Haloalkanes and Haloarenes Test

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Haloalkanes and Haloarenes Test - 65

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Weekly Quiz Competition

Question 1
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Which of the following cannot undergo nucleophilic substitution under ordinary conditions?

A
Chlorobenzene

B
Tert-butyl chloride

C
Iso-propyl chloride

D
None of the above

Solution

Chlorobenzene undergoes nucleophilic substitution under a drastic condition because due to resonance C-Cl bond acquire some double-bond character as a result substitution of -Cl by nucleophile becomes difficult ordinary condition.
Question 2
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The IUPAC name of the given compound is:

A
$$1-bromo-1'-chloro-4,\,4'-bicyclobutane$$

B
$$4-(4'-chlorocyclobutyl)-1-bromocyclobutane$$

C
$$3-bromo-3'-chloro-1,1'-bicyclobutane$$

D
$$4-(4'-bromocyclobutyl)-1-chloro cyclobutane$$
Question 3
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In an $$S_N1$$ reaction on chiral centres, there is:

A
$$100$$% retention

B
$$100$$% inversion

C
$$100$$% racemization

D
Inversion more than retention leading to initial racemization

Solution

In an $$S_N1$$ reaction on chiral centres, there is inversion more than retention leading to initial racemization.

Thus if we start with a pure enantiomer and carry out $$S_N1$$ substitution on chiral carbon, the product will be racemic. This is because a planar carbocation intermediate is obtained. The nucleophile can attack from either side of this intermediate.

Option D is correct.
Question 4
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Directions For Questions

Alkyl halides and alcohols easily undergo nucleophilic substitution either through $$S_N1$$ or $$S_N2$$ mechanism. The relative case of these two processes depends upon the nature of the substrate (alkyl group) as well as leaving group, nature of nucleophile and also upon the nature of the solvent.
$$S_N1$$ mechanism involves the formation of carbocation as intermediate while $$S_N2$$ mechanism involves the formation of a transition pentavalent state. $$S_N1$$ is the main mechanism in $$3^0$$ alkyl halides and alcohols, while $$S_N2$$ mechanism is the path adopted by most of the $$1^o$$ alkly halides; $$2^o$$ alkyl halides may follow $$S_N1$$ as well as $$S_N2$$.

...view full instructions

Which of the following solvent will give maximum yield for an alkyl halide undergoing $$S_N1$$ mechanism?

A
Water

B
Ethanol

C
Diethyl ether

D
n-hexane

Solution

The $$S_N1$$ path is strongly favoured by the presence of polarising solvents which stabilizes both cation and anion effectively.

More polar is a solvent, easier is $$S_N1$$ path. The polarity of the given solvents is water > ethanol > ethoxyethane > n-hexane.

Option A is correct.
Question 5
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Which of the following is the best method for synthesis of 1-bromo-3-chlorobenzene?

A

B

C

D

Solution

As, $$Br$$ and $$Cl$$ are o,p directing, they cannot be used directly for chlorination of bromobenzene. As $$NO_2$$ is meta directing firstly $$Cl$$ is placed over 2,4 position of nitrobenzene, then reduction is done using $$Zn/HCl$$ to produce, 1,3 dichlorobenzene. In the last step one $$Cl$$ is replaced by $$Br$$ using $$Cu_2Br_2$$
Question 6
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In the above reactions, which of the following is/ are products?

A
$$1,2$$- Dichlorobenzene

B
$$1,3$$- Dichlorobenzene

C
$$1,4$$- Dichlorobenzene

D
Both A and C

Solution

$$\begin{array}{l}\text { Cl}^{+}\text{ electrophile is formed during reaction of FeCl }_{3} \text { and } \mathrm{Cl}_{2} \text { . }\\\text { Now, in chlorobenzene chlorine increases the } \\\text { eleetron density at ortho and para position through } \\\text { resonance (as chlorine is ortho-para directing group) }\\\text{Hence, option D is correct.}\end{array}$$
Question 7
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An organic compound which produces a bluish-green coloured flame on heating in presence of copper is:

A
chlorobenzene

B
benzaldehyde

C
anlline

D
benzoic acid

Solution

An organic compound which produces a bluish green flame when heated on a copper wire is chlorobenzene. This is called Beilstein's test and this test is used to defect the presence of halogen in an organic compound. Green colour of the flame is due to formation of copper halide..
Question 8
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On treating a mixture of two alkyl halides with sodium metal in dry ether, $$2-$$ methyl propane was obtained. The alkyl halides are:

A
$$2$$- chloropropane and chloromethane

B
$$2$$- chloropropane and chloroethane

C
chloromethane and chloroethane

D
chloromethane and $$1$$- chloropropane

Solution

Hence, option A is correct.
Question 9
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Haloarenes react with halogens in the presence of ferric salt as catalyst to give ortho and para dichloro benzene. Which of the following products predominates in ortho and para isomers in this case?

A
Ortho-dichlorobenzene

B
Para-dichlorobenzene

C
Meta-dichlorobenzene

D
A and C both

Solution

Haloarenes react with halogens in the presence of ferric salt as catalyst to give ortho and para dichloro benzene.

Out of ortho and para positions para isomer predominates on ortho isomer due to steric factor.

Option B is correct.
Question 10
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In the reaction $$C_{7}H_{8}\xrightarrow {3Cl_{2}/\Delta}X\xrightarrow {Br_{2}/Fe} Y \xrightarrow {Zn/HCl}Z$$.

The compound $$Z$$ is :

A
o-bromotoluene

B
m-bromotoluene

C
p-bromotoluene

D
3-bromo-2, 2, 6-trichlorotoluene

Solution

$$Z$$ is m-bromotoluene.

Toluene is heated with chlorine to form (trichloromethyl)benzene. The aromatic nucleus is then brominated with bromine in presence of iron to form 1-bromo-3-(trichloromethyl)benzene.

Treatment with $$Zn/HCl$$ gives m-bromotoluene (compound $$Z$$).

So, option B is correct.