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AMINES CONTENTS • Prior knowledge • Structure and classification • Nomenclature • Physical properties • Basic properties • Nucleophilic properties • Amino acids • Peptides and proteins AMINES Before you start it would be helpful to… • know the functional groups found in organic chemistry • know the arrangement of bonds around atoms • recall and explain nucleophilic substitution reactions STRUCTURE & CLASSIFICATION Structure Contain the NH2 group Classification H R N: H R H primary (1°) amines R secondary (2°) amines R R N: R R R tertiary (3°) amines N: + N R R quarternary (4°) ammonium salts Aliphatic methylamine, ethylamine, dimethylamine Aromatic NH2 group is attached directly to the benzene ring (phenylamine) NOMENCLATURE Nomenclature Named after the groups surrounding the nitrogen + amine C2H5NH2 ethylamine (CH3)2NH dimethylamine (CH3)3N trimethylamine C6H5NH2 phenylamine (aniline) PREPARATION Amines can be prepared from halogenoalkanes Reagent Aqueous, alcoholic ammonia Conditions Reflux in aqueous, alcoholic solution under pressure Product Amine (or its salt due to a reaction with the acid produced) Nucleophile Ammonia (NH3) Equation C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr ( or C2H5NH3+Br¯ ) PHYSICAL PROPERTIES The LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them ... NUCLEOPHILES - provide a lone pair to attack an electron deficient centre CHEMICAL REACTIONS - WEAK BASES Water Amines which dissolve in water produce weak alkaline solutions CH3NH2(g) Acids + H2O(l) CH3NH3+(aq) + OH¯(aq) Amines react with acids to produce salts. C6H5NH2(l) + HCl(aq) ——> C6H5NH3+Cl¯(aq) phenylammonium chloride This reaction allows one to dissolve an amine in water as its salt. Addition of aqueous sodium hydroxide liberates the free base from its salt C6H5NH3+Cl¯(aq) + NaOH(aq) ——> C6H5NH2(l) + NaCl(aq) + H2O(l) AMINO ACIDS Structure Amino acids contain 2 functional groups amine NH2 carboxyl R1 H2N COOH C COOH R2 They all have a similar structure - the identity of R1 and R2 vary H H2N C H H COOH H2N C CH3 COOH AMINO ACIDS – OPTICAL ISOMERISM Amino acids can exist as optical isomers If they have different R1 and R2 groups Optical isomers exist when a molecule Contains an asymmetric carbon atom H H2N Asymmetric carbon atoms have four different atoms or groups attached C COOH CH3 Two isomers are formed - one rotates plane polarised light to the left, one rotates it to the right H Glycine doesn’t exhibit optical isomerism as there are two H attached to the C atom H2N C COOH H GLYCINE 2-aminopropanoic acid AMINO ACIDS - ZWITTERIONS Zwitterion • a dipolar ion • has a plus and a minus charge in its structure (see below) • amino acids exist as zwitterions • give increased inter-molecular forces • melting and boiling points are higher R1 H3N+ C R2 COO¯ AMINO ACIDS - ACID-BASE PROPERTIES • amino acids possess acidic and basic properties • this is due to the two functional groups (see above) • COOH gives acidic properties • NH2 gives basic properties • they form salts when treated with acids or alkalis. R1 H2N C R2 COOH AMINO ACIDS - ACID-BASE PROPERTIES Acidic properties: with H+ HOOCCH2NH2 + H+ ——> HOOCCH2NH3+ with HCl HOOCCH2NH2 + HCl ——> HOOCCH2NH3+ Cl¯ Basic properties: + OH¯ ——> ¯OOCCH2NH2 + H2O with OH¯ HOOCCH2NH2 with NaOH HOOCCH2NH2 + NaOH ——> Na+ ¯OOCCH2NH2 + H2O PEPTIDES - FORMATION & STRUCTURE Amino acids can join together to form peptides via an amide or peptide link 2 amino acids joined dipeptide 3 amino acids joined tripeptide many amino acids joined polypeptide a dipeptide PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis • attack takes place at the slightly positive C of the C=O • the C-N bond is broken • hydrolysis with water is very slow • hydrolysis in alkaline/acid conditions is quicker • hydrolysis in acid/alkaline conditions (e.g. NaOH) will produce salts with HCl H+ NaOH OH¯ NH2 NH2 COOH COOH becomes becomes becomes becomes NH3+Cl¯ NH3+ COO¯ Na+ COO¯ PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H2N H C CO NH CH3 C H CH3 CO NH C CH3 Which amino acids are formed? COOH PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H2N C H CO NH CH3 C CH3 CO NH C COOH CH3 H H H H2N C CH3 COOH + H2N C H CH3 COOH + H2N C CH3 COOH PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H2N H C CO NH CH3 C H H CO NH C CH3 Which amino acids are formed? COOH PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H2N C H CO NH CH3 H CO NH C H2N C CH3 COOH CH3 H H H 2x C COOH + H2N C H COOH PROTEINS • are polypeptides with high molecular masses • chains can be lined up with each other • the C=O and N-H bonds are polar due to a difference in electronegativity • hydrogen bonding exists between chains dotted lines ---------- represent hydrogen bonding