Class 12 Chemistry Revision Notes for Chapter 12 - Amines - Free PDF Download

Section – A (1 Mark Questions)

1. Arrange the following: In decreasing order of the pKb values:

C2H5NH2, C6H5NHCH3, (C2H5)2NH and C6H5NH2

Ans. Decreasing order of pKb is as follows: (Low pKb value indicates a stronger base)

C6H5NH2 > C6H5NHCH3 > C2H5NH2 > (C2H5)2NH

2. Arrange the following: In increasing order of basic strength:

C6H5NH2, C6H5N(CH3)2, (C2H5)2NH and CH3NH2 

Ans. C6H5NH2 < C6H5NHCH3 < CH3NH2 < (C2H5)2NH

3. Arrange the following: In increasing order of solubility in water:

C6H5NH2, (C2H5)2NH, C2H5NH2.

Ans. C6H5NH2 < (C2H5)2NH < C2H5NH2

4. Convert toluene into p-toluidine

Ans.

5. Complete the following reaction.

Ans.  Benzene diazonium chloride reacts with phenol in which the phenol molecule at its para position is coupled with the diazonium salt to form p-hydroxyazobenzene. This type of reaction is known as coupling reaction.

6. What is the best reagent to convert nitrile to primary amine?

Ans. Nitriles on reduction with lithium aluminium hydride (LiAlH4) or catalytic hydrogenation, produce primary amines.

$R-C\equiv N\xrightarrow[Na(Hg)/C_{2}H_{5}OH]{H_{2}/Ni}R-CH_{2}-NH_{2}$

7. Explain why MeOH is stronger acid than MeNH2?

Ans. In alcohols, the hydrogen atom is attached to more electronegative oxygen atom whereas nitrogen of amines is less electronegative. After the loss of H+ ion, the negative charge is more easily accommodated on oxygen than in case of nitrogen in amines. Hence, amines have lesser tendency to lose H+ ions, so they are less acidic than alcohols.

$Me-NH_{2}\rightarrow Me-\bar{N}H+H^{+}$

$Me-O-H\rightarrow Me-O^{-}+H^{+}$

8. Why do amines behave as nucleophiles?

Ans. Due to the presence of a lone pair of electrons on nitrogen atom, amines behave as nucleophiles

9. What is hinsberg reagent ?

Ans. Benzene sulphonylchloride (C6H5SO2Cl)

10. Out of aniline and benzylamine which one give azo dye test ?

Ans. Aniline

Section – B (2 Marks Questions)

11. Write following conversions:

(i) nitrobenzene → acetanilide

Ans. 

12. How will you bring out the following conversion?

Ans. Complete conversion of above reaction can be shown as

13. Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis?

Ans. The Gabriel phthalimide reaction involves the nucleophilic attack of the phthalimide anion on alkyl halide and the alkyl group is attached to nitrogen. Since aryl halides do not undergo nucleophilic substitution, therefore, aromatic primary amines cannot be prepared by this reaction.

14. Describe a method for the identification of primary, secondary and tertiary amines. Also write chemical equations of the reactions involved.

Ans. Add Hinsberg reagent, C6H5SO2Cl. Primary amines will react to form a compound soluble in KOH. 2o amines will form a salt insoluble in KOH while 3o amines will not react.

15. Why is benzenediazonium chloride not stored and is used immediately after it’s preparation?

Ans. Benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at 273-278K. Nitrous acid is produced in the reaction mixture by the reaction of sodium nitrite with hydrochloric acid. The conversion of primary aromatic amines into diazonium salts is known as diazotization. Due to its instability, the diazonium salt is not generally stored and is used immediately after its preparation.

Benzenediazonium chloride is a colorless crystalline solid. It is readily soluble in water and is stable in cold but reacts with water when warmed. It decomposes easily in the dry state.

16. How are the following conversions carried out?

(i) CH3CH2Cl to CH3CH2CH2NH2

(ii) Benzene to aniline

Ans. 

17.  Write the main products of the following reactions:

Ans. 

18.  Complete the following reactions:

(i)  CH3CH2NH2 + CHCl3 + alc.KOH →

(ii) 

Ans. 

19.  Give chemical tests to distinguish between the following pairs of compounds:

(i) Benzene to aniline

(ii) Ethylamine and dimethylamine

Ans. (i) Aniline gives white or brown precipitate with bromine water.

Ethylamine does not react with bromine water.

(ii) When heated with an alcoholic solution of KOH and CHCl3, ethylamine gives foul smelling ethyl isocyanide. Dimethylamine does not give this test.

20.  Give possible explanation for each of the following:

(a) The presence of a base is needed in the ammonolysis of alkyl halides.

(b) Amides are more acidic than amines.

Ans. (a) To remove HX formed so that the reaction shifts in the forward direction.

Due to +R effect, availability of lone pair of electron on N of –NH2 group decreases. As a result, amide is much weaker base than amines. Because of the positive charge on N, as a result of resonance, N can easily lose a proton and behaves, as a weak acid.

PDF Summary - Class 12 Chemistry Amines Notes (Chapter 12)

Introduction

Amines are ammonia-based organic compounds that have one or more alkyl or aryl groups attached to the nitrogen atom. Amines play a variety of roles in living creatures, including bioregulation, neurotransmission, and predator protection. Many amines are employed as pharmaceuticals and treatments because of their great biological activity.

Alkaloids are a class of physiologically active amines that are primarily produced by plants to protect them from insects and other animals. Although some alkaloids are used medicinally (as pain relievers), they are all poisonous and can lead to death if consumed in high amounts.

Classification

Primary, secondary, and tertiary amines have one, two, or three alkyl or aryl groups linked to nitrogen, respectively.

Four alkyl or aryl bonds connect a nitrogen atom in quaternary ammonium ions. Just like in simple ammonium salts like ammonium chloride, the nitrogen atom has a positive charge. An example of quarternary ammonium salt is as follows:

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Structure of Amines

The tetrahedral geometry of ammonia is slightly deformed. One of the tetrahedral sites is occupied by a single pair of nonbonding electrons. The bulky lone pair compresses the $H - N - H$  bond angles to ${107^ \circ }$  from the "ideal" $s{p^3}$  bond angle of ${109.5^ \circ }$  in this geometry, which is represented by $s{p^3}$  hybridization of nitrogen. Because the bulky methyl groups expand the angle slightly, trimethylamine shows less compression.

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The mirror counterpart of a tetrahedral amine with three distinct substituents (including a lone pair) is non-superimposable. However, because the enantiomers interconvert rapidly, we cannot usually resolve such an amine into two enantiomers. The lone pair transfers from one face of the molecule to the other during this conversion, which is called nitrogen inversion.

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Inversion of configuration is not conceivable in quarterary ammonium salts with asymmetric nitrogen atoms because there is no lone pair to undergo nitrogen inversion.

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Amines that are unable to achieve the $s{p^2}$ -hybrid transition state for nitrogen inversion exhibit chirality as well. If the nitrogen atom is enclosed in a tiny ring, for example, it is unable to achieve the ${120^ \circ }$  bond angles that allow inversion to occur.

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Physical Properties

State

Lower aliphatic amines are gaseous compounds with a fishy odour. Primary amines with three or more carbon atoms are liquid, while those with more than three carbon atoms are solid.

Aniline and other aryl amines are generally colourless, but due to air oxidation, they become coloured over storage.

Dipole Moment

Because 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.

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Solubility

$N - H$  bonds in primary and secondary amines allow them to create hydrogen bonds. Pure tertiary amines cannot form hydrogen bonds because they lack $N - H$  bonds. They can accept hydrogen bonds from molecules that have $O - H$  or $N - H$  bonds, though.

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As a result of their ability to establish hydrogen bonds with water, lower aliphatic amines are soluble in water. However, when the molar mass of amines increases, solubility decreases due to the increased size of the hydrophobic alkyl component. Higher amines are essentially water insoluble.

Boiling Point

The $N - H$  bond is less polar than the $O - H$  bond because nitrogen is less electronegative than oxygen. As a result, amines and alcohols with similar molecular weights form weaker hydrogen bonds. The boiling temperatures of primary and secondary amines are lower than those of alcohols, but higher than those of ethers with equal molecular weights. Tertiary amines have lower boiling temperatures than primary and secondary amines with equal molecular weights because they lack hydrogen bonds.

Preparation of Amines

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Alkyl Halides

Hoffmann’s Ammonolysis Method

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

1. A combination of goods is produced by the method. 

2. Tertiary alkyl halide isn't recommended because it produces an alkene. 

3. The reaction is initiated by a bimolecular substitution $({S_{{N^2}}})$ .

Example 1:

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Gabriel’s Phthalimide Synthesis

After treating phthalamide with $KOH$ , alkyl phthalamide is produced by heating potassium phthamide with alkyl halide. The hydrolysis that follows generates only primary amine. To improve the yield of primary amine, alkyl phthalamide is also treated with hydrazine.

Example 2:

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Grignard Reagent

The primary amine is obtained using the Grignard reagent or a trialkyl borane after chloramine treatment.

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

Alcohols

Sabatier Reaction

Under pressure, alcohols and ammonia are heated in the presence of a catalyst, such as copper chromite or alumina. It is possible to obtain a blend of items.

Example 3:

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Carbonyl Compounds

Reductive Amination

In the presence of ammonia, a number of aldehydes and ketones are reduced to amines. 

The reduction process can be catalysed (hydrogen $20 - 150\;atm$  over Raney nickel $40 - {150^ \circ }C$  or by use of sodium cyanohydridoborate, $NaB{H_3}CN$ ).

Example 4:

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Example 5:

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Example 6:

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Example 7:

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Carboxylic Acids and Derivatives

Schmidt Reaction

Primary amine is produced by treating a mixture of carboxylic acid and hydrazoic acid with cold concentrated sulphuric acid.

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

Example 8:

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Curtius Reaction

The acyl azide is pyrolyzed to create isocyanates in this process. As a result of the following hydrolysis, amine is produced.

Example 9:

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Example 10: 

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Hofmann’s Degradation of Amides

Bromine and a strong aqueous KOH or NaOH solution are used to warm the amide.

1. The product is a primary amine with one less carbon than amide. 

2. The intermediate is alkyl isocyanate, and the rearrangement is intramolecular.

Example 11:

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Hydrolysis of Amides and Isocyanides

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

Example 12:

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Example 13:

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

Hydrolysis of Isocyantes also yields amines.

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

Decarboxylation of Amino Acids

When amino acids are heated with barium hydroxide, primary amine is produced. If there is no $\beta $ -hydrogen in the alkyl group, quaternary ammonium hydroxide decomposes into alcohol on heating, but if there is $\beta $ -hydrogen, Hofmann elimination occurs.

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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 14:

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Example 15:

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Example 16:

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Example 17:

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Example 18:

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Reduction of Nitro Compounds

NItroalkanes are commonly converted to amines using tin and hydrochloric acid or lithium aluminium hydride. For such a reduction, ${H_2}$  and a catalyst are also used.

Example 19:

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Example 20:

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Example 21:

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Example 22:

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Example 23:

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Reactions of Amines

Sulphonylation – Hinsberg Test 

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

1. The amine mixture is separated using the reaction. 

2. Due to the presence of active hydrogen, primary and secondary amines react to form matching sulphnoamides, however tertiary amine does not. 

3. N-alkyl benzene sulphonamide, a primary amine product, dissolves in $KOH$ and forms a water soluble salt.

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Nitrous Acid Test

1. When a primary amine reacts with HNO3, it usually produces alcohol. As a result of the reaction, diazonium salt is formed as an intermediate. 

2. Nitrosoamine is a yellow oily liquid produced by secondary amine. 

3. Tertiary amine dissolves in cold $HN{O_2}$  to generate an unstable salt, which decomposes into nitrosoamine and alcohol when heated.

Example 24:

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Example 25: 

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Example 26:

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Carbylamine Reaction

When primary amine is treated with chloroform and alcoholic $KOH$ , it produces unpleasant or pungent alkyl carbylamine vapours (isocyanide).

The reaction is known as a test for primary amine:

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Treatment with $\mathbf{C{S_2}/HgC{l_2}}$ 

1. When primary amine is heated with CS2, dithiocarbamic acid is formed, which is then destroyed by mercuric chloride to create alkyl isothiocyanate. The reaction is known as Hofmann's Mustard Oil Reaction. 

2. Although secondary amine produces dithiocarbamic acid, mercuric chloride does not destroy it. 

3. Tertiary amine is unaffected by CS2.

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Acylation

N-substituted amide is formed when primary and secondary amines react with acid halide or anhydride. The reaction is used to protect the ring in aniline.

Example 27:

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Example 28:

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Example 29:

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Hofmann’s Exhaustive Methylation and Elimination

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With damp $A{g_2}O$ , quaternary ammonium iodides are transformed to hydroxides. Heat causes the hydroxide to be eliminated, resulting in tertiary amines and alkenes. The fact that the least substituted alkene is the primary product is a significant component of this elimination. Hofmann's Elimination is the name given to this process.

Example 30:

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Example 31:

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Metal Ions

Amines form coordination compounds with metal ions.

Example 32:

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Preparation of Aromatic Amines

Besides the methods in the preparation of aliphatic amines, following methods can also be used.

Reduction of Nitro Compounds

Vapour – Phase Reduction

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

Example 33:

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Catalytic Hydrogenation

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

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Ammonolysis of Aryl Halides

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Phenol and Ammonia

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Reduction of Azo Compounds

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Reactions of Aromatic Amines

Diazonium Salt and Reactions

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Oxidation

Depending on the conditions, aromatic amines are easily oxidised to a variety of compounds. Aniline is converted to p-benzoquinone through controlled oxidation with $N{a_2}C{r_2}{O_7}$  and ${H_2}S{O_4}$ . In fact, p-benzoquinone is generated when a substance is exposed to air.

If oxidation is not controlled, aniline black, a black dye, is produced.

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Halogenation

The amine group $(N{H_2})$  is a powerful activator and ortho/para director. On typical bromination in ${H_2}O$ , it generated 2,4, 6-tribromo aniline.

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The $ - NHCOC{H_3}$  group is a mild ortho and para director, and it can be used to make ortho and para derivatives by reducing the activating power of amino groups by acetylation of aniline.

Example 34:

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Nitration

Because the nitrating mixture is oxidising and $N{H_2}$  is a strong activating group, typical nitration of aniline is not done. Furthermore, at 288 K, aniline is protonated to generate the meta-directing anilinium ion. As a result, in addition to ortho and para derivatives, a considerable number of meta derivatives are generated.

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The nitration reaction can be regulated and the p-nitro derivative obtained as the primary product by shielding the $ - N{H_2}$  group with an acetylation reaction with acetic anhydride.

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Sulphonation

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Note: Normally, the $ - N{H_2}$  group linked to benzene is incapable of forming the zwitter ion, but $ - S{O_3}H$  is extremely acidic. As a result, sulphanilic acid produces the zwitter ion. When the $ - N{H_2}$  group is linked to benzoic acid in any location, for example, no zwitter ion is generated.

Analysis of Amines

Amines are distinguished primarily by their basicity. An amine is nearly often a water-insoluble chemical that dissolves in cold dilute hydrochloric acid or a water-soluble compound whose aqueous solution colours litmus blue. The Hinsberg test is the best way to determine whether an amine is primary, secondary, or tertiary. In the presence of aqueous potassium hydroxide, the amine is shaken with benzenesulfonyl chloride. Substituted sulfonamides are formed by primary and secondary amines, but not by tertiary amines. A primary amine's monosubstituted sulfonamide has an acidic hydrogen linked to nitrogen. This amide is converted to a soluble salt by reacting with potassium hydroxide. Because there is no acidic hydrogen in the disubstituted sulfonamide from a secondary amine, it is insoluble in the alkaline reaction mixture. The way an amine reacts to nitrous acid can help determine its classification. Primary aromatic amines, in particular, have a distinct behaviour: treatment with nitrous acid transforms them to diazonium salts, which generate brightly coloured azo compounds when treated with beta-naphthol.

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Class 12 Chemistry Amines Revision Notes

Our Amines Class 12 Chemistry revision notes cover the important concepts of this chapter. Amines are the ammonia derivatives. It has one or more than one hydrogen atom that is replaced with an alkyl group. Amines are categorized into primary, secondary, and tertiary. This classification is done based on the hydrogen atoms present in the ammonia molecule that has been substituted with the alkyl group.

How Are Amines Prepared?

Amines are prepared by reducing the nitro compounds. Here, an alkyl or the benzyl halide reacts with the ethanolic solution of ammonia and goes through the nucleophilic substitution reaction. Here the halogen atom will be replaced with the amino group. Amines are also prepared by reducing the nitrites or when the amides are reduced.

The Gabriel phthalimide synthesis is the process that is used for the alkyl amines and not for the aromatic amines. Hoffmann bromamide degradation is also used to prepare amines.

The three classes of the aliphatic amines form an H-bond along with water. This makes the lower aliphatic amines to be soluble in water.

Reaction of Amines

Amines are basic. These react with the acid to form a salt. The aliphatic and the aromatic primary and secondary amines react with the acid chlorides, esters, and anhydrides with the nucleophilic substitution reaction. This is the acylation reaction.

The aliphatic and the primary aromatic amines, when heated with chloroform and with ethanolic potassium hydroxide, form an isocyanide or a carbylamine. These are substances that smell foul.

The secondary and the tertiary amines do not show any kind of reaction. This is the carbylamine reaction or the isocyanide test. This test is used to find out the primary amines.

Alkyl Amines

The factors influence the stability of the substituted ammonium cations in the polar solvents. This affects the basic amine nature, which is a combination of the electron releasing H bonding and the steric factors. As compared to the aminoalkyl, the amines are a stronger base.

Aromatic Amines

The main characteristics of the aromatic amine are dependent on its withdrawing group and its electron releasing. To distinguish between the primary, the secondary, and the tertiary amines, the p-Toluenesulfonyl chloride is used. The process of Acylation controls the reactivity of the aromatic amine.

Primary Aliphatic Amines

This is a form of amine that is highly unstable alkyl diazonium salt. This gets decomposed to give alcohol and evolved out N2. The primary kinds of alkyl diazonium salts are stable for a short time in the solution at a low temperature.

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