This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Amines Physical Properties”.
1. Which of the following amines is not a gas?
Explanation: Lower members of aliphatic amines exist as gases at ordinary temperature. The higher members with three or more carbon atoms are liquid and still higher ones are solid.
2. What is the characteristic odour of relatively lower aliphatic amines?
a) Fruity odour
b) Fishy odour
c) Rotten egg smell
Explanation: Most of the amines have an unpleasant odour. The smell of lower amines is similar to that of ammonia with a fishy odour.
3. Which of the following best describes aniline in pure form?
a) Colourless liquid
b) White waxy solid
c) Brown gas
d) Yellowish gas
Explanation: Aniline and other aryl amines are usually colourless, but they become coloured on storage due to atmospheric oxygen. Aniline develops a yellow to brown colour dur to this reason.
4. The intermolecular hydrogen bonds, if any, in amines is formed between _______
a) N-H and N-H
b) N and N-H
c) Alkyl carbon and N-H
d) Alkyl H and N-H
Explanation: Primary and secondary amines are involved in intermolecular molecular hydrogen bonding between nitrogen of one and hydrogen of another molecule.
5. What is the correct order of boiling points of the isomeric amines where A=ethylmethylamine, B=propylamine and C=trimethylamine?
a) A > B > C
b) C > B > A
c) B > C > A
d) B > A > C
Explanation: A, B and C are respectively the 2°, 1° and 3° isomers of the compound C3H9N. Primary and secondary amines have two and one hydrogen atoms respectively for hydrogen bonding. Whereas, tertiary amines do not have intermolecular association due to absence of H for hydrogen bonding.
6. Which of the following has a lower boiling point than ethanamine?
d) Methanoic acid
Explanation: All given compounds have similar molecular masses and can be compared. Ethanamine has a higher boiling point than propane because it is a polar molecule and forms intermolecular hydrogen bonds. However, it has a lower boiling point than ethanal, ethanol and methanoic acid as the O-H bonds in these compounds is more polar than the N-H bond in ethanamine. This makes the hydrogen bonds stronger.
7. If the boiling point of diethylamine is 329 K, predict the boiling point of ethyldimethylamine.
a) 310 K
b) 329 K
c) 340 K
d) 351 K
Explanation: Diethylamine is a 2° amine whereas, ethyldimethylamine is a 3° amine. In 3° amines, there are no hydrogen atoms available for intermolecular hydrogen bonding. Therefore, the tertiary amines have lowest boiling point among the primary and secondary counterparts.
8. If the boiling point of n-C4H9NH2 is 351 K, what will be the boiling point of n-C4H9OH?
a) 329 K
b) 301 K
c) 351 K
d) 390 K
Explanation: Alcohols have higher boiling points than amines of comparable molecular masses. This is because the electronegativity of N is lower than that of O, which results in the OH bond in alcohols being more polar and hence stronger (in hydrogen bonding) than the N-H bond in amines.
9. If A is the boiling point of propanamine and B is the boiling point of butanamine, what is the correct relation between the two?
a) A > B
b) B > A
c) A = B
d) A >> B
Explanation: Butanamine is a larger molecule than ethanamine. As the carbon chain increase, the magnitude of van der Waals forces increases resulting in the increase in boiling point.
10. All aliphatic amines exist as associated molecules due to intermolecular hydrogen bonding.
Explanation: Only primary and secondary amines form intermolecular hydrogen bonds as they have individual at least one hydrogen atom to form bonds with. Tertiary amines do not have intermolecular association due to absence of hydrogen atom available for hydrogen bonding.
11. Which of the following amines are insoluble in water?
Explanation: Lower aliphatic amines are soluble in water as they easily form hydrogen bonds with water molecules. Higher amines containing six or more carbon atoms are insoluble in water due to a large hydrophobic alkyl/aryl part. Therefore, aniline is insoluble in water.
12. If ‘x’ ml of butan-1amine and ‘y’ ml of butan-1-ol is completely soluble in 100ml of water each, what is the relation between x and y?
a) x > y
b) x < y
c) x = y
d) x + y = 100
Explanation: Alcohols are more polar than amines of comparable molecular masses and form stronger intermolecular hydrogen bonds with water. Therefore, the solubility of butan-1-ol is more than that of butan-1-amine.
13. Aniline is insoluble in which of the following solvents?
Explanation: Aromatic amines have a very large hydrocarbon part (benzene ring) which tends to retard the formation of hydrogen bonds with water. However, they are soluble in organic solvents like ethers, alcohols and benzene.
14. Which of the following is least soluble in water?
Explanation: Lower amines are more soluble because of the smaller hydrophobic alkyl part. Also, alcohols and carboxylic acids have higher solubility than amines because of the greater polarity of OH bond than NH bond.
15. Primary amines are more soluble than tertiary amines of same formula.
Explanation: Solubility in water requires the presence of free hydrogen in the structure of a compound to form hydrogen bonds. Tertiary amines do not have any hydrogen attached to the N atom and are not soluble in water.
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