Hydrocarbons Test

Result

Hydrocarbons Test - 24

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Weekly Quiz Competition

Question 1
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Direction (Q. Nos. 1 - 8) This section contains 8 multiple choice questions. Each question has four choices (a), (b), (c) and (d), out of which ONLY ONE option is correct.
Q. How many distinct alkynes exist for C₆H₁₀which gives effervescence on heating with Na?

A
5

B
2

C
3

D
4

Solution

Hence D
Question 2
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Which of the following improperly describes the physical property of an alkyne?

A
Relatively non-polar

B
Insoluble in alcohols and ethers

C
Less dense than water

D
Nearly insoluble in water

Solution

Alkynes are generally nonpolar molecules with little solubility in polar solvents, such as water. Solubility in nonpolar solvents, such as ether and acetone, is extensive. Like the alkanes and alkenes, alkynes of four or fewer carbon atoms tend to be gases.
Question 3
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What two atomic or hybrid orbitals overlap to form the carbon-carbon σ (sigma) bond in ethyne?

A
sp³-sp³

B
s-s

C
sp-sp

D
sp-p

Solution

The correct answer is option C
The chemical bonding in acetylene (ethyne) (C₂H₂) consists of sp-sp overlap between the two carbon atoms forming a sigma bond, as well as two additional pi bonds formed by p-p overlap. Each carbon also bonds to hydrogen in a sigma s-sp overlap at 180 degree angles.
Question 4
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Which of the following statements correctly describes the general reactivity of alkynes?

A
Alkynes are more reactive than alkenes

B
Alkynes reacts as an electrophile

C
Unlike alkenes, alkynes do not undergo electrophilic addition reaction

D
An alkene is electron rich molecule, therefore react as a nucleophile

Solution

The correct answer is option D
The clouds of electrons surrounding the sigma bond makes an alkyne an electron-rich molecule. They are therefore nucleophiles that react with electrophiles. Thus alkynes, like alkenes, undergo electrophilic addition reactions because of their weak pi bonds. When a proton adds to an alkyne, a vinylic cation is formed.
Question 5
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How many different isomeric alkynes on catalytic hydrogenation gives the same 3-ethyl hexane?

A
2

B
3

C
4

D
6
Question 6
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An organic compound X (C₆H₁₃Br) is optically active. X on treatment with (CH₃)₃COK in (CH₃)₃COH gives Y (C₆H₁₂), a major product. Y on treatment with Br₂— CCI₄ in the presence of FeBr₃ gives a dibromide which on further treatment with NaNH₂ gives C₆H₁₀ which is still optically active. Hence, X and Y respectively are

A

B

C

D
Question 7
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When 2,3-dibromobutane is treated with KOH in ethanol, 2-bromo-2-butene is formed which does not undergo further dehydrobromination to form 2-butyne under similar condition because

A
H and Br are not trans

B
there is no further B— H available

C
vinyl bromide are very less reactive in E2 elimination reaction

D
reactant is more stable than product

Solution

Because vinyl bromide is less reactive and Alc. KoH is not that strong a base; we need a strong base like NaNH₂ .
Question 8
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When vicinal dibromide is heated with KOH in ethanol (~ 200°C), double dehydrohalogenation takes place giving alkyne. Which of the following fails to give alkyne according to this procedure?

A

B

C

D

Solution

The correct answer is option C
CH³)²BrC*–CHBr–CH²–CH³
C* has no H attached to it hence the next Bromine cannot perform dehydrohalogenation using this Carbon. Therefore, triple bond formation is not possible in this compound.
Question 9
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Direction (Q. Nos. 9 - 12) This section is based on Statement I and Statement II. Select the correct answer from the codes given below.
Q.
Statement I : When 1-butyne is treated with NaNH₂, sodium salt is formed.
Statement II : NH₃ is w eaker acid than .

A
Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

B
Both Statement I and Statement II are correct and Statement II is not the correct explanation of Statement

C
Statement I is correct but Statement II is incorrect

D
Statement II is incorrect but Statement I incorrect
Question 10
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Statement I : Treatment of either 1,2-dibromobutane or 2,2-dibromobutane with NaNH₂ gives the same 1-butyne.
Statement II : NaNH₂ forms salt with terminal alkyne which makes possible the above observation.

A
Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

B
Both Statement I and Statement II are correct and Statement II is not the correct explanation of Statement I

C
Statement I is correct but Statement II is incorrect

D
Statement II is incorrect but Statement I incorrect
Question 11
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Statement I : Propyl lithium on reaction with 1-bromo-1-pentyne gives 4-octyne.
Statement II : Propyl lithium acts as a very strong nucleophile, brings about SN₂ reaction of halides.

A
Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

B
Both Statement I and Statement II are correct and Statement II is not the correct explanation of Statement I

C
Statement I is correct but Statement II is incorrect

D
Statement II is incorrect but Statement I incorrect
Question 12
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Statement l : C₆H₅MgBr with propyne gives 1-phenyl propyne.

A
Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

B
Both Statement I and Statement II are correct and Statement II is not the correct explanation of Statement I

C
Statement I is correct but Statement II is incorrect

D
Statement II is incorrect but Statement I incorrect