Specification from OCR Explain the basicity of amines in terms of proton acceptance by the nitrogen lone pair. Describe the reactions of amines with acids to form salts Describe the preparation of: i) aliphatic amines by substitution of halogenalkanes with excess ethanolic ammonia ii) aromatic amines by reduction of nitroarenes using tin and conc. hydrochloric acid Describe the synthesis of an azo-dye State the use of reactions in the formation of dyestuffs

Amines Amines are essentially molecules of ammonia One or more of the hydrogen atoms have been replaced with an alkyl group.

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

NOMENCLATURE (CH3)2NH dimethylamine (CH3)3N trimethylamine Nomenclature Named after the groups surrounding the nitrogen + amine C2H5NH2 ethylamine (CH3)2NH dimethylamine (CH3)3N trimethylamine C6H5NH2 phenylamine (aniline)

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.

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

Amines as bases Bases are proton acceptors. Amines don’t actually accept protons, they donate a lone pair to the hydrogen atom to form a dative bond. Ammonia and bases can do this with any suitable acid to give a salt.

BASIC PROPERTIES Bases The lone pair on the nitrogen atom makes amines basic; RNH2 + H+ ——> RNH3+ 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; RNH2 + H+ ——> RNH3+ 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 C6H5 N: H

BASIC PROPERTIES C6H5 N: CH3 N: Bases The lone pair on the nitrogen atom makes amines basic; RNH2 + H+ ——> RNH3+ 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 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

BASIC PROPERTIES The strength of a weak base will depend on whether the substituents are electron- relasing or withdrawing Compound Formula Comments ammonia NH3 methylamine CH3NH2 methyl group is electron releasing phenylamine C6H5NH2 electrons delocalised into the ring strongest base > > weakest base

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)

The preparation of amines

1. PREPARATION OF ALIPHATIC AMINES Aliphatic amines can be prepared from substitution of halogenoalkanes with excess ethanolic ammonia Reagent Excess alcoholic ammonia Conditions Reflux in aqueous, alcoholic solution under pressure Product Amine Nucleophile Ammonia (NH3) Equation C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr

WHY IS THE AMMONIA IN EXCESS?? Reagent Aqueous, alcoholic ammonia (in EXCESS) Conditions Reflux in aqueous , alcoholic solution under pressure Product Amine Nucleophile Ammonia (NH3) Equation e.g. C2H5Br + 2NH3 (aq / alc) ——> C2H5NH2 + NH4Br (i) C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr (ii) HBr + NH3 (aq / alc) ——> NH4Br Mechanism Notes The equation shows two ammonia molecules. The second ammonia molecule ensures the removal of HBr, because otherwise the salt ethylammonium bromide C2H5NH3+Br¯ will be formed.

NUCLEOPHILIC SUBSTITUTION WHY IS THE AMMONIA IN EXCESS?? Why excess ammonia? A large excess ammonia also ensures that further substitution doesn’t take place - see below Problem Amines are also nucleophiles (lone pair on N) and can attack another molecule of halogenoalkane 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)2NH diethylamine, a 2° amine (C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N triethylamine, a 3° amine (C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯ tetraethylammonium bromide a quaternary (4°) salt

2. Reduction of nitrobenzene to give phenylamine Conditions are reflux, this is important in the production of compounds called azo-dyes.

H2N C COOH R H H2N C COOH H H2N C COOH CH3 H AMINO ACIDS carboxyl COOH Structure Amino acids contain 2 functional groups amine NH2 carboxyl COOH They all have a similar structure - the identity of R varies H2N C COOH R H H2N C COOH H H2N C COOH 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 H2N C COOH CH3 H H2N C COOH H GLYCINE 2-aminoethanoic acid

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 H3N+ C COO¯ R2 R1

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. H2N C COOH R2 R1

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

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

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

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

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

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

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 -CONH2 Nomenclature White 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 Preparation Acyl chloride + ammonia CH3COCl + NH3 ——> CH3CONH2 + HCl ethanoyl chloride ethanamide

AMIDES - CHEMICAL PROPERTIES Hydrolysis general reaction CH3CONH2 + H2O ——> CH3COOH + NH3 acidic soln. CH3CONH2 + H2O + HCl ——> CH3COOH + NH4Cl alkaline soln. CH3CONH2 + NaOH ——> CH3COONa + NH3 Identification 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: CH3CONH2 + 4[H] ——> CH3CH2NH2 + H2O