Amino Acids, Isomerism and Condensation Polymers

OPTICAL ISOMERISM

• Form of stereoisomerism in which a molecule has a chiral centre attached to 4 different groups

• Non-superimposable mirror images (enantiomers)• Note in the two enantiomers have the same

Molecular formula and Structural formula, but differ by the arrangement of atoms in 3D space

Amino Acids

Amino acids Exist as Optical enantiomers

• Unless R = H (glycine) the central atom in amino acids is a chiral centre. So two enantiomers exist

AMINO ACIDSCarboxylic Acid AND Amine functionality

• Behave as acids (donate a proton) and bases (accept protons)

Zwitterion ionsThe amino acids will undergo self neutralisation and hence in water will exist as a zwitterion

The pH of a solution of an amino acid will be 7 if the R group is an alkyl group

exists as in water so pH of solution is 7

If the R group contains a COOH group the pH of the amino acid will be <7 as the COOH in the acid group will become COO- and H+ (increases H+ in solution)

exists as and H+ in water so pH of solution is < 7

If the R group contains an NH2 group the pH of the amino acid will be >7 as the NH2 in the acid group will become NH3

+ (decreases H+ in solution)

e exists as and OH- in water so pH of solution is >7

Physical Properties 1

• MUCH HIGHER MP THAN EXPECTEDIntermolecular forces MUCH HIGHER than expected as both Hydrogen Bonds AND Ionic Interactions present

Physical Properties 2

• MUCH MORE WATER SOLUBLE THAN EXPECTEDBoth Hydrogen bonds and electrostatic interactions form with water molecules

Isoelectric Point• The pH at which only the zwitterion existsThis means that all COOH and NH2 groups in the side group exist as COOH and NH2

Only the COOH and NH2 group in the amino acid are charged (as COO- and NH3

+)

ABOVE the isoelectric Point all NH2 groups = NH2 and all COOH groups = COO-

BELOW the isoelectric Point all NH2 groups = NH3+ and all COOH groups = COOH

If R contains NH2 the isoelectric point will be >7If R contains COOH the isoelectric point will be <7If R is only C and H then the isoelectric point will be 7

Condensation Reactions

A reaction in which water is formed during the reaction

Important Condensation Reactions• Amide FormationCarboxylic acid + Amine Amide + water

• Ester FormationCarboxylic acid + Alcohol Ester + water

Condensation Polymerisation• Man made Polyamide formationDicarboxylic acid and Diamine Polyamide + water

Polyamide with repeat unit showing

Nylon and KevlarFor NylonDicarboxylic acid = Hexan-1,6 dioic acidDiamine = 1,4 diaminohexane

For KevlarDicarboxylic acid = Benzene 1,4 dicarboxylic acidDiamine = 1,4 diaminobenzene

Manmade Polyester formationDicarboxylic acid and Diol Polyester + water

TeryleneFor TeryleneDicarboxylic acid = Benzene 1,4 dicarboxylic acidDiol = Ethan-1,2-diol

Poly(lactic Acid)• Monomer = 2-hydroxypropanoic acid (Lactic acid)This acid has dual functionality so is both an alcohol AND a carboxylic acid so will undergo self polymerisation

Naturally occurring polyamides• Proteins (or polypeptides)Monomers = Amino Acids (amine and acid functionality so undergoes self condensation polymerisation

Repeat condensation process to get polymer

Hydrolysis of amides (Addition of H2O)• Amide hydrolysis in acid solutionsAmide + water Carboxylic acid + Salt of Amine

Amide hydrolysis in acid solutionsAmide + water Carboxylate salt + Amine

Hydrolysis of esters (Addition of H2O)• Ester hydrolysis in acid solutionsEster + water Carboxylic acid + Alcohol

Ester hydrolysis in acid solutionsEster + water Carboxylate salt + Alcohol

Hydrolysis of polyamides (Addition of H2O)• Poly amide Amide hydrolysis in acid solutionsPolyAmide + water Dicarboxylic acid + Salt of Diamine

Amide hydrolysis in acid solutionsAmide + water Dicarboxylate salt + Diamine

Hydrolysis of Polyesters (Addition of H2O)• Polyester hydrolysis in acid solutionsPolyester + water Dicarboxylic acid + Diol

Polyester hydrolysis in acid solutionsPolyester + water Dicarboxylate salt + Diol

Disposal of Polymers 1

• Condensation Polymers vs Addition PolymersCondensation Polymers are often BiodegradableAddition Polymers are NOT Biodegradable

Plastic Bags are being made from Starch or Poly(lactic acid). Lactic acid and Starch are naturally occuring molecules so the bags are biodegradable.Also Lactic acid and starch are obtained from renewable sources (plants)

Disposal of Polymers 2• Condensation Polymers will hydrolyse in water to

produce compounds that will dissolve in water (Hence Wash away)

Condensation Polymers contain C=O bond so will absorb ir light (sunlight) so photodegrade