Haloalkanes & Haloarenes Test

Result

Haloalkanes & Haloarenes Test - 8

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Weekly Quiz Competition

Question 1
1 / -0.25

The atom which defines the structure of a family of organic compounds and their properties is called ___________

A
Resonating structure

B
Homologous structure

C
Functional group

D
Nucleophile

Solution

Explanation: The atom which defines the structure of a family of organic compounds and their properties is called a functional group. Functional groups are specific groups of atoms or bonds within molecules that are responsible for the characteristic chemical reactions of those molecules.
Question 2
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In S_N 1 reaction, racemisation occurs if the reaction occurs at a stereogenic centre, however, 50:50 mixture of enantiomers are rarely obtained, why?

A
Usually one enantiomer is more stable than other

B
Retention of configuration is always favoured in reaction

C
Hydroxylic solvent favour retention of configuration

D
There is steric hindrance to the approach of nucleophile from the front side as some of the leaving group are not departed completely which favour inversion of configuration

Solution

In S_N 1 reaction, some of the substrate, even after ionisation, remains associated as an intimate ion pair preventing nucleophilic attacks from front side and gives inverted product.

Due to partial reaction of nucleophile with intimate ion pair, net inversion of configuration is observed although predominant reaction occurs at planar carbocation giving racemic products.
Question 3
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Pick out the most reactive alkyl halide for an S_N 1 reaction.

A

B

C

D

Solution

Tertiary halide would be most reactive in S_N 1 reaction.
Question 4
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Pick out the compound which reacts fastest in the presence of AgNO₃ .

A
(CH₃ )₃ CCI

B
(CH₃ )₂ CHCH₂ CI

C
(CH₃ )₂ CHCI

D
CH₃ CH₂ CI

Solution

Tertiary halide would react at fastest rate with AgNO₃ as reaction will proceed by S_N 1 mechanism.
Question 5
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From the following, pick out the explanation for why molecule Y hydrolyses faster than molecule Z.

A
Less steric hindrance

B
Neighbouring group participation

C
Greater carbocation stability

D
Better solvent

Solution

Y forms a tertiary carbocation which rearranges to a further more stable benzylic carbocation.
Question 6
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Which of the following statement(s) is/are true for S_N 1 reaction?
I. The rate of S_N 1 reaction depends on concentration of alkyl halide
II. The rate of S_N 1 reaction depends on concentration of nucleophile
III. S_N 1 reactions of alkyl halides are favoured by non-polar solvents

A
Only I

B
Only II

C
Only III

D
Both I and III

Solution

Rate of S_N 1 reaction is directly proportional to concentration of alkyl halide but independent of concentration of nucleophile. Non-polar solvents play no role in S_N 1 reaction.
Question 7
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From the following, pick out the potential energy profile for a S_N 1 reaction.

A

B

C

D

Solution

S_N 1 reaction proceeds via carbocation intermediate which is being indicated here by two transition states and a small trough between reactant and product. Also the second transition state must be below first transition state in S_N 1 reaction
Question 8
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The most and least reactive electrophiles respectively in a S_N 1 reaction are

A
Both I and.II

B
Both II and III

C
Both I and III

D
Both III and IV

Solution

Electron donating —CH₃ gro up stabilises benzylic carbocation, hence increases reactivity of corresponding substrate. Electron withdrawing —NO₂ decreases stability of carbocation, decreases reactivity of corresponding substrate in S_N 1 reaction
Question 9
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Pick out the strongest substrate(s) for a S_N 1 reaction.

A
Only I

B
Only II

C
Only III

D
Only IV

Solution

It forms a secondary carbocation which, by hydride shift, rearranges to a tertiary carbocation.
Question 10
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Pick out the following factor(s) which promote a S_N 1 reaction :
I. Temperature
II. Concentration of nucleophile
III. Concentration of alkyl halide
IV. Aprotic solvents

A
Both I and II

B
Both II and III

C
Both III and IV

D
Both I and III

Solution

Increasing temperature increases rate of any reaction. Also, rate of S_N 1 reaction is directly proportional to concentration of alkyl halide but independent of nucleophile and aprotic solvent.