Amines

The order of basic strength in case of methyl substituted amines

(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ 2° > 1° > 3°

The order of basic strength in case of ethyl substituted amines

(C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ 2° > 3° > 1°

Order of boiling points of isomeric amines

1° > 2° > 3°

The order of basicity of aliphatic amines

1° > 2° > 3°

Why are the boiling points of primary and secondary amines greater than tertiary?

1° and 2° amines are associated through intermolecular hydrogen bonding. This intermolecular association is more in 1° amines than in 2° amines. Tertiary amines do not have intermolecular association due to the absence of hydrogen atom.

The order of basicity of amines in the gaseous phase

3° > 2° > 1° > NH₃

Carbylamine reaction (isocyanide test)

Aliphatic and aromatic primary amines on heating with chloroform and alcoholic potassium hydroxide form foul smelling isocyanides or carbylamines. This reaction is known as carbylamines reaction or isocyanide test and is used as a test for primary amines. Secondary and tertiary amines do not give this reaction.

Acylation

Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides and esters in presence of base like pyridine to form substituted amides. This reaction is known as acylation.

Preparation of amines by ammonolysis of alkyl halides (Hoffmann's Ammonolysis)

Alkyl halides when treated with alcoholic solution of ammonia at 373 K, undergo nucleophilic substitution reaction to give a mixture of 1°, 2° and 3° amines and quaternary ammonium salt (R₄N⁺X⁻). This process of cleavage of C-X bond by ammonia is called ammonolysis

Basic character of amines

Amines react with acids to form salts. This reaction shows that they are basic in nature. R-NH₂ + HX → R-NH₃⁺X⁻

Alkylation of amines hoffmann's ammonolysis

Amines react with alkyl halides undergo nucleophilic substitution reaction to form a mixture of secondary and tertiary amines and quaternary ammonium salt.

Benzoylation

Amines react with benzoyl chloride (C₆H₅COCl) to form N-alkylbenzamide

Bromination

Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6- tribromoaniline.

Sulphonation

Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product. Sulphanilic acid contains both acidic and basic groups and so it forms internal salts called zwitter ions. (dipolar ion)

Aromatic diazonium salts general formula

ArN₂⁺X⁻ Named by suffixing diazonium to the name of the parent hydrocarbon followed by the name of anion

Diazotization

Aromatic diazonium salts are prepared by treating an aromatic primary amine with Nitrous acid (which is prepared by mixing NaNO₂ and HCl) at 273 - 278K (0-5° C). The conversion of primary aromatic amines into diazonium salts is known as diazotization.

Why can't tertiary amines be acylated

As there is no H bonded to nitrogen

Why is ammonolysis not suitable for the preparation of aryl amines?

Because aryl halides have low reactivity towards nucleophilic substitution

Nitration

Direct nitration of aniline with conc. HNO₃ and conc. H₂SO₄ gives a mixture of ortho, meta and para nitroanilines. In strongly acidic medium, aniline is protonated to form the anilinium ion which is meta-directing. So a large amount (47%) of meta-isomer is formed.

Why are alkyl amines stronger bases than ammonia?

Due to the electron releasing nature of alkyl group (R) in alkyl amines, it (the alkyl group) pushes electrons towards nitrogen and thus makes the unshared electron pair more available for protonation. Also the substituted ammonium ion formed from the amine gets stabilized by the +I effect of the alkyl group. Hence, alkyl amines are stronger bases than ammonia.

Preparation of p-nitroaniline

For the preparation of p-nitroaniline, the -NH₂ group is first deactivated by acetylation. The acetanilide thus formed is nitrated followed by hydrolysis

Why do amines act as Lewis bases?

In amine, there is an unshared pair of electrons on nitrogen atom and hence it can donate this electron pair. So it acts as a Lewis base

Factors that affect the basic strength of amines (3)

Inductive effect solvation effect steric hindrance

Basic character of amines can be expressed in terms of

Kb and pKb

Reaction of NH₂ group

NH₂ group is ortho and para directing and a powerful activating group. So the reactions are very vigorous.

Preparation of amines by reduction of nitriles

Nitriles on reduction with lithium aluminium hydride (LiAlH₄) or catalytic hydrogenation produce primary amines. This reaction is used for ascending in amine series.

Preparation of amines by reduction of nitro compounds

Nitro compounds when reduced by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum or by reduction with metals in acidic medium, we get amines. R-NO₂ + H₂ → (Pd/ethanol) R-NH₂

Gabriel phthalimide synthesis

Phthalimide when treated with ethanolic solution of potassium hydroxide to form potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine.

Reaction with nitrous acid

Primary aliphatic amines react with nitrous acid (prepared by mixing sodium nitrite and HCl) to form alcohols with the liberation of nitrogen gas. Aromatic primary amines react with nitrous acid at 0 to 5°C (273-278 K) to form aromatic diazonium salts

Hinsberg's test for primary amines

Primary amines react with benzenesulphonyl chloride to form N-alkyl benzenesulphonamide, which is soluble in alkali

Order of reactivity of halides with amines

RI > RBr > RCl

Hinsberg's test for secondary amines

Secondary amines react with benzene sulphonyl chloride to give N,N-dialkylbenzene sulphonamide, which is insoluble in alkali , as it does not contain any hydrogen atom attached nitrogen atom, it is not acidic.

Two types of reactions given by benzene diazonium chloride

Substitution reactions involving replacement of diazonium group. Coupling reactions involving the retention of diazo group acts as an electrophile.

Hinsberg's test for tertiary amines

Tertiary amines do not react with benzene sulphonyl chloride.

Preparation of amines by reduction of amides

The amides on reduction with lithium aluminium hydride give amines.

Why are aryl amines are less basic than ammonia

This is because in aryl amines, the -NH₂ group is attached directly to the benzene ring. So the lone pair electrons present in the nitrogen atom enter into the benzene ring and the following resonating structures are formed. So the electron pairs are less available for protonation and hence aryl amines are less basic.

Why are aliphatic amines are stronger bases than ammonia?

This is due to the +I effect of alkyl groups leading to greater electron density on the nitrogen atom. But aromatic amines are weaker bases than ammonia. This is due to the electron withdrawing nature of the aryl group.

Hoffmann bromamide degradation reaction

This method is used for the conversion of an amide to a primary amine with one carbon atom less than that present in the amide. In this method, an amide is treated with bromine and ethanolic solution of NaOH to give an amine. R-CO-NH₂ + Br₂ + 4NaOH → Na₂CO₃ + 2NaBr + 2H₂O CH₃-CO-NH₂ + Br₂ + 4NaOH → CH₃-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O

Reaction with benzene sulphonyl chloride (Hinsberg's Test)

This test is used for the distinction of primary, secondary and tertiary amines and also for the separation of a mixture of amines. Benzenesulphonyl chloride (C₆H₅SO₂Cl) is known as Hinsberg's reagent

Replacement by halide or cyanide ion

When a diazonium salt is treated with hydrogen halide in presence of cuprous halide, we get halobenzene. This reaction is called Sandmeyer's reaction.

Coupling reaction

When benzene diazonium chloride is treated with phenol or aniline, the para position of it is coupled with the diazonium salt to form p-hydroxyazobenzene or paminoazobenzene. This type of reaction is known as coupling reaction. This is an example of electrophilic substitution reaction. [ADD DIAGRAM]

Replacement by fluoride ion Balz-Schiemann reaction

When benzenediazonium chloride is treated with fluoroboric acid (HBF4),benzene diazonium fluoroborate is formed which on heating decomposes to give flourobenzene.

Replacement by H

When benzenediazonium chloride is treated with reducing agents like hypophosphorous acid (phosphinic acid) or ethanol, we get benzene. C₆H₅N₂⁺Cl⁻ + H₃PO₂ + H₂O → C₆H₆ + N₂ + H₃PO₃ + HCl

Replacement by hydroxyl group

When benzenediazonium chloride is warmed with water, we get phenol. C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + H₃PO₃ + HCl

Replacement by -NO₂ group

When benzenediazonium fluoroborate is heated with aqueous sodium nitrite solution in the presence of copper, the diazonium group is replaced by -NO₂ group

How does the inductive effect, solvation effect and steric hindrance of the alkyl group decide the basic strength of alkyl amines in the aqueous state.

When the alkyl group is small, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than CH3 group, there will be steric hindrance to H-bonding. Therefore, the size of the alkyl group also determines the order of basic strength.

Replacement by iodide ion

When the diazonium salt solution is treated with potassium iodide, iodobenzene is formed

IUPAC name of amine

alkanamine / arenamine In the case of secondary and tertiary amines, the alkyl group containing the maximum number of carbon atom is taken as the parent chain and the other alkyl groups are taken as substituents.

Why is reduction with iron and hydrochloric acid preferred

because FeCl₂ formed gets hydrolysed to release hydrochloric acid during the reaction. Thus, only a small amount of hydrochloric acid is required to initiate the reaction

Why can't aromatic primary amines be prepared by Gabriel phthalimide synthesis?

because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide

Why are lower aliphatic amines soluble in water?

because they can form hydrogen bonds with water molecules

For the preparation of cyanobenzene

benzenediazonium salt is treated with KCN in presence of cuprous cyanide

Basic character of an amine depends on the

ease of formation of the cation by accepting a proton from the acid

Preparation of o-bromoaniline and p-bromoaniline

first reduce the activating power of NH₂ group by protecting it through acetylation with acetyl chloride or acetic anhydride. Then the resulting acetanilide is brominated by Br₂ in acetic acid followed by hydrolysis, we get p-bromoaniline as the major product

Higher amine: soluble or insoluble in water?

insoluble

The greater the size of the ion, ___________ will be the solvation and the ______ stabilised is the ion. So the basicity also __________

lesser less decreases

Amines are soluble in

organic solvents like alcohol, ether and benzene

2 resonating structures of anilinium ion

other side

Benzenesulphonyl chloride can be replaced by

p-toluenesulphonyl chloride

Major product if excess of ammonia is used (hoffmann)

primary amine

Why does aniline not undergo Friedel-Crafts reaction

since it forms salt with anhydrous aluminium chloride, which is used as catalyst in the reaction, where nitrogen has a positive charge and deactivated for further reaction.

Greater the value of Kb

smaller will be the pKb value and stronger will be the base

As the stability of the cation ↑

the basicity ↑

Basicity of substituted aniline

the electron releasing groups like -OCH₃, -CH₃ increase basic strength while electron withdrawing groups like -NO₂, -SO₃H, -COOH, -X etc. decrease the basic strength

Gabriel phthalimide synthesis is used for

the preparation of aliphatic primary amines only

Benzenediazonium chloride is prepared by

the reaction of aniline with nitrous acid at 273-278K

Disadvantage of ammonolysis

yielding a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt

Structure of amines (3)

• Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp³ hybridised and the geometry of amines is trigonal pyramidal. • Each of the three sp³ hybridised orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the composition of the amines. • The fourth orbital of nitrogen in all amines contains an unshared pair of electrons. Due to the presence of unshared pair of electrons, the angle C-N-E is the angle less than 109.5.

Solubility of amines ↓ with

↑ in molar mass of amines due to increase in size of the hydrophobic alkyl part.

Basic nature of aliphatic amines ↑ with

↑ in the number of alkyl groups. But this trend is followed only in gas phase.