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Chemical Properties of Alkynes MCQ - Practice Questions with Answers

Edited By admin | Updated on Sep 25, 2023 25:24 PM | #NEET

Quick Facts

Hydration, Hydroboration and Oxidation of Alkynes is considered one the most difficult concept.
Hydrohalogenation and Halogenation of Alkynes is considered one of the most asked concept.
15 Questions around this concept.

Solve by difficulty

Predict the correct intermediate and product in the following recation.

$H_{3}C-C\equiv CH \underset {HgSO_{4}}{\overset {H_{2}O, H_{2}SO_{4}}{\longrightarrow}} \:\:\:\underset {(A)}{Intermediate}\rightarrow \underset {(B)}{product}$

$A:H_{3}C-\underset {O}{\underset {||}{C}}=CH_{3}B:H_{3}C-C\equiv CH$

$A:H_{3}C-\underset {SO_{4}}{\underset {|}{C}}=CH_{2}B:H_{3}C-\underset {O}{\underset {|}{C}}-CH_{3}$

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Concepts Covered - 4

Hydrohalogenation and Halogenation of Alkynes

Hydrohalogenation
Addition of one molecule of halogen gives vinyl halide which then adds another molecule of hydrogen halide to form gem-dihalide. This addition follows Markownikoff's rule. The reaction occurs as follows:

For example:

Halogenation
Alkenes combine with gaseous chlorine or bromine in the dark to form di or tetrahalides. Here the addition is Anti Markownikoff's. The reaction occurs as follows:

For example:

Hydration, Hydroboration and Oxidation of Alkynes

Hydration
Alkynes cannot be hydrated more easily than alkenes because of their low reactivity towards electrophilic addition reactions. The reaction occurs as follows:

Mechanism

For example:

Hydroboration-Oxidation reaction
Alkynes react with BH₃ (in THF) and finally converted into carbonyl compounds. This method is useful for preparing aldehyde from terminal alkyne, which is otherwise not possible by hydration. The reaction occurs as follows:

For example:

Reaction with Carbonyls and Oxidative Coupling

Reaction with carbonyls
This reaction is very useful for the preparation of alcohols. In this reaction, a salt like NaNH₂ is used which produces a carbanion as follows:

$\mathrm{CH\equiv CH\: \overset{NaNH_{2}}{\longrightarrow}\: CH\equiv C^{-}}$

Now this carbanion reacts with the carbonyl group. Here, the carbanion binds with the carbonyl carbon and carbon-oxygen bond shifts to the oxygen giving it a negative charge. Now we use H₃O⁺ to bind with O^- and thus forms the OH or alcohol.

The complete mechanism is given below:

Reaction with HOCl and Polymerisation Reaction

Addition of Hypochlorous acid
Alkynes when passed into hypochlorous acid solution forms dichloroacetaldehyde. The reaction occurs as follows: