This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Structure of Amines”.
1. Which of the following is not an amine?
Explanation: Amines are clearly defined as ‘derivatives’ of ammonia obtained by replacement of one or more H atoms by an alkyl/aryl group. Therefore, NH3 itself is not an amine, but the base compound for all amines.
2. Amines are derivatives of ______
Explanation: There are three hydrogen atoms in ammonia. When one or more of these is substitutes by an alkyl/aryl group, the resultant compound is an amine. Hence, they are derivatives of NH3.
3. How many lone pairs of electrons does the nitrogen atom of amines have?
Explanation: The nitrogen of NH3 forms sp3 hybridised orbitals and also a valency of three (as it is attached to three hydrogen atoms). This results is one the orbitals having a lone pair of electrons, resulting in one unshared electron pair on nitrogen.
4. What is the geometry of ammonia molecule?
a) Trigonal planar
b) Square planar
Explanation: The nitrogen atom of ammonia undergoes hybridisation to form 4 sp3 orbitals. Three of these overlap with s orbital of H forming three N-H bonds. This results in a pyramidal geometry with N atom at the apex and three H atoms at the corners of a triangle base.
5. What is the expected geometry of CH3-NH-CH3?
a) Square planar
b) Trigonal planar
c) Trigonal pyramidal
d) Trigonal bipyramidal
Explanation: CH3-NH-CH3 is an amine in which two hydrogen atoms are replaced by methyl groups. The hybridisation of N atom of amines is same as that of ammonia, and is also expected to have a pyramidal geometry, with N at the apex and the groups CH3, H and CH3 occupying the corners of a trigonal base.
6. What is the bond angle in ammonia molecule?
Explanation: The bond angle for a standard tetrahedral geometry is 109.5°. But the ammonia molecule contains a lone electron pair on nitrogen. As it is closer to the N atom than the other orbitals, it creates a repulsive effect and pushes the Hydrogen atoms close to each other, thus reducing the bond angle to 107°.
7. What is the C-N-C bond angle in the following compound?
Explanation: The shown compound is an amine with three methyl group. This induces a steric effect between the three alkyl groups, and results in a slight increase in bond angle from 107° (bond angle in ammonia).
8. An amine with two or more alkyl groups does not have a lone pair of electron on nitrogen atom.
Explanation: Amines are derivatives of ammonia. Nitrogen of NH3 is sp3 hybridised with one orbital having unshared pairs of electrons as it bonds to only three H atoms. Similarly, amines also have an electron lone pair on nitrogen.
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