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THE CHEMISTRY OF AMINES A guide for A level students

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  1. THE CHEMISTRY OF AMINES A guide for A level students 2008 SPECIFICATIONS KNOCKHARDY PUBLISHING

  2. KNOCKHARDY PUBLISHING AMINES INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available. Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at... www.knockhardy.org.uk/sci.htm Navigation is achieved by... either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard

  3. AMINES • CONTENTS • Prior knowledge • Structure and classification • Nomenclature • Physical properties • Basic properties • Nucleophilic properties • Amino acids • Peptides and proteins • Amides • Check list

  4. 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

  5. H H R R R N: R N: R N: + R NR H R R R STRUCTURE & CLASSIFICATION Structure Contain the NH2 group Classification primary (1°) amines secondary (2°) amines tertiary (3°) amines quarternary (4°) ammonium salts Aliphaticmethylamine, ethylamine, dimethylamine AromaticNH2 group is attached directly to the benzene ring (phenylamine)

  6. NOMENCLATURE NomenclatureNamed after the groups surrounding the nitrogen + amine C2H5NH2ethylamine (CH3)2NH dimethylamine (CH3)3N trimethylamine C6H5NH2phenylamine (aniline)

  7. PREPARATION Amines can be prepared from halogenoalkanes ReagentExcess, alcoholic ammonia (WHY USE EXCESS?) ConditionsReflux in excess, alcoholic solution under pressure ProductAmine (or its salt due to a reaction with the acid produced) NucleophileAmmonia (NH3) EquationC2H5Br + NH3 (alc) ——> C2H5NH2 + HBr ( or C2H5NH3+Br¯ )

  8. PREPARATION Amines can be prepared from halogenoalkanes ReagentExcess, alcoholic ammonia (WHY USE EXCESS?) ConditionsReflux in excess, alcoholic solution under pressure ProductAmine (or its salt due to a reaction with the acid produced) NucleophileAmmonia (NH3) EquationC2H5Br + NH3 (alc) ——> C2H5NH2 + HBr ( or C2H5NH3+Br¯ ) WHY USE EXCESS AMMONIA? Ammonia attacks halogenoalkanes because it has a lone pair and is a nucleophile. The amine produced also has a lone pair C2H5NH2 so can also attack a halogenoalkane; this leads to the formation of substituted amines. Using excess ammonia ensures that all the halogenoalkane molecules react with the ammonia before having the chance to react with any amines produced.

  9. PHYSICAL PROPERTIES The LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them ... LEWIS BASES - they can be lone pair donors BRØNSTED-LOWRY BASES - they can be proton acceptors RNH2 + H+ ——> RNH3+ NUCLEOPHILES - provide a lone pair to attack an electron deficient centre

  10. PHYSICAL PROPERTIES Boiling point Boiling points increase with molecular mass Amines have higher boiling points than corresponding alkanes because of their intermolecular hydrogen bonding Quarternary ammonium salts are ionic and exist as salts Solubility Lower mass compounds are soluble in water due to hydrogen bonding with the solvent. Solubility decreases as the molecules get heavier. Soluble in organic solvents.

  11. BASIC PROPERTIES Bases The lone pair on the nitrogen atom makes amines basic; RNH2 + H+ ——> RNH3+ a proton acceptor Strengthdepends on the availability of the lone pair and its ability to pick up protons • the greater the electron density on the N, the better it can pick up protons • this is affected by the groups attached to the nitrogen

  12. BASIC PROPERTIES Bases The lone pair on the nitrogen atom makes amines basic; RNH2 + H+ ——> RNH3+ a proton acceptor Strengthdepends on the availability of the lone pair and its ability to pick up protons • the greater the electron density on the N, the better it can pick up protons • this is affected by the groups attached to the nitrogen electron withdrawing substituents (benzene rings) decrease basicity as the electron density on N is lowered and the lone pair is less effective H C6H5 N: H

  13. BASIC PROPERTIES Bases The lone pair on the nitrogen atom makes amines basic; RNH2 + H+ ——> RNH3+ a proton acceptor Strengthdepends on the availability of the lone pair and its ability to pick up protons • the greater the electron density on the N, the better it can pick up protons • this is affected by the groups attached to the nitrogen electron withdrawing substituents (benzene rings) decrease basicity as the electron density on N is lowered and the lone pair is less effective electron releasing substituents (CH3 groups) increase basicity as the electron density is increased and the lone pair is more effective H C6H5 N: H H CH3 N: H

  14. BASIC PROPERTIES Measurementthe strength of a weak base is depicted by its pKb value the smaller the pKb the stronger the base the pKa value can also be used; it is worked out by applying pKa + pKb = 14 the smaller the pKb, the larger the pKa. Compound Formula pKbComments ammonia NH3 4.76 methylamine CH3NH2 3.36 methyl group is electron releasing phenylamine C6H5NH2 9.38 electrons delocalised into the ring strongest basemethylamine > ammonia > phenylamineweakest base smallest pKb largest pKb

  15. CHEMICAL REACTIONS - WEAK BASES Water Amines which dissolve in water produce weak alkaline solutions CH3NH2(g) + H2O(l) CH3NH3+(aq) + OH¯(aq) Acids 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)

  16. CHEMICAL REACTIONS - NUCLEOPHILIC Due to their lone pair, amines react as nucleophiles Reagent Product Mechanism haloalkanes substituted amines nucleophilic substitution acyl chlorides N-substituted amides addition-elimination

  17. NUCLEOPHILIC SUBSTITUTION HALOALKANES Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes to produce a 2° amine. This too is a nucleophile and can react further producing a 3° amine and, eventually an ionic quarternary ammonium salt. C2H5NH2 + C2H5Br ——> HBr + (C2H5)2NHdiethylamine, 2° amine

  18. NUCLEOPHILIC SUBSTITUTION HALOALKANES Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes to produce a 2° amine. This too is a nucleophile and can react further producing a 3° amineand, eventually an ionic quarternary ammonium salt. C2H5NH2 + C2H5Br ——> HBr + (C2H5)2NHdiethylamine, 2° amine (C2H5)2NH + C2H5Br ——> HBr + (C2H5)3Ntriethylamine, 3° amine

  19. NUCLEOPHILIC SUBSTITUTION HALOALKANES Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes to produce a 2° amine. This too is a nucleophile and can react further producing a 3° amine and, eventually an ionic quarternary ammonium salt. C2H5NH2 + C2H5Br ——> HBr + (C2H5)2NHdiethylamine, 2° amine (C2H5)2NH + C2H5Br ——> HBr + (C2H5)3Ntriethylamine, 3° amine (C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯tetraethylammonium bromide a quaternary (4°) salt

  20. NUCLEOPHILIC SUBSTITUTION HALOALKANES Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes to produce a 2° amine. This too is a nucleophile and can react further producing a 3° amine and, eventually an ionic quarternary ammonium salt. C2H5NH2 + C2H5Br ——> HBr + (C2H5)2NHdiethylamine, 2° amine (C2H5)2NH + C2H5Br ——> HBr + (C2H5)3Ntriethylamine, 3° amine (C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯tetraethylammonium bromide a quaternary (4°) salt UsesQuarternary ammonium salts with long chain alkyl groups are used as cationic surfactantsin fabric softening e.g. [CH3(CH2)17]2N+(CH3)2 Cl¯

  21. R1 H H H2N C COOH H2N C COOH H2N C COOH CH3 H R2 AMINO ACIDS StructureAmino acids contain 2 functional groups amine NH2 carboxyl COOH They all have a similar structure - the identity of R1 and R2 vary

  22. H H H2NCCOOH H2NCCOOH CH3 H 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 • Asymmetric carbon atoms have four • different atoms or groups attached • Two isomers are formed - one rotates plane • polarised light to the left, one rotates it to the right • Glycine doesn’t exhibit optical isomerism as • there are two H attached to the C atom GLYCINE 2-aminoethanoic acid

  23. R1 H3N+ C COO¯ R2 AMINO ACIDS - ZWITTERIONS Zwitterion•a dipolar ion •has a plus and a minus charge in its structure •amino acids exist as zwitterions •give increased inter-molecular forces •melting and boiling points are higher

  24. R1 H2N C COOH R2 AMINO ACIDS - ACID-BASE PROPERTIES •amino acids possess acidic and basic properties • this is due to the two functional groups • COOH gives acidic properties • NH2 gives basic properties • they form salts when treated with acids or alkalis.

  25. AMINO ACIDS - ACID-BASE PROPERTIES Basic properties: with H+HOOCCH2NH2 + H+ ——> HOOCCH2NH3+ with HCl HOOCCH2NH2 + HCl ——> HOOCCH2NH3+ Cl¯ Acidic properties: with OH¯ HOOCCH2NH2 + OH¯ ——> ¯OOCCH2NH2 + H2O with NaOH HOOCCH2NH2 + NaOH ——> Na+ ¯OOCCH2NH2 + H2O

  26. 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

  27. 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 NH2 becomes NH3+Cl¯ H+ NH2 becomes NH3+ NaOH COOH becomes COO¯ Na+ OH¯ COOH becomes COO¯

  28. H H CH3 NH C COOH H2N C CO NH C CO CH3 CH3 H PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis Which amino acids are formed?

  29. H H CH3 NH C COOH H2N C CO NH C CO CH3 CH3 H H H CH3 H2N C COOH H2N C COOH H2N C COOH CH3 CH3 H PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis + +

  30. H H H NH C COOH H2N C CO NH C CO CH3 CH3 H PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis Which amino acids are formed?

  31. H H H H H NH C COOH H2N C CO NH C CO H2N C COOH H2N C COOH CH3 CH3 H CH3 H PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis 2 x +

  32. 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

  33. AMIDES Structure derivatives of carboxylic acids amide group is -CONH2 NomenclatureWhite crystalline solids named from the corresponding acid (remove oic acid, add amide) CH3CONH2 ethanamide (acetamide) C2H5CONHC6H5 N - phenyl propanamide - the N tells you the substituent is on the nitrogen Nylons are examples of polyamides PreparationAcyl chloride + ammonia CH3COCl + NH3 ——> CH3CONH2 + HCl ethanoyl chloride ethanamide

  34. AMIDES - CHEMICAL PROPERTIES Hydrolysis general reaction CH3CONH2 + H2O ——> CH3COOH + NH3 acidic soln. CH3CONH2 + H2O + HCl ——> CH3COOH + NH4Cl alkaline soln. CH3CONH2 + NaOH ——> CH3COONa + NH3 IdentificationWarming an amide with dilute sodium hydroxide solution and testing for the evolution of ammonia using moist red litmus paper is used as a simple test for amides. Reduction Reduced to primary amines: CH3CONH2 + 4[H] ——> CH3CH2NH2 + H2O

  35. THE CHEMISTRY OF AMINES THE END © 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING