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Test: Alkynes Preparation & General Properties - NEET MCQ


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Test: Alkynes Preparation & General Properties - Question 1

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 C6H10 which gives effervescence on heating with Na?

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 1

Hence D

Test: Alkynes Preparation & General Properties - Question 2

Which of the following improperly describes the physical property of an alkyne?

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 2

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.

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Test: Alkynes Preparation & General Properties - Question 3

What two atomic or hybrid orbitals overlap to form the carbon-carbon σ (sigma) bond in ethyne?

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 3

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.

Test: Alkynes Preparation & General Properties - Question 4

Which of the following statements correctly describes the general reactivity of alkynes?

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 4

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.
 

Test: Alkynes Preparation & General Properties - Question 5

How many different isomeric alkynes on catalytic hydrogenation gives the same 3-ethyl hexane?

Test: Alkynes Preparation & General Properties - Question 6

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

Test: Alkynes Preparation & General Properties - Question 7

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

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 7


Because vinyl bromide is less reactive and Alc. KoH is not that strong a base; we need a strong base like NaNH2 .

Test: Alkynes Preparation & General Properties - Question 8

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?

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 8

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.

*Multiple options can be correct
Test: Alkynes Preparation & General Properties - Question 9

Direction (Q. Nos. 9 -12) This section contains 4 multiple choice questions. Each question has four choices (a), (b), (c) and (d), out of which ONE or MORE THAN ONE are correct.

Q. In which of the following reaction(s), an alkyne product is formed?

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 9

The correct answer are options B & C
(A) An alkyne would be formed if both the bromines are on adjacent carbons.
(B) CaC₂ + H₂O → C₂H₂ + Ca(OH)₂
(C) Mg₂C₃ + H₂O → C₃H₄ + Mg(OH)₂
(D) Al₄C₃ + H₂O → CH₄ + Al(OH)₃
Therefore alkynes are produced only in (B) and (C).

*Multiple options can be correct
Test: Alkynes Preparation & General Properties - Question 10

Which of the following reagents gives an effervescence when reacted with 1-butyne?

*Multiple options can be correct
Test: Alkynes Preparation & General Properties - Question 11

Which of the following reagent(s) can be used to distinguish between 1-hexyne and 2-hexyne?

Detailed Solution for Test: Alkynes Preparation & General Properties - Question 11

The correct answers are Options A and B. 
AgNO3 is called Tollen's reagent. 
It reacts with terminal alkynes (1-hexyne) but not with alkenes (1-hexene).
           
2CH≡CCH2CH2CH3 + 2AgNO3 + 2NH4OH——› 
           1-hexyne
2CH≡CCH2CH2CH3Ag + 2NH4NO3 + 2H2O

Terminal alkynes (hex-1-yne) react with ammoniacal 
Cu2Cl2 solution. Tollens reagent, etc.

*Multiple options can be correct
Test: Alkynes Preparation & General Properties - Question 12

Terminal alkyne being therm odynamically less stable than an internal alkyne, instead when 2,2-dibromo butane is treated with NaNH2,1-butyne is formed as major product, not 2-butyne because

Test: Alkynes Preparation & General Properties - Question 13

Direction (Q. Nos. 13 - 16) 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 .

Test: Alkynes Preparation & General Properties - Question 14

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.

Test: Alkynes Preparation & General Properties - Question 15

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.

Test: Alkynes Preparation & General Properties - Question 16

Statement l : C6H5MgBr with propyne gives 1-phenyl propyne.

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