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Haloalkanes and Haloarenes

NEET Chemistry · 51 questions · Page 4 of 6 · Click an option or "Show Solution" to reveal answer

Q31
Which of the following reaction is an example of ammonolysis?
A C6H5COCl + C6H5NH2 → \to C6H5CONHC6H5
B C6H5CH2CN ⟶[H]\overset{{[H]}}\longrightarrow C6H5CH2CH2NH2
C C6H5CH2CN ⟶HCl\overset{{HCl}}\longrightarrow C6H5N+\mathop N\limits^ + H3Cl−-
D C6H5CH2Cl + NH3 → \to C6H5CH2NH2
Correct Answer
Option D
Solution

Ammonolysis of alkyl halides is the reaction of an alkyl halide with NH3 which leads to the preparation of amines.

C6H5CH2Cl + NH3 →\to C6H5CH2NH2

Q32
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : SN2\mathrm{S}_{\mathrm{N}} 2 reaction of C6H5CH2Br\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br} occurs more readily than the SN2\mathrm{S}_{\mathrm{N}} 2 reaction of CH3CH2Br\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}. Reason (R) : The partially bonded unhybridized p-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring. In the light of the above statements, choose the most appropriate answer from the options given below :
A Both (A) and (R) are correct but (R) is not the correct explanation of (A)
B Both (A) and (R) are correct and (R) is the correct explanation of (A)
C (A) is not correct but (R) is correct
D (A) is correct but (R) is not correct
Correct Answer
Option B
Solution

To determine the correct answer, we need to analyze both the Assertion (A) and the Reason (R) independently, as well as see if the Reason (R) provides the correct explanation for the Assertion (A).

Assertion (A) Analysis: Assertion (A) states that

SN2\mathrm{S}_{\mathrm{N}} 2

reaction of

C6H5CH2Br\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br}

occurs more readily than the

SN2\mathrm{S}_{\mathrm{N}} 2

reaction of

CH3CH2Br\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}

. In an

SN2\mathrm{S}_{\mathrm{N}} 2

reaction, the rate depends on the nucleophile as well as the substrate. Specifically,

SN2\mathrm{S}_{\mathrm{N}} 2

reactions proceed through a backside attack mechanism, leading to the formation of a transition state where the nucleophile and the leaving group are simultaneously bonded to the carbon atom.

The benzyl group

C6H5CH2−\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2-

is more stable for

SN2\mathrm{S}_{\mathrm{N}} 2

reactions compared to the ethyl group

CH3CH2−\mathrm{CH}_3 \mathrm{CH}_2-

because the benzyl carbon's partial positive charge in the transition state is stabilized by the resonance of the phenyl ring.

Reason (R) Analysis: Reason (R) explains that the partially bonded unhybridized p-orbital that develops in the trigonal bipyramidal transition state is stabilized by conjugation with the phenyl ring.

This means that the presence of the phenyl ring can delocalize and stabilize the charge in the transition state, facilitating the

SN2\mathrm{S}_{\mathrm{N}} 2

reaction. Combining Assertion (A) and Reason (R): Given that the phenyl ring in

C6H5CH2Br\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br}

can indeed stabilize the transition state through conjugation more effectively than an ethyl group, both the Assertion (A) and Reason (R) are correct.

Furthermore, Reason (R) provides a valid explanation for Assertion (A), reinforcing why the

SN2\mathrm{S}_{\mathrm{N}} 2

reaction occurs more readily for

C6H5CH2Br\mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{Br}

. Thus, the most appropriate answer is: Option B: Both (A) and (R) are correct and (R) is the correct explanation of (A)

Q33
The organic chloro compound, which shows complete stereochemical inversion during a SN2 reaction, is
A (C2H5)2CHC
B (CH3)3CCl
C (CH3)2CHCl
D CH3Cl
Correct Answer
Option D
Solution
SN2{S_N}2

reaction is favoured by small groups on the carbon atom attached to halogen. So, the order of reactivity is

CH3Cl>(CH3)2CHCl>C{H_3}Cl > {\left( {C{H_3}} \right)_2}CHCl >
              (CH3)3CCl>(C2H5)2CHCl\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\left( {C{H_3}} \right)_3}CCl > {\left( {C{}_2{H_5}} \right)_2}CHCl

NOTE :

SN2{S_N}2

reaction is shown to maximum extent by primary halides. The only primary halides given is

CH3Cl,C{H_3}Cl,

so the correct answer is

(d).(d).
Q34
An 'Assertion' and a 'Reason' are given below. Choose the correct answer from the following options : Assertion (A) : Vinyl halides do not undergo nucleophilic substitution easily. Reason (R) : Even though the intermediate carbocation is stabilized by loosely held p-electrons, the cleavage is difficult because of strong bonding.
A Both (A) and (R) are correct statements but (R) is not the correct explanation of (A)
B Both (A) and (R) are correct statements and (R) is the correct explanation of (A)
C (A) is correct statement but (R) is a wrong statement
D Both (A) and (R) are wrong statements.
Correct Answer
Option C
Solution

Due to resonance C - Cl bond strength increses, that is why C - Cl bond do not break easily and don't show nucleophilic substitution reaction easily.

Q35
The correct pair(s) of the ambident nucleophiles is(are) (A) AgCN/KCN (B) RCOOAg/RCOOK (C) AgNO2/KNO2 (D) AgI/KI
A (B) and (C) only
B (B) only
C (A) and (C) only
D (A) only
Correct Answer
Option C
Solution

Ambident ligands are those which have two donor sites but at once it has the ability to donate either site.

Hence in AgCN/KCN, AgNO2/KNO2, CN-, NO2- have two donar sites act as ambident Nu-.

Q36
Elimination of bromine from 2-bromobutane results in the formation of-
A equimolar mixture of 1 and 2-butene
B predominantly 2-butene
C predominantly 1-butene
D predominantly 2-butyne
Correct Answer
Option B
Solution

The formation of

22

-butene is in accordance to Saytzeff's rule. The more substituted alkene is formed.

Q37
Bottles containing C6H5l and C6H5CH2I lost their original labels. They were labelled A and B for testing A and B were separately taken in test tubes and boiled with NaOH solution. The end solution in each tube was made acidic with dilute HNO3 and then some AgNO3 solution was added. Substance B gave a yellow precipitate. Which one of the following statements is true for this experiment?
A A and C6H5CH2I
B B and C6H5I
C Addition of HNO3 was unnecessary
D A was C6H5I
Correct Answer
Option D
Solution
C6H5I  ⟶NaOH  C6H5ONa  ⟶HNO3/H+{C_6}{H_5}I\,\,\overset{{NaOH}}\longrightarrow \,\,{C_6}{H_5}ONa\,\,\overset{{HN{O_3}/{H^ + }}}\longrightarrow
C6H5OH  ⟶AgNO3  {C_6}{H_5}OH\,\,\overset{{AgN{O_3}}}\longrightarrow \,\,

No yellow ppt.

C6H5CH2I  ⟶NaOH  C6H5CH2ONa  ⟶HNO3/H+{C_6}{H_5}C{H_2}I\,\,\overset{{NaOH}}\longrightarrow \,\,{C_6}{H_5}C{H_2}ONa\,\,\overset{{HN{O_3}/{H^ + }}}\longrightarrow
C6H5CH2OH  ⟶AgNO3.{C_6}{H_5}C{H_2}OH\,\,\overset{{AgN{O_3}}}\longrightarrow .
   \,\,\,

yellow ppt. Since benzyl iodide gives yellow ppt. hence this is compound

BB

and

AA

was phenyl iodide

(C6H5I).\left( {{C_6}{H_5}I} \right).
Q38
Reaction of trans-2-phenyl-1-bromocyclopentane on reaction with alcoholic KOH produces
A 4-phenylcyclopentene
B 2-phenylcyclopentene
C 1-phenylcyclopentene
D 3-phenylcyclopentene
Correct Answer
Option D
Solution

Hughes and Ingold proposed that bimolecular elimination reactions take place when the two groups to be eliminated are trans and lie in one plane with the two carbon atoms to which they are attached i.e.

E2 reactions are stereoselectively trans.

Q39
Given below are two statements: Statement - I: High concentration of strong nucleophilic reagent with secondary alkyl halides which do not have bulky substituents will follow SN2\mathrm{S}_{\mathrm{N}}{ }^2 mechanism. Statement - II: A secondary alkyl halide when treated with a large excess of ethanol follows SN1\mathrm{S}_{\mathrm{N}}{ }^1 mechanism. In the light of the above statements, choose the most appropriate from the options given below:
A Both Statement I and Statement II are true.
B Statement I is true but Statement II is false.
C Both Statement I and Statement II are false.
D Statement I is false but Statement II is true.
Correct Answer
Option A
Solution

Statement - I: Rate of

SN2∝[R−X][Nu−]\mathrm{S}_{\mathrm{N}} 2 \propto[\mathrm{R}-\mathrm{X}]\left[\mathrm{Nu}^{-}\right]
SN2\mathrm{S}_{\mathrm{N}} 2

reaction is favoured by high concentration of nucleophile

(Nu−)(\mathrm{Nu}^{-})

& less crowding in the substrate molecule. Statement - II: Solvolysis follows

SN1\mathrm{S}_{\mathrm{N}} 1

path. Both are correct Statements.

Q40
Which of the following is the correct order of decreasing SN2 reactivity?
A RCH2X > R3CX > R2CHX
B RCH2X > R2CHX > R3CX
C R3CX > R2CHX > RCH2X
D R2CHX > R3CX > RCH2X
Correct Answer
Option B
Solution

In

SN2{S_N}2

mechanism transition state is pentavelent.

For bulky alkyl group it will have sterical hinderance and smaller alkyl group will favour the

SN2{S_N}2

mechanism. So the decreasing order of reactivity of alkyl halides is

RCH2X>R2CHX>R3CXRC{H_2}X > {R_2}CHX > {R_3}CX
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