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 ACIDS Carboxylic Acid AND Amine functionality Behave as acids (donate a proton) and bases (accept protons)

Zwitterion ions The 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 NH 2 group the pH of the amino acid will be >7 as the NH 2 in the acid group will become NH 3 + (decreases H + in solution) e exists as and OH - in water so pH of solution is >7

Physical Properties 1 MUCH HIGHER MP THAN EXPECTED Intermolecular forces MUCH HIGHER than expected as both Hydrogen Bonds AND Ionic Interactions present

Physical Properties 2 MUCH MORE WATER SOLUBLE THAN EXPECTED Both Hydrogen bonds and electrostatic interactions form with water molecules

Isoelectric Point The pH at which only the zwitterion exists This means that all COOH and NH 2 groups in the side group exist as COOH and NH 2 Only the COOH and NH 2 group in the amino acid are charged (as COO - and NH 3 + ) ABOVE the isoelectric Point all NH 2 groups = NH 2 and all COOH groups = COO - BELOWthe isoelectric Point all NH 2 groups = NH 3 + and all COOH groups = COOH If R contains NH 2 the isoelectric point will be >7 If R contains COOH the isoelectric point will be <7 If 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 Formation Carboxylic acid + Amine  Amide + water Ester Formation Carboxylic acid + Alcohol  Ester + water

Condensation Polymerisation Man made Polyamide formation Dicarboxylic acid and Diamine  Polyamide + water Polyamide with repeat unit showing

Nylon and Kevlar For Nylon Dicarboxylic acid = Hexan-1,6 dioic acid Diamine = 1,4 diaminohexane For Kevlar Dicarboxylic acid = Benzene 1,4 dicarboxylic acid Diamine = 1,4 diaminobenzene

Manmade Polyester formation Dicarboxylic acid and Diol  Polyester + water

Terylene For Terylene Dicarboxylic acid = Benzene 1,4 dicarboxylic acid Diol = 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 H 2 O) Amide hydrolysis in acid solutions Amide + water  Carboxylic acid + Salt of Amine Amide hydrolysis in acid solutions Amide + water  Carboxylate salt + Amine

Hydrolysis of esters (Addition of H 2 O) Ester hydrolysis in acid solutions Ester + water  Carboxylic acid + Alcohol Ester hydrolysis in acid solutions Ester + water  Carboxylate salt + Alcohol

Hydrolysis of polyamides (Addition of H 2 O) Poly amide Amide hydrolysis in acid solutions PolyAmide + water  Dicarboxylic acid + Salt of Diamine Amide hydrolysis in acid solutions Amide + water  Dicarboxylate salt + Diamine

Hydrolysis of Polyesters (Addition of H 2 O) Polyester hydrolysis in acid solutions Polyester + water  Dicarboxylic acid + Diol Polyester hydrolysis in acid solutions Polyester + water  Dicarboxylate salt + Diol

Disposal of Polymers 1 Condensation Polymers vs Addition Polymers Condensation Polymers are often Biodegradable Addition 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