CBSE Class 12 Chemistry Revision Notes Chapter 13
Class 12 Chemistry Chapter 13 Notes: Amines
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Chapter 13 of Class 12 Chemistry is on Amines. Amines are ammonia-based organic compounds with one or more alkyl or aryl groups attached to the nitrogen atom. Amines have essential function in living creatures, including bioregulation, neurotransmission, and predator protection. Many amines are used as pharmaceuticals and treatments because of their biological phenomena.
Generally, Alkaloids are a class of physiologically active amines primarily produced by plants to save them from insects and animals. Therefore some alkaloids are used medicinally (as a painkiller). They are poisonous and can lead to death if consumed in high amounts.
In this unit, students will learn about amines and diazonium salts. Amines are one of the essential classes of organic compounds that can be obtained whenever the hydrogen atoms of ammonia molecules are replaced with an alkyl or aryl group.
Extramarks Class 12 Chemistry Chapter 13 Notes cover the entire topic accurately as per the current NCERT textbook syllabus. Along with the chapter notes, students can access other study materials by registering on Extramarks’ website.
Key Topics Covered in the Class 12 Chemistry Chapter 13 Notes: Amines
Structure of Amines:
As per the Chemistry Class 12 Chapter 13 Notes, Nitrogen has five valence electrons and so is trivalent with a lone pair. As per VSEPR theory, nitrogen in amines is sp3 hybridised. Due to the presence of lone pairs, its shape is pyramidal instead of tetrahedral, which is a general structure for most sp3 hybridised molecules. The three sp3 hybridised orbitals of nitrogen lap over with the orbitals of hydrogen or carbon depending upon the configuration of amines. The C-N-H angle seen in amines is less than 109 degrees, because of the presence of a lone pair, which is a characteristic angle of the tetrahedral geometry. The angle of amines is about 107 degrees.
Occurrence of Amines:
Naturally, amines are formed in proteins, vitamins, hormones, etc., and they are also prepared synthetically to make polymers, drugs, and dyes.
Amines are Classified as primary (1o), secondary (2o), and tertiary (3o) depending upon the number of hydrogen atoms that are replaced by alkyl or aryl groups in ammonia molecules.
Primary, secondary, and tertiary amines have one alkyl group, two alkyl groups, or three alkyl or benzyl groups linked to nitrogen.
When one of the hydrogen atoms of ammonia molecule gets replaced by an alkyl or aryl group.
For example, Methylamine CH3NH2 and Aniline C6H5NH2.
2. Secondary Amines:
Two organic substituents change the hydrogen atoms of the ammonia molecule, forming an amine.
For example, Dimethylamine (CH3)2NH, Diphenylamine (C6H5)2NH.
When 3 of the hydrogen atoms are changed by an organic substituent, it could be an aryl or aromatic group.
For example, include Trimethylamine N(CH3)3 and Ethylenediaminetetraacetic acid (EDTA).
These are the secondary or tertiary amines in an aromatic ring structure—for example, Piperidine (CH2)5NH, Aziridines C2H5N.
Nomenclature of Amines by IUPAC System:
Common Names of Amines:
In the standard system, an aliphatic amine is named by prefixing the alkyl group to amine, i.e. alkylamine as one word. We are considering primary amines. In secondary and tertiary amines, whenever two or more groups are the same, the prefix di or tri is added to them before the name of the alkyl group. In the case of mixed amine, alkyl groups are written in alphabetical order.
In the IUPAC system, amines are named alkanamines, derived by replacement by ‘e’ of alkane by the word amine, examples: Methanamine (CH3NH2) and Ethan amine (C2H5NH2). The longest chain containing an amino group is selected for naming higher member hydrocarbons. The amino group attached to the C atom is given a lower number. In case, where more than one amino group is present at various positions in the parent chain, their positions are determined by giving specific numbers to the carbon atoms bearing –NH2 and by giving suitable prefix such as di, tri etc., attached to the amine. However the letter ’e’ of the suffix of the hydrocarbon part is retained.
For example: (Ethane 1,2 diamine) H2N-CH2-CH2-NH2
Every alkyl group bonded to the N atom is classified as an N-alkyl group.
Students can refer to our Class 12 Chemistry Chapter 13 Notes for further understanding of the IUPAC system. Our subject matter experts have explained the topic with many more examples.
The NH2 group is directly attached to the benzene ring in aryl amines. C6H5NH2 is the simplest example of aryl amine. It is also known as aniline. According to the IUPAC system, the suffix ‘e’ of arene is replaced by ‘amine’ for naming aryl amines.
For example, C6H5-NH2 is named benzenamine.
Preparation of Amines:
The following methods are used to prepare amines:
- Reduction of nitro compounds.
- Ammonolysis of alkyl halides.
- Reduction of nitriles.
- Reduction of amides.
- Gabriel phthalimide synthesis.
- Hoffmann bromamide degradation reaction.
Reduction of nitro compounds:
Compounds of Nitro are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and by reducing with metals in an acidic medium.
R- NO2 + 3H2 → R-NH2 + 2H2O, In the presence of Ni, Pt or Pd
Ar- NO2 + 3H2 → Ar- NH2 + 2H2O
Nitro alkanes can also be reduced to the alkanamines. Reduction with iron scrap as well as the hydrochloric acid is favoured as FeCl2 formed gets hydrolysed to give hydrochloric acid during reaction. Thus, only a small quantity of hydrochloric acid is needed to initiate the reaction. Nitro (-NO2 )compounds can also be reduced with active metals like Fe, Sn, Zn etc., with concentrated HCL.
R- NO2 + 3H2 → R- NH2 + 2H2O, In the presence of Sn/HCl
Ar- NO2 + 3H2 → Ar- NH2 + 2H2O
Ammonolysis of alkyl halides:
Alkyl halides undergo nucleophilic substitution by the SN2 mechanism, with NH3 forming primary amines. A nucleophile can efficiently cleave the carbon-halogen bond in alkyl or benzyl halides.
Therefore, an alkyl or benzyl halide reaction with an ethanolic solution of ammonia undergoes a nucleophilic substitution reaction where the halogen atom is always replaced by an amino (–NH2) group. This cleavage of the C–X bond with the help of the ammonia molecule is known as ammonolysis.
The reaction is executed in a sealed tube at 373 K. The primary amine thus formed behaves as a nucleophile. It can further react with an alkyl halide to form secondary and tertiary amines, and finally, quaternary ammonium salt. The ammonium salt can achieve the free amine by reacting with a strong base.
- A primary amine is formed as the main product when ammonia is taken in excess.
- When alkyl halide is used in excess, the quaternary ammonium salt is formed as the main product.
This method is unsuitable for preparing aryl amines because aryl amines are relatively less reactive than alkyl halides towards nucleophilic substitution reactions.
Reduction of nitriles:
Nitriles produce primary amines on reduction with lithium aluminium hydride (LiAlH4) or catalytic hydrogenation. This reaction is used for an ascent of the anime series, i.e., to prepare amines containing one carbon atom that is more than the starting amine.
Reduction of amides:
On reduction with lithium aluminium hydride, the amides give amines.
Gabriel phthalimide synthesis:
Gabriel synthesis is used to prepare primary amines. Phthalimide, when mixed with ethanolic potassium hydroxide produces potassium salt of phthalimide. On heating with alkyl halide, which is followed by alkaline hydrolysis, it produces the corresponding primary amine. This method cannot synthesise primary aromatic amines because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.
Hoffmann bromamide degradation reaction:
Under this section of Class 12 Chemistry Chapter 13 Notes, students learn the reaction Hoffmann developed preparing primary amines by mixing an amide with bromine in an aqueous solution of sodium hydroxide. In this reaction, migration of an alkyl or aryl group takes place from the carbonyl carbon of the amide to the nitrogen atom. The amine so formed contains one carbon less than that present in the amide.
Physical Properties of Amines:
Physical state and smell:
The lower aliphatic amines are gaseous with a fishy smell. Primary amines with three or higher carbon atoms are liquids at room temperature, whereas higher ones are solids. Among arylamines, the lower members are liquid while the higher are solids. Aniline and other arylamines are usually colourless, but they get coloured if stored in an open area because of atmospheric oxidation.
Lower aliphatic amines are able to form hydrogen bonds with water molecules. That is why they are soluble in water. An enhancement in the size of the hydrophobic alkyl part increases amines’ molar mass, which decreases its solubility in water. Higher amines are naturally insoluble in water.
Some organic solvents such as, alcohol, benzene, and ether readily dissolve amines. Since, Alcohols have higher polarity as compared to amines, they form stronger intermolecular hydrogen bonds.
Primary and secondary amines often engage in intermolecular association due to polar hydrogen bonding between the nitrogen of one molecule and hydrogen of the other molecule.
In case of primary amines the intermolecular association is more prominent as compared to secondary due to the presence and availability of two hydrogen atoms. There is no intermolecular association in tertiary amines due to the absence of a free hydrogen atom for bonding. The sequence of boiling point of amines is as follows: Primary > Secondary > Tertiary.
Students may refer to various study materials including Class 12 Chemistry Chapter 13 Notes to ace their examinations. All our study materials are available on our official website www.extramarks.com once you register on it.
The chemical reactions of amines:
Amines are active due to the presence of electronegativity difference between N & H and the presence of lone pairs.
- Basic character of amines:
Amines react with acid to form salts.
Structure-basicity relationship of amines:
(a) Alkanamines versus ammonia:
The basicity of amines in the gaseous state: 3° amino > 2° amino > 1° amine > ammonia.
The basicity order of aliphatic amines-1° > 2° > 3°
The order of primary strength in case of substituted amines:
(b) Arylamines vs ammonia:
In the case of substituted benzylamines, electron releasing groups such as –OCH3, –CH3 increase basic strength, whereas electron-withdrawing groups such as –NO2, –SO3H, –COOH, –X decrease primary strength.
Due to the lone pair of electrons on nitrogen, amines exhibit a fundamental character. Therefore, they are more robust bases than water and are protonated by water in their aqueous solutions. Aromatic amines are far less essential than ammonia and aliphatic amines. The NH2 group is attached directly to the benzene ring in aniline or other arylamines. The unshared electron pair on the nitrogen atom is in conjugation with the benzene ring and hence making it less available for protonation. Aniline is considered to be a resonance hybrid of the following structures.
Amines react with alkyl halides to form amines of higher class. In this reaction, the amine acts as a nucleophile.
RNH2 + RX → R2NH + RX → R3N + RX →R4N+X–
Primary amine changes into secondary, tertiary and tetra alkyl ammonium halide.
Aliphatic, aromatic primary and secondary amines undergo acylation through nucleophilic substitution reactions with acid chlorides, anhydrides & esters to form an amide. At the time of the reaction, the acid generated can form salts of the amine.
RNH2 + CH3COCl → RNHCOCH3 + HCl
- Carbylamine reaction:
On heating the Aliphatic and aromatic primary amines with chloroform and ethanolic potassium hydroxide creates isocyanides or carbonyl amines which are foul-smelling substances.
RNH2 + CHCl3 +3KOH → RNC + 3KCl + 3H2O
- Reaction with nitrous acid:
Primary aliphatic amines derived from aliphatic diazonium salts on reaction with nitrous acid, which is unstable, decompose to yield a mixture of nitrogen gas and alcohols.
RNH2 + HNO2 → R-N2+-Cl– + H2O→ ROH + N2 + HCl
Aromatic amines give diazonium salts on reaction with nitrous acid at low temperatures.
- Reaction with aryl sulfonyl chloride:
Under CBSE and with the help of Class 12 Chemistry Chapter 13 Notes, students learn about Benzenesulfonyl chloride, also called Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides.
(a) The chemical reaction of benzene sulphonyl chloride yields N-ethylbenzenesulphonyl amide on reaction with a primary amine.
(b) N, N-diethyl benzene sulphonamide is formed in the reaction with a secondary amine with benzene sulphonyl chloride.
(c) Tertiary amines and benzene sulphonyl chloride do not react.
- Electrophilic substitution
Benzene ring in aniline is so activated that on reaction with aqueous bromine, all the ortho and para positions get substituted, resulting in the formation of a white precipitate of 2,4,6 tribromo aniline.
Direct nitration of aniline yields its oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline protonates to form the anilinium ion, meta directing.
Aniline reacts with concentrated sulphuric acid to form an anilinium hydrogen sulphate which, on heating with sulphuric acid at 453 – 473 K, produces p amino benzene sulphonic acids, commonly known as sulphanilic acid, as the primary product.
Students are advised to refer to our Class 12 Chemistry Chapter 13 Notes for a more detailed explanation about Electrophilic substitution.
The diazonium salts have the standard Formula – RN2 + X– where R represents an aryl group & X- ion, the Cl–, Br–, HSO4–, BF4–, etc. The N2 group is a nitrogen atom connected to another Nitrogen by a triple bond called the diazonium.
The name of the parent hydrocarbon is added as a prefix & the name of anion is set after diazonium. example: benzene diazonium chloride etc.
1° aliphatic amines from volatile alkyl diazonium salts & 1° aromatic amines from arene diazonium salts (stable at low temperature for a short time).
Methods of Preparation of Diazonium Salts:
As explained in CBSE solutions Class 12 Chemistry Chapter 13 Notes, the reaction of aniline with nitrous acid at 273-278 K produces Benzenediazonium chloride. Nitrous acid is formed in the reaction mixture by reacting sodium nitrite with hydrochloric acid. Diazotization is the term coined for converting primary aromatic amines into diazonium salts.
Benzenediazonium chloride is a colourless crystalline solid. It is readily soluble and stable in the cold but reacts with water when warmed. It decomposes quickly in a dry state. Some diazonium salts, such as Benzenediazonium fluoroborate, are water-insoluble and stable at room temperature.
Reactions involving displacement of nitrogen:
- Replacement by halide or cyanide ion: The chloride bromide and cyanide nucleophiles can quickly be introduced into the benzene ring in the presence of copper ions. This reaction is known as the Sandmeyer reaction.
Also referred to as the Gatterman reaction, It involves the nucleophilic substitution of diazonium salt to produce aryl halide in the presence of copper powder. Sandmeyer’s reaction is better than Gattermann’s reaction.
- Replacement by iodide ion:
Iodobenzene is synthesised when a diazonium salt solution is treated with potassium iodide.
ArN2+Cl– +KI → Ari + KCl + N2
- Replacement by fluoride ion:
Benzene diazonium fluoroborate is produced by treating arene diazonium chloride with fluoroboric acid, which on heating decomposes to yield aryl fluoride.
- Replacement by Hydrogen:
Diazonium salts are reduced to benzene when treated with ethanol or hypophosphorous acid.
Image Source: NCERT textbook
- Replacement by hydroxyl group:
If the temperature of the diazonium salts solution increases to 283 K, the salt gets hydrolysed to phenol.
ArN2+Cl– + H2O → ArOH + HCl + N2
- Replacement by nitro (–NO2) group:
The Diazonium group changes to the –NO2 group when the diazonium fluoroborate is heated with aqueous NaNO2 in Cu.
Reactions involving retention of diazo group:
A chemical reaction between Benzene diazonium chloride & phenol will lead to coupling between the two molecules at the para position of phenol via -N=N- bond. And P-hydroxyazobenzene is formed as a product.
Image Source: NCERT textbook
Azo coupling of sulfanilamide acid and N-(1-Naphthyl)ethylenediamine
Importance of Diazonium Salts in the formation of Aromatic Compounds:
These diazonium salts are an excellent intermediate. Therefore, they are helpful in a reaction where direct substitution is impossible. For example, aryl fluorides, aryl iodides, cyanobenzene, and chlorobenzene cannot be synthesised by direct substitution but are easily synthesised when the diazo group is introduced.
Amines Class 12 Notes will help revise the chapter and get an idea about the main topics covered in the Chapter. These Class 12 Chemistry Chapter 13 Notes will help students to cover the main topics of the Class 12 CBSE Chemistry syllabus. For competitive examinations such as JEE Mains, NEET, and CBSE board exams students should visit the Extramarks’ website and access various study resources to prepare for these examinations.
Class 12 Chemistry Chapter 13: Exercise & Answer Solutions
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CBSE solutions Class 12 Chemistry Chapter 13 Notes are explained in detail by subject matter experts at Extramarks. In addition to Chapter 13, students can access NCERT solutions and chapter notes for all other Chemistry chapters of Class 12.
NCERT Exemplar Class 12 Chemistry Chapter 13
All solutions and problems in our Class 12 Chemistry Chapter 13 Notes are given to help students prepare for their final examinations. These example questions from NCERT exemplars are a little more complex and cover every concept covered in each of the Class 12 Chemistry chapter.
Students will fully understand the concepts covered in each chapter by practising from the NCERT exemplar for Chemistry Class 12.
Key Features of Class 12 Chemistry Chapter 13 Notes
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