THE CHEMISTRY OF AMINES A guide for A

THE CHEMISTRY OF AMINES A guide for A level students KNOCKHARDY PUBLISHING

KNOCKHARDY PUBLISHING AMINES INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A 2 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 A 2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at. . . www. argonet. co. uk/users/hoptonj/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

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

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 NH 2 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, dimethylamine Aromatic NH 2 group is attached directly to the benzene ring (phenylamine)

NOMENCLATURE Nomenclature Named after the groups surrounding the nitrogen + amine C 2 H 5 NH 2 ethylamine (CH 3)2 NH dimethylamine (CH 3)3 N trimethylamine C 6 H 5 NH 2 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 (NH 3) Equation C 2 H 5 Br + NH 3 (aq / alc) ——> C 2 H 5 NH 2 + HBr ( or C 2 H 5 NH 3+Br¯ )

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 RNH 2 + H+ ——> RNH 3+ NUCLEOPHILES - provide a lone pair to attack an electron deficient centre

PHYSICAL PROPERTIES 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.

BASIC PROPERTIES Bases The lone pair on the nitrogen atom makes amines basic; RNH 2 + H+ ——> RNH 3+ a proton acceptor Strength depends 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

BASIC PROPERTIES Bases The lone pair on the nitrogen atom makes amines basic; RNH 2 + H+ ——> RNH 3+ a proton acceptor Strength depends 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 C 6 H 5 N: H

BASIC PROPERTIES Bases The lone pair on the nitrogen atom makes amines basic; RNH 2 + H+ ——> RNH 3+ a proton acceptor Strength depends 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 C 6 H 5 N: H electron releasing substituents (CH 3 groups) increase basicity as the electron density is increased and the lone pair is more effective H CH 3 N: H

BASIC PROPERTIES Measurement the strength of a weak base is depicted by its p. Kb value the smaller the p. Kb the stronger the base the p. Ka value can also be used; it is worked out by applying p. Ka + p. Kb = 14 the smaller the p. Kb, the larger the p. Ka. Compound Formula p. Kb ammonia NH 3 4. 76 methylamine CH 3 NH 2 3. 36 methyl group is electron releasing phenylamine C 6 H 5 NH 2 9. 38 electrons delocalised into the ring strongest base smallest p. Kb Comments methylamine > ammonia > phenylamine weakest base largest p. Kb

CHEMICAL REACTIONS - WEAK BASES Water Amines which dissolve in water produce weak alkaline solutions CH 3 NH 2(g) Acids + H 2 O(l) CH 3 NH 3+(aq) + OH¯(aq) Amines react with acids to produce salts. C 6 H 5 NH 2(l) + HCl(aq) ——> C 6 H 5 NH 3+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 C 6 H 5 NH 3+Cl¯(aq) + Na. OH(aq) ——> C 6 H 5 NH 2(l) + Na. Cl(aq) + H 2 O(l)

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

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. C 2 H 5 NH 2 + C 2 H 5 Br ——> HBr + (C 2 H 5)2 NH diethylamine, 2° amine

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. C 2 H 5 NH 2 + C 2 H 5 Br ——> HBr + (C 2 H 5)2 NH + C 2 H 5 Br ——> HBr + (C 2 H 5)3 N diethylamine, 2° amine triethylamine, 3° amine

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. C 2 H 5 NH 2 + C 2 H 5 Br ——> HBr + (C 2 H 5)2 NH + C 2 H 5 Br ——> HBr + (C 2 H 5)3 N + C 2 H 5 Br ——> (C 2 H 5)4 N+ Br¯ diethylamine, 2° amine triethylamine, 3° amine tetraethylammonium bromide a quaternary (4°) salt

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. C 2 H 5 NH 2 + C 2 H 5 Br ——> HBr + (C 2 H 5)2 NH + C 2 H 5 Br ——> HBr + (C 2 H 5)3 N + Uses C 2 H 5 Br ——> (C 2 H 5)4 N+ Br¯ diethylamine, 2° amine triethylamine, 3° amine tetraethylammonium bromide a quaternary (4°) salt Quarternary ammonium salts with long chain alkyl groups are used as cationic surfactants in fabric softening e. g. [CH 3(CH 2)17]2 N+(CH 3)2 Cl¯

AMINO ACIDS Structure Amino acids contain 2 functional groups amine NH 2 carboxyl COOH R 1 H 2 N C R 2 They all have a similar structure - the identity of R 1 and R 2 vary H H 2 N C H H COOH H 2 N C CH 3 COOH

AMINO ACIDS – OPTICAL ISOMERISM Amino acids can exist as optical isomers If they have different R 1 and R 2 groups Optical isomers exist when a molecule Contains an asymmetric carbon atom H H 2 N C COOH CH 3 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 H H 2 N 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 R 1 H 3 N + C R 2 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 • NH 2 gives basic properties • they form salts when treated with acids or alkalis. R 1 H 2 N C R 2 COOH

AMINO ACIDS - ACID-BASE PROPERTIES Acidic properties: with H+ HOOCCH 2 NH 2 with HCl. HOOCCH 2 NH 2 + HCl + H+ ——> HOOCCH 2 NH 3+ Cl¯ Basic properties: with OH¯ HOOCCH 2 NH 2 + OH¯ ——> ¯OOCCH 2 NH 2 + H 2 O with Na. OH HOOCCH 2 NH 2 + Na. OH ——> Na+ ¯OOCCH 2 NH 2 + H 2 O

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. Na. OH) will produce salts with HCl H+ Na. OH OH¯ NH 2 COOH becomes NH 3+Cl¯ NH 3+ COO¯ Na+ COO¯

PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H 2 N H C CO NH CH 3 CO NH C CH 3 Which amino acids are formed? COOH

PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H 2 N C H CO NH CH 3 CO NH C COOH CH 3 H H 2 N C CH 3 COOH + H 2 N C H CH 3 COOH + H 2 N C CH 3 COOH

PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H 2 N H C CO NH CH 3 C H H CO NH C CH 3 Which amino acids are formed? COOH

PEPTIDES - HYDROLYSIS Peptides are broken down into their constituent amino acids by hydrolysis H H 2 N C H CO NH CH 3 H CO NH C H 2 N C CH 3 COOH CH 3 H H H 2 x C COOH + H 2 N 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

AMIDES Structure derivatives of carboxylic acids amide group is Nomenclature -CONH 2 White crystalline solids named from the corresponding acid (remove oic acid, add amide) CH 3 CONH 2 ethanamide (acetamide) C 2 H 5 CONHC 6 H 5 N - phenyl propanamide - the N tells you the substituent is on the nitrogen Nylons are examples of polyamides Preparation Acyl chloride + ammonia CH 3 COCl + ethanoyl chloride NH 3 ——> CH 3 CONH 2 + HCl ethanamide

AMIDES - CHEMICAL PROPERTIES Hydrolysis general reaction acidic soln. alkaline soln. Identification CH 3 CONH 2 + + + H 2 O ——> CH 3 COOH + NH 3 H 2 O + HCl ——> CH 3 COOH + NH 4 Cl Na. OH ——> CH 3 COONa + NH 3 Warming 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: CH 3 CONH 2 + 4[H] ——> CH 3 CH 2 NH 2 + H 2 O

REVISION CHECK What should you be able to do? Recall the structural types and nomenclature of amines Explain and understand the basic properties of amines Recall and explain the nucleophilic properties of amines Recall the structure and general properties of amino acids, proteins and peptides Understand why amino acids can exhibit optical activity Work out the products from the hydrolysis of a peptide Recall the structure, nomenclature, preparation and simple properties of amides CAN YOU DO ALL OF THESE? YES NO

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THE CHEMISTRY OF AMINES THE END © 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING