Organic Chemistry Multiple Choice Questions on “Nucleophilic Substitution Reaction”.
1. Which of the following compound shows the correct decreasing order of solvolysis with aqueous ethanol?
The correct choice is:
a) III > II > I > IV
b) III > II > IV > I
c) II > III > IV > I
d) III > I > IV > II
Answer: b
Clarification: The solvation of above compound can be compared by this trend, i.e. Solvation trend: 3O halide > 2O halide > 1O halide > vinylic halide. So, (III) molecule is an example of 3O halide, (II) molecule is an example of 2O halide, (IV) molecule is an example of 1O halide and (I) molecule is an example of vinylic halide.
2. Which of the following reaction will go faster if the concentration of the nucleophile is raised?
a) I and III
b) II and IV
c) I and II
d) I, II and IV
Answer: c
Clarification: I & II reaction undergo via SN2 mechanism. III & IV reaction undergo via SN1 mechanism. As we know the rate of reaction in SN2 reaction is directly proportional to the concentration of nucleophile.
3. SN1 reaction undergoes through a carbocation intermediate as follows:
[R = t-Bu, iso-Pr, Et, Me] (X = Cl, Br, I)
The correct statements are:
I. The decreasing order of rate of SN1reaction is t-BuX > iso-PrX > EtX > MeX
II. The decreasing order of ionization energy is MeX > EtX > iso-PrX > t-BuX
III. The decreasing order of energy of activation is t-BuX > iso-PrX > EtX > MeX
a) I & II are correct
b) I & III are correct
c) II and III are correct
d) I, II & III are correct
Answer: a
Clarification:
(1) Formation of carbocation is RDS, more stable the cation more easily it will form. Hence rate order is t-BuX > iso-PrX > EtX > MeX
(2) Formation of carbocation CH3 is most difficult hence ionization energy is highest. Similarly, EtX has lesser ionization energy than MeX and so on.
(3) Order of energy of activation is MeX > EtX > iso-PrX > t-BuX.
4. Which statement is incorrect about the following reaction?
a) The rate of these reaction depends on both [R—I] and [131I-]
b) Loss of optical activity was twice as fast as the gain of radioactivity
c) Each molecule undergoing substitution suffers Inversion of configuration
d) The Final solution has radioactive iodine only
Answer: d
Clarification:
Here the rate of reaction is directly proportional to the concentration of RI and (131I-), so statement is correct.
If any molecule undergoes substitution, then it forms inversion product due to SN2 reaction. Hence statement is correct.
For statement, below reaction one (+) molecule converts into (–) molecular and it also converts one optical reaction of (+) molecule.
Final solution contains both Iodine (I131) and normal (I). So, we can say this is the only incorrect statement about the above reaction.
5. Which one is an excellent substrate for SN2 reaction?
a)
b)
c) CH3 – CH2 – Cl
d)
Answer: a
Clarification: Alpha-halo carbonyl undergoes fastest SN2 reaction due to the coupling of π* of CO bond and π* of C–X bond.
6. Which of the following curve correctly represents SN1 vs SN2?
a)
b)
c)
d)
Answer: b
Clarification: As we know rate order in SN2 is CH3X > 1o > 2o > 3o halide and rate order in SN1 is CH3X ooo halide. So according to the following representation.
