Chemical Properties of Alkynes

Alkynes are hydrocarbons with a minimum of one carbon-carbon triple bond. They form cornerstones in many chemical processes, from industrial synthesis to organic reactions. Their unique structure imparts distinct chemical properties, hence quite valuable both in a laboratory and an industrial setup. For example, the formation of an alkyne triple bond makes the molecule very reactive and gives quite a different type of structure from the more flexible alkanes and alkenes.

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This Story also Contains

1. Alkynes
2. Key Chemical Reactions of Alkynes
3. Hydration, Hydroboration and Oxidation of Alkynes
4. Reaction with Carbonyls and Oxidative Coupling
5. Reaction with HOCl and Polymerization Reaction
6. Relevance and Applications of Alkyne Chemistry
7. Some Solved Examples
8. Summary

In real life, pharmaceuticals, agrochemicals, and the creation of advanced materials include alkynes in their synthesis. That is the power of alkyne chemistry manipulation: letting the chemist construct molecules with precision and tune their properties for desired applications. This article will present the exciting chemical properties of alkynes, including hydration, hydroboration, oxidation, reaction with carbonyls, oxidative coupling, reactions with HOCl and polymerization reactions. By the end, you will be deep inside how alkynes behave and their importance in real-world applications.

Alkynes

Alkynes are hydrocarbons with a carbon-carbon triple bond. This feature makes them rather stand aloof in properties from alkanes and alkenes. The simplest member of this family is acetylene, ${\mathrm{C}}_2{\mathrm{H}}_2$which is an alkyne that has extremely wide applications in welding and cutting of metals owing to its very high temperature of combustion. The geometry is linear because of the triple bond, with considerable electron density, which makes alkynes rather reactive. This reactivity may be harvested in a number of chemical reactions that use alkynes as a valuable intermediate in organic synthesis. Understanding basic properties, such as the acidity of alkynes due to sp hybridized carbon, sets the stage for the whole richness of their reactions.

Key Chemical Reactions of Alkynes

Hydration, Hydroboration and Oxidation of Alkynes

Note: Hydration of alkynes follows the same mechanism as alkenes with the slight difference that it is usually induced through mercuric sulfate in acid conditions to yield ketones. Hydroboration adds borane on the triple bond, hence CIS-addition, resulting in CIS-alkenes, which might be oxidized into aldehydes or ketones. Some of the oxidation reactions, especially those involving potassium permanganate, are known to cleave the triple bond into carboxylic acids.

Hydrohalogenation
The 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
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 are 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

Alkynes react with carbonyl compounds in the presence of strong bases to form alkynyl alcohols, a critical step in many organic syntheses. Alkyne molecules may be coupled by oxidative coupling—facilitated by catalysts like copper salts—to a conjugated diyne, of importance in materials science and synthetic chemistry.

Reaction with carbonyls
This reaction is very useful for the preparation of alcohol. In this reaction, a salt like${\mathrm{NaNH}}_2$ is used which produces a carbanion as follows:

$\mathrm{CH}\equiv \mathrm{CH}\stackrel{{\mathrm{NaNH}}_2}{\to }\mathrm{CH}\equiv \mathrm{C}$

Now this carbanion reacts with the carbonyl group. Here, the carbanion binds with the carbonyl carbon, and the carbon-oxygen bond shifts to the oxygen giving it a negative charge. Now we use ${\mathrm{H}}_3{\mathrm{O}}^{+}$ to bind with O^- and thus form the OH or alcohol.

The complete mechanism is given below:

Reaction with HOCl and Polymerization Reaction

The reaction of alkynes with hypochlorous acid HOCl results in the addition of chlorine and a hydroxyl group across the triple bond to form chlorohydrins. Polymerization reactions of alkynes, particularly in the presence of special catalysts like Ziegler-Natta, can form polyacetylenes with special electronic properties for conductive polymers and advanced materials applications.

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

For example:

Polymerization
Alkynes polymerize to give the following compounds. The reactions occur as follows:

Relevance and Applications of Alkyne Chemistry

The chemical properties of alkynes thus have huge implications for industry and academics. It is in this area of chemistry that the synthesizing of complex molecules from simpler alkyne precursors, very useful in pharmaceuticals, agrochemicals, and materials science, immensely helps the industry. Hydration of alkynes into ketones and aldehydes lies central to the synthesis of many drugs and fine chemicals. Within the academic circle, alkynes are important in understanding the mechanisms of reactions and in developing new synthetic methodologies. Their unique reactivity helps construct complex molecular architecture for a chemist and therefore paves the way for further improvements in medicinal chemistry and nanotechnology. In addition, polymerization of alkynes has been used in building new materials applied in electronics, namely OLEDs (organic light-emitting diodes) and solar cells.

Recommended topic video on (Chemical Properties of Alkynes)

Some Solved Examples

Example 1
Question: Which of these will not react with acetylene?

1. NaOH
2. AmmoniacalAgNO:
3. Na
4. HCl

Solution:

Except for NaOH, acetylene reacts with the other three.

$$\begin{array}{r}\mathrm{CH}\equiv \mathrm{CH}\underset{\text{liq}{\mathrm{NH}}_3}{\overset{\mathrm{Na}}{\to }}\mathrm{CH}\equiv \mathrm{CNa}\\ \mathrm{CH}\equiv \mathrm{CH}\stackrel{+\mathrm{HCl}}{\to }{\mathrm{CH}}_2=\mathrm{CHCl}\stackrel{+\mathrm{HCl}}{\to }{\mathrm{CH}}_3-{\mathrm{CHCl}}_2\\ \mathrm{CH}\equiv \mathrm{CH}\underset{+{\mathrm{NH}}_4\mathrm{OH}}{\overset{+\mathrm{Ag}{\mathrm{NO}}_3}{\to }}\mathrm{AgC}\equiv \mathrm{CAg}+{\mathrm{NH}}_4{\mathrm{NO}}_3\end{array}$$

OH^- cannot extract a proton from acetylene because $\mathrm{HC}\equiv {\mathrm{C}}^{-}$ is a stronger base than OH^-. In fact, the hydrolysis of calcium carbide is used for the preparation of acetylene.

Hence, the answer is the option (1).

Example 2
Question:

The major product of the following reaction is :

${\mathrm{CH}}_3\mathrm{C}\equiv \mathrm{CH}\underset{(\text{ii)}DI}{\overset{\text{(i) DCl( equiv.})}{\to }}$

1) ${\mathrm{CH}}_3\mathrm{CD}(\mathrm{I})\mathrm{CHD}(\mathrm{Cl})$
2) ${\mathrm{CH}}_3\mathrm{CD}(\mathrm{Cl})\mathrm{CHD}(\mathrm{I})$
3) ${\mathrm{CH}}_3{\mathrm{CD}}_2\mathrm{CH}(\mathrm{Cl})(\mathrm{I})$
4) (correct) ${\mathrm{CH}}_3\mathrm{C}(\mathrm{I})(\mathrm{Cl}){\mathrm{CHD}}_2$

Solution:

Solution

As we have learned,

Addition of Hydrogen halide on alkyne -

The reaction takes place according to Markovnikov's rule. There is an addition of two equivalents of the acid and a geminal dihalide is obtained.

Now, the given reaction occurs as

Hence, the option number (4) is correct.

Example 3
Question: The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is:

1. (iv) < (i) < (ii) < (iii)
2. (i) < (ii) < (iii) < (iv) (correct)
3. (iv) < (i) < (iii) < (ii)
4. (iv) < (iii) < (ii) < (i)

Solution:

Reactivity $\propto -\mathrm{M}$ group present at $\mathrm{O}/\mathrm{P}$ position.

As the number of -M group increases the reactivity towards nucleophilic substitution reaction increases.

Correct order: (i) < (ii) < (iii) < (iv)

Summary

The carbon-carbon triple bond characterizes the alkynes and confers on them a rich array of chemical properties, which have applications in several reactions. Hydration, hydroboration, reactions with carbonyls, and polymerization all show the reactivity of alkynes underpinning their importance in both industrial and academic contexts. Alkynes are highly reactive, hence useful in organic synthesis, because of their straight chain and high electron density.