This set of Organic Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Physical Properties of Amines”.
1. Which of the following compound is expected to be most basic?
Explanation: Alkyl groups are electron donating (EDG). So, the alkyl group stabilizes the positive charge on the attached nitrogen atom.
2. A solution of methyl amine shows which type of property with litmus paper?
a) Turns blue litmus red
b) Turns red litmus blue
c) Does not affect red or blue litmus
d) Bleaches litmus
Explanation: Methyl amine changes red litmus blue, this litmus paper test shows basic nature of methyl amine.
3. Which of the following statement is true about methyl amine?
a) Methyl amine is slightly acidic
b) Methyl amine is less basic than NH3
c) Methyl amine is stronger base than NH3
d) Methyl amine forms salts with alkali
Explanation: Presence of alkyl group increases electron density on nitrogen atom due to +I effect. Thus, basic nature increases.
4. By the presence of a halogen atom in the ring, what is the effect of this on basic properties of aniline?
Explanation: By the presence of a halogen atom in the ring, basic properties of aniline is increased because it is more electronegative so donation of electron will be easy, so basicity increases.
5. What is the characteristic smell of liquid amines?
a) Fish like smell
b) Rotten egg smell
c) Fruity smell
d) Bitter almond smell
Explanation: Amines possess a characteristic ammonia smell, liquid amines have a distinctive “fishy” smell.
6. Which of the following have less melting point than amine?
c) Carboxylic acid
Explanation: Hydrogen bonding significantly influences the properties of primary and secondary amines. Thus, the melting points and boiling points of amines are higher than those of the corresponding phosphines, but generally lower than those of the corresponding alcohols and carboxylic acids, ether.
7. Which of the following amine has highest boiling point?
a) Butyl amine
Explanation: Primary and secondary amines have higher boiling because they can engage in intermolecular hydrogen bonding. The boiling points of tertiary amines, which cannot engage in hydrogen bonding because they have no hydrogen atom on the nitrogen atom, and d is correct option because it has more molar mass than other primary, secondary and tertiary amine, more +I effect of propyl group.
8. What is the correct decreasing order of the basic character of the three amines and ammonia?
a) NH3 > CH3NH2 > C2H5NH2 > C6H5NH2
b) C2H5NH2 > CH3NH2 > NH3 > C6H5NH2
c) C6H5NH2 > C2H5NH2 > CH3NH2 > NH3
d) CH3NH2 > C2H5NH2 > C6H5NH2 > NH3
Explanation: All of the amines are more basic than ammonia, but primary and secondary amines are the most basic.
9. Unpleasant smelling carbylamines are formed by heating alkali and chloroform with which of the following?
a) Any amine
b) Any aliphatic amine
c) Any aromatic amine
d) Any primary amine
10. Which statement is an incorrect statement?
a) Amines form hydrogen bond
b) Ethyl amine has higher boiling point than propane
c) Methyl amine is more basic than ammonia
d) Dimethyl amine is less basic than methyl amine
Explanation: In methyl amine only one electron releasing group is present but in dimethyl amine two electron releasing groups are present which increase the basicity higher in dimethyl amine.
11. Which of the following is most basic?
Explanation: In dimethyl amine two electron releasing groups are present which increase the basicity higher in dimethyl amine.
12. If methyl is alkyl group, then which order of basicity is correct?
a) R2NH > RNH2 > R3N > NH3
b) R2NH > R3N > RNH2 > NH3
c) RNH2 > NH3 > R2NH > R3N
d) NH3 > RNH2 > R2NH > R3N
Explanation: Alkyl groups are electron donating (EDG), so they stabilize the positively charge ammonium ions, i.e., the methyl ammonium ion is more stable than the parent ammonium ion because the alkyl group stabilizes the positive charge on the attached nitrogen atom. In dimethyl amine two electron releasing groups are present which increase the basicity higher in dimethyl amine.
13. Among the following, which one is the strongest base?
Explanation: Alkyl groups are electron donating (EDG) and NO2 group is electron withdrawing group. So, the alkyl group stabilizes the positive charge on the attached nitrogen atom.
14. Which of the following is true characteristic feature of aniline?
a) Colourless solid
b) Brown coloured solid
c) Colourless liquid
d) Brown coloured liquid
Explanation: Pure aniline is a colourless liquid, but the colour of aniline changes to dark brown on storage due to atmospheric oxidation.
15. The correct order of basicity of amines in water is which of the following?
a) (CH3)2NH > (CH3)3N > CH3NH2
b) CH3NH2 > (CH3)2NH > (CH3)3N
c) (CH3)3N > (CH3)2NH > CH3NH2
d) (CH3)3N > CH3NH2 > (CH3)2NH
Explanation: Basicity of amines increase with increase in number of −CH3 groups (or any group which cause +I effect), due to increase in electron density on N atom. As a rule, the basicity of 3° amine should be more than that of 2° amine, but actually it is found to be lesser than 2° amines. This is due to more solvation by water in secondary amine. Hence the correct order of basicity is: (CH3)2NH > (CH3)3N > CH3NH2.
Sanfoundry Global Education & Learning Series – Organic Chemistry.
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