RI 2025 Nitrogen Compounds Notes

-1- Raffles Institution Year 6 H2 Chemistry 2025 Lecture Notes 20 Nitrogen Compounds – Amines, Amides, Amino Acids & Proteins Content Amines (exemplified by ethylamine and phenylamine) (i) their formation (ii) salt formation (iii) other reactions of phenylamine Amides (exemplified by ethanamide) (i) formation from acyl chlorides (ii) neutrality of amides (ii) hydrolysis (under acidic and basic conditions) Amino acids (exemplified by aminoethanoic acid) (i) their acid and base properties (ii) zwitterion formation Proteins (i) formation of proteins (ii) hydrolysis of proteins Condensation Polymers (i) polyamides and polyesters Learning outcomes Candidates should be able to: (a) describe the formation of amines as exemplified by ethylamine (through amide and nitrile reduction) and by phenylamine (through the reduction of nitrobenzene) (b) describe the reaction of amines in the formation of salts (c) describe and explain the basicity of primary, secondary and tertiary amines in the gaseous phase (interpret as Lewis bases) (d) explain the relative basicities of ammonia, ethylamine and phenylamine, in aqueous medium, in terms of their structures (e) describe the reaction of phenylamine with aqueous bromine (f) describe the formation of amides from the condensation reaction between RNH2 and R'COCl (g) explain why an amide is neutral in terms of delocalisation of the lone pair of electrons on nitrogen (h) describe the chemistry of amides, exemplified by the following reactions: (i) hydrolysis on treatment with aqueous alkali or acid (ii) reduction to amines with lithium aluminium hydride (i) describe the acid/base properties of amino acids and the formation of zwitterions [knowledge of isoelectric points is not required] (j) describe the formation of peptide (amide) bonds between -amino acids, and hence explain protein formation (k) describe the hydrolysis of proteins

-2- Amines RNH2 1 – Introduction 1.1 What are amines?  Amines are derivatives of ammonia in which one or more hydrogen atoms of the ammonia molecule have been substituted by alkyl or aryl groups.  Amines are classified as primary, secondary or tertiary amines depending on the number of alkyl or aryl groups attached to the nitrogen atom. Type of amine Ammonia (not amine) Primary (1°) Secondary (2°) Tertiary (3°) Structure Number of R groups attached to N 0 1 2 3 1.2 Nomenclature  In the common names of amines, the names of the alkyl/aryl groups bonded to nitrogen are given first, followed by the suffix -amine. primary (1o) amine secondary (2o) amine tertiary (3 o) amine Aliphatic amines CH3–NH2 methylamine CH3CH2–NH2 ethylamine CH3CH2CH2–NH2 propylamine N-methylethylamine (ethylmethylamine) N,N-dimethylethylamine (ethyldimethylamine) H2N–CH2CH2–NH2 ethane-1,2-diamine (phenylmethyl)amine N H diethylamine N N-methyldiethylamine (diethylmethylamine) Aromatic amines  Aromatic amines are amines in which the nitrogen atom is attached directly to the aromatic ring. phenylamine (aniline) 2-methylphenylamine N-methylphenylamine diphenylamine N,N-dimethylphenylamine H CH3 CH3CH2 N CH3 CH3 CH3CH2 N CH2 NH2 NH2 NH2 CH3 N CH3 H N H N CH3 CH3

-3-  In naming amines with more complicated structures or in compounds in which other functional groups are of higher priority, the –NH2 group becomes the prefix (substituent) and is called the amino group. H2N–CH2CH2–COOH H2N–CH2CH2–OH 3-aminopropanoic acid 2-aminoethanol 4-aminobenzoic acid  Quaternary ammonium salts (R4N+X–) are the organic equivalent of ammonium compounds (e.g. NH4Cl). They have four alkyl/aryl groups bonded to the nitrogen atom. (CH3)4N+Cl– tetramethylammonium chloride  Cyclic amines exist. Some examples are given below. HN pyrrolidine piperidine coniine  Some naturally occurring amines. 1.3 Structure of amines  Similar to ammonia, the N atom of most amines is sp3 hybridised.  Consider the ethylamine molecule. CH3CH2–NH2 What is the shape of the ethylamine molecule with respect to the N atom?  There are 4 electron pairs around the N atom in an amine molecule.  They must be directed in a tetrahedral manner in order to minimise repulsion.  Since there are 3 bond pairs and 1 lone pair, the shape of the amine molecule is trigonal pyramidal with respect to the N atom. N H N H H H CH3CH2 N .. H2N CO2H CH3 CH3 CH3 N CH3 + Cl

-4- Amines RNH2 2 – Physical Properties  Amines are polar compounds. 2.1 Boiling point  The smaller aliphatic amines are gases (e.g. CH3NH2 and CH3CH2NH2) or liquids with low-boiling points (e.g. CH3CH2CH2NH2 and CH3CH2CH2CH2NH2). (a) For a particular class of amines, boiling point generally increases with increasing Mr. Amine Mr melting point/ oC boiling point / oC Reason  larger and more polarisable electron cloud  increasing strength of instantaneous dipole- induced dipole interactions CH3NH2 31 –94 – 6 CH3CH2NH2 45 –81 17 CH3CH2CH2NH2 59 –83 48 CH3CH2CH2CH2NH2 73 –51 78 C6H5NH2 93 –6 184 (b) Among isomeric amines, boiling point increases in the following order: 3o < 2o < 1o amines. Amine Class boiling point / oC Points to note CH3CH2CH2–NH2 1o amine 48  instantaneous dipole-induced dipole forces  permanent dipole-permanent dipole forces  hydrogen bonding CH3CH2–N–CH3 H 2o amine 34  instantaneous dipole-induced dipole forces  hydrogen bonding  less polar than CH3CH2CH2–NH2  weaker permanent dipole-permanent dipole forces (compared to 1° amines) (CH3)3N 3o amine 3  instantaneous dipole-induced dipole forces  permanent dipole-permanent dipole forces  no hydrogen bonding among the molecules. (c) In general, amines have higher boiling points than alkanes of similar electron cloud size. Compound Mr boiling point / oC predominant intermolecular forces CH3CH2NH2 45 17  hydrogen bonding CH3CH2CH3 44 –42  instantaneous dipole-induced dipole forces contains N–H bond? hydrogen bonding among own molecules? 1o amine 2o amine 3o amine

-5- (d) In general, amines have lower boiling points than alcohols of similar electron cloud size. Compound Mr boiling point / oC Reason  the N atom is less electronegative than the O atom  the N–H bond is less polar than the O–H bond  the N–HN hydrogen bond is weaker than the O–HO hydrogen bond CH3CH2NH2 45 17 CH3CH2OH 46 78.5 CH3CH2CH2NH2 59 48 CH3CH2CH2OH 60 97 2.2 Solubility  All three classes of amines can form hydrogen bonds with water.  Why are the lower members very soluble in water? o their molecules can interact with water molecules via hydrogen bonding o their molecules have relatively small non-polar hydrocarbon portions  The higher members have poor solubility in water because they have relatively large non-polar hydrocarbon portions. Borderline solubility in water is reached at about six carbon atoms. Hydrogen bonding between trimethylamine (a 3o amine) and water. 2.3 Odour  The smell of the early members of the amine series is similar to that of ammonia, though with a slightly fishy character.  Dimethylamine and trimethylamine are found in rotting fish and are partly responsible for its peculiar smell.  Putrescine (butane-1,4-diamine) and cadaverine (pentane-1,5-diamine) are found in decaying animal flesh. 2.4 Aromatic amines 