Haloalkanes and Haloarenes Test

Result

Haloalkanes and Haloarenes Test - 31

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Weekly Quiz Competition

Question 1
1 / -0

Which of the given pair of $${ S }_{ N }1$$ reaction would you expect to proceed faster? Explain your answer.

A
$${3}^{o}$$-alcohol > $${2}^{o}$$-alcohol

B
$${3}^{o}$$-alcohol < $${2}^{o}$$-alcohol

C
$${3}^{o}$$-alcohol = $${2}^{o}$$-alcohol

D
None of these

Solution

Option A is the correct answer as tertiary alcohol reacts faster than secondary alcohol which in turn reacts faster than primary alcohol towards $$S_N1$$ reaction.
Question 2
1 / -0

Which of the following chlorides is highly reactive in $${{ S }_{ N }}1$$ reaction, even solvolysed in cold water?

A

B

C

D
None of the above

Solution

Chlorocycloheptane is more reactive in $$S_{N}1$$ reaction.

As water is a polar protic solvent which has more number of highly polarised hydrogen atoms which react with hydrogen attached to an electronegative atom resulting in cleavage of $$Cl$$.

As for size increases, the basicity decreases and nucleophilicity increases which makes the reaction to occur more reactively.

The correct option: A
Question 3
1 / -0

The alkyl halides required to prepare $$(CH_3)_2CH-CH_2-CH_2-CH_3$$ by Wurtz reaction are:

A

B

C

D
Question 4
1 / -0

Give the decreasing order of $$S_N1$$ reactions of the following compounds in $$H_2O$$.

A
(II) > (I) > (III)

B
(III) > (I) > (II)

C
(II) > (III) > (I)

D
(III) > (II) > (I)

Solution

Reactivity towards $$S_N1$$ reaction mechanism $$\propto$$ stability of carbocation.
The stability order of carbocation is:
(II)> (III)> (I)
Explanation:
(II) structure has more no. of hyperconjugating hydrogen atom than (III) structure.
(I) structure is least stable because there is no Inductive effect.
Hence, the correct option is C.
Question 5
1 / -0

Give the decreasing order of $$S_N1$$ reactions of the following compounds in $$H_2O$$.

A
1>2>3

B
2>3>1

C
3>2>1

D
2>1>3
Question 6
1 / -0

Assertion: $$E1cB$$ reaction is favoured by stabilisation of carbanion and poor leaving group
Reason: The reaction is kinetically of the second-order and unimolecular

A
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

B
Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

C
Assertion is correct but Reason is incorrect

D
Assertion is incorrect but Reason is correct

E
Both Assertion and Reason are incorrect

Solution

$$E1cB$$ reaction is a second-order and unimolecular reaction. The reaction proceeds in two steps. In the first step, the base abstracts an acidic hydrogen atom from the beta carbon atom to form the carbanion transition state, which is stabilized by resonance. In the second step, the lone pair of electrons on the anion moves to the neighbouring atom forming a pi bond with elimination of leaving group. The rate law for this reaction can be given as
$$Rate\quad =k\left[ concentration\ of\ substrate \right] \left[ concentration\ of\ base \right] $$
Hence, greater the stability of carbanion intermediate faster will be reaction. Hence, both assertion and reason are correct.
Question 7
1 / -0

Product $$(C)$$ is:

A

B

C

D
None of the above

Solution

Bromobenzene reacts with methyl bromide in presence of anhydrous aluminium chloride to form a mixture of o-bromotoluene and p-bromotoluene. Aryl halides undergo electrophilic aromatic substitution reaction in the ortho and para positions. Thus, halogens are ortho-para directing groups due to +M and + E effects. Halogens are deactivating due to strong -I effect.
Question 8
1 / -0

Assertion: $$1^o$$ allylic halides are more reactive than $$1^oRX$$ in $$S_N1$$ reaction
Reason: Allylic carbocation intermediate is stabilised by resonance

A
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

B
Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

C
Assertion is correct but Reason is incorrect

D
Assertion is incorrect but Reason is correct

E
Both Assertion and Reason are incorrect

Solution

Reactivity of $$S_N1$$ reaction is compared by the stability of its carbocation intermediate.
$$1^o$$ allylic halides are more reactive than $$1^o$$$$RX $$ in $$S_N1$$ reaction due to the formation of the allylic carbocation.
Allylic carbocation intermediate is stabilised by resonance.
Hence correct option is A
Question 9
1 / -0

Assertion : Rate of ethanolysis of $$1^o$$ halide by $$S_N1$$ mechanism is fast.
Reason : Carbocation is stabilised by resonance.

A
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

B
Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

C
Assertion is correct but Reason is incorrect

D
Assertion is incorrect but Reason is correct

E
Both Assertion and Reason are incorrect

Solution

The ethanolysis of primary alkyl halide generally proceeds
via $$S_N2$$ mechanism. In case it proceeds via $$S_N1$$ mechanism, the intermediate
carbocation is unstable, since it has no adjacent carbon atom for resonance.
Hence, both assertion and reason are incorrect.
Question 10
1 / -0

Which of the following aromatic compound is least reactive towards electrophilic substitutions?

A

B

C

D

Solution

Pyridine is least reactive towards ES because pyridine is resonance stabilised as shown in first figure.
Because of withdrawal of $${ e }^-$$'s from the ring by $$N$$ atom, the ring is deactivated, thereby resembling the benzene ring in nitrobenzene. So, it is least reactive towards ES reaction.
$$(b)$$ and $$(d)$$ are resonance structures so ES reaction takes place easily.
ES reaction in pyrrole, thiophene, and furan takes place at $$C-2$$ or $$C-5$$ positions because there are three resonating structures when $${E}^{\oplus }$$ attacks at $$C-2$$ or $$C-5$$ and two resonating structures when $${E}^{\oplus }$$ attacks at $$C-3$$.