Bihar Board - Class 12 Chemistry - Chapter 13: Amines Short Answer Question

                                  Short Question Answer

Question. 1. Alkyl Amines are a stronger base than Ammonia.

Answer. The inductive effect of the attached carbon chain to an amine will raise the energy of the lone pair or the nitrogen, because carbon. On amines will withdraw less electron density from an atom of interest than hydrogen (in ammonia). That explains why alkyl amines are stronger bases than ammonia.

Question. 2. Ethylamine is more basic than Aniline.

Answer. Because, less electron is available in the nitrogen atom in C6H5NH2. It can easily dissociate to donate a proton, H+ by breaking the -N-H  bond in the amine group that attaches to the phenyl group. Hence ethylamine is more basic than aniline.

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Question. 3. Alkyl amines are stronger bases than ammonia and aryl amines,

Answer. Alkylamines are stronger bases than ammonia and arylamines. This can be explained by the fact that alkyl groups are electron-releasing relative to hydrogen and thus provide greater electron availability at nitrogen. On the other hand, arylamines are weaker than ammonia as the aromatic ring in these compounds is electron withdrawing as compared to hydrogen and thus reduces the electron density at nitrogen.

Question. 4. Amines are less acidic than alcohols of comparable molecular masses.

Answer. Oxygen is more electronegative than nitrogen and its atom is smaller than that of nitrogen. As a result, it pulls the bonding electrons of the O-H bond towards itself. This imparts acidic character (though very weak) to the alcoholic hydrogen. In the case of amines. N-H bond is not as polar as O-H bond. As a result, N atom tends to pick up a proton at its lone-pair of electrons rather than push the H-atom out as a proton.

Question.5. Aliphatic amines are stronger bases than aromatic amines.

Answer. In aromatic amines (or aryl amines), the-NH2  The group is directly bonded to the aromatic ring. Aromatic ring is electron withdrawing. The alkyl group in aliphatic amines is electron releasing. So, the presence of an aromatic ring reduces the electron density at N atom and the presence of an alkyl group in aliphatic amines increases electron density at the N atom. This makes the aryl amines/aromatic amines weaker bases than the aliphatic amines.

Question. 6.Classify the following amines as primary, secondary or tertiary :

(iii) (C2H5)2 CHNH2
(iv) (C2H5)2 NH

Answer. (i) Primary amine,
(ii) tertiary amine,
(iii) Primary amine,
(iv) Secondary amine.

Question. 7. What is a coupling reaction ?

Answer. The reaction of of diazonium salts with phenols in basic medium (pH 9-10) and with amines in acidic medium (pH 4-5) to give corresponding azo (-N=N-)  Dyes are called coupling reactions.

Question. 8. Name one chemical test to distinguish between an aromatic primary amine and an aliphatic primary amine.

Answer. Azo dye test.

Question. 9. What is the directive influence of the amino group in arylaminés ?

Answer. o , p-directing.

Question. 10. Mention two important uses of sulphanilic acid.

Answer. It is used in the manufacture of (i) dyes, (ii) Sulpha drugs.

Question. 11. Aniline gets coloured by standing in air for a long time. Why ?

Answer. Due to the strong electron donating effect (+R effect) of NH2 group, the electron density on the benzene ring increases. As a result, aniline is easily oxidized on standing in air for a long time to form coloured products.

Question 12.  Why is alkylamine more basic than ammonia?

Answer:  Due to electron releasing inductive effect (+1) of the alkyl group, the electron density on the nitrogen atom increases and thus, it can donate the lone pair of electrons more easily than ammonia.

Question .13.  How may methyl amine be preferentially converted to methyl isocyanide?

Answer: 

By carbylamine reaction:

Question.14.  Complete the following reaction equations:

(i) C6H5N2Cl +H3PO2+H2O

(ii) C6H5NH2+Br2 (aq)

Answer: 

(i) C6H5N2Cl +H3PO2+H2O C6H6+N2+H3PO3+HCl

Question15. Describe a method for the identification of primary, secondary and tertiary amines. Also write chemical equations of the reactions involved.

Answer:

 Hinsberg's test:

This is an excellent test for distinguishing between primary, secondary and tertiary amines. The amine is treated with benzene sulphonyl chloride (Hinsberg’s reagent) in the presence of excess aqueous potassium hydroxide solution.

Question.16. Lower aliphatic amines are soluble in water while aromatic amines are not. Give a reason.
Answer:
Lower aliphatic amines are soluble in water because of their tendency to form intermolecular hydrogen bonding with water. While aromatic amines because of high molar mass and steric effect are unable to form intermolecular hydrogen bonding and are insoluble in water.

Question 17. Write a short note on Schotten Baumann reaction.
Answer:
Aromatic acid chloride reacts with phenol and aniline in presence of aqueous NaOH or pyridine. The reaction is known as the Schotten Baumann reaction.

C6H5NH2+C6H5COCl C6H5NHCOC6H5+HCl

                                                    N-phenyl benzamide

Question 18. What is ‘aniline black’ ? Write its two uses.
Answer:
When aniline is oxidized by acidic KMnO4, a black substance is formed which is known as aniline black.
Uses:

• In manufacturing of black color.

• In manufacturing of useful compounds.

Question 19. Ethyl amine is more basic than ammonia, why ?

Answer:

The value of Ka=4.5 10-4 for ethyl amine and for ammonia it is 1.810-5. Larger is the Kb value, more basic is the amine and vice-versa. In ethylamine the availabil-ity of lone pair of electrons on nitrogen atom increases due to the +1 inductive effect of the ethyl group. Hence, this lone pair of electrons can easily accept a proton. This explains why ethyl amine is more basic than ammonia.

Question 20. Write only chemical equation for the following conversions:
(i) Conversion of methyl cyanide into C6H5NH2.
(ii) Conversion of C6H5NH2 into chlorobenzene.
Answer:

(i) CH3-CN+4HCH3-CH2-NH2

(ii) C6H5NH2C6H5N2ClC6H5Cl+N2

Question 21. What is Mendius' reaction ?

Answer: Reduction of alkyl cyanides by sodium and alcohols yield primary amine. This reaction is called the Mendius reaction.

R-CN+4[H]R-CH2-NH2

CH3CN +4 [H]CH3CH2 NH2

                                                      Ethyl amine

Question 22. Why is aniline less basic than ethyl amine ?

Answer:

Aniline is less basic than ethylamine as due to resonance of the benzene nucleus, the lone pair of electrons of the nitrogen atom is attracted towards the nucleus and gets delocalised in the ring. Thus, the electron pair is liberated with more difficulty  in aniline than ethyl amine. Hence, its basic property is less than ethyl amine. The delocalisation of electron as a result of resonance is shown as follows:

Question 23. Complete the following acid-base reactions and name the products :

(i) CH3CH2CH2NH2+HCl ?

(ii) (C2HCl)3N +HCl ?

Answer:

Question 24. What is the difference between Alkyl nitrite and nitroalkane ?
Answer:
Nitrous acid exist in two isomeric forms:

Thus, two alkyl derivatives of nitrous acid are formed.

Thus, it is clear that in nitroalkane, alkyl radical is linked to nitrogen, whereas in alkyl nitrite alkyl radical is linked to oxygen atom. Alkyl nitrite is ester whereas nitroalkane is the derivative of paraffin.

Question.25.  Give IUPAC names of the following compounds : 

Answer: 

(a) IUPAC name: Methyl prop-2-en-1-amine 

(b) IUPAC name: Phenyl acetamide 

(b) Ethanoyl chloride to Ethanenitrile.