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.