom with four different groups bonded around it, so that the molecule has no line of symmetry.

Optical Isomers

Optical isomerism is a type of stereoisomerism where molecules have the same molecular formula but a different spatial arrangement of atoms. The presence of a chiral centre leads to two possible isomers that are non-superimposable mirror images of each other. These are called optical isomers.

Condensation Polymers

Condensation polymers form when a water molecule is removed from the species of a reaction. Polyesters and polyamides can be formed in this way.

Polyesters

Polyesters are formed when a dicarboxylic acid and a diol react together, producing an ester linkage, -COO-.

Terylene (PET)

Terylene (PET) is a common polyester made from ethane-1,2 diol and 1,4-benzenedicarboxylic acid.

Polyamides

Polyamides are condensation polymers generally formed in a reaction between a dicarboxylic acid and a diamine. A molecule of water is removed, producing an amide linkage.

Kevlar

Kevlar is another common polyamide made from benzene-1,4 dicarboxylic acid and 1,4-diaminobenzene.

Hydrolysis

The ester groups in polyesters and the amide groups in polyamides can be broken down by acid or base hydrolysis. The ester and amide linkages are polar, so they can be broken in the presence of water.

Ester hydrolysis is the reverse reaction to esterification, converting esters back into alcohols and carboxylic acids. This process is done by adding water, but can be carried out under different conditions to produce different products. For polymers, the ester linkage is broken and the polyester is converted back into the diol and dicarboxylic acid.

Extending Carbon Chain Length

The carbon chain in organic molecules can be extended by C C bond formation. This is useful in synthesis to form new molecules. Nitriles can be used to add in the extra carbon, and then other reactions can be carried out to change the nitrile group into another functional group.

Reactions of Nitriles

The nitriles formed above can be used as a precursor to other products. Amines can be produced by the reduction of nitriles.

This reduction reaction is called hydrogenation. The reaction requires a combination of hydrogen with a nickel catalyst (catalytic hydrogenation).

Friedel-Crafts Reactions

A C-C bond can be introduced into an aromatic compound using Friedel-Crafts Alkylation or Friedel-Crafts Acylation. Both reactions occur in the presence of a halogen carrier to make the strong electrophile and allow electrophilic substitution to occur.

Friedel-Crafts Acylation

Friedel-Crafts acylation substitutes an acyl group for a hydrogen atom on the benzene ring. The electrophilic reactive intermediate is produced from a reaction between the acyl chloride and an aluminium chloride catalyst.

Friedel-Crafts Alkylation

Friedel-Crafts alkylation substitutes an alkyl group for a hydrogen atom on the benzene ring. The electrophilic reactive intermediate is produced from a reaction between a haloalkane and an aluminium chloride catalyst.

Experimental Techniques

Synthesis pathways are used to make organic compounds. They involve a variety of preparatory and purification techniques that have been introduced throughout this course.

These include:

● Reflux

● Distillation

● Melting point determination

● Boiling point determination

● Washing and drying

● Recrystallisation

● Solvent extraction

Organic Preparation

The first step of synthesis is to prepare your organic compound. Common techniques for preparation are reflux and distillation.

Heating under Reflux

Reflux apparatus is used to continually heat the contents of the flask to allow reactions like the oxidation of primary alcohols to proceed all the way to the formation of carboxylic acids. The condenser helps ensure the vapours condense and return to the flask for further heating. This ensures the product vapours can not escape.

Distillation

Distillation apparatus is used to separate liquids with different boiling points. The round-bottomed flask is heated and the liquid with the lower boiling point will evaporate first. It rises out of the flask and into the attached tubing which is surrounded by a condenser. The condenser causes the vapour to cool and condense back into a liquid, which is then collected in a separate flask.

Recrystallisation

Recrystallisation is a purification technique that uses the differing solubilities of solid and impurities in different solvents.

Melting Point Analysis

Determining the melting point of a compound and comparing it to a databook value is a way of testing its purity. The purer a substance, the closer to the databook melting point value it will be. If a sample has a low purity, the melting point will take place over a range of temperatures.

Synthetic Routes

Synthesis pathways are needed to convert starting materials into a target product. This can sometimes be achieved through single-step reactions, but other times multistep pathways are required. The functional groups in a desired product can be identified and then different transformations between functional groups can be used to get to the desired product.