Amines Class 12 Notes NEET Chemistry (Free PDF Download)

Introduction

Amines are organic compounds made composed of amino group and one or more alkyl or aryl groups linked to the nitrogen atom. Amines have many functions in living things, including bioregulation, neurotransmission, and predator defence. Because of their high biological activity, several amines are used as medications and therapies.

Alkaloids are a type of physiologically active amine produced by plants to defend themselves against insects and other animals. While some alkaloids are used medicinally (as pain relievers), they are all dangerous and can cause death if ingested in large quantities.

Classification

One, two, or three alkyl or aryl groups are connected to nitrogen in primary, secondary, and tertiary amines, respectively.

Type	Examples
Primary $({1^ \circ })$  $R - N{H_2}$	Structure of cyclohexylamine and tert-butylamine
Secondary $({2^ \circ })$  ${R_2}NH$	Structure of n-ethylamine and piperidine
Tertiary $({3^ \circ })$  ${R_3}N$	Structure of N,N-diethylaniline and Quinuclidine

In quaternary ammonium ions, four alkyl or aryl bonds join a nitrogen atom. The nitrogen atom has a positive charge, much like in simple ammonium salts like ammonium chloride. The following is an example of quaternary ammonium salt:

Structure of Tetraethylammonium Iodide

Structure of Amines

Ammonia's tetrahedral geometry is slightly distorted. A single pair of nonbonding electrons occupy one of the tetrahedral positions. In this geometry, the bulky lone pair compresses the $H - N - H$ bond angles to ${107^ \circ }$ from the "ideal" $s{p^3}$ bond angle of ${109.5^ \circ }$, which is represented by $s{p^3}$ nitrogen hybridization. Trimethylamine has less compression because the bulky methyl groups extend the angle slightly.

Structure of Ammonia and Trimethylamine

Non-superimposable is the mirror counterpart of a tetrahedral amine with three different substituents (including a lone pair). We can't normally resolve such an amine into two enantiomers since the enantiomers interconvert so quickly. During this process, known as nitrogen inversion, the lone pair moves from one side of the molecule to the other.

Since there is no lone pair to undergo nitrogen inversion in quarterary ammonium salts with asymmetric nitrogen atoms, inversion of configuration is not possible.

Structure of Quaternary Ammonium Salts

Amines that can't establish the $s{p^2}$ -hybrid transition state for nitrogen inversion also have chirality. If the nitrogen atom is confined in a small ring, it will be unable to attain the ${120^\circ} $ bond angles required for inversion.

Physical Properties

State

Lower aliphatic amines are odorless gaseous molecules that smell like fish. Liquid primary amines have three or more carbon atoms, while solid primary amines have more than three carbon atoms.

Aniline and other aryl amines are normally colourless, but they become coloured over time owing to air oxidation.

Dipole Moment

Since the significant dipole moment of the lone pair of electrons contributes to the dipole moments of the $C - H$  and $H - N$  bonds, amines are highly polar.

Solubility

Hydrogen bonds can be formed by $N - H$ bonds in primary and secondary amines. Because they lack $N - H$ bonds, pure tertiary amines cannot generate hydrogen bonds. However, hydrogen bonding from molecules with $O - H$ or $N - H$ bonds can be accepted.

Hydrogen Bonding in Primary, Secondary and Tertiary Amines

Lower aliphatic amines are soluble in water due to their propensity to form hydrogen bonds with water. However, as the molar mass of amines rises, the size of the hydrophobic alkyl component grows larger, reducing solubility. Higher amines are largely insoluble in water.

Boiling Point

Since nitrogen is less electronegative than oxygen, the $N - H$ bond is less polar than the $O - H$ bond. As a result, hydrogen bonding between amines and alcohols of identical molecular weights are weaker. Primary and secondary amines have lower boiling temperatures than alcohols, but are higher than ethers with similar molecular weights. Because they lack hydrogen bonds, tertiary amines have lower boiling temperatures than primary and secondary amines with comparable molecular weights.

Preparation of Amines

Preparation of Amines

Alkyl Halides

Hoffmann’s Ammonolysis Method

It's an ammonolysis case. An alkyl halide and an ethanolic ammonia solution are heated to ${100^ \circ }C$ in a sealed tube.

1. The process creates a combination of items.

2. Because it forms an alkene, tertiary alkyl halide is not recommended.

3. A bimolecular substitution $({S_{{N}}2})$ sets the stage for the reaction.

Gabriel’s Phthalimide Synthesis

Alkyl phthalimide is made by heating potassium phthalimide with alkyl halide after it has been treated with $KOH$. Only primary amine is produced by the subsequent hydrolysis. Alkyl phthalimide is also treated with hydrazine to enhance the yield of primary amine.

Example:

Reaction of Gabriel’s Phthalimide Synthesis

Grignard Reagent

After chloramine treatment, the primary amine is produced using the Grignard reagent or a trialkyl borane.

$RMgX + ClN{H_2} \to R - N{H_2} + MgXCl$ 

Alcohols

Sabatier Reaction

Alcohols and ammonia are heated under pressure in the presence of a catalyst like copper chromite or alumina. A mixture of goods is possible to gain.

Example:

Example of Sabatier Reaction

Carbonyl Compounds

Reductive Amination

A number of aldehydes and ketones are converted to amines in the presence of ammonia.

Hydrogen $20 - 150\;atm$ over Raney nickel $40 - {150^\circ} C$ or sodium cyano hydrido borate, $NaB{H_3}CN$, can catalyse the reduction process.

Carboxylic Acids and Derivatives

Schmidt Reaction

A mixture of carboxylic acid and hydrazoic acid is treated with cold concentrated sulphuric acid to form primary amine.

$RCOOH + {N_3}H\xrightarrow[{{H_2}S{O_4}}]{{Cold\;conc.}}RN{H_2} + C{O_2} + {N_2}$ 

Example:

Schmidt Reaction 

Curtius Reaction

In this procedure, the acyl azide is pyrolyzed to produce isocyanates. Amine is formed as a result of the following hydrolysis.

Example:

Curtius Reaction

In the first step nitrogen gas evolves and carbon dioxide is produced along with the amine in the second step.

Hofmann’s Degradation of Amides

The amide is warmed with bromine and a strong aqueous KOH or NaOH solution.

1. The final product is a primary amine with one fewer carbon atoms than amide.

2. Alkyl isocyanate is intermediate, and the rearrangement is intramolecular.

Example:

Reaction of Hofmann Degradation

Hydrolysis of Amides and Isocyanides

Primary amine is given on the hydrolysis of N-substituted amide and isocyanide.

Example:

Hydrolysis of Amides

Example:

$R - NC\xrightarrow[\Delta ]{{KOH,\;{H_2}O}}R - N{H_2} + HCOOK$ 

Hydrolysis of Isocyantes also yields amines.

$R - N = C = O\xrightarrow[\Delta ]{{{H_3}{O^ + }}}R - N{H_2}$ 

Decarboxylation of Amino Acids

Primary amine is formed when amino acids are heated with barium hydroxide. On heating, quaternary ammonium hydroxide decomposes into alcohol if there is no $\beta $ -hydrogen in the alkyl group, but if there is, Hofmann elimination occurs.

Degradation Reaction of Amino Acids

Nitrogen Compounds

Reduction of Cyanides, Isocyanides, Oximes, Imines

Nitrile $( - CN)$ , oxime $( = NOH)$ , imine $( = NH)$ , enamine etc, on catalytic reduction with ${H_2}$ , nickel gives corresponding amine.

Example:

Reaction of Cyanide 

Example:

Reaction of Oxime

Example:

Reaction of Imine

Reduction of Nitro Compounds

Tin with hydrochloric acid or lithium aluminium hydride are widely used to convert nitroalkanes to amines. ${H_2}$ and a catalyst are also utilised in this reduction.

Reactions of Amines

Sulphonylation – Hinsberg Test 

Treatment with benzene sulphonyl chloride or p-toluene sulphonyl chloride (Hinsberg reagent).

1. The reaction is used to separate the amine mixture.

2. Primary and secondary amines react to generate matching sulphonamides in the presence of active hydrogen, while tertiary amine does not.

3. The main amine product, N-alkyl benzene sulphonamide, dissolves in $KOH$ and creates a water-soluble salt.

Hinsberg Test 

Nitrous Acid Test

1. When a primary amine interacts with HNO3, alcohol is frequently the result. As an intermediate, diazonium salt is generated due to the reaction.

2. Secondary amine produces nitrosoamine, a yellow oily liquid.

3. A soluble salt forms when the tertiary amine is dissolved in cold $HN{O_2}$.

Example:

$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{N}_{2}+\mathrm{H}_{2} \mathrm{O}$

Example:

$\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~N}-\mathrm{N}=\mathrm{O}+\mathrm{H}_{2} \mathrm{O}$

Carbylamine Reaction

When primary amine is treated with chloroform and alcoholic $KOH$, it creates pungent alkyl carbylamine vapours that are unpleasant or pungent.

The reaction is referred to as a primary amine test:

Reaction Between Chloroform and Alcohol 

Treatment with $C{S_2}/HgC{l_2}$ 

1. Dithiocarbamic acid is generated when primary amine is heated with carbon sulphide which is subsequently destroyed by mercuric chloride to produce alkyl isothiocyanate. Hofmann's Mustard Oil Reaction is the name of the reaction.

2. Mercuric chloride does not degrade dithiocarbamic acid, which is produced by secondary amine.

3. Carbon sulphide has no effect on the tertiary amine.

Reaction of Carbon Sulphide with Primary, Secondary and Tertiary Amine

Acylation

When primary and secondary amines react with acid halide or anhydride, N-substituted amide is produced. In aniline, the reaction is employed to protect the ring.

Hofmann’s Exhaustive Methylation and Elimination

Quaternary ammonium iodides are converted to hydroxides by $A{g_2}O$. The hydroxide is removed by heat, resulting in tertiary amines and alkenes. The fact that the least substituted alkene is the major product is a key factor in this process. This procedure is known as Hofmann's Elimination.

Example:

Hofmann Elimination Reaction

Metal Ions

Amines form coordination compounds with metal ions.

Preparation of Aromatic Amines

In addition to the methods for making aliphatic amines, the following methods can be employed.

Reduction of Nitro Compounds

Vapour – Phase Reduction

The reducing agents used were: \[CuO\] on $Si{O_2}$ or $V - Pt$ Catalyst.

Example:

Vapour Phase Reduction 

Catalytic Hydrogenation

The reducing agents used were ${H_2},\;Pd - C/Et - OH$ 

Catalytic Hydrogenation 

Ammonolysis of Aryl Halides

Ammonolysis of Aryl Halide 

Reduction of Azo Compounds

Reaction of azo Compounds

Reactions of Aromatic Amines

Diazonium Salt and Reactions

Reaction of Aromatic Amine

Reaction of Diazonium Salt

Oxidation

Aromatic amines can be quickly oxidised to a range of chemicals depending on the conditions. Through controlled oxidation using $N{a_2}C{r_2}{O_7}$ and ${H_2}S{O_4}$, aniline is transformed to p-benzoquinone. When a chemical is exposed to air, p-benzoquinone is produced.

Aniline black, a black dye, is produced if oxidation is not controlled.

Oxidation Reaction of Aniline 

Halogenation

$(N{H_2})$ is a strong activator and ortho/para director. It produced 2,4, 6-tribromo aniline after normal bromination in ${H_2}O$.

Halogenation Reaction

By lowering the activating effect of amino groups by acetylation of aniline, the $ - NHCOC{H_3}$ group can be exploited to produce ortho and para derivatives.

Nitration

Typical aniline nitration is not done because the nitrating mixture is oxidising and $N{H_2}$ is a strong activating group. Aniline is also protonated at 288 K to produce the meta-directing anilinium ion. As a result, a large number of meta derivatives are produced in addition to ortho and para derivatives.

Nitration Reaction 

By shielding the $ - N{H_2}$ group with an acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p-nitro derivative obtained as the main product.

Reaction of Aniline with Pyridine 

Sulphonation

Sulphonation Reaction

Note: 

The $- N{H_2}$ group attached to benzene is normally incapable of producing the zwitter ion, whereas $- S{O_3}H$ is highly acidic. Sulphanilic acid forms the zwitter ion as a result. There is no zwitter ion produced when the $- N{H_ 2}$ group is coupled to benzoic acid in any place.

Analysis of Amines

Amines is distinguished primarily by their basicity. An amine is a water-insoluble chemical that dissolves in cold dilute hydrochloric acid or a water-soluble molecule that turns litmus blue in an aqueous solution. The best approach to tell if an amine is primary, secondary, or tertiary is to use the Hinsberg test. The amine is shaken with benzenesulfonyl chloride in the presence of aqueous potassium hydroxide. Primary and secondary amines, but not tertiary amines, create substituted sulfonamides. The monosubstituted sulfonamide of a primary amine has an acidic hydrogen connected to nitrogen. By interacting with potassium hydroxide, this amide becomes a soluble salt. The disubstituted sulfonamide from a secondary amine is insoluble in the alkaline reaction mixture because it lacks an acidic hydrogen. The way an amine reacts with nitrous acid can aid in classifying it. Primary aromatic amines, in particular, have a unique behaviour: when they are treated with nitrous acid, they become diazonium salts, which when combined with beta-naphthol produce vividly coloured azo compounds.

Hinsberg Test 

Solved examples:

1. The best reagent for converting-2-phenylpropanamide into 1-phenylethanamine is.

\[\begin{array}{*{20}{l}}{\left( {\rm{A}} \right){\rm{ excess~ }}{{\rm{H}}_{\rm{2}}}{\rm{/Pt}}}\\{\left( {\rm{B}} \right){\rm{ NaOH /B}}{{\rm{r}}_{\rm{2}}}}\\{\left( {\rm{C}} \right){\rm{ NaB}}{{\rm{H}}_{\rm{4}}}{\rm{/methanol}}}\\{\left( {\rm{D}} \right){\rm{ LiAl}}{{\rm{H}}_{\rm{4}}}{\rm{/ether}}}\end{array}\]

Correct Answer: Option (B)

Explanation: 

The Hoffmann bromamide reaction, also known as the degradation reaction, begins with the treatment of the primary amide group with halogen (Br), and then the halogen substituted amide product is converted to primary amine with the release of carbon dioxide gas.

The reaction is shown below.

1-phenylethanamine

Thus, option (B) is correct.

2. Which of the following compounds will not undergo azo coupling reaction with benzene diazonium chloride?

(A) Aniline 

(B) Phenol 

(C) Anisole 

(D) Nitrobenzene

Correct Answer: Option (D)

Explanation:

A weak electrophile, such as Diazonium cation, readily reacts with electron-rich compounds, such as hydroxyl groups and amine groups. They do not react with electron removing groups such as nitro groups. As a result, nitrobenzene will not undergo azo coupling with benzene diazonium chloride. Thus, option (d) is correct.

Importance of Amines in Class 12 Chemistry

This chapter is all about the organic compounds containing an amino group (-NH2) in the main chain. These compounds can be differentiated into primary, secondary, and tertiary classifications based on the position of the amino group in the carbon chain. The position of this functional group is defined by the carbon atom it is attached to.

In proceeding further in this chapter, students will learn how the amines are formed from amides, nitriles, and from alkyl compounds. There are different ways to synthesise these compounds. This chapter will explain all the reaction steps and reagents used along with the intermediate products. You will find out how the transition of these organic compounds leads to the formation of amines.

Every reaction will be explained in detailed steps so that the students can understand the mechanism of developing or substituting the amino group for a particular reactant organic compound.

This chapter is crucial for the NEET aspirants as they will understand how to achieve the desired amine from a given reactant organic compound. By understanding the concept and mechanism of the synthesis reactions, students will be able to write equations and answer fundamental questions.

To revise this chapter quickly, one can refer to the Amines notes on Vedantu prepared by our experts. These notes will provide a concise format of these reactions and other fundamental concepts of this chapter.

Benefits of Amines NEET Notes

• The Amines NEET notes are prepared in a concise format so that students preparing for the exam can easily revise the chapter in an organized way. All the conceptual theories, definitions, reactions, explanations, etc., can be found in an easy-to-comprehend format in these notes.

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Download Amines Class 12 Notes Free PDF

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