This set of Chemistry Question Bank for Class 12 focuses on “Haloalkanes & Haloarenes – Chemical Reactions – 2”.
1. As the stability of carbocation formed in the first step of SN1 reaction increases, the rate of the reaction _________
c) remains same
d) may increase or decrease
Explanation: Greater the stability of the carbocation, greater will be the ease of formation from alkyl halide and faster will be the reaction rate.
2. The SN1 reaction cannot be carried out in which of the following media?
a) Acetic acid
Explanation: SN1 reactions are carried out in polar protic solvents like water, alcohol and acetic acid. These solvents promote the ionisation step by stabilising the ions by solvation.
3. If a compound does not affect the plane of plane polarised light when passed through it, the compound is said to be _________
a) optically active
b) optically inactive
Explanation: When the plane of plane polarised light is rotated either to the left or right when passed through a compound, it is said to be optically active. Else, it is optically inactive.
4. Which of the following compounds does not have a stereocenter?
Explanation: A stereocenter is an asymmetric carbon that has four different groups attached to it. Propan-2-ol has two methyl groups attached to the carbon.
5. Which of the following has a chiral carbon atom?
Explanation: A chiral carbon atom is that which has four different substituent groups attached to it. 2-Bromopentane has one of each H, CH3, C3H7 and Br groups attached to its C atom.
6. What is the chirality of 2-Chlorobutane?
c) Superimposable mirror images
Explanation: 2-Chlorobutane has four different groups attached to the tetrahedral carbon and is chiral as it forms non-superimposable mirror images.
Explanation: The C atom that is attached to the OH group in compound A is a stereocenter and makes the compound A chiral. On the other hand, compound B does not have any asymmetric carbon.
8. A chiral compound with an ‘X’ group attached to the stereocenter has been replaced by a ‘Y’ group. The product obtained rotates the plane of polarised light in the direction opposite to that of the original compound. Name the process that has taken place.
d) Inversion or retention
Explanation: Since the product rotates the plane of polarised light in some direction, it cannot be racemic. The product formed is a completely new compound from the original one and may have different or same optical behaviour as compared to the original compound.
9. The process of separation of a racemic mixture into the respective d and l optical isomers is called __________
Explanation: Racemisation is the process of converting optical isomers into a racemic mixture and separating them from the mixture is known as resolution.
Explanation: It can be seen that the relative spatial arrangement of bonds at the stereocenter remain the same even after the transformation. Hence, this transformation is said to take place with a retention of configuration.
11. When a reaction proceeds with the retention of configuration, the optical activity of the product will remain same as that of the original reactant.
Explanation: For example, when (-)-2-methylbutan-1-ol is heated with conc. HCl, the product formed is (+)-1-chloro-2-methylbutane, which has opposite optical behaviour to that of the reactant.
12. What will be the effect on the plane of plane polarised light when it passes through a solution of dlbutan-2-ol?
a) Clockwise rotation
b) Anti-clockwise rotation
c) Zero rotation
d) 180° rotation
Explanation: dlbutan-2-ol represents a mixture of dbutan-2-ol (dextro) and lbutan-2-ol (laevo), and is called a racemic mixture. There optical rotation due to one isomer will cancel the rotation due to the other and will have no effect on the polarised light passed through it.
13. When (-)-2-bromooctane reacts with potassium hydroxide over an SN2 mechanism, what will be the product formed?
Explanation: The OH nucleophile attaches itself opposite to where the halide was present but one the same carbon. This results in an inversion of configuration and hence changes in the direction in which the plane of polarised light rotates.
14. The heating of 1-Chlorobutane along with alcoholic KOH results in ________
a) elimination of hydrogen from α-carbon atom
b) elimination of hydrogen from β-carbon atom
c) elimination of hydrogen from γ-carbon atom
d) no reaction
Explanation: When a haloalkane with a β-hydrogen atom is heated with alc. KOH, it results in a β-elimination reaction to form alkenes.
15. The major product formed from the dehydrohalogenation of 2-Bromopentane is pent-2-ene.
Explanation: According to Zaitsev’s rule, the major product will be that alkene which has more number of alkyl groups attached to the double bonded carbon atoms.
Sanfoundry Global Education & Learning Series – Chemistry – Class 12.
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